GTA
Support Equipment For PCP/HPA/CO2 and springers ,rams => 3D printing and files => Topic started by: subscriber on November 02, 2023, 03:36:10 AM
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One of the many aspects worthy of an organized study is the effect of pellet to LDC bore clearance on muffling effectiveness. There is a common belief that baffle bore size matters a great deal, and that making the bores 1 mm larger than necessary is unacceptably wasteful of suppression capability. The idea with this set of LDCs is to enable anyone with a printer to test this premise for themselves.
The three LDCs have a very generic baffle design, using 60 degree included angle cones. The OD is 1", and the overall length is just over 4". The STLs attached in the ZIP file are identical, except for the bore diameters. They should print with bores having 0.5 mm; 1 mm and 1.5 mm radial clearance with a 5.5 mm diameter pellet, respectively.
The actual bore diameters are expected to print at 6.5; 7.5 and 8.5 mm, respectively. All baffle bores are the same in any given LDC - there is no taper or step in diameter. The front end has the bore diameter embossed. Hopefully that will show well enough in the prints to distinguish them from each other.
These LDCs are intended for .22 caliber up to 30 FPE. The threads are 1/2-20, and should be usable off the printer.
I predict a spread of 4 dB, smallest to largest; for this set of LDCs. But I could be wrong...
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I forgot to mention; these mufflers need to be printed standing vertical on the printer platen, with the threads uppermost.
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I forgot to mention; these mufflers need to be printed standing vertical on the printer platen, with the threads uppermost.
.5 on the plate now 48mins lol.
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Mufflers are illegal in Canada, but I will be curious to see the results of dB testing....
Bob
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Hey there Sub! I am back (limited for now) and see your designs and ideas have progressed elegantly.
I am especially intrigued by how total printing a moderator now (rather than just an insert) is a more efficient use of resources as well as design optimization.
Would you have files using an arrangement that shuttles off the air blast mainly to one side, or even above/below the pellet path? I thought I saw something regarding that for a GK1, but cannot locate it now.
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Mufflers are illegal in Canada, but I will be curious to see the results of dB testing....
Bob
Good to know I can straight pipe my trucks up there. Lol...
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Thanks, Mike
Charlie, I will answer your questions later.
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Charlie,
If I interpret your fully printed question correctly, I think a printed insert that lives in a screw capped aluminum can is the best embodiment, for any given individual. The can is durable, and the insert can be swapped easily, quickly and cheaply to try other ideas.
However, unless a number of people have the same a standard can, they cannot all print and test the same set of inserts. It is here where the self-contained threaded "insert" comes in handy. Anyone with a printer and curiosity can participate.
The fragility of the self-contained insert would be a detraction for many. Yet, that fragility is seen as a plus by others.
On re-reading your other question, I realize that you are talking about the offset mufflers I designed for the Huben pistol. Those downward vents are internal - why I mention this will become apparent near the bottom of this reply.
Huben pistol offset mufflers:
https://www.gatewaytoairguns.org/GTA/index.php?topic=210109.0
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=210109.0;attach=440569;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=210109.0;attach=440571;image)
I misinterpreted your vent question earlier; and wrote this. Might be amusing:
I have air stripper designs that divert air sideways. In the context of shrouded PCPs and add-on mufflers I have railed relentlessly against side vents, because and leak path on the side that would reduce noise towards the front, will increase loudness to the side. Some people prefer to make the sound less directional, or quieter to the front; so that works for them.
If the holes in the side, rear, top or bottom of the shroud or muffler are so small and so few as to not be louder away from the muzzle, I doubt they can flow enough air to make the front end quieter. Perhaps such vents should tested in an organized manner; similar to the baffle bore diameters :)
If the rear or side vents blow into a space between two tubes that are closed to the outside, then such vents add value. But the second tube can add weight; if it is metal. So printing such a design might be an option.
I have done so many designs lately. Some of them are spur of the moment exercises, such as the micro-baffle "insert" I designed yesterday, while I was doing the ones for this thread - see image below.
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It is likely that the spread in sound reduction would be greater for longer "test tubes", than the 2 dB differences so far reported between the 4.2" long 6.5 and 8.5 mm bore "test tubes".
It was suggested that the typical length for a 1" OD .22 muffler was 6.5". So, I cut and spliced the 4+" ones to stretch them to 6.5".
If anyone wants to test this theory, the two STLs for the longer versions are attached in the same ZIP file. See image below.
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Thank you Sub! I will send on your replies to the guy who printed AUO inserts, appreciated.
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MY bore test.
OK 2 shots per bore test size. gun without moderation 97.2dB
8.5/ 95.7dB
7.5/96.7dB
6.6/96.2dB
I'll run another couple test with ammo fit for testing. namely apollo pellets.
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Thanks, Mike
You are not the only person to see what seems like a glitch with respect to bore size and measured report, as a function of diameter.
I hole to see more people participate.
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My results from a few days ago:
Bore Test 1"x4" 8.5mm bore 1.5mm radial clearance
-11.2 -9.0 -9.5 -7.6
ave -9.3
Bore Test 1"x4" 7.5mm 1.0mm radial clearance
-6.0 -4.4 -3.8 -6.5
ave -5.2
Bore Test 1"x4" 6.5mm 0.5mm radial clearance (clipped)
-8.0 -8.7 -8.5 -6.3
ave -7.9
My relative numbers are somewhat different, but quietest to loudest matches Mikes.
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Very interesting!
Thanks, Bob
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One thing is apparent, there's a wide variation between shots, statistically. To get a decent estimate of the true average value, more shots should be taken. The sample set should be larger, like more than 22, the larger the set the better. This presents it's own problems, but could be managed. Unfortunately, this is a tough ask of volunteers. I'd hazard a guess, that the numbers presented are interesting, but not statistically representative. I ran radar simulations for my job, and my takeaway was small data sets lead to biased and unreliable results. I needed thousands of trials to reduce bias errors. Ideally we'd like good mean values and reliable estimates of the standard deviation to evaluate LDC performance.
Actually, even better would be statistical performance over frequency as some LDC's sound better than others due to their spectral distributions. That would be a serious study and require a bit of work. It's not a casual effort.
I'm not discouraging your efforts as I think they're important. I do think that if you want meaningful results, one would need a prescribed procedure for the experiments and a whole lot more data taken. My two cents.
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Thanks for your input, Bruce
I agree that more data is better. So, more volunteers would be another way to see if they see the same trends. Making a test onerous and specifying how it must be done to be meaningful makes it more like work, so reducing the probability of unpaid volunteers. That said, I would be grateful for your participation at any level you see fit. Just don't saturate you meter so all readings are artificially limited to the same range.
Tone and quality of sound is important. A dull thud is often preferred to a sharp snap; even if the dull sound meters slightly higher.
To quote Paul Harrell; "not enough of a difference to make a difference" comes to mind. If one can't hear which is louder and the difference in meter readings is within measurement noise, then that suggests baffle bore clearance is not as important as some claim it is. Which is the purpose of this study: Not to claim one thing or another, but to test if there is a signal when only one aspect is varied. My opinion is that if baffle bore clearance is very important factor then it will show a signal that drowns out measurement noise. So far, we are not seeing that.
There is an explanation for why the three bore sizes may not reduce loudness in rank order. Some time ago I posted on GTA about a US patent for reducing concussion from field artillery. It worked by increasing the volume of gas that leaks by the shell, near the chamber throat, thus making the initial blast while the shell "uncorked" from the muzzle less of a square wave; essentially by reducing the rate of pressure change at the muzzle relative to the standing air near the muzzle, by means of pre-compressing the air in that region slightly using the controlled extra gas leakage. I think that more air leakage from our experimental mufflers may be working on that principle, as paradoxical as that sounds. I wish I could find my post or the patent fast, to share it with you.
This post by Neil Clague about muffler baffle bore clearance may be interesting, considering the above context:
https://www.airgunnation.com/threads/fx-stackable-on-wildcat.85990/#post-86004
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Not sure exactly what I can do. Personally, I think digitizing the sound and post processing it is going to reveal the most valuable information. Afterwards, one could collapse or weight the values into some "meaningful value". A "sound meter" does the collapse immediately, which destroys information. I'm not an audio engineer, and don't have anything besides my PC for these experiments. However, doing signal processing is what I used to do for a living, so writing code for that isn't all that hard, given having data.
I'll check out Neil's post on baffle clearance.
There's a lot of hooey on the forums, based on not much data. The guys like Neil and other manufacturers tend to have a visceral feeling for what works, usually based on trying a lot of stuff, and practical applications of science. They have done hundreds, if not thousands of experiments, which build up that experience. And they get a lot feedback from their customers. It's a unique advantage that they have over us. But all is not lost. Many of us have motivation, or just plain curiosity to make things better.
To prove any assertions about the optimal baffle bore clearance will require that the bias errors of the mean due to small sample size are far less than what we are trying to measure. Otherwise we will be fooled into thinking things are significant, when they are not. Also, the standard deviation should be measured. If it is huge, there will be a large variation in perception from shot to shot. So some shots will alarm the neighbors, and others will not.
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Found this. https://www.cdc.gov/niosh/surveyreports/pdfs/349-11a.pdf (https://www.cdc.gov/niosh/surveyreports/pdfs/349-11a.pdf) Yeah, not a casual effort. That system received a US patent. Uses software that I've used in the past, but I'm not about to go and buy a license. B&K makes a system, which appears to be a quote only, that's means it is far more expensive than I want to pay. https://www.bksv.com/-/media/literature/Product-Data/bp0006.ashx (https://www.bksv.com/-/media/literature/Product-Data/bp0006.ashx)
That being said, they seem to revolve around a good microphone or three, (described in the report) and sampling rates on the order of 200 KHz. These systems were developed to determine exposure to acoustic impulsive noise, specifically for the shooter, and range personnel, as opposed to others. The microphone was located 1 meter from the muzzle, perpendicular to the shot, as well as 15cm away from each ear.
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Thank, Bruce
That is some sophisticated equipment and methods.
I found the technology I mentioned above, where a deliberate quantity of gas is leaked ahead of the projectile to reduces concussion from the gas behind it. I posit that the same principle may be used to reduce the sharp edge of the "air slap" emanating from an airgun moderator front cap bore: https://techlinkcenter.org/technologies/weapon-blast-attenuation/a2cef892-1ab8-45d2-80c8-c29857621326
A method to attenuate the blast wave/blast overpressure from a weapon discharge by leaking an effective quantity of propellant gas from behind the projectile, into the precursor gas flow in front of the projectile, while the projectile is still in the weapon barrel—whereby the exit pressure ratio of the projectile from the weapon is surprisingly reduced by about 95%—resulting in a reduction of from about 51.6 to about 58.2% in the peak pressure level/sound wave which impacts the user or crew of the weapon.
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Airgun or PB report not an easy thing to measure quantitatively as there are strict requirements on the linearity of the front end. Saturate the front end, or driving it into the non-linear region can cause desensitization in the case of the former, or actually creating more spectral energy in places where it wasn't for the latter. Based on my limited reading, I'd go with 24 bit sampling simply for the dynamic range requirements. Don't think a 16 bit ADC is adequate, although if one is willing to give up sensitivity at the low end, maybe it could work.
As for the method of attenuation, it sound interesting. Would be tough to engineer, I'd imagine. Is there any merit to the FlowMaster method for automotive mufflers? That's dividing the airflow in half, phase shifting one branch by 180 degrees and recombining the flows. (Destructive interference principle.) I'd guess that would only work if the sound was relatively narrow band. That could be done passively.
Broadband cancellation can be done, but it's a lot harder. Maybe it could be done actively. Might be interesting electronic acoustic project. Tiny microphones, powerful tiny speakers, a big battery, and one heck of a lot of digital signal processing. Probably state of the art kind of work. Not sure how many airgun shooters would want to electrify their moderators! One more battery to go bad on you... Nonetheless an interesting idea.
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People have been working on active noise control, since the 1936 patent came out. Algorithms that remain stable for impulsive noise are being worked on. Apparently, it is not an easy problem to solve. Everyone, from car makers, to headphone makers would like a solution that worked better.
https://www.cambridge.org/core/journals/apsipa-transactions-on-signal-and-information-processing/article/recent-advances-on-active-noise-control-open-issues-and-innovative-applications/9E27156562A24026ECD6F6A49A54F53A (https://www.cambridge.org/core/journals/apsipa-transactions-on-signal-and-information-processing/article/recent-advances-on-active-noise-control-open-issues-and-innovative-applications/9E27156562A24026ECD6F6A49A54F53A)
https://pubmed.ncbi.nlm.nih.gov/24363493/ (https://pubmed.ncbi.nlm.nih.gov/24363493/)
Not going to lie, I understand the basics of this, but this is far from my expertise.
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I think that the closest we have to destructive interference in airgun moderators is Tesla Valves; turning the air flow against itself. With an engine muffler you don't want to restrict the gas flow; you just want to reduce the pressure vibration to a state of steady flow. The latter would also work for an airgun muffler, but mainly it is the pellet's "flow" than must not be restricted.
I think that active sound cancellation, including electronically needs to listen to the sound in order to respond to it. A shot's sound has rapid onset and changes continually after that, before dying out. Probably not enough time for a system to track the sound and cancel it. Unless it can learn from the previous shot sounds.
An engine muffler is dealing with gas pulses from multiple firing events. There is a frequency that depends on the engine speed and number of cylinders (and ducting branches from them). I think it is easier to arrange destructive interference of sound waves where there is repetition in the sound driver. A gunshot is a multi-spectrum square wave with some reverberation. More like a car crash than the engine sound. Although a vehicle crash is in some ways like banging on a drum; hard enough to damage it. Perhaps a machinegun sound could be managed more like an engine muffler. If you are familiar with the sound of a Gatling at 4000 rounds per minute, that has a much more musical sound to it.
Rather than state what equipment would be ideal for this project and then giving up because we don't have access to it; all I insist on is not saturating the microphone; and that the meter placement is consistent. If people cannot tell the difference between shots using different moderators by ear, then frankly, the accuracy of the sound meter matter much less. The reason for measurements is to predict what people hear, and are likely going to find acceptable/unacceptable.
One reason to care about absolute loudness is to determine the threshold where people notice a difference between sounds. Important, but a different study. I am more interested in relative measurements produced by multiple unbiased people, using available equipment. I am very interested in their subjective impressions, as ultimately, it is how loud your wife thinks your airguns are that matters. Perhaps trumped by how loud the neighbor's wife thinks they are.
The point of this study is to examine the school of thought that, for maximum sound attenuation, muffler baffle bores should be as tight as possible. Based on what I have seen, baffle bores can be larger, with a smaller loss in performance than one might expect. The fact that mapping this function may show kinks in the graph due to the effect of "pre-curser gas" ahead of the projectile adds another dimension, but points the same way: That the ideal baffle bore diameter for caliber and power may not be "as tight as you can make it" (short of clipping baffles)
From that noise cancellation paper, snoring represents some of the same challenges as a gunshot. It changes too rapidly to track; and has multiple components to the sound:
"Another challenge is that a snore is a fast time-varying and intermittent noise. This requires fast convergence algorithms such as those introduced in Section II(L). Owing to the broadband nature of snore, feedforward ANC with a reference sensor is required. This may cause causality problems if the snorer and bed partner are too close to shorten the acoustic delay. In addition, a snorer does not always snore so a robust snore detector to turn the ANC algorithm on and off is needed.
Furthermore, ANC systems do not completely reduce primary noise owing to many physical limitations. In the snore ANC application, which is often used in quiet bedrooms, still annoying residual noise may need special treatment."
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There is the urge to reduce any problem or solution to a single number. In this case, a peak dB reading for a gunshot - or more precisely, and airgun shot. Muffled, VS bare barrel; or compared to other mufflers.
When it comes to airgun mufflers, there is a second factor that is arguably more important than the peak amplitude: Once the peak amplitude falls into perhaps the 85 to 90 dB range; what matters most is if the residual sound still has a "crack" to it, typical of a firearm gunshot sound.
Based on feedback from people who have tested my custom airgun muffler designs (not the test mufflers posted in the OP) it is the reduction in the crack sound that is most appreciated.
The crude time/amplitude sketch below has two "charts", with the same peak amplitude. "A" has a very abrupt edge, with an echo; and would be more objectionable than "B", despite "B" carrying on longer. So, measuring peak dB is very useful when one is more concerned about hearing damage to the person shooting the gun, than in the context of "backyard friendly" airgun use. Except for very loud airguns, I am suggesting that a muffler that removes the "crack" may be preferred to one that peaks at a few dB less, but that still has a definite "gunshot" signature to it.
Why then do I ask for dB measurements for this project? To satisfy those that insist on objective measurement; and hold that a 2 dB difference is significant. Based on the available readings, it seems that this three muffler test may have uncovered something about baffle bore diameter and projectile clearance that was missed before. Something that a two baffle bore diameter test cannot show. If anything, the results suggest the trial be expanded further than +1 and + 2 mm on baffle bore diameters, with with 0.5 mm increments. Perhaps that can wait until someone who has access to better test equipment and facilities volunteers to run such a trial.
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Impulsive active noise cancellation is state of the art, it's hard. But likely not impossible. It will take quality sensors, transducers, advanced algorithms and fast processing. That being said, getting 60 dB reduction is a pipe dream. If you do a lot of hard work probably the best one could do is maybe 35-40dB. First starting out, might get 20dB. I've done some similar work in signal cancellation and got close to 38dB reduction, but in the end, one is limited by the information in the dataset, the time duration and your estimation errors.
Simply taking measurements is a whole lot easier. Yes, I agree that the microphone cannot be saturated, actually there should be at least 10-20 dB of head room before saturation to be have reasonably low distortion.
The issue really comes down to experimental technique. If one is a diligent experimentalist, one can get good results. But multiple people using different techniques or distances tend to make these studies just qualitative rather than quantitative. It's not that qualitative data is bad, but it simply isn't quantitative. So if you really want to know a quantitative answer, the setups have to be exactly documented - like writing a lab report sort of thing. So someone else could actually exactly duplicate the results, with the same moderator (and ideally, the same airgun). That includes the shooting range characteristics, dimensions and pictures, since I'm sure they are all wildly different. If they are "set up wrong", whatever that means, they could impact the results. Also, each AG has a different pitch and tenor, and they probably are altered by our tuning efforts. This is an N variable problem, and it would seem that one would like to minimize the amount of variables in the beginning. Then we could explore the differences between AG's. I'm very open to suggestions on this. Just blurting out some thoughts. But if all we want to do is A/B tests that's ok.
But the tests are most likely are affected by their acoustic environments. No we don't have to do this in an acoustic dead room, but we do need to be sure the methodology of data gathering is such that we don't have people shooting along a wall or fence, with an acoustic reflector, or in enclosed unattenuated spaces. That's why photos, drawings with dimensions and things like that are important. Ideally, the ranges would be in open fields, with no acoustic reflectors. Not everyone has access to those areas. All the ones local to me have turned into developments. :(
I appreciate what you are trying to do. And everyone wants to turn a complex problem into a simple number. I don't doubt you have come up with some nice hardware, it's been apparent from your various postings. At the moment I don't have a decent way to capture data the way I would like, but I am looking into it. I could capture data at 96KHz with a card that I have, but my microphone is limited to maybe 40 KHz. I don't know, I have to review the data. How the heck I synchronize the data collection, I have to think about that. I don't want to have any eclipsing losses - that is sampling after the shot has started. I need to guarantee that I collect the whole shot. Most of the systems I see, don't prevent eclipsing loss. That becomes a variable which can vary up to 10dB, which needs to be eliminated. If you start sampling in the middle of a symmetric pulse, you lost 3dB, as an example.
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Why then do I ask for dB measurements for this project? To satisfy those that insist on objective measurement; and hold that a 2 dB difference is significant. Based on the available readings, it seems that this three muffler test may have uncovered something about baffle bore diameter and projectile clearance that was missed before. Something that a two baffle bore diameter test cannot show. If anything, the results suggest the trial be expanded further than +1 and + 2 mm on baffle bore diameters, with with 0.5 mm increments. Perhaps that can wait until someone who has access to better test equipment and facilities volunteers to run such a trial.
There very well could be a sweet spot for the clearance needed. To find it empirically, will take some very methodical and repeatable testing. Not all breakthroughs come from pondering things, sometimes you just plain got to try it out and see what works better. Wasn't that what Edison said, success is 2% inspiration and 98% perspiration?
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AGN has a tough crowd. They can be pretty harsh. Sorry you are dealing with that. Things are slower here, but at least we don't pick many fights here. The moderators tolerate a lot less "stuff" here and aren't afraid of the ban hammer.
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There very well could be a sweet spot for the clearance needed. To find it empirically, will take some very methodical and repeatable testing. Not all breakthroughs come from pondering things, sometimes you just plain got to try it out and see what works better.
I agree 100%.
I predict that the ideal bore size depends on power level and how it is achieved: A short burst of air at higher air pressure, VS long valve dwell at lower air pressure.
If you set up your .22 PCP to shoot your favorite pellets at 30 FPE, then fact that a different pellet at a different tune and velocity might favor another muffler bore diameter may be moot.
I would argue that any dB differences less than 3 dB are academic, but some people on AGN forum are adamant that, "every dB matters". Reminds me of a Monty Python song :)
To complicate matters academically, my muffler designs usually do not have constant bore diameters from end to end, unlike the ones designed for this study. I usually choose a baffle bore diameter for the first baffle, then step the bores open to form a 0.5 degree included angle cone. So, the longer the muffler, the wider the front baffle bore gets. This is purely to guard against potential angular misalignment, and prevent clipping. If I were to taper the three muffler set baffle bores, that would add another wrinkle to the test. It also might confuse the general audience, so I went with short mufflers and constant baffle bore sizes per muffler.
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I took your advice and tapered the bore in my moderators. Think it's only a 1/4 degree though. Since I can machine stuff, I can run closer tolerances if I want to.
As a radar engineer I fought for every dB in receiver sensitivity. It made a huge operational difference. Like the difference between crashing and dying or not. So I understand where some folks are coming from. I'm not sure that a dB matters for this application.
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I predict that the ideal bore size depends on power level and how it is achieved: A short burst of air at higher air pressure, VS long valve dwell at lower air pressure.
This effectively is describing the characteristics of the air burst, the impulse shape, which has an extremely high influence on whether it sounds objectionable or not. A true impulse has energy spread everywhere. Fast attack signals have a lot of spectral splatter as well.
By having a shaped pulse, ie it's envelope, we can alter its sonic acceptability. Think about this as a filtering problem (and I am not sure this is a good analogy,) if we could 1) shape the pulse (air blast), we could contain the spectral information to a region, and 2) we could design a filter network to suppress that frequency range. Writing this down has given me an idea to look into. Don't know if it will lead anywhere...
What I found was quite interesting, basically a chapter in a book where acoustic stuff is related to electromagnetics. Now, I know nothing (not much any ways) about acoustics, but I used to know e-mag cold. But I'm rusty and out of practice. Need to read some more and see if my idea pans out. Might not be earth shaking, but could help me understand a methodology to make things better.
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Bruce, Since the sound measurement discussion has come up in a number of threads, I researched it a bit and did some of my own measurements. Here are some observations, though with your data processing background you may be entering at a higher level.
Initially, I worried about the response time of the microphone, sampling rate of the soundcard, etc. I set up an OK microphone (imm-6 Dayton), a 96 kHz soundcard, and captured a bunch of traces using Audacity SW. Based on the short duration of the transient, I was thinking that the long sampling windows of the sound level meters (Fast=125ms, Impulse= 30ms I think) must not be applicable.
I then started looking at the other side of the problem, the human hearing response (psycho-acoustics). First, there is very little data on the hearing response to impulse sounds. There are studies on hearing thresholds of tonal sounds, there are studies on ear damage from impulse sounds, but very little on the ability to hear an impulse sound. What I did find is that the ability to hear short duration sounds drops off below 200 ms and especially below 50-100ms. Down below 50ms, the ear acts like an integrator and it is the energy, not the peak that is important.
So now I'm back to thinking that the 125ms Fast setting on the sound meter may not be too bad and that I probably don't need to worry as much about capturing the sub-ms details of the waveform.
I would be interested to hear your conclusions.
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I see the difference in pressure x duration as taking place in a sealed system. Sure, there is some noise generated before the projectile leaves the muzzle, but mostly it is the volume of air expelled and the muzzle pressure that drives muffler design.
To achieve a given FPE requires a specific average pressure. Using very high input pressure and a short valve dwell drives the bulk of projectile acceleration near the breech end results. This results in a lower volume of air exiting the muzzle at lower pressure. Using a low airtank (or regulator) pressure and a long valve dwell to achieve the required average pressure, ends up with a greater air volume leaving, at a higher muzzle pressure.
The difference in sound is obvious when you shoot an unregulated PCP from a tank pressure so high that the velocity is lightly depressed for that hammer strike. Then continue shooting until the velocity has peaked and dropped below the starting velocity. The latter shots are loud, if with a lower tone - less crisp than shooting very short valve duration.
I wonder if the valve duration sound is emanating from the airtank at the input end, because it can't be coming from the muzzle, until the projectile exits. All I can say is the reason why a short burst of higher pressure air is more efficient is the same reason its muzzle blast is less objectionable: The air expansion ratio is higher, if the valve closes earlier. If the valve closes after the projectile has passed half the barrel length, efficiency suffers. Closing the valve as the projectile exits the muzzle has terrible efficiency and makes any kind of muffler work very hard.
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Bruce,
If you were simply stating that the projectile uncorking from the muzzle produced very high rise rates and that the way the ambient air is slapped by that is what produces the objectionable noise, I agree.
It is like a belly flop making a slapping sound, compared to a streamline dive. If only we can taper the air pressure leaving the barrel muzzle to make it less of a square wave our ears could not mind so much. Other than tuning the PCP for lower muzzle pressure, the only other way is to use a muffler - assuming that a four foot barrel if off the table :)
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Stan,
The sound duration effect on human perception has a wrinkle:
A moderator takes the edge of muzzle blast, and lowers the peak by stretching the event duration. So, are we making a very short loud sound seem louder by stretching the duration?
One clue about how sharp and loud the original sound was is the presence or absence of noticeable echo, and the duration of the echo. If I hear a gunshot in the distance, it is not how many dB it is at my location that draws my attention, but the duration of the echo. Ditto for videos of guns being fired. Gunshot sound capture has the level suppressed automatically, and it does not hurt our ears to watch the video, the same way watching an arc welding video does not hurt our eyes. But there are other clues about how loud or bright the sound or light are.
Obviously echo is very much dependent on what surfaces are in the vicinity, but for a given location, the more obvious the echo, the louder the sound is assumed to be at its source. The video below is of a trebuchet launching a supersonic projectile. The crack in the open field is obvious and a "rolling echo" is reflected back for a long time, even in the absence of hard surfaces such as walls. I don't need a dB meter to tell me that passers by will think someone is shooting a firearm not far away from them. Generally, this sound is exactly the opposite of what we want from our airguns:
Copy and paste link to start at the point of interest, 14 minutes 44 second in:
www.youtube.com/watch?v=gdXOS-B0Bus&t=884s
https://youtu.be/gdXOS-B0Bus?feature=shared&t=884
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If only we can taper the air pressure leaving the barrel muzzle to make it less of a square wave our ears could not mind so much.
Shaping the sound/air pressure is the idea.
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Stan, measuring the sound through a filter will distort the original information, but one can get some kind of repeatable information, a value. But if we want to understand how to modify the moderator to get the characteristics we'd like, I'd personally go for minimal filtering (obviously we need to prevent inadvertent Nyquist foldover). I'm starting to think about the moderator as an acoustic filter. The size, shape, and spacing of the baffles are changing the acoustic filter response.
I need to set up some acoustic sampling rig in the next few days. My casual aural observation is that the report with and without a projectile is pretty close for a limited set of subsonic shots. I'd like to instrument that to know if that is true or not. If it is "mostly true", it would make designing an acoustic filter, aka a moderator a little easier. If I could make a filter analogue then I could test it with different pulse shapes and amplitudes, an audio workbench, so to speak.
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This chapter gives me some hope that I could somehow design an acoustic filter using my former E-mag knowledge. https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electromagnetics_and_Applications_(Staelin)/13:_Acoustics/13.02:_Acoustic_waves_at_interfaces_and_in_guiding_structures_and_resonators
Used to design microwave filters using apertures in waveguides. Guess what a moderator is? An acoustic waveguide. What we would like is a Gaussian like shape, as opposed to impulses or a reverse sawtooth. |\
Now to see if I still can do this stuff, it's been decades...
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I think an airgun moderator is more than just an acoustic filter. I think the last baffle chambers are closer to pure acoustic filters, while the fist baffle chambers function primarily to partition off and expand discreet volumes of air to lower their bulk pressure. Thus turning the large pop at the barrel muzzle into several smaller pops of diminishing intensity. The air pressure oscillation from all of the pops are superimposed and filtered to some extent by the chambers down stream. Then the sound is filtered and partially blocked as you move towards the front.
Obviously, there is no sharp cut-off in function when considering a moderator that has only four chambers.
A related study that could be very useful and fun is to make moderators that have the same cylindrical internal volume, then compare performance based on the number of baffles and their spacing.
The thread in the PCP gate "Taming the bark of an Avenger" has a large body of work in which many moderator designs were tried. It was there that I developed my own benchmark that I dubbed "plume baffles". There the spacing and angle of the conical baffles shifts continually. The early baffles are not slanted as much and are closer together. This is because the design tracks the bulk pressure of the air as it flow behind the projectile radially towards the ID of the casing.
As the air moves forwards it expands and needs more room. This goes along with a drop in bulk pressure. Bulk pressure is what drives the air radially to be more effectively engaged with the next baffle "wall"; away from the exit. It is this function of bulk pressure that makes some moderators more effective with a projectile than dry firing.
If there are too many baffles, spaced too close together, I think that air flow takes a shortcut between baffle bores, rather than spreading out to be more effectively engaged by the baffle "walls". After the bulk pressure has dropped and airflow down the baffle bores is at its maximum velocity, bulk air has considerable forward momentum. This important, because there is less force spreading the air radial and more inertial resistance to spreading radially. What my increasing baffle slant does, is help the air flow stay attached to the back side of the cones, rather than break away to continue on down the baffle bores as a narrow "jet". The lower bulk pressure and the faster the air flow at lower pressure, the more the baffles need to look like a rocket nozzle.
We assume that the same amount of air is released dry firing, but the resistance of the pellet probably affects valve duration, and the shape of the leading column of air entering the muffler. Certainly, the presence of a projectile should act as a moving plug to help drive air into the baffle space, towards the ID of the outer tube wall. So, any study that insist on the scientific method to represent shooting airgun projectiles, should include shooting airgun projectiles - in my opinion.
Shallow cone angles drive longer baffle spacing as a mathematical certainty. Shallow baffle spacing will work early on where the bulk air pressure is higher. But there will be less expansion volume between baffles that are greatly slanted per inch of muffler length. As muffler length is not "free", and most people prefer shorter mufflers, it seems to make sense to me, to start with more upright baffles cones, and then slant them more towards the front. Because it helps with air management and because it helps with sound filtering:
I see the sound filtering aspect of muffler design as multi-dimensional:
The way I slant my baffle cones should also reduce pressure waves bouncing between parallel surfaces, compared to say all 60 degree cones (as used in the test mufflers of the OP).
Then, I see spacing of the cones as part of noise filtering: If you want to hear people talking on the other side of a door, you not only get close to the door; you put your ear close to the key hole. Here, key hole is an analog for the baffle bore ahead of the noisy air in any give baffle chamber. The closer you are to the hole in the baffle, the wider the angle of sound that escapes through that hole. The further away from the hole, the smaller the hole seems to the radiated sound source.
An analog of this would be looking out of a window through a small gap between curtains. If you stand far away from the curtain, you see mostly curtain with very little information seen through the gap in the curtain. If you stand close enough to the gap in the curtain, the curtain block almost none of what can be seen on the other side. I see this as angular filtering from a radiating source, as a function of stand-off distance. This perhaps explains why Weihrauch moderators are so effective, despite having only three chambers. The first is a large chamber (capacitor) to drop bulk pressure. The next is a flow resistor (that works like its electrical analog). The last is another larger expansion chamber that has a very long stand-off distance, to make it seem as if the baffle bore at the end is very tight. Certainly with regard to noise trying to escape the chamber.
So, I think that modelling a muffler would be pretty tricky, but useful if you can do it. A test of a math model, or set of models is if they predict what happens when you test actual moderators. If the math model does not predict the empirical results, then the math model may be incomplete. Some important factors may be missing, because no one knew they were important. Or the concept of how mufflers work is incomplete, or dare I say, wrong.
The fact that there are so many baffles types that work really well, suggests there are several overlapping principles at play. Some design approaches favor one principle, and other designs favor one or more other principles.
"More baffle are better" is countered by looking at the three chamber Weihrauch approach.
You must "not use constant baffle spacing" is countered by the benchmark of quiet, the Marauder.
Similar arguments can be made for baffle angle. Some conical and some flat designs that work very well, can be found with either approach. My plume style baffles seem to fall in the middle somewhere; and seem to work rather well. Not just at lowering dB, but at reducing the inherent irritating "crack" signature. Handsome is as handsome does. Technology is most interesting, if it works. Else it is interesting only to avoid emulating it. Yes, one learns more from failure and fixing it, than accidental success...
End of chapter 3 billion :)
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You've obviously been thinking about this longer than I have. Not sure that all your theories are correct, but you certainly have more practical experience.
I'm thinking that under some conditions using filter theory could be beneficial. I very well could be wrong, but I think tuning the aperture size and spacing would very well change the filter response, aka user perception. If I were to jump deep into this, I'd start with simple straight baffles, so the math is easier, and design an acoustic filter to see how it turns out. I'd like to determine it's impulse response. Angled baffles might be a perturbation relative to straight ones. A lot of EMag problems can be solved this way, so I'm guessing acoustic ones might be solved similarly. Obviously, there's limits to the aperture size as the projectile needs to pass unimpeded.
Still mulling this over. It's an interesting problem.
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Don't know how far I will get, but I installed the CfdOF workbench in FreeCAD. This gives me CFD analysis. I have used the FEM workbench to do finite element analysis.
CfdOF might give me some insight. Or it could be a totally frustrating experience... I'll find out in a while. I will make a series of constant aperture baffles in a tube, and attempt to puff an impulse (really a square pulse, I hope) into it and see what happens. And no, I don't quite know what to do, so I'll be making a series of dumb mistakes - but maybe I will learn something.
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Well, If you first model a bare muzzle, it should look like this.
https://www.youtube.com/watch?v=HZD-IgMvt54 (https://www.youtube.com/watch?v=HZD-IgMvt54)
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Bruce,
If color and light are models for filtering, where some parts of the spectrum are reflected and some absorbed, would the resonance frequency of each expansion chamber in an airgun muffler predict which frequencies would be amplified and which absorbed?
From a musical perspective, a one inch long air tube would have a pretty high resonant frequency, at one atmosphere. Inside a muffler the pressure would start out at several atmosphere, then decay to level still above an atmosphere when the air leaves the front bore. Higher air pressure should raise the resonant frequency. I suspect that for baffle bore lengths of 0.5 to 2" a lot of that will be in the ultrasound range. If I am right, the question is what does that do to the other frequencies?
An expert on AGN forum states that all airgun muffler are doing is converting audible frequencies to inaudible ones, like a dog whistle. I think there are designs where that principle is used and others not. My toroidal baffle design that traps air by spinning it almost certainly shift the vibrations created up in frequency. This, while Weihrauch's large, long three chamber designs offer plenty of expansion volume, and produce a low tone. The latter have "hair curlers" with felt to absorb audible higher frequency sound.
As for my mental models, they are based on observation and an attempt to explain what is seen, based on scientific principles. Only, I am not sure if I am locking on to the right principles.
An airgun muffler drops the bulk pressure of the air exiting the barrel muzzle, and blocks, filters and converts sound. Other than contain noise, a muffler generate less sound than the uncorking of a bare muzzle, by dividing one large bang into several smaller ones.
We know that higher muzzle pressure is louder, yet that is not all there is too it. If a 30 FPE .22 PCP has a muzzle pressure of 300 PSI the task of making the sound comfortable would seem daunting, considering how loud a balloon pop is at only 3 PSI. So, clearly air volume and the "size" of the "speaker cone" where it couples to the air also matter.
Perhaps it is as simple as calculating the energy in the air when it is released abruptly. Basically pressure x volume. So explaining how a balloon can be so loud, when its "muzzle pressure" is so low. A bit like comparing the report from a 12 gauge bird shot load to a .223 centerfire rifle. Both are very loud, but one is a small very sharp sound, compare to a larger less sharp sound. Unless the size of the bore is also part of that equation, in this case, a smal VS larger speaker cone. On the other hand, higher pressure gas will leave the muzzle more radially behind the projectile than low pressure gas. One can see a ball of gas leave a gun muzzle directly behind the projectile; then that gas turns into more of a linear jet as the pressure drops. Yes, part of that gas plume shape is due to the projectile being in the way of the gas.
It would be great if you could model what happens inside an airgun muffler to lead to more effective designs that are smaller, lighter and cheaper to produce. They should also not degrade grouping ability, nor shift it excessively from bare muzzle. Sometimes such changes are due to a shift in barrel harmonics and can be restored via velocity tuning. This can happen with any muffler, but is generally worse with long heavy mufflers; so shorter and lighter is less likely to cause harmonic problems. All this assumes the bores are not so tight as to contact the pellet, and that air currents inside the muffler are not going to steer or buffet the projectile to the point of grouping degradation. Usually, the air stripping function of mufflers improves groups, so that is what we like to see.
Other than what happens to the air inside a muffler, the outer casing also generates and transmits sound directly to the air. That is a related field of study that is important. Currently, wrapping the casing in something like a foam rubber sleeve is a known method to avoid adding casing noise to air noise. The casing material certainly matters. I was informed that some of my insert designs that were printed in slightly flexible TPU were quieter and had a less sharp sound than the same mufflers printed in PETG.
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Bruce,
If color and light are models for filtering, where some parts of the spectrum are reflected and some absorbed, would the resonance frequency of each expansion chamber in an airgun muffler predict which frequencies would be amplified and which absorbed?
This is not an active system, it is passive. Therefore there is no amplification. However, each expansion chamber does have a resonant frequency range. By appropriately staggering and weighting these resonant frequencies we can change the overall frequency response. I would think, in general, the resonant frequencies are controlled by the cavity length of an expansion chamber, and we can weight the individual filters by judicious choices of the apertures. So far this sound a lot like conventional filter design.
From a musical perspective, a one inch long air tube would have a pretty high resonant frequency, at one atmosphere. Inside a muffler the pressure would start out at several atmosphere, then decay to level still above an atmosphere when the air leaves the front bore. Higher air pressure should raise the resonant frequency. I suspect that for baffle bore lengths of 0.5 to 2" a lot of that will be in the ultrasound range. If I am right, the question is what does that do to the other frequencies?
Well, if the chamber dimension is too small, it tends to cut off propagation for lower frequencies. In waveguides, for frequencies below cut off, the lower frequencies decay at an exponential rate. The lower the frequency, the faster it decays. So small chambers are sort of a high pass filter, at least in microwaves, and I suspect this applies to acoustic propagation as the math is similar. But it isn't that simple, because in the case of a moderator we have to let the projectile pass through... And sound does leak through the hole. So this complicates things a bit.
An expert on AGN forum states that all airgun muffler are doing is converting audible frequencies to inaudible ones, like a dog whistle. I think there are designs where that principle is used and others not. My toroidal baffle design that traps air by spinning it almost certainly shift the vibrations created up in frequency. This, while Weihrauch's large, long three chamber designs offer plenty of expansion volume, and produce a low tone. The latter have "hair curlers" with felt to absorb audible higher frequency sound.
I don't know about experts on forums, I tend to take a lot of things said with a grain of salt. Heard plenty of blowhards in my career, not to mention on forums. But following up on waveguides below cut off, this tends to support that the filters may be reducing sound at lower frequencies. It's unclear there is any up conversion in frequency at all, I don't remember the spectral response with and without a moderator. If there are longer chambers, then more low frequencies will pass. Felt will attenuate high frequencies in any of the designs.
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An airgun muffler drops the bulk pressure of the air exiting the barrel muzzle, and blocks, filters and converts sound. Other than contain noise, a muffler generate less sound than the uncorking of a bare muzzle, by dividing one large bang into several smaller ones.
Not sure I agree with converting sound, but I am in agreement with the gist of the rest.
It would be great if you could model what happens inside an airgun muffler to lead to more effective designs that are smaller, lighter and cheaper to produce. They should also not degrade grouping ability, nor shift it excessively from bare muzzle. Sometimes such changes are due to a shift in barrel harmonics and can be restored via velocity tuning. This can happen with any muffler, but is generally worse with long heavy mufflers; so shorter and lighter is less likely to cause harmonic problems. All this assumes the bores are not so tight as to contact the pellet, and that air currents inside the muffler are not going to steer or buffet the projectile to the point of grouping degradation. Usually, the air stripping function of mufflers improves groups, so that is what we like to see.
Other than what happens to the air inside a muffler, the outer casing also generates and transmits sound directly to the air. That is a related field of study that is important. Currently, wrapping the casing in something like a foam rubber sleeve is a known method to avoid adding casing noise to air noise. The casing material certainly matters. I was informed that some of my insert designs that were printed in slightly flexible TPU were quieter and had a less sharp sound than the same mufflers printed in PETG.
Not going to mislead you, this is one heck of a stretch for me. I don't know a thing about computational fluid dynamics. But I know some things that might be transferable. At the moment, I am just starting out with the tool and don't know how to use it yet. I suspect I will have to watch about 10 tutorial videos before doing much. And using open source tools always has the risk that it simply won't or can't do what is needed. That being said, I will start watching the vids and hope to be able to use the tool in some basic fashion. I self taught myself FEA, so maybe CFD won't be so hard.
Totally agree that the materials matter. They will transfer the internal acoustic energy to the outside world by vibrating as a cylindrical speaker. It may not be an efficient speaker, but it will make sound. It makes sense that a material that is flexible and in a sense lossier would help. Increasing thickness or material type would help. If the material were denser, it would shift the resonant frequency higher. But for that matter, so would adding stiffening ribs. As I recall, you sometimes add them in. They will shift resonant frequency higher.
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Regarding frequency translation. This image https://www.airgunnation.com/attachments/with-jpg.394810/ (https://www.airgunnation.com/attachments/with-jpg.394810/) was used to claim frequency translation of a particular moderator design. The claim is technically not true. What it does show is the moderator is a passive device, otherwise know as a passive acoustic filter. In no frequency bin is there amplification for the moderated response as opposed to the corresponding unmoderated frequency bin input. This makes sense as the moderator (the filter) is un-powered by either mechanical, electrical, or pneumatic inputs. A passive filter device cannot amplify frequencies, it only can attenuate them. Now the author of the post seems to have a LOT of practical experience in the subject area, but any claims of acoustic amplification cannot be true due to conservation of energy.
One method of designing microwave filters is called the high Z - low Z method, also known as stepped impedance. It consist of high impedance and low impedance structures, be they transmission lines, or waveguide cavities. If this were to be directly translated into acoustics it might look like cavities of some length and different diameters perhaps with apertures. For a moderator to be practical this isn't helpful. No one wants to tote around a 4" diameter moderator. But, like in waveguides and other structures, we can do some magic to make it appear like it is wider. It is sort of a virtual wideness. As a matter of fact, it would seem to me, that angled cones and tesla like structures behave exactly like virtual widening of the cavities, allowing a more compact design. There are other design techniques that would work as well, I don't want to bore you on basic filter design or microwave filter design. There's also coupled resonator design methodology. BTW, this is stuff that I learned around 1985 or so.
So, if I were to continue the thought, perhaps the desired frequency response could be synthesized in an unfolded design at first and validated. Perhaps then the folded design would be introduced. There can be a blurring between the two, as obviously the unfolded design is undesirable. But perhaps one could see if the general approach yielded any interesting results.
So, what exactly is the design goal? Mouse fart quiet is not a technical filter design parameter! Is there a filter response or rough attenuation that is desired? Because that is how filters are designed. Are we looking for a low pass, high pass, or bandpass type response? Is 20 dB attenuation sufficient? (Not aware of any that are that good. Is there a moderator with 20 dB attenuation?) What is a desirable cut off frequency? In the beginning, we can just use WAG's for a starting point. Anyways, this is interesting, at least to me.
Couldn't find my copy of Matthaei, Young and Jones, but found a link on the internet archive. Not light reading...
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Bruce,
With regard to the chart you posted above; some of the charts from the same source; with and without moderators seem more convincing when it comes to the frequency shift claim. It is not my claim, so I am not going to try to defend it, but I assume you noticed the scales are not the same on the Y-axis? The charts that seem more convincing for frequency shifting are produced using very small volume mufflers for the power level of the shot. Anyway, what you are looking to do does not hinge on such "shape shifting".
I am not usually impressed by a muffler producing less than a 10 dB drop, compared to bare muzzle, but that statement should be seen in context. Reducing 120 dB to 110 is relatively easy. A greater than 20 dB drop from bare muzzle is routing with PBs, but they go from eardrum busting to barely hearing safe. So, an impressive drop, but still much louder than considered acceptable for airguns.
For backyard friendly airguns, I have seen very few moderators drop below 70 dB, and then the absolute accuracy of measurement comes into question. At such a low dB readings, noise from the trap can easily dominate. So, a drop from 90 to 75 dB seems possible. But 80 to 65 dB seems very unlikely.
In other words, while we look at the drop in dB, we should try to define how quiet, is quiet enough. If the reading is between 70 and 80 dB it is probably low enough. Even around 90 dB for the moderated sound, the nature of the sound is often more important.
People like to argue numbers, because sound quality is too subjective. Even with only a 4 dB reduction, the nature of the sound can make all the difference:
At the video below; the first muffler on the Huben pistol is only 2.75" long. If I remember correctly, it only offers a 4 dB drop over bare muzzle. But compare the nature of the sound to the bare muzzle at the end of the video. The latter has an echo, with a definite "gunshot" signature, while the short muffler produces a loud, but much more pleasant sound. The second muffler in the video is longer than the first, yet it sounds harsher. I can't explain that, and I designed both mufflers.
https://player.vimeo.com/video/873521032
So, asking for muffler dB or dB reduction specs is actually difficult, because they matter in context with the nature of the sound, to me. Despite many people caring a whole lot about one muffler winning by 2 dB over another. Usually, it is because they are selling the quieter one.
I started this thread about muffler bore clearance because I wanted evidence to support what a few of us know already: Making the bore a littler larger loses very little in performance; and it may actually result in a more pleasing sound. Maybe even a little quieter in some cases - as discussed a few posts up. If the terrible loss in performance was real, should we open the baffle bores by 1 mm, then no one would suggest shooting a .117 from a .22 muffler; or a .22 from a .25 muffler. Clearly, that is a very common practice.
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@subscriber "but I assume you noticed the scales are not the same on the Y-axis?"
Indeed I did. Which is exactly the reason I asserted that the moderator is a passive device and incapable of any amplification, only attenuation. In all cases, the moderated frequency, bin by bin, is less than the un-moderated frequency bin. The moderator attenuates the input by its transfer function, which is the plot of the frequency response vs frequency. The scale of the two plots is different, which fools us into thinking there is amplification. There isn't any amplification, which is line with conservation of energy.
Had the plots been the same offset and scale, this would have been obvious. If I had access to the point data, for both plots, I could plot it and show you. The moderated output power for each frequency is less than or equal to the input un-moderated power for the corresponding frequency. It has to be, it is a passive system! Actually, if I had those points, I could plot the transfer function of the moderator. It is just the difference (in dBs) between each and every frequency bin, plotted vs frequency. The transfer function would be very interesting indeed.
BTW, had there been ANY amplification, it is only due to the statistical variation between the shots. We all know shots vary. If one did 10K trials of each, and they were averaged correctly (not in dB's) it would be extremely unlikely so see any apparent amplification. Had you truly seen amplification, you would have invented a "perpetual motion" machine, which to my knowledge is not possible with any passive system.
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From the chart posted, it looks like the moderator was most effective below 5kHz with the upper frequencies relatively unaffected. Not sure what a frequency shift would be. This looks like bandpass.
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I am not usually impressed by a muffler producing less than a 10 dB drop, compared to bare muzzle, but that statement should be seen in context. Reducing 120 dB to 110 is relatively easy. A greater than 20 dB drop from bare muzzle is routing with PBs, but they go from eardrum busting to barely hearing safe. So, an impressive drop, but still much louder than considered acceptable for airguns.
For backyard friendly airguns, I have seen very few moderators drop below 70 dB, and then the absolute accuracy of measurement comes into question. At such a low dB readings, noise from the trap can easily dominate. So, a drop from 90 to 75 dB seems possible. But 80 to 65 dB seems very unlikely.
In other words, while we look at the drop in dB, we should try to define how quiet, is quiet enough. If the reading is between 70 and 80 dB it is probably low enough. Even around 90 dB for the moderated sound, the nature of the sound is often more important.
People like to argue numbers, because sound quality is too subjective. Even with only a 4 dB reduction, the nature of the sound can make all the difference:
The language of filter design is desired transfer function. Above you indicate some numbers like 70dB, which on the surface isn't yet meaningful to me. Relative to what? What was the bare muzzle reading? Is this a single number, rather than a spectral plot?
At the video below; the first muffler on the Huben pistol is only 2.75" long. If I remember correctly, it only offers a 4 dB drop over bare muzzle. But compare the nature of the sound to the bare muzzle at the end of the video. The latter has an echo, with a definite "gunshot" signature, while the short muffler produces a loud, but much more pleasant sound. The second muffler in the video is longer than the first, yet it sounds harsher. I can't explain that, and I designed both mufflers.
So, asking for muffler dB or dB reduction specs is actually difficult, because they matter in context with the nature of the sound, to me. Despite many people caring a whole lot about one muffler winning by 2 dB over another. Usually, it is because they are selling the quieter one.
I started this thread about muffler bore clearance because I wanted evidence to support what a few of us know already: Making the bore a littler larger loses very little in performance; and it may actually result in a more pleasing sound. Maybe even a little quieter in some cases - as discussed a few posts up. If the terrible loss in performance was real, should we open the baffle bores by 1 mm, then no one would suggest shooting a .117 from a .22 muffler; or a .22 from a .25 muffler. Clearly, that is a very common practice.
Perhaps if we could get a spectral plots of the pleasing vs non pleasing moderators AND the un-moderated input, we actually might be able to determine what the differences in transfer function might be. Without any quantitative measurements we are reduced to waving out arms about. There has to be something, some characteristic that makes something better sounding, right? I'm still maintaining that a scientific/engineering approach can get us closer to the goal.
From a point of view of changing the clearances, I have no issue with what you are saying. Actually I think the aperture size could have a very strong influence as it changes the acoustic coupling into the next chamber. That means it changes the characteristics of the transfer function - which could make things sound better or worse. In my opinion, there is no reason the aperture has to be either uniform, or tapered, it could have a pattern so the transfer function is more pleasing. Besides the obvious limit of allowing the projectile to get through unimpeded, it could be larger than necessary for passage, and make the sonic qualities more favorable.
I have no idea if this is true, but perhaps more pleasing is simply a lower cut off frequency for the low pass filter. I also think you are right, a couple of dB doesn't matter that much, IF the filter has a more pleasing shape (transfer function). Although it wouldn't be bad if it had higher attenuation, and be pleasing.
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From the chart posted, it looks like the moderator was most effective below 5kHz with the upper frequencies relatively unaffected. Not sure what a frequency shift would be. This looks like bandpass.
I guess a basic moderator structure would be a high pass, simply because of the hole through it! Didn't look at it carefully enough to plot any differences.
Can't imagine there is a frequency shift, it's not an active device. The transfer function is probably lumpy, simply because it wasn't designed as a filter.
Could be a bandpass, though, but I don't think it is likely. I would bet this filter is periodic in a sense and lets through ultrasonic frequencies. In microwave structures like this, the frequency response repeats about some magic frequency related to the dimensions of the structure. So there would be periodic stop and pass bands. Most of the time this doesn't matter, since those frequencies are not of interest, or in this case, we humans simply can't hear them.
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Ok. Math time. If I properly understand the analogy, we can compute the cut off frequency of a circular wave guide as:
fc = ( 1.8412 * cs) / ( 2 * pi * r) where cs = speed of sound, and r is the radius of the inside of the waveguide.
Since the speed of sound is 343 m/sec, I will use the radius in m.
Lets take as an example, ID = 25mm, so r = 12.5mm or 12.5x10-3 m
I get fc = 8040 Hz. So the tube cuts off frequencies below 8040 Hz. Basically there should be sort of a rough ramp, with the maximum attenuation at DC, up to 8040 Hz, where the sound is unattenuated by the tube diameter. I will have to get back to you all on this, to let you know the slope of the curve. I want to say 20dB/decade, but actually I don't know. The info I seek is buried in an old college text book which I have yet to find.
The larger the diameter, the lower the cut off frequency. Thus narrow diameter moderators "should" have less low frequencies, if we prevent the shell from vibrating. Larger diameter moderators should pass more lower frequencies. I am pretty sure this passes the reality test so far. Small apertures will tend to pass higher frequencies, but fortunately, they are limited in their length, so some low frequencies do get through.
I am going to have to go pound the books for a while, I'm not smart enough to do any of this on the fly...
But I will say, the tube itself is a high pass filter, as it has a low frequency cut off. This means the low frequencies are being selectively attenuated, but not high frequencies.
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Curiously, if the tube diameter = 12mm, the cut off frequency is 16.7 KHz. Which is nearly out of hearing range. Now that's an interesting thought! Just a simple straight tube.
I need to figure out the attenuation per unit length, might not be enough to be reasonable. I don't think it will work, but hey, its dead simple to try. Slowly getting there.
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We've been doing it wrong all along. I just tried a 1/2" copper pipe, and I couldn't even hear the hammer drop!
Of course, I'm kidding, I actually appreciate your trying to use established methods and apply them to our audio issue. The closest thing I've dealt with are bass ports on loudspeakers -- "tuned" to allow more bass through than a sealed box would. Not sure that is applicable.
I have a question when you say there is no amplification -- because these are only passive devices --- If I blow through a soda straw, it is low amplitude white noise, more or less. If I blow the same into whistle, or a flute, I surely get an amplitude increase in one of the "frequency bins." Overall energy must be conserved, so must be other frequencies getting attenuated as well. Is an unmoving hunk of metal or plastic (the whistle) "active"?
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We've been doing it wrong all along. I just tried a 1/2" copper pipe, and I couldn't even hear the hammer drop!
Of course, I'm kidding, I actually appreciate your trying to use established methods and apply them to our audio issue. The closest thing I've dealt with are bass ports on loudspeakers -- "tuned" to allow more bass through than a sealed box would. Not sure that is applicable.
I have a question when you say there is no amplification -- because these are only passive devices --- If I blow through a soda straw, it is low amplitude white noise, more or less. If I blow the same into whistle, or a flute, I surely get an amplitude increase in one of the "frequency bins." Overall energy must be conserved, so must be other frequencies getting attenuated as well. Is an unmoving hunk of metal or plastic (the whistle) "active"?
Bass ports are applicable, I'd think. As for doodling around with this, like you I have an interest in this (and a bunch of other weird stuff).
Whistles are interesting. Actually, I know little about them. I just went to Wikipedia and a lot of what's there isn't all that digestible. Perhaps what you are saying is true, I'm not sure at this point. It seems they rely on small apertures and chaotic flow. These apertures or cavities will have high resonant frequencies. But I am not sure if there is amplification. If you blow weakly into a whistle it doesn't work that well, if I remember correctly.
Now it is possible, if the cavity has high enough Q (quality factor) large fields (or sounds) can build up. But they are still passive devices, but they were pumped over time. Obviously can't get more total power than you put in. However, if due to a high Q cavity, you can pump it with energy from the frequencies around it, it gives the appearance of amplification. Saying it a different way, if you collapse the energy of a bunch of frequencies into one, yes the amplitude will be greater in that bin, but less elsewhere. So perhaps for a whistle mechanism one could do some spectral management. If the whistle is ultrasonic, and we were able to efficiently pump or excite it, then I'd think it was possible for energy to be transferred from audible frequencies to the ultrasonic. I don't know if the process is efficient or not. If not a whistle, I think I'd state at no time can there be an increase in spectral power between input and output. Maybe the whistle is non-linear, which can act as a mixer. But this is total speculation on my part, I just don't know. Mixers can translate energy from one frequency to another.
For a passive system, which includes moderators and whistles, the total summation of all the power contained in all the bins, has to be less than the total summation of power contained in all the input frequency bins.
Yeah, I wish we were doing it all wrong. ;) Lots of people have worked on this, over, what a century now. It doesn't help that details on this technology are not widely available. So it's hard to just look stuff up. Yes you can read some patents, but I mean engineering papers on the subject.
So we are left with copying original designs from last century, diddling around with variations, empirically trying stuff, or making feeble attempts to jump out of the box. I'm only jumping out of the box because it seemed that maybe I could transfer some knowledge from a different field. Maybe it's the wrong thing to do, or it won't lead anywhere. But perhaps it is worth a try.
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Bruce,
I was not very clear with all the dB numbers I offered: The "70 dB" was not relative to anything, it was absolute. So not a reduction of 70 dB, but the loudness of a very quiet muffler equipped airgun report. Was that obtained by standard PB measurement method, 1 meter to the side of the muzzle, 1.5 meter off the ground? Almost certainly not.
So, rather than provide dB numbers and then argue about the calibration of the measurement system, it helps to shoot a number of different mufflers side by side, and compare them to each other and the bare muzzle.
As I am not the one doing the testing, I cannot control the set-up; only suggest it. Most important to me is avoiding sound levels that saturate the meter, or are too close to its rate full scale capability to be "accurate". So, when I suggest standing the meter off by 5 meters to avoid meter limitations, the absolute readings are going to be a lot lower, than if they were measured at 1 m with a proper meter.
I am playing catch-up reading the rest of this thread, so I wanted to clear up the obvious confusion I created above, first.
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This might not work, because I may have over looked something. That's always the case for thinking out loud in public.
A round waveguide pipe is a high pass filter, whose cut off frequency is determined by some geometric constants, the speed of sound, and the radius. I have given the equation previously. You can test out this idea, by holding a 6" long tube to your ear, say an LDC. A narrower one will sound different that a wider one. Both will sound a lot like noise, but the pitch of the larger diameter one will have a lower tone. This is because the cut off frequency of the larger diameter tube is lower in frequency. This means you will hear lower tones or lower frequency noise.
The power transferred is dependent on frequency. Actually, if we plot the attenuation as a function of the log of the frequency, we get a straight line, that has a slope of 20dB/decade. The attenuation goes down as the frequency goes up. At the cut off frequency, the attenuation is 3 dB. After that, the attenuation asymptotically approaches 0.
Now, what happens if we open up the bore diameter to a diameter d1 that is larger than the bore diameter, for a length L1? We get a low pass filter response for that section of the filter. What does a low pass filter do? It lets through low frequencies up to its cut off frequency and attenuates frequencies higher than the cut off. It also has a slope of -20dB per decade, the opposite of the high pass filter.
If we cascade these filters, the composite response is just the addition of the two transfer functions (in dB's). We now have a bandpass filter. Pretty much like Stan said. Now I found some equations, that I still need to prove to myself are "good enough", but I put together a program that plots the expected response.
Here are two graphs. I'm not saying this is realizable or practical at the moment, but it is something that at least remotely makes sense to me. If we have a tube with an OD of say 30mm and have a bore through it with an ID of 12mm, the HPF (high pass filter) cut off is 16.8KHz. If we then carve out a section in the middle of the moderator and open up the diameter to 25mm for 65 mm, the low pass filter (LPF) that is formed has a cut off frequency of about 500Hz. If we combine the two filters the minimum attenuation is expected to be 31 dB. I don't think there is any harm in putting the LPF in the center of the LDC, with the narrow bore tube leading to it.
If we do the same experiment, but instead we have a bore of 8mm, rather then 12mm and changing nothing else, the cut off frequencies for both filters change, they get further apart. Guess what, we get 41 dB of attenuation! At the very least it seems promising. Anyways, that's my days work on this subject.
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Bruce,
I was not very clear with all the dB numbers I offered: The "70 dB" was not relative to anything, it was absolute. So not a reduction of 70 dB, but the loudness of a very quiet muffler equipped airgun report. Was that obtained by standard PB measurement method, 1 meter to the side of the muzzle, 1.5 meter off the ground? Almost certainly not.
So, rather than provide dB numbers and then argue about the calibration of the measurement system, it helps to shoot a number of different mufflers side by side, and compare them to each other and the bare muzzle.
As I am not the one doing the testing, I cannot control the set-up; only suggest it. Most important to me is avoiding sound levels that saturate the meter, or are too close to its rate full scale capability to be "accurate". So, when I suggest standing the meter off by 5 meters to avoid meter limitations, the absolute readings are going to be a lot lower, than if they were measured at 1 m with a proper meter.
I am playing catch-up reading the rest of this thread, so I wanted to clear up the obvious confusion I created above, first.
I pretty much knew the moderator didn't offer 70dB attenuation, at least not in the pass band. This is why I asked, what was the unmoderated value. From those two values we can determine attenuation. And I agree with most of your perspective on testing. Can't saturate the meter! Anyways, I made a little progress on my ideas, as shown in the previous post. At least there is some kind of indicator that sort of points in the direction to head.
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Ok, you've piqued my curiosity. What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work? I just used 50 and 50 with overall length 165.
Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore? Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance? My reason for asking is to simplify or overcome 3D printing limitations.
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Bruce,
A number of your statements have prompted answers and ideas; pasted and addressed below:
Perhaps if we could get a spectral plots of the pleasing vs non pleasing moderators AND the un-moderated input, we actually might be able to determine what the differences in transfer function might be.
This is a great idea. Bob (TorqueMaster) has several spectral analyses of different muffler sounds. It might take him some digging to find good examples. From my perspective, a pleasing sound produces not just a lower amplitude full spectrum magnitude/time chart, but it starts with a taper, rather than at peak amplitude.
In my opinion, there is no reason the aperture has to be either uniform, or tapered, it could have a pattern so the transfer function is more pleasing.
If we look at the exhaust cones on the latest passenger jet engines, that V-notched shape may be inspirational. That shape apparently reduces noise pollution considerably - by producing a less abrupt shockwave, rather than filtering out noise. See attached images.
I will try to conjure something meaningful that incorporates the V-notch concept, that is also 3D printable. Speaking of printable, flat baffles are tricky to print, so I use cones in my designs. I am convinced of the effectiveness of progressively slanting cones, in managing air flow as the bulk pressure decays, based on lots of test results - much of which Bob TM produced; followed up by happy customers buying prints of my designs, many as printed by Bob. Many printed by interested parties for their own use.
I have it on good authority that TKO mufflers use flat baffles, to very good effect. So there is at least one counterpoint to using cones. When it comes to "air stripping" turbulent air from the pellet path, cones are deemed superior to flat baffles. I presume this "common knowledge" is backed by test results and optimization.
Can't imagine there is a frequency shift, it's not an active device.
If I bang on a drum (skin), twice per second, what is the frequency of the sound coming off the drum? If it were 2 Hz, then a small drum would not sound different from a large one; yet they clearly do.
My point is that a muffler does not just filter the sound of a bare airgun muzzle blast; a different sound is generated inside the muffler, by each abrupt expansion event that occurs in successive chambers, starting with the first chamber. All of those thumps are applied to the air inside, making it ring at its natural frequency. Ditto for the structure of the casing. Some of that casing vibration is re-applied to air in surrounding chambers. The baffle bore communicates between chambers potentially exciting or damping the other chambers, by virtue of the degree of phase shift between them.
Even if all the chambers are identical, the fact that the bulk pressure decays from inlet to outlet means the natural frequency of each chamber is lower than the one before it. The natural frequency of all chambers drops as time passes, due to the drop of the bulk air pressure inside over somewhere around perhaps 1/3 second; with perhaps the first one or two 1/10 of a second being the most significant from a sound signature perspective.
Muffler that drive deliberate turbulence to slow down the exit of air from the front end, probably produce a lot of white noise, or hiss. So the model for what happens when air has pressure waves bouncing between walls, may not apply to high turbulence flow device. Or not apply to the same extent.
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Starting with the 6.5 mm bore muffler attached to the OP, I did the simplest design modification I could think of, to emulate the jet engine V-notches seen at the turbine and bypass fan exhausts, in the images above.
The new V-notched baffle bores span the (as printed) ID of 6.5 mm at the sharp tips, reaching 8.5 mm ID that the rounded notch "roots". So, spanning the total 2 mm baffle bore diameter range of the three mufflers in the OP, but having a non-circular exit.
There are 9 notches at each baffle bore, so that none of the facets are parallel - as would be the case for an even number of facets. See images below.
There are two variants to this design. V1 has the same 1.2 mm wide flat annular ring on the leading baffle face, as the mufflers in the OP. V2 extends the baffle forwards to a sharp edge (to a point, after the notches are cut).
V2's baffle bore leading edge shape will be limited by the slicer and printer, in terms of how sharp those points will be, but the flat surface of V1 is in the same area is avoided. The leading edge of a baffle acts as the exit of the previous baffle chamber, so while the scale of these jagged details is small, the effect may or may not be significant.
I predict that V1 and V2 will produce more hiss than the original set of three mufflers, but will not outperform them in dB or sound quality. If, on the other hand the V-notched baffles provide a signal that suggests an improvement, then trying to optimize that will be worth the effort. Up till now, the closest I have come to aircraft engine nozzle notches are V-structures I use as permanent printing supports for overhanging aspects of a design - see last image for that.
The STL files for both V1 and V2 are attached in one ZIP file, below.
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Ok, you've piqued my curiosity. What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work? I just used 50 and 50 with overall length 165.
Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore? Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance? My reason for asking is to simplify or overcome 3D printing limitations.
What you have shown is the idea I modeled. As I mentioned, it's not clear that I have done all my homework on the correctness of my sources, ie I found some stuff on the Internet.
I don't think one has to have square edges exactly. Perturbation theory would indicate that minor chamfered or filleted edges wouldn't change the answer much. Unfortunately, at the moment I don't know how to model that analytically. The only way would be to simulate it numerically, and I'm not yet up to speed there.
The only caveat I can think of is that the main tube impedance is not matched to the LPF. That would take some work to verify.
I was going to print something quite similar, since it's dead simple. Ran out of time yesterday doing research, analysis and chasing down math errors.
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Ok, you've piqued my curiosity. What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work? I just used 50 and 50 with overall length 165.
Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore? Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance? My reason for asking is to simplify or overcome 3D printing limitations.
I neglected to answer one of your questions. The derivation only applies to circular cylinders, not square or rectangular.
Something else that I ignored for this is the possibility of multiple propagating modes in circular guides. Might need to add some features for mode control, but hey it's rather early in the design cycle for me. I thought it was exciting to start to see a design moving in the right direction. It's expected that some iteration may be required.
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@subscriber it's hard to address your posts! Lots of good stuff there. I can't effectively answer much on the phone, but may comment later. I hope some of the stimulus I provided gets your creative juices going. Hadn't thought about jet engine noise management, but this stuff is all similar. Good to see some effort into trying this out. When I get a chance I will take a look at the stl files.
Based on what I've seen, a multiplicity of short chambers of the same diameter and differing lengths correspond to multiple low pass filters of different cut off frequencies. I'd wager that they interact with each other rather than be totally independent of each other. Until one understands the interaction, I'd stay with a single longer chamber, which does actually have a lower frequency cut off. But this is conjecture at the moment. It should be possible to design multi-pole filters, for more attenuation, but at least for me, I need to walk before I run. And finally, one can perform complex filter synthesis which gives even more options. But as you well know, sometimes the options are not physically realizable within the packaging constraints. But in general, simple structures are better and potentially easier to construct.
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I did a simple man test of basic aerosol can mufflers, found they did the trick on low power, most sound was resonating from cylinder ping.
(https://www.gatewaytoairguns.org/GTA/gallery/albums/Back_Roads_Basement/Dir_1/main_7711.jpg)
(https://www.gatewaytoairguns.org/GTA/gallery/albums/Back_Roads_Basement/Dir_1/main_7710.jpg)
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I did a simple man test of basic aerosol can mufflers, found they did the trick on low power, most sound was resonating from cylinder ping.
(https://www.gatewaytoairguns.org/GTA/gallery/albums/Back_Roads_Basement/Dir_1/main_7711.jpg)
(https://www.gatewaytoairguns.org/GTA/gallery/albums/Back_Roads_Basement/Dir_1/main_7710.jpg)
Makes sense. Yeah I'd expect that baby to ring! Tried a dense rubber foam sleeve to damp the ping? Might make it a little more tolerable. Or even wrap it with a wet towel...
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Nice!
I think that's one of the basic questions for LDC designs. How much noise is coming from the front opening and how much is coming through the walls. If you were to overlay your aerosol can over the video I posted in #39, I would guess all of the muzzle turbulance is gone by the time it hit the end of your can.
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@Back_Roads moderator is a giant low pass filter, with ringing walls. It would be very effective with some damping, I'd bet. For my simplified calculations, the walls are assumed to be infinitely stiff, but unfortunately for us, they aren't.
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Another thought, the cavity is so large it is possible to excite other modes. These modes could resonate at undesired frequencies. It is possible to design some internal structures that favor a single mode.
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Another thought, the cavity is so large it is possible to excite other modes. These modes could resonate at undesired frequencies. It is possible to design some internal structures that favor a single mode.
Something as simple as a depinger and inner wrap sound dampener IMO.
I do have a sorta name brand LDC 2nd so no name attached, that rang, I gave it an outer wrap of heavy self adessive vinyl to deaden the ping.
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Another thought, the cavity is so large it is possible to excite other modes. These modes could resonate at undesired frequencies. It is possible to design some internal structures that favor a single mode.
Something as simple as a depinger and inner wrap sound dampener IMO.
I do have a sorta name brand LDC 2nd so no name attached, that rang, I gave it an outer wrap of heavy self adessive vinyl to deaden the ping.
Can't argue with what works!
If something like this were printed, I think it's possible to do something to the diameter that would reduce pinging of the structure. Or, at least it is possible to do in microwaves, hoping there's an acoustic analogue.
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Printing something. We'll see if it works at all. There's nothing like the acid test of reality to shatter all your dreams. I'm not sure that I printed the best of my options. It might be it is better to locate the expansion chamber sooner. I'll test it on a low power 2240 first, no need to blow it up first try!
Anyone got a recommendation for a not too expensive microphone I can plug into my PC?
Here's an off the wall question, anyone got a recommendation for a mini speaker, oh maybe only 12mm wide? I was thinking of mounting at the input of the LDC, and sweeping a tone, and measuring the output at the exit. I could measure with and without the LDC. Poor man's filter response. A while back I programmed an Arduino to make a stepped frequency test tone. If I could use it to power a speaker, then I could measure the relative output of the moderator. Shooting is fun and all, but I think the measurement would have less noise in it using a swept source.
Edit: the ultimate test is always shooting through an LDC!
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Ok, you've piqued my curiosity. What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work? I just used 50 and 50 with overall length 165.
Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore? Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance? My reason for asking is to simplify or overcome 3D printing limitations.
I neglected to answer one of your questions. The derivation only applies to circular cylinders, not square or rectangular.
Something else that I ignored for this is the possibility of multiple propagating modes in circular guides. Might need to add some features for mode control, but hey it's rather early in the design cycle for me. I thought it was exciting to start to see a design moving in the right direction. It's expected that some iteration may be required.
Three actually.
Is there a preferred length for the 12mm (or 8mm) bore ahead of the chamber?
Is there a preferred length for the 12mm (or 8mm) bore after of the chamber?
And I asked, for these cavities, does the exterior shape matter? Does the math require that moderator outside shape is circular, and the cross-sections look like pipes, or thick pipes? My understanding was that only the cavity shape and dimensions mattered, but I may have just assumed that.
I would usually print such a thing vertically, but the "upper" inner flat chamber wall will not be very clean (droopy) if I did that. Next choice is print horizontally, which would be pretty clean with the exterior as a cylinder, but better if the printbed side was flattened.
Curious what your design looks like and what orientation you're printing it. And how it performs.
I use this mic on my phone. Nothing fancy, but it had pretty good reviews from folks who sounded like they knew what they were talking about. It's disco now, but maybe it's still available under a different name.
https://www.amazon.com/gp/product/B08DJ7V859
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Ok, you've piqued my curiosity. What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work? I just used 50 and 50 with overall length 165.
Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore? Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance? My reason for asking is to simplify or overcome 3D printing limitations.
I neglected to answer one of your questions. The derivation only applies to circular cylinders, not square or rectangular.
Something else that I ignored for this is the possibility of multiple propagating modes in circular guides. Might need to add some features for mode control, but hey it's rather early in the design cycle for me. I thought it was exciting to start to see a design moving in the right direction. It's expected that some iteration may be required.
Three actually.
Is there a preferred length for the 12mm (or 8mm) bore ahead of the chamber?
Is there a preferred length for the 12mm (or 8mm) bore after of the chamber?
And I asked, for these cavities, does the exterior shape matter? Does the math require that moderator outside shape is circular, and the cross-sections look like pipes, or thick pipes? My understanding was that only the cavity shape and dimensions mattered, but I may have just assumed that.
I would usually print such a thing vertically, but the "upper" inner flat chamber wall will not be very clean (droopy) if I did that. Next choice is print horizontally, which would be pretty clean with the exterior as a cylinder, but better if the printbed side was flattened.
Curious what your design looks like and what orientation you're printing it. And how it performs.
I use this mic on my phone. Nothing fancy, but it had pretty good reviews from folks who sounded like they knew what they were talking about. It's disco now, but maybe it's still available under a different name.
https://www.amazon.com/gp/product/B08DJ7V859
Sorry about not answering all your questions.
1. I don't know actually. I just finished printing one with the big cavity in the middle. But it may be better to have the cavity shifted elsewhere.
2. Same.
3. The exterior shape (first order) shouldn't matter. The primary acoustic properties should be dominated by the interior dimensions, if the material is stiff enough. But, I'm not an ME.
I ended up with a design like attached. I printed it vertically, threads up so I had to accommodate for the overhang. I'll find out within the day if this is just a stupid idea or not... Believe me, I will let everyone know, good or bad. I printed in draft mode and used 50% infill, simply to reduce print time. It isn't beautiful, but hopefully it sort of works, at least for low power.
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Thanks for the answers! Your design is what I had in mind for the 2nd generation if it didn't mess up the calculations -- 45 degree angled end walls on the chambers. Which, if you used two or more chambers, with a 2+ mm space between them -- it starts looking like...conical moderator baffles inside a cylindrical shell. I have a hunch this single chamber will work to some degree.
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I just chased out the threads with a tap and dry fired it on my 2240.
It is noticeably quieter than my slim LDC. It was lower in pitch and sounded more like a snort than firing. The action of the hammer and valve are a lot more pronounced. It doesn't sound like a gun, more like something pneumatic.
Think I made a mistake on it for the seat of the threads, but this unit impresses me. No, a 2240 is not a high power PCP. I'll fire it with a pellet to see if there is any significant difference in tone. Then I will try it with my PP700SA.
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Anyone got a recommendation for a not too expensive microphone I can plug into my PC?
I think you said you had a 96Khz sound card. This is the mic I got for these type of measurements.
www.amazon.com/gp/product/B00ADR2B84
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I can report that firing a 15gr Daisy HP pellet sounded identically the same, save for the pellet slap when it hit my rubber mulch target. It's a lot more pleasant to shoot, especially in my office. I used a 7.5mm bore for the narrow section. I'd bet it printed slightly small, which actually makes it a more effective moderator. Sort of a snort, burp like sound then whap. I would need to set up something a bit less casual to check for clipping or sliding down the moderator. The pdf file is what I printed.
All in all, I'm astonished really. Definitely going to have a celebratory beer tonight.
Later on, I will try out the same moderator on my PP700SA. Beats me what velocity it shoots, I need to chrony it. The PP700 is a whole lot louder and more powerful than a 2240.
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Anyone got a recommendation for a not too expensive microphone I can plug into my PC?
I think you said you had a 96Khz sound card. This is the mic I got for these type of measurements.
www.amazon.com/gp/product/B00ADR2B84
Thanks Stan. I will check it out.
Right now, my ears are not deceiving me, this unit I just printed is loads quieter than the slim LDC I posted in one of my threads. It's lower pitched as well. No sharp sounds at all.
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I've been using Audacity (free) for the capture and analysis.
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Good job, Bruce
What you have there may be the longer last stage of a multi chamber LDC, where the latter is taking the muzzle blast of a higher power PCP. In other words, the previous chambers drop the pressure significantly, to feed the last long chamber a more gentle pop (or series of pops), that is closer to a stream. This theory will collapse, if your single chamber LDC is as quiet at 30 FPE as a multi chamber LDC.
How important is the significant length of the 7.5 mm ID sections? My latter designs make those short, because the solid material around them seems like wasted expansion volume, in any given length casing. Also, I have concerns about the pellet being steered aerodynamically by the air stream down such a narrow tube; but one thing at a time. Certainly, the long narrow sections represent resistors and the open chamber a capacitor, to act as an AC filter.
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Good job, Bruce
What you have there may be the longer last stage of a multi chamber LDC, where the latter is taking the muzzle blast of a higher power PCP. In other words, the previous chambers drop the pressure significantly, to feed the last long chamber a more gentle pop (or series of pops), that is closer to a stream. This theory will collapse, if your single chamber LDC is as quiet at 30 FPE as a multi chamber LDC.
How important is the significant length of the 7.5 mm ID sections? My latter designs make those short, because the solid material around them seems like wasted expansion volume, in any given length casing. Also, I have concerns about the pellet being steered aerodynamically by the air stream down such a narrow tube; but one thing at a time. Certainly, the long narrow sections represent resistors and the open chamber a capacitor, to act as an AC filter.
The initial reason for the lengths at 7.5mm is to ensure the section of pipe is truly acting as a high pass filter. If it is short, like a mm or so, it really doesn't act as one. To be effective as a HPF, we would need the low frequencies to die out. Technically we need to determine say 5 to 10 length constants, which I have yet to calculate. The field (acoustic or EM) will decay exponentially with a length constant l1. l1 is dependent on the guide dimensions and the frequency. In 5-6 length constants the energy has basically vanished. Since I haven't determined the value yet, I conservatively chose rather long values. After a determination is made, I can see if it makes sense to shorten up those skinny sections.
I'd think the long narrow section would be predominantly inductive rather than resistive, and the open areas could be thought of as capacitive. At least that's the microwave analogue. In the microwave world, we make can make filters from distributed inductors and capacitors. Resistive elements are infrequently used.
I'm leery of the narrow tubes myself. Seems like it's just waiting for something bad to happen there. Steering, crashing, you name it. Honestly, I gritted my teeth when I pulled the trigger with the pellet, I was expecting something not good to happen, but things were fine (as far as I can tell).
But the thing that really makes this particular design work is the ratio of the squares of the two diameters, the higher the ratio, the lower the low frequency cut off. And the narrower the tube, the higher the HPF cut off is. That's how the math works out. But there's still a lot of topological tricks that still could be played, so not to worried about that.
I wish there was an more effective way to push out the HPF cut off than bore diameter. Going to have to think about that. I really need an acoustic simulator, but I don't have one. At the moment, I'm a long way off from running CFD. I can't even get the examples to run! I want to be able to see the effects of multiple sections, and their interactions.
That all being said, I think there's still some gain to be made with this simplistic approach. But to make more real progress, will require better tools. Then I could visualize turbulent flow and could mess with some more of the more interesting structures and get some quantifiable results. On the other hand, simulations can take a long time. Sometimes it's faster to try it...
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I've been using Audacity (free) for the capture and analysis.
I have used it before. Reinstalled it on my laptop a couple of days ago. You wouldn't happen to know what the allowed windowing functions are? Never mind, I found them. It seems you can't put in custom filters (that easily).
I would strongly recommend NOT using boxcar, or rectangular windows (another name for the same thing). Spent my whole life telling colleagues never to use it. You get high spectral splatter, especially for signals with high dynamic range. This splatter masks other smaller signals. As a radar engineer my job was to find picowatt signatures in milliwatt noise. You just can't do it with rectangular windows. Other window functions are far, far better. Hamming are not bad, but out of what's available, I'd use the Blackman-Harris. I used to use modified Kaiser windows.
Hmm, I just read, may be a use for rectangular windows is for impulse analysis... I'll keep that in mind, although my experience is warning me otherwise...
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Bruce,
As you can see from the attached images; I took a wire EDM to your PDF design. Then, while keeping as much of your original dimensions intact as I could, I recaptured expansion volume at the front and rear of the tube, keeping the long 7.5 mm bore tubes at your design lengths.
The 25 mm ID section is still 75 mm long, but with a "soft wall" that communicates into the new secondary spaces.
While this design may violate your principles, my approach is that you have to worry less about the frequency of reflected waves, if the reflections are weaker; and more diffuse. That is what I expect the short support ribs to do - act as diffusers. And "confuse" the resonant frequency of the 25 mm ID tube.
It would be great, if you could throw this chimera into the mix for your testing purposes. Ditto, anyone else that is curious about how your single chamber LDC works - please add my interpretation to the mix.
STL attached below
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I forgot to mention; these mufflers need to be printed standing vertical on the printer platen, with the threads uppermost.
I have the 8.5 version in hand, just printed for me by a friend in his spare time. Am establishing no-pellet, to-the-ear shooting noise baseline for now, will try to get at least the 6.5 printed soon for comparo's sake. If promising will find an appropriate venue and set up the meter and shoot real pellets through...thx!
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Thanks, Charlie
Any chance you could test the 7.5 mm one too, if you are using some sort of dB meter on the 6.5 and 8.5 mm versions? There appears to be something interesting going on with bore size that was not expected; hence my question.
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So, Chimera 1 is the moderate hack of Bruce's test piece. Chimera 2 is how I would optimize it for reflected wave damping with some surfaces having the 45 degree aspects flipped; and some flat. Then further restricting the flow area into the quench zones between the support structures neat the ends of the 7.5 mm bore "tubes". Then adding fillets in may places to make like easier on the printer; and outside esthetics.
STLs for both Chimera 1 and 2 attached below
Image in next post compares Chimera 1 and 2. If you are inclined to print only one of these two, go with version 2 please.
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Image below compares Chimera and 2:
C-1 and C-2 designations are embossed in front cap.
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If I did this right, and I'm not sure if it is correct, I calculate approximately 4dB attenuation of the lower frequencies per mm of tube length, due to the small tube ID structure. Which is why I think the ID small sections need to have some length. But I see some weird stuff that I need to sort through if I have appropriate formulations. A lot of the references use different ways of talking about the same things, and use different variable names which makes things tough to sort out. My old book is still missing in action, it might be in storage. Have to say understanding EM and acoustics after a 25 year pause is tough, especially for an old dog. It's like getting slapped in the face, repeatedly. But it's fun trying to dust off the cobwebs.
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Went off to where I store
junk stuff and rummaged through some old dusty boxes. I found my ancient copy of Matthaei, Young and Jones, which was originally copyrighted in 1964. I bought that book in 1985 for the princely sum of $79. That was big money for a book then.
In it are some really old school methods of design, a lot of which were prevalent in the 50's and 60's, before people had much access to computational methods. (Before wide spread adoption of computing available to the public.) There are some interesting filter synthesis methods in there. I will browse through the book looking for ideas that may be transferable to acoustics.
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Image below compares Chimera and 2:
C-1 and C-2 designations are embossed in front cap.
Printing a C-2, just because. Has some nice design elements in it. Just wanted to mention that I appreciate your efforts, and your design abilities. You have an artistic flair, which is nice to see.
I goosed up the hammer on my PP700, so it is a bit noisier. My LDC is letting through more noise than I'd like. Looking forward to trying this C-2. And good idea on printing on the designation! I need to learn how to do that in my version of CAD.
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Thanks for your eager persistence, Bruce
Without the identifying characters on the front end, confusing too many similar mufflers would be easy. Especially the set of three from the OP study. One might just assume that the rank order of dB reduction is smallest to largest bore diameter.
Speaking of which, OldSpook on AGN tested the same exact muffler set as Mike (miksatx), and saw the same rank order anomaly, when the meter was placed 3 m to the side of the muzzle. Ahead of the muzzle, the results followed the direction of increasing baffle bore diameter. So, meter placement matters, probably because more high frequency noise is heard ahead of the muzzle, than to the side.
OldSpook's sound recordings showed a significant lowering of the report's tone, with reduced sharpness, between the 6.5 mm muffler bore and the 7.5 mm bore. With a smaller change between the 7.5 mm bore and the 8.5 mm bore: https://www.airgunnation.com/members/oldspook.34264/#profile-post-309
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Well, drat. Print failure after 4-7/8". Somewhere around layer 435 out of 588. The print fell over. That's it for this evening. :( I'll try again tomorrow morning. Next time I will reapply the hair spray on the sheet. Haven't had an LDC or anything else fall over before. Guess there's always a first time.
Maybe I will saw this C-2 in half. It's not good for anything else.
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Bruce,
It would be interesting to see what the print looks like inside.
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Thanks for your eager persistence, Bruce
Without the identifying characters on the front end, confusing too many similar mufflers would be easy. Especially the set of three from the OP study. One might just assume that the rank order of dB reduction is smallest to largest bore diameter.
Speaking of which, OldSpook on AGN tested the same exact muffler set as Mike (miksatx), and saw the same rank order anomaly, when the meter was placed 3 m to the side of the muzzle. Ahead of the muzzle, the results followed the direction of increasing baffle bore diameter. So, meter placement matters, probably because more high frequency noise is heard ahead of the muzzle, than to the side.
OldSpook's sound recordings showed a significant lowering of the report's tone, with reduced sharpness, between the 6.5 mm muffler bore and the 7.5 mm bore. With a smaller change between the 7.5 mm bore and the 8.5 mm bore: https://www.airgunnation.com/members/oldspook.34264/#profile-post-309
Persistence is one of my character failings...
Interesting stuff. Did an audio recording of a few shots but not satisfied with what I got. I need to play with Audacity more. I was getting a lot of background noise due to the mic gain being turned up, and my 3d printer running in the background. I generated a spectrum, but didn't think it was good due to the background. Slow incremental progress.
As for the failed print, maybe I will add a brim next time.
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Bruce,
It would be interesting to see what the print looks like inside.
I think so too. Will use a slitting saw on it tomorrow. Unfortunately printed it black, which is tough to photograph. Next one I will print in red PETG. For testing that's fine.
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So, the idea of piping more air to the front of the muffler, directly from the blast chamber, without "pointing" the duct openings directly at the frontal exit, to avoid transmitting shockwaves directly out, inspired the monstrosity below.
It looks like Nicola Tesla and Werner Von Braun's love child - if such a thing were possible. The bypass tubes would act to some degree as a Tesla Valve, although 12 tubes each with a 3 mm bore diameter are not going to flow that much air. But the idea is to pipe a controlled volume of air into the last expansion chamber, so that it can leave ahead of the projectile (and the air) travelling down the sequential baffle bores; but without its energy, or more specifically, shock being being transmitted directly out the frontal bore. So the momentum of the piped air will need to stagnate in the last chamber, and turn around before leaking more gently out of the front bore.
Can this happen in the amount of time it takes the projectile to leave the front bore? This would seem like expecting the air to exit very suddenly, but yet merge with the ambient air more gently, is asking for a lot...
Will this contraption do anything useful or interesting? I don't know, but there is a way to find out. The zipped STL is attached for anyone wanting to have a go.
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I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?
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I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?
I'd think one might have a lot more flow with your idea of an outer sleeve. Might have 4 dividers, maybe six, rather than so many little pipes.
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I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?
Quite honestly, because that was the idea that woke me; begging to be brought to life. An outer tube with a space between tubes would have been so much easier to model. If there is a good reason; I wanted the lower duct entrances to point upwards at 30 degrees. I also wanted to be able to change the duct ID and number of ducts easily in Solid works. Probably possible the way you suggested. If there was one "excuse", it might be the amount of added material.
So, no compelling reason, other than Bruce has been telling us about the acoustic damping properties of long skinny ducts. So, that must be the reason this was the vision that popped into my mind. Certainly, there will be no looking at this monster and confusing it for something else :)
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I'd think one might have a lot more flow with your idea of an outer sleeve. Might have 4 dividers, maybe six, rather than so many little pipes.
Quite right, but the idea was "some flow", rather than as much as possible.
Modelling that part was intense work. It was a challenge, and took considerable jiggery-pokery to complete. So, fun and showing off might have been factors.
But, mostly, Bruce, I am going to credit you with the idea that long skinny tubes have acoustic damping properties. And here they are....
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I'd think one might have a lot more flow with your idea of an outer sleeve. Might have 4 dividers, maybe six, rather than so many little pipes.
Quite right, but the idea was "some flow", rather than as much as possible.
Modelling that part was intense work. It was a challenge, and took considerable jiggery-pokery to complete. So, fun and showing off might have been factors.
But, mostly, Bruce, I am going to credit you with the idea that long skinny tubes have acoustic damping properties. And here they are....
I'll bet it was a bit of work. How about an outer tube with radial sectors to partition them, I'd imagine it would be a lot easier to model, and it would preserve the small tube aspect without being onerous to model. On the other hand, it seems you rather enjoy it! The sectors could be fed by holes coming in at similar angles from the main chamber.
At the moment, your design ability far outstrips my ability to analyze or print things. If I am lucky, the Chimera-2 will print in about 4 hours. I will try variable layers primarily in the thread region. I didn't like how 1.27mm pitch threads came out with 0.3mm layer height, they tapped ok, but it wasn't easy starting the tap.
I took the liberty of slicing open the failed print from yesterday. I used a 1/16" slitting saw and roughly sawed it in half lengthwise. Had to clean up the edges, but otherwise isn't bad.
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Bruce,
That thing looks almost organic. :)
Well, I may or may not produce a cleaner looking bypass design; but I have added a bracing structure to the middle of the tube span.
Another reasons for the ducts that seems obvious: I want to feed the air through the ducts fast to the front chamber; ahead of the projectile. If the air first filled an annular plenum, it would lag behind while pressurizing that space.
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V3 "aims" the lower vent openings at the middle of the upstream baffle bore, to more effectively dissipate the shockwave inside the previous expansion chamber.
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Finished printing the Chimera-2. Sprayed the platen with hairspray and used a brim. So this one stayed attached.
The C-2 is close in tone to mine, although truthfully, I'd give a slight edge to the C-2. I'd like both to be lower toned, but that's because I'm spoiled by either one of these LDCs on a 2240. That sounds like a low snort, not a shot. Perfect for indoor shooting.
Lot more high frequency content on a PP700. Lower pitch usually means bigger diameter. I could comfortably go to 32mm OD. Beyond that gets unwieldy.
Need to get a better microphone, the built in one on my laptop isn't very good. I'll check out Stan's suggestion.
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https://www.daytonaudio.com/product/1117/imm-6-idevice-calibrated-measurement-microphone
Note it has a trrs 4 ring plug
I was using it with an older laptop so I used this USB soundcard
-https://www.amazon.com/gp/product/B00KGK0YZ0
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Anyone got a recommendation for a not too expensive microphone I can plug into my PC?
I think you said you had a 96Khz sound card. This is the mic I got for these type of measurements.
www.amazon.com/gp/product/B00ADR2B84
How do you use it? Kind of puzzling from the listing.
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https://www.daytonaudio.com/product/1117/imm-6-idevice-calibrated-measurement-microphone
Note it has a trrs 4 ring plug
I was using it with an older laptop so I used this USB soundcard
https://www.amazon.com/gp/product/B00KGK0YZ0
You posted while I was writing mine. The link you provided helps.
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It shows up as a mic input in windows and Audacity. You just need to make sure the 96 khz is selected in both if that's what you are using.
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It shows up as a mic input in windows and Audacity. You just need to make sure the 96 khz is selected in both if that's what you are using.
Is the calibration file downloadable from a browser?
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I didn't use it but yes, if I remember, it is on the Dayton website.
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Lower pitch usually means bigger diameter. I could comfortably go to 32mm OD. Beyond that gets unwieldy.
Does the length of the tube affect the pitch? I would have thought it had more effect on tone than the diameter, as in "tuned length" intake and exhaust runners; or organ pipe.
Are you tempted to evaluate V-3 of the long tube bypass monster? STL here: https://www.gatewaytoairguns.org/GTA/index.php?topic=211053.msg156489512#msg156489512
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Lower pitch usually means bigger diameter. I could comfortably go to 32mm OD. Beyond that gets unwieldy.
Does the length of the tube affect the pitch? I would have thought it had more effect on tone than the diameter, as in "tuned length" intake and exhaust runners; or organ pipe.
Are you tempted to evaluate V-3 of the long tube bypass monster? STL here: https://www.gatewaytoairguns.org/GTA/index.php?topic=211053.msg156489512#msg156489512
It actually is a function of both the area of the tube and its length. But since it's area, that goes up as a square of the radius, whereas for length it is just linear. So double the radius, the frequency goes down by a factor of 4, where as doubling the length, the frequency goes down as a factor of 2. So a minor change in radius is a significantly faster way to lower the frequency. Or at least that's how it is for the low pass filter that I studied. Recirculating designs might look a lot longer, so there is that.
Had enough printing for the day. I'll take a look at your file, though. Printing bothers my eyes, so I can't do it a lot in my office.
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The multi tube design reminds me of how Bose gets tons of base response from a tiny speaker.
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The multi tube design reminds me of how Bose gets tons of base response from a tiny speaker.
If that bypass tube design does something useful, I'll find a nicer way to package it.
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I was dying of curiosity about that last design, so I did print it last night. It looks like a Buck Rogers/Mad Max sort of thing. It has a big bore at 8.5mm which printed around 8.3mm, which is cavernous compared to my earlier prints. PrusaSlicer warned me that it might have issues with tipping over and suggested a brim and supports. I only added a brim. It was printed in draft mode 0.3mm layer, with variable layers for the threads. Required no tapping, which was nice. Saved me a trip to the shop for the tap. 4hr 44min, so like most 3D printing, kind of slow.
I'll say it works better than I expected, although it's a bit louder than the C-2. Doesn't seem to be much sonic difference with adjusting the hammer spring on the PP700SA. This is with dry firing. In previous LDCs, there didn't seem to be a noticeable aural difference between dry and live fire, so I didn't bother using a pellet.
I ordered the electret microphone. Hopefully I can set up a better rig for taking audio measurements so I would have a chance at repeatable measurements.
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Doing it now on my new printer, my 3 s1pro failed to print it.
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Interesting, Bruce. Try them with pellets, when you get a chance.
Yes, I grabbed the 8.5 m bore when I meant to grab the 7.5 mm. Could easily modify the bore after the fact, if anyone cares.
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3-D printing potentially allows for some really unconventional procedural surfaces and shapes - things with complexity that really defy modeling and are typically generated mathematically instead. For example, different infill patterns with 3-D prints. Some of those are very complex, and some new ones very fractal like.
What about filling the space with something like infill structure? For example, what if you had a bore surrounded by hundreds of small tubes radiating outwards and all angled at whatever is optimal for diverting flow? I recognize a lot of volume is lost, but this could be just for one section like one baffle could have some kind of diffusing characteristic. I think I saw one moderator that actually has a 3-D printed sponge like material that breaks up shockwaves I guess
How about taking a look at biological systems that might be well adapted - for example branching bronchial passages with little chambers like alveoli at the ends?
There’s gotta be something special waiting to be discovered there if anybody can do it it seems like it would be you guys.
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Interesting, Bruce. Try them with pellets, when you get a chance.
Yes, I grabbed the 8.5 m bore when I meant to grab the 7.5 mm. Could easily modify the bore after the fact, if anyone cares.
As you might imagine, the sound is subjective. Think it is snappier and perhaps longer duration with max max, than C2. Maybe there's reverberation with mad max. C2 has a shorter duration and seems a little quieter. Both could use some additional lowering of the pitch, but maybe that's just a personal preference.
For the above test, I used a 22 cal barrel and pellets for the PP700SA.
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How about taking a look at biological systems that might be well adapted - for example branching bronchial passages with little chambers like alveoli at the ends?
As I was reading your description I thought, "that sounds like lungs". Then I saw the bit above.
Interesting idea. Someone skilled in infill placement would have to figure out how to do it. The infill pattern used by the guy making the foam filled mufflers is called "gyroid". My skill would end at producing CAD for the tubes, or vents feeding the infill.
Of course, lungs are flexible, so the inside should be printed with TPU or other rubbery material.
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Thanks, Bruce
Perhaps the tubular bypass idea should be bypassed, for now. As a concept, it seems much easier to control bypass air leakage via baffle bore diameter.
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Cobbled up something. Not that pretty, but worth a try. I put in a lot of vents towards down range. The vent sizes are small and only should pass ultrasonic frequencies. However, the area of all the vents combined is comparable with the bore area. Going to give it a try. It's wide, a 40mm fat boy. May or may not work. It's a print over night kind of deal... Kind of like the C-2, but with forward micro-vents. I'm trying long 1.5mm diameter vents. Think they only pass stuff around 130 KHz, nearly 7 times higher than most of us can hear.
I'll know in the morning, that's if I get a good print. If it fails, umm, I'm not sure I want to try another, its over an 8 hour print on my machine. I don't have that much patience.
In the meantime, I need to make an aluminum insert adapter for one of my LDCs. It has a 1/2-20 internal thread, and a 3/4-20 external thread with a flange. It's also has threads to attach to a CF tube. Maybe I will start out with a simple version that doesn't thread into a CF tube. Then I only have to do 2 20 TPI threads, an internal and an external.
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Glad to see someone else generating and exploring ideas, Bruce
I am not going to predict the performance, even though prediction is a hobby of mine. My only comment is about the ability of FDM printing to create small holes: Small holes tend to close in a lot, percentage area wise.
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Dang, woke up to a plate of red spaghetti :(. Printed 153mm worth out of 166mm. Got two threads at the top, and the LDC fell over.
I don't know. Used to have an over adhesion problem, now under adhesion. I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.
Might redesign the chambers to have less ribs and more air. Had 15 degree sectors with 20 degree spacing. Probably will double the sector width and make the ribs slightly wider.
Is there a way to add external support? Add a brim and supports from there?
I'd attach this file but it's way too large for this forum, zipped it's 18MB.
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Bummer
Should have a brim. Some of the other printer guys might give better advice.
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Bummer
Should have a brim. Some of the other printer guys might give better advice.
I figured at 40mm diameter it didn't need one, but I was wrong. This thing needs external supports to prevent rocking/vibrating. Wouldn't be surprised if the top was waving around while printing. Guess that's why they have other kinds of printers.
If I kick another one off, it will have a brim for sure. Going to increase the sector size, though. Too much filament usage. The reject print weighs 103 grams. I have to reduce print time, 9 hours is just too long, especially for a prototype.
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As another thought, someone mentioned TPU I can't imagine being able to complete something too complicated with it. At least when I tried, I got a LOT of stringing. Can't imagine it would resist gravity that well either. But, it got me to think. What if one changed the filament properties? Say we selected a denser material? That may make the printed object act either more ideally, or less.
The only filament that I know of with significantly higher density is one with tungsten powder in it. Anyways, just some rambling on my part.
Really should get back to my CAD program and figure out where my FEA stuff is. I'd love to be able to sim one of these.
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Brim. You could also model in some supports.
Also check your bed leveling. If the first layer gap has opened up and the first layer isn't getting squished down, that will affect adhesion.
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Brim. You could also model in some supports.
Also check your bed leveling. If the first layer gap has opened up and the first layer isn't getting squished down, that will affect adhesion.
Good advice. I will check the bed again.
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Dang, woke up to a plate of red spaghetti :(. Printed 153mm worth out of 166mm. Got two threads at the top, and the LDC fell over.
I don't know. Used to have an over adhesion problem, now under adhesion. I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.
Might redesign the chambers to have less ribs and more air. Had 15 degree sectors with 20 degree spacing. Probably will double the sector width and make the ribs slightly wider.
Is there a way to add external support? Add a brim and supports from there?
I'd attach this file but it's way too large for this forum, zipped it's 18MB.
Definitely a brim. If you use supports, choose "bed only" so they are not internal and inaccessible. If it's cylindrical, I doubt external supports will be attached automatically, you'd need to design and attach them. Shouldn't really be necessary though.
I use PETG on a PEI textured bed and it sticks like the dickens, no worries until I get over roughly 5:1 height to width. How fast are you printing? That would obviously affect the forces the print sees. I print pretty slowly ~30mm/s on average. Also if you are overextruding a bit, sometimes a glob on top will get smacked by the printhead and knock the print over.
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Dang, woke up to a plate of red spaghetti :(. Printed 153mm worth out of 166mm. Got two threads at the top, and the LDC fell over.
I don't know. Used to have an over adhesion problem, now under adhesion. I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.
Might redesign the chambers to have less ribs and more air. Had 15 degree sectors with 20 degree spacing. Probably will double the sector width and make the ribs slightly wider.
Is there a way to add external support? Add a brim and supports from there?
I'd attach this file but it's way too large for this forum, zipped it's 18MB.
Definitely a brim. If you use supports, choose "bed only" so they are not internal and inaccessible. If it's cylindrical, I doubt external supports will be attached automatically, you'd need to design and attach them. Shouldn't really be necessary though.
I use PETG on a PEI textured bed and it sticks like the dickens, no worries until I get over roughly 5:1 height to width. How fast are you printing? That would obviously affect the forces the print sees. I print pretty slowly ~30mm/s on average. Also if you are overextruding a bit, sometimes a glob on top will get smacked by the printhead and knock the print over.
That's exactly what happened. Glob above the z height got smacked. I saw it happen. And for the third time. What gets me is this hasn't happened before. Don't recall seeing globs. The globs were pretty small, only 0.5mm or less. But that's enough to make full contact with the nozzle.
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That's exactly what happened. Glob above the z height got smacked. I saw it happen. And for the third time. What gets me is this hasn't happened before. Don't recall seeing globs. The globs were pretty small, only 0.5mm or less. But that's enough to make full contact with the nozzle.
I have a purely anecdotal, but similar situation -- I had a print fail at the same place in the print (near the end of the print, of course) at least twice. Maybe it even got me a third time. Printhead either stuck to, or ran into the print and knocked it loose. This was an item I'd printed many times before, so the failures were thought to be a fluke, until they weren't. These failures were all using the same gcode file. I made a trivial change to the slicer parameters, enough to change the print path a little bit, resliced, and the resulting gcode printed as well as always on the first try. I never figured out what caused the "bad spot" -- I just chalked it up to a slicer glitch. Maybe something about that particular path created a blob big enough to cause an issue, I dunno.
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Bob, @TorqueMaster,
What might such a change look like? Say changing from aligned seams to random?
These printers seem to require a lot of odd tweaking. My mill and lathe generally don't. Is this because of the materials, the way they are built or some technology limitations? I've printed several LDCs and none have run into this issue. They were the same height, but not as wide. They would have been easier to knock over as the base was narrower.
Used the same slicer. Hmm, I recall some updates to the profile were done. After that, I started to get blobs. Or it's coincidence. Makes me suspicious. I may see if I can redownload the slicer and decline any further changes. Only problem is this effect doesn't seem to be obvious until the layer count is high, which makes for a frustratingly long test cycle. Maybe I can make a simpler object that shows the problem in less time. Say 10mm OD and 45mm high.
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Printed Chimera 1&2. Chimera 1 clipped 2 pellets it’s out for now. Reprinting it on my other printer.
Chimera2
Gun Vet Compact shooting 18ftlb 770fps gun w/o moderator 3 shots
91.5 92.7 92.6 dB
W moderator
88.3 88.3 89.5 dB
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Printed Chimera 1&2. Chimera 1 clipped 2 pellets it’s out for now. Reprinting it on my other printer.
Chimera2
Gun Vet Compact shooting 18ftlb 770fps gun w/o moderator 3 shots
91.5 92.7 92.6 dB
W moderator
88.3 88.3 89.5 dB
The New Chimera1 print same problem clipping at the muzzle.
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Thanks, Mike
I wonder if the long skinny tubes has the airflow trying to flip the pellet end over end? I am vary of using such long tubular sections in anything but an experimental LDC.
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Thanks, Mike
I wonder if the long skinny tubes has the airflow trying to flip the pellet end over end? I am vary of using such long tubular sections in anything but an experimental LDC.
Why would it do that? Why doesn't it do that in other designs? The pellet should have sufficient gyroscopic stability to travel in very close to a straight line, especially near the muzzle.
But I do agree that this sort of design is experimental. My last working LDC with a long small ID tube inside isn't showing any signs of clipping. Just looked at the exit for signs of clipping and I don't see any.
My printer and I seem to be having a spat. Can't get a good print out of it. Have numerous issues to sort through. Going to be out of action with respect to printing new ideas for a bit.
But I did get a microphone. So I'll try to get a test set up working.
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A waisted pellet's skirt helps it to fly nose first due to air flowing front to back in normal flight. When the pellet is in a narrow tube that does not constrain the pellet (compared to what happens while the pellet is in the barrel), the dominant air flow is back to front, due to the high pressure air still behind the pellet. That airflow acting on the pellet skirt tries to flip the pellet end over end.
Yes, the spinning pellet is gyroscopically stiff on its axis, so it does not actually flip, but it can get bumped and then oscillate in yaw until the pellet is out of the tube. If the extent of the yaw is large enough, the pellet could contact the tube and then bounce to an even greater yaw angle.
What Mike reports could start in the first narrow tube section, then cause a crash as the pellet enters the second narrow section.
The degree of spin stabilization depends on the barrel twist and the pellet velocity. How much the air from behind might affect the pellet depends on the airgun tune; as in pressure and volume of that air; with or without hammer bounce adding extra air.
If there is a little axial misalignment between barrel bore and plastic tube for any reason, that could help kick off the initial yaw that is then exaggerated by the back to front air flow.
A long skinny tube with a definitive but narrow clearance between tube ID and pellet, that allows air to flow rapidly forwards between bore and pellet over a long distance, is the very opposite of an air stripper. If we accept why air strippers are useful to minimize air blast from causing pellet wobble, then anything that exaggerates the problem air strippers are intended to alleviate, is "bad". In extreme cases, bad enough to cause clipping.
If my explanation does not satisfy; the fact that it happened is evidence that it is possible :). It may be on the edge of happening with other skinny tube designs, but depends on the wobble frequency of a given pellet type whether a crash results or not.
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Bob, @TorqueMaster,
What might such a change look like? Say changing from aligned seams to random?
These printers seem to require a lot of odd tweaking. My mill and lathe generally don't. Is this because of the materials, the way they are built or some technology limitations? I've printed several LDCs and none have run into this issue. They were the same height, but not as wide. They would have been easier to knock over as the base was narrower.
Used the same slicer. Hmm, I recall some updates to the profile were done. After that, I started to get blobs. Or it's coincidence. Makes me suspicious. I may see if I can redownload the slicer and decline any further changes. Only problem is this effect doesn't seem to be obvious until the layer count is high, which makes for a frustratingly long test cycle. Maybe I can make a simpler object that shows the problem in less time. Say 10mm OD and 45mm high.
Changing from aligned seam to random would surely change up the print paths. Back in the day, before cura 5.x, I could change the linewidth slightly, say from 0.45mm to 0.46mm and that would stir things up a little bit. That might still work, it sort of depends how much of you print is walls, and how much is "infill."
Slicing software is still in it's infancy, so unforseen things could happen with combined settings, of which there are quite a few to keep track of. Every update fixes things, but could muck up others. For a free product, Cura does pretty darn well at keeping things working while improving. I generally stick with a version that's doing well, and try upgrades when a cool new feature is worth trying. On Linux it lets me install multiple versions, hopefully windows does too. I can always switch which version I'm using.
I'm not a machinist, but I think it's safe to say there's a lot more going on in software to determine the printhead "toolpath" than your other machines.
Some updates in the past have caused blobbing. Moisture will also cause it, PETG absorbs it over time, a good roll could "go bad" sitting on the machine for a few days, depends on your RH. A dehydrator would dry it out.
Also, are you sure you are in a good state of tune? Extruder calibrated, percent flow adjusted to not over or underextrude (by very much) ? Failures like globbing at the top could be overextrusion dragging along on the nozzle, building up more, the taller it gets, until it's a big enough blob to impede the nozzle.
Someone also mentioned getting a good first layer, with good "stick." That should help in safely smoothing any small blobs that block the printhead, but not if they are larger.
Good luck, keep trying!
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Wave cancellation:
https://physics.aps.org/articles/v16/196#:~:text=Cavities%20at%20the%20sides%20of,new%20techniques%20for%20protecting%20coastlines.&text=Goodbye%20wave.
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Wave cancellation:
https://physics.aps.org/articles/v16/196#:~:text=Cavities%20at%20the%20sides%20of,new%20techniques%20for%20protecting%20coastlines.&text=Goodbye%20wave.
These techniques work fairly well for narrow bands of wave lengths. It's harder to do for wider bandwidths.
It's one of the reasons I wanted to search for my old fashioned book on waveguide filters, as there are some wide band filters in there, and a methodology to design them, that is not brute force numerical simulation.
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A long skinny tube with a definitive but narrow clearance between tube ID and pellet, that allows air to flow rapidly forwards between bore and pellet over a long distance, is the very opposite of an air stripper. If we accept why air strippers are useful to minimize air blast from causing pellet wobble, then anything that exaggerates the problem air strippers are intended to alleviate, is "bad". In extreme cases, bad enough to cause clipping.
If this does happen, then it would seem easy enough to add small holes into the narrow tube to act as a stripper. So we get our cake and eat it too? The small holes would empty into the surrounding cavity? Worth a try?
Honestly don't know if what you have said is valid, (it's outside of my experience) but it would seem one could test it out.
It's unclear to me what happens as the bore decreases. Air is a strange fluid. There can be turbulence effects, viscosity, and other things, which may or may not affect behavior, which strongly depend on the dimensions (or relative dimensions) of the structure under consideration. That's a fancy way of saying, "it's complicated". Or maybe it isn't. Complicated may mean that the problem is not fully understood.
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Gavin,
Some airgun air strippers are narrow perforated tubes, such as used by Cometa - image below. So, venting air from behind the pellet in the long skinny tube by means of perforations into a secondary cavity is an option.
Don't the perforation change the tube's resonant characteristics? Not the fundamental frequency perhaps, but the strength of the waves? On the other hand, damping waves bouncing in that tube is surely a good thing.
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Thanks, Charlie
Any chance you could test the 7.5 mm one too, if you are using some sort of dB meter on the 6.5 and 8.5 mm versions? There appears to be something interesting going on with bore size that was not expected; hence my question.
Just saw this, will see. I now permanently reside at a difficult-to-shoot venue ... but if I can figure it all out will meter them then. Thx.
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To study the effects of pellets travelling down "skinny tubes" with high pressure air behind them, print up a series with ID's varying from 0.01" radial clearance, up to pellet caliber radial clearance. Somewhere in that series I predict "blown groups" due to the air from behind trying to flip the pellet in the tube. I am not suggesting the pellet actually flip, but that it is likely to wobble greatly. It should be obvious that a pellet cannot actually flip inside a tube that has a narrower bore than the length of the pellet.
This effect drops off once the clearance between pellet and tube ID is so large that the air velocity down the tube past the pellet is reduced. Certainly, when the air flow in the tube matches the pellet velocity, then spin stabilization should be enough to keep it stable. This speed matching of air happens when the pellet is a tight fit in the barrel (or tube), and when the tube area is large enough to accommodate the volume of air at a lower flow rate. As we can't make a printed plastic tube as close and smooth sliding fit with the pellet, the trouble starts when a significant volume of air leaks past the pellet, at much higher speed than pellet velocity.
Most of the skinny tubes in such a series of IDs are likely to shift the point of impact too, by aerodynamically steering the pellet; relative to bare muzzle.
If the printed tube is long and flexible such that it whips around, the effect of steering to a different POI will be worse. It may be repeatable, but should act like poor barrel harmonics on a whippy barrel. If the skinny bore tube is stiff due to a large OD, then the mass of the device could still shift barrel harmonics and POI.
Now, if you mount the skinny tube to a PCP that has a shroud and its own air stripper up stream, the negative effects of a skinny tube will be smaller, than if it were mounted directly to the barrel muzzle. This is simply because the volume and energy in the air directly from the muzzle is much greater.
If you doubt that this skinny tube pellet instability effect exists at all, shoot them pellet from a smooth bore tube that has an ID that is 0.001 or 0.002" smaller than the pellet head. This smooth bore barrel then steps up to the skinny tube where the tube no longer supports and guides the pellet. Then the skirt still acts to direct the pellet nose forwards, except the airflow in the second tube passes the pellet and the skirt tries to correct the pellet's attitude by flipping it over. Yes, we have rifled barrels, but that does not remove the conflict that occurs between air flow direction and spin stabilization.
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Perforation of the tube can change resonance depending on the hole size. But I'd think it would be a perturbation of the frequency if the holes are relatively small. The holes should act as high frequency sonic leaks, or perhaps even as whistles. The energy that leaks out is removed from the main slug of air. Having not run any numbers, couldn't say if it is a good or effective strategy to pursue.
I'd agree that too narrow tubes aren't good, as there could be unwanted guiding even without contact. Would be nice to know the clearance, but that's the whole point of this thread! I suspect that even without tilt error that the clearance could depend on both the bore (duh) and the length and maybe the properties of air. A helium LDC might be a lot different, even at the same pellet velocity.
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On what subscriber is saying , I have found a large tube on the end of a barrel that is open say 1/2" or there abouts, like the cocking aid on a springer pistol, or a ldc with the muzzle end removed along with its baffles, throws accuracy out the window.
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On what subscriber is saying , I have found a large tube on the end of a barrel that is open say 1/2" or there abouts, like the cocking aid on a springer pistol, or a ldc with the muzzle end removed along with its baffles, throws accuracy out the window.
Weights, like LDC's or large tubes at the end of a muzzle, definitely can affect accuracy. I've heard that sometimes other weights strategically place on the barrel can help tame the barrel harmonics. Our long skinny barrels are prone to issues like that.
Are tensioned barrels prone to the same effect as you describe? Is the effect less or more?
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Finally got my LDC to print! I have two skinny tubes, separated by a large wide expansion chamber. This is the bigboy, at 40mm OD. I also added 24 1.5mm whistle vents that go from the expansion chamber out away from the shooter. The area of the whistle vents is comparable with the bore area. I also used the area that I previously left solid. They are radial sectors, with some rounded ends to use up some air energy.
Been playing with adding vents in the narrow bores, but haven't finished that - sort of an air stripper idea.
One thing that I have on my 1/2-20 adapters are rather large flanges. I turned the adapters on my lathe, so they are guaranteed perpendicular to the bore. I seat the LDC's to the flange. My PP700SA flange is 25mm in diameter. Another thing that can help, if you have the equipment, is to face off the end of the LDC in a lathe. Even with a decent print, there are little bumps and things on that surface that can slightly cant the LDC.
In case you are wondering, the bigboy is quieter than my previous try. I now need to get some relative quantitative acoustic measurements.
The bigboy is at my absolute limit for size. Now I have to do internal stuff, to further reduce the sound. Tone isn't bad indoors, but I think it could be improved a bit more. The bigboy has a 7.25mm bore. I shot through it a couple of times with a pellet, but I don't know if there's any steering going on, I need to set things up in a vise to check that.
Hmm, close examination shows a small chip at the muzzle exit. Arggh. I need to measure carefully the bore ID. My digital caliper says 7.00mm, not sure I believe that. The CAD file says 7.25mm. I'll go get my micrometer and my inside hole tools.
Also haven't faced off the thread area yet, which might alleviate the issue. The thread area has a slight rock on a flat surface, whereas the muzzle area does not rock. This means the bore of the LDC was not quite mounted true. Or, the pellet was processing or tumbling. Or something else entirely!
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With the small hole gauges and a micrometer, near the exit I get 7.24mm. But further in the bore it seems smaller, around 7.23mm. This could be measurement error, deep hole measurement isn't easy in metal, and in plastic with layers it's harder to gauge what is the proper feel. Going to say this is not conclusive. When I set it for size at ~5mm deep, and then slowly moved the gauge downward, the gauge did catch on an apparent shelf or narrowing. It's quite repeatable. Playing around some more, I'd say the bore is not uniform. Setting the gauge to slightly slip at 5mm deep, also catches on the way up. In other words, the very end is smaller. I get 7.00mm and it was compressing, so it is narrower than that. If I wanted it to be dead on, I guess I would have to run a reamer through it. At least my FDM printer (and filament) is not quite good enough to maintain the correct bore size.
But from a conservative approach, I'd say this is too narrow of a bore for this length, with a non guaranteed flat bottom mating surface. 7.5mm might work.
Or I could get an "L" reamer at 0.2900", which is 7.366mm. At least an L reamer is 6" long. The cheapo 7.3mm reamers from China are only 100mm long total.
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Cleaning up the bore of the 3D print may be a challenge. You could line it.
Example: https://www.grainger.com/product/APPROVED-VENDOR-Tubing-Seamless-4NTD5
But I think the general concern remains. A pellet being pushed down a loose fitting tube is likely to suffer in accuracy. The barrel bore and the tube are unlikely to be concentric. The pressure in that eccentric annular gap is probably not uniform so the pellet will likely see some side load, and it may vary from pellet to pellet.
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I am thinking these LDC's need more clearance for printing defects. There is something in the first narrow bore that could deflect or alter the pellet path. I could easily see this being a large contributing issue.
As printed, the narrow hole did not pass a J drill, which is 7.04mm. That means the bore was narrowed by 0.21mm from the design point. I started the J, then worked up to L, going through K. The bore is clean now, except for a sort of hanger of filament on the inside of the entrance tube. It does protrude into the bore. Will need a special deburring tool to cut that off. Maybe I can get a Noga debur tool in there. Something like an RC2200 might work if it has the reach.
I lucked out, the Noga was just long enough to do the job. All clear all the way through. Bore is roughly 7.366mm, give or take what ever spring back the plastic has. I'll measure it when I get a chance.
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Cleaning up the bore of the 3D print may be a challenge. You could line it.
Example: https://www.grainger.com/product/APPROVED-VENDOR-Tubing-Seamless-4NTD5
But I think the general concern remains. A pellet being pushed down a loose fitting tube is likely to suffer in accuracy. The barrel bore and the tube are unlikely to be concentric. The pressure in that eccentric annular gap is probably not uniform so the pellet will likely see some side load, and it may vary from pellet to pellet.
I think with sufficient clearance this is not an issue. The real question, which has yet to be answered is what is the minimum sufficient clearance that maintains the original accuracy.
These printed devices are tough to chuck up in a lathe and be straight. This particular size is outside of my collet range, (max collet I have is 32mm) so I need to deal with 3 jaw, or preferably 4 jaw chucks, which take a while to set up. I did a quick and dirty set up in my 3 jaw and found it wobbled by 2-3 mm as it rotated. I used a bump wheel to straighten it, but as soon as I put any load, like a light touch with a sharp cutter, the cylinder went out of true again. I could make a collar, sort of a clamp around the LDC and clamp that. It would need to be about 2" in diameter, maybe a 2" long, and bored out to 40mm in diameter. Then I would slit it so it could clamp on the plastic. Straight chuck jaws on plastic tend to just mash the plastic. I had wanted to true the threaded end, but so far my attempts have failed, due to the cylinder run out (for what ever cause).
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With the small hole gauges and a micrometer, near the exit I get 7.24mm. But further in the bore it seems smaller, around 7.23mm. This could be measurement error, deep hole measurement isn't easy in metal, and in plastic with layers it's harder to gauge what is the proper feel. Going to say this is not conclusive. When I set it for size at ~5mm deep, and then slowly moved the gauge downward, the gauge did catch on an apparent shelf or narrowing. It's quite repeatable. Playing around some more, I'd say the bore is not uniform. Setting the gauge to slightly slip at 5mm deep, also catches on the way up. In other words, the very end is smaller. I get 7.00mm and it was compressing, so it is narrower than that. If I wanted it to be dead on, I guess I would have to run a reamer through it. At least my FDM printer (and filament) is not quite good enough to maintain the correct bore size.
But from a conservative approach, I'd say this is too narrow of a bore for this length, with a non guaranteed flat bottom mating surface. 7.5mm might work.
Or I could get an "L" reamer at 0.2900", which is 7.366mm. At least an L reamer is 6" long. The cheapo 7.3mm reamers from China are only 100mm long total.
The bore issue has been discussed many times -- maybe not in this thread though. It's a side-effect of FDM printing that small circular holes in the XY axis will have some shrinkage. Half a nozzle size reduction in diameter is about what's expected for these bores. Smaller holes, 1mm and under, may seal up completely. Larger holes are less affected. I've read that square holes, hex holes, etc., are also less/un affected by this shrinkage, since they are made up of straight lines, not a constant curving.
Cura has a setting to try to compensate (hole expansion) which might solve it with no side-effects. I haven't tried it. I know my models are printing on FDM, so I increase the hole size a bit in the CAD to compensate.
If your "tube" is not a constant ID that's tougher to diagnose -- it should be pretty steady, only off by some backlash error. It's possible on layers with more or less material surrounding the tube, the holes could close in more or less. Finer tuning (less overextrusion) might help, or it could be another of those FDM peculiarities to watch out for if fit is critical.
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Deleted repost.
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@TorqueMaster Bob, I was less surprised about the narrowing of ID, since I've experienced it before. But more surprised about the uniformity of the bore. Using a Yuasa small bore gauge (the kind that look like expanding wine glasses) and setting it to the current bore, and then dragging it showed that there were sections that varied in ID, a bit more than I would have expected. I suppose this is all due to the mechanical slop in the printer system. It's not huge variation, but in bores, where we are probably cutting it too close perhaps, the non-uniformity might matter.
At least for my future LDC's, I'm going to run a drill bit or reamer down the bore to ensure it is the right size, just like I do with a tap for my 1/2-20 threads. It doesn't take much time and ensures there's fewer problems.
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For what it is worth, the 6.5, 7.5 and 8.5 mm bores of the mufflers attached to the OP have CAD dimensions of 6.8, 7.8 and 8.8 mm, in anticipation of shrinkage.
Non-circularity would seem to come from the X/Y movements, or the smoothness of the filament feed. Try printing a 7.5 mm rod so you can look at the surface on the outside. If it is X/Y axis movements causing non-uniform tube ID, the same movements should occur producing an OD of that diameter. If there is mechanical ringing, then the same ripple should show on the OD. If it is random layer variation, it might be filament extrusion rate variation; or the filament itself.
If the printer is belt driven, placement of the part on the platen might affect how badly the mechanism vibrates in one spot VS the other, as different lengths of tight VS slack belt would be involved.
Of course, I am not printer expert, so the above diagnostic is speculation.
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@TorqueMaster Bob, I was less surprised about the narrowing of ID, since I've experienced it before. But more surprised about the uniformity of the bore. Using a Yuasa small bore gauge (the kind that look like expanding wine glasses) and setting it to the current bore, and then dragging it showed that there were sections that varied in ID, a bit more than I would have expected. I suppose this is all due to the mechanical slop in the printer system. It's not huge variation, but in bores, where we are probably cutting it too close perhaps, the non-uniformity might matter.
How much variation did you measure? My ender 3 is no high end machine, and I expect mechanical slop of ~0.05mm. Printed uniform tubes, inside and out, shouldn't be much worse than that. I think it may be how much "stuff" is exterior to the bore affecting the hole diameter.
I'm going to try printing this -- actually just the top part -- to see if I feel any "steps" in the bore. I expect it may be narrower where the print is widest, as shown by my crude lines on the cutaway view. My text disappeared -- it's CAD at 7mm bore diameter.
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Bob, I destroyed the evidence on the bigboy, by drilling it out. But I still have two failed prints (just fished out from the trash) that should have similar issues. I will measure them and see what I get. The exit dimensions should be similar.
In my case, the small diameter tube near the AG muzzle had some multiple strands of filament cutting across the bore near the edge, as a WAG, it protruded maybe as much as 0.5mm into the bore. I hadn't noticed it before. This could have caused some instability in the pellet flight.
Also the bore at the exit of the LDC was less than 7mm, possibly due to first layer elephant foot. Further in, the bore was larger. After the second shot, that was clipped. After drilling out the holes to 7.3mm, (and internal deburring) I have not seen any evidence of clipping. The filament creating the chord (mentioned above) was not removed by drilling. I had to insert a Noga inside debur tool to remove those strands.
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Elephants foot and loose strands are two more potential issues for sure. I do my first layer with a bit of "horizontal expansion" to counteract e-foot caused by getting a good squish.
I printed the test part, and I do feel ridges inside it as depicted in my lines drawn on the cutaway. Moving in from one direction, it gets hung up on them, and from the other direction I feel the bumps, but they do not impede movement. Similar to trying to go up a step vs. falling down a step. I do not have tools to measure the size of the steps. I may cut it open.
Anyway, the suspicion is correct -- the hole diameter is affected by the thickness of material outboard of it, and can contribute to uneven/unsmooth bores.
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Elephants foot and loose strands are two more potential issues for sure. I do my first layer with a bit of "horizontal expansion" to counteract e-foot caused by getting a good squish.
I printed the test part, and I do feel ridges inside it as depicted in my lines drawn on the cutaway. Moving in from one direction, it gets hung up on them, and from the other direction I feel the bumps, but they do not impede movement. Similar to trying to go up a step vs. falling down a step. I do not have tools to measure the size of the steps. I may cut it open.
Anyway, the suspicion is correct -- the hole diameter is affected by the thickness of material outboard of it, and can contribute to uneven/unsmooth bores.
Thanks for doing the test. Interesting results. Most of the time, this minutia doesn't matter. But as they say, in pursuit of performance, nuances can make the difference.
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I was able to save one of my failed prints, the one with long skinny tubes that died on the variable layers (where the threads were). There was enough meat in that design to drill the correct hole for tapping 1/2-20 threads. So the side that screws onto the muzzle is 10mm short. Nearly all the print was done at 0.3mm spacing, so called draft mode. As you might imagine, the squish was a little less, and a J drill only cut minor nurbs off, but not the bore. A K drill actually took off minor amounts of material. The L drill did remove material. There were also filaments cutting a chord across the bore on the AG muzzle end. Once again, had to use the Noga to internally debur the material. Since the failed print had blobs and a gouge in it, I chucked it in my lathe and turned the blobs off, and faced the end. I was able to tap the hole about 11mm deep. My adapters are a little long than that, but I was able to test my straight tube design.
It is a little higher pitched than the bigboy, but not that bad. It has a bunch of vents out the front. From an acoustic point of view, they should only pass ultrasonic frequencies, no matter how many there are, as long as they don't resonate by some other means. I could test that by simply blocking the small holes to see if it was quieter. Duct tape might work.
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Bruce,
What size are the little holes in CAD, and if you probe them with gauge pins or drill bit shanks?
Thanks
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Bruce,
What size are the little holes in CAD, and if you probe them with gauge pins or drill bit shanks?
Thanks
In CAD, they are 1.5mm. Due to elephant foot, the opening is less than #60 or 0.040". If I turn the drill bit, it goes through, and the rest of the bore gives some minor resistance to turning. I need to find my pin vise to do much more, as turning such a small bit by hand is just flirting with a broken bit.
I'm trading off slight elephant foot for good base adhesion. Losing a print after 6 out of 6.5 inches are printed, vs minor elephant foot, I would select the elephant foot every time. Or, if I were to remember - to accommodate it in CAD by opening up the first layer or two. If I were better at this, I'd remember this finesse stuff every time.
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Thanks, Bruce
If elephant foot bothers you, why not add a 45 degree chamfer in the CAD file, to both the OD and each hole edge? Make the chamfer slightly larger than the expected contraction in CAD, print and see if that helps. I am guessing that a 1 mm chamfer will be plenty, and that 0.5 mm might be enough for most hole edges.
Very few engineering parts have a 90 degree corners. They are chamfered or have a radiused corner for a bunch of good reasons. If you don't break the corners in CAD, then use a chamfering tool or file after printing. The latter is probably wise anyway to remove bumps and cob webs anyway.
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At the time, I was battling adhesion issues, so adding chamfers was not a good idea, since they would reduce plate surface area.
On metal parts, if I have access, I do try to either chamfer edges or ease the edges, because they are sharp. Trying out an off the wall idea of mine, I often leave out those features, since they are mostly niceties. However, a chamfer or relief might be useful around small holes to reduce narrowing. For this application, I'd think chamfered vents would be cosmetic.
For me my next step might be to form structures that circulate the air. A likely location might be inside the radial sections.
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Chamfering the part in CAD is a matter of degree. What I was proposing would cancel the closing in of holes, without reducing contact area with the platen.
The opening of the last bore that the pellet travels through should be cleared of plastic that closed that hole down during printing, at least to the extent that a square hole edge is restored. If platen adhesion is a problem, then cleaning the hole edge after printing seems prudent. Else, a minimalist chamfer that is added to the CAD file could be used. OD flare and vent hole edge condition are much less important than the primary projectile path.
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Agreed with concentrating on the bore chamfer. As you said, a minor chamfer of 1mm is all that is actually needed, maybe a little more for cosmetics.
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Seems this thread went dormant, both here and on AGN. Are there any conclusions? My printer bed is not the flattest, despite my best leveling efforts. Also had some computer changes, and have finally gotten back to getting my FreeCAD and printing setup working again. Kind of hard changing computing platforms - a lot of stuff changed.
Has this effort been overtaken by porous moderators? They seem kind of neat - kind of muffled and muted, at least from the sound files that were offered.
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Bruce,
Based on the number of participants, the conclusion is that bore to projectile to baffle bore radial clearance should not be reduced below 1 mm. The gain in performance below that is too small to justify the clipping risk. The latter is almost verbatim what OldSpook concluded. As I had proposed 1 mm radial clearance as a good ballpark value, the exercise seems to have confirmed my views.
The fact that some of the smaller clearance data was louder than larger clearance data has been attributed to "too little data". I can live with that; although I see more going on. My point was that very little is lost by making baffle bore radial clearances larger; within the 0.5 to 1 mm range. The results of the study supports that contention.
The effectiveness of porous moderator tube lining looks very promising:
https://www.airgunnation.com/threads/porous-moderator-design-test.1302564/
If you saw the video at the link above of the porous moderator outer tube, with and without foil tape wrapping, the reduction in sound with the tape is very significant. It seems to vindicate my position on shroud and moderator side vents being as effective as pin striping. As in, nonsense marketing. I say this because the porous moderator was full of small side vents and it was very loud until that leak path was closed off by means of the tape. A few small side vents don't do much harm, but they don't do any good either. Those that state the vents need to be used in conjunction with a fibrous stuffing material are really just blocking off air flow on the inside, making my point for me.
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Yeah, my bigboy had clipping right at the exit of the LDC, it was supposed to be 7.25mm, but obviously closed in a little. After drilling out with an L drill to 0.2900" (7.366mm) I didn't see anymore clipping. It had those forward vents, aka dog whistles. But it wasn't spectacular in performance either. It was quieting, but not enough. The porous model seemed to have lots of places for the air to loose energy, simply by its structure.
Not quite sure how to do the porous slicing for mine, and keep it intact upon firing. Probably will take a different approach for the model. Might print it all with a hard shell, simply to avoid having to wrap it up. Back to the drawing board...
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All I can advise is to ask those doing porous fill how to set it up. You could add a hard shell later, if they can't be printed at the same time. As the porous fill is "fill", it seems that having 100% dense regions around that to print at the same time should be easy.
I think the porous material is not just able to accept air and slow it down, the surface makes a very poor sound reflector. I have tried designing various porous wall structures, but the part files become massive. Using the slicer to define those porous areas is much more data efficient. That is out of my league, as I design stuff for others to print. My direct printing experience is negligible by comparison.
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Think I figured it out. It's done in the slicer.
Set the perimeters = 0. Use gyroid for the fill. This makes everything gyroid infill (practically speaking). I need to put in a bigger nozzle, but I will try this out with the existing 0.4mm and use 0.3mm draft mode. I'm going to try 70% infill. Hope the thing doesn't blow up on me. I'll try it on the 2240 first, should be safer.
Same design as before, just porous. Then I'll wrap it. I'll figure out a shell later. Don't think it will be very hard to add the shell.
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Seems there's a bug in the latest firmware for my printer. After only 30 minutes I got a thermal anomaly alarm. I remembered that I had gotten a couple before, but they all were after a firmware update. This alarm paused the print. There also was excess material on the print. Resuming the print resulted in a second pause, so I cancelled the print. I found a note on Reddit about excessive extruder current causing this problem. I edited the gcode to reduce the current by 5%, but it was late at that point. I'll give it a try later today. Draft mode current was something like 538 mA originally.
This printer seems to require more attention than I'd like to run well. I don't want the printer to be the hobby, I want the printed parts to assist my hobbies.
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Bruce,
What printer are you using?
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I have a Prusa i3 MK3S+. I bought it because I didn't want to fiddle with stuff and wanted to just print. Turns out to be one of the worst designs in terms of ergonomics. Never seen a piece of kit that is so difficult to assemble, service or repair. Everything is in the way of something else. Kind of have to be a contortionist to work on it. Have to stop ranting about it, or I'll get worked up over it :).
Think the thermistor in the hot end has a problem. The wire was pinched somehow. But the printer passes all the static tests fine. Printer self test is fine, the Thermal management cal worked ok, PID calibrated ok. The thermistor is within reason, resistance wise. Dynamically, the printer claims a thermal anomaly and quits within a few minutes of a print. Right now, the printer is dead in the water. Pretty frustrating.
I ordered a new thermistor, but honestly, I'm not sure this will fix it. If it doesn't, I'm going to dump it, or shoot it, because it has been an aggravating thorn in my side. (I'll probably put it aside for a while, and ponder if I should buy a Bambu Labs unit.)
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People that have the Bambu seem to like them.
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The Bambu Labs printers certainly seem quite capable. Not quite ready to jump ship, but getting awfully close.
The Prusa hasn't been that easy, it's ergonomics are quite bad, especially for assembly and service. I'm astonished about it really. Every time I try to do something, either visibility or access is blocked. Removing a nozzle is a lot more difficult than it should be, since you can't immobilize the hot end, because you just can't get a wrench on it, because &^^& is in the way. Not well thought out, in my opinion.
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It sounds like it was made by Ford Motor Company ;)
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It sounds like it was made by Ford Motor Company ;)
Actually, one could add most car manufacturers today. They are designed for minimal assembly cost, rather than ease of service. When you have to remove the front clip to replace a headlight methinks they've gone too far. Headlight replacement used to be trivial, not any more! Believe I heard that's what was needed to be done in an Audi. I wouldn't know, I can't afford one, nor would I want a car made that way.
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It sounds like it was made by Ford Motor Company ;)
Actually, one could add most car manufacturers today. They are designed for minimal assembly cost, rather than ease of service. When you have to remove the front clip to replace a headlight methinks they've gone too far. Headlight replacement used to be trivial, not any more! Believe I heard that's what was needed to be done in an Audi. I wouldn't know, I can't afford one, nor would I want a car made that way.
I could somewhat understand if it was a a "forever" LED headlamp assembly, but no, you are probably correct. I noticed a buddies 20xx Maxima had dim headlights and offered to install fresh bulbs...until I read about the process -- removing the front fascia/bumper or taking a wheel/fender liner off were the "easy" options. Not a 2 minute driveway job. Call me when one burns out!