.22 LDC bore clearance study print files

[subscriber]

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...

[subscriber]

I forgot to mention; these mufflers need to be printed standing vertical on the printer platen, with the threads uppermost.

[miksatx]

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.

[rsterne]

Mufflers are illegal in Canada, but I will be curious to see the results of dB testing....

Bob

[mrbulk]

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.

[Insanity]

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...

[subscriber]

Thanks, Mike

Charlie,  I will answer your questions later.

[subscriber]

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






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. 

[subscriber]

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.

[mrbulk]

Thank you Sub! I will send on your replies to the guy who printed AUO inserts, appreciated.

[miksatx]

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.

[subscriber]

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.

[TorqueMaster]

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. 

[subscriber]

Very interesting!

Thanks, Bob

[WobblyHand]

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.

[subscriber]

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

[WobblyHand]

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.

[WobblyHand]

Found this.  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 

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.

[subscriber]

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.

[WobblyHand]

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.