Taming the bark of an Avenger

[subscriber]

Thanks Bob,

This is very interesting and useful.  Including your impressions.

The fact that the Fatso is not much better than anything else suggests the residual sound has more to do with reflected waves escaping, than an absence of expansion volume to further lower the pressure.  That some sort of non-reflective surface would help.  Hence how common felt and "hair curlers" are in airgun LDCs.

I have a couple of ideas.  Might take some time to generate something sensible.

It is also possible that what is left of the sound is coming from the shroud itself.  Perhaps you could wrap the shroud in a layer of paper, then a few wraps of duct tape over the paper.  The paper is just to stop the tape from sticking to your shroud.

The air tube should be similarly wrapped.  If a wrapped shroud and air tube make the system quieter when using the plume style LDC, then that is significant info.  I know a lot of people think sound comes from a "speaker", and muzzle blast comes from the muzzle, but the fact is that the shroud and air tubes emit a loud ping too.  Once the muzzle is quieter than these pings,  all LDCs will be equally loud.

You might start by wrapping the external LDC with duct tape and testing just that.  Wrapping directly is preferred, so that the tap and outer wall are in intimate contact.  3 full wraps should do.  If it was a metal casing, that would almost certainly make a difference when wrapped like that.  The plastic wall won't ring like metal, but it is actually more flexible, thus very likely to be acting as a "speaker cone".

[subscriber]

Bob,

I have created a version of the "Plume" .22 Avenger LDC that uses a 1 mm thick felt wrap around an open framework.  That framework is then capped with a tube from the front, holding the felt in place, and acting as the "pressure vessel".   This felt is what I have in mind:  www.amazon.com/Otylzto-Fabric-Assorted-Squares-Nonwoven/dp/B08T7LLYCP
PM me your address and I can have amazon ship it to you - assuming your are up for printing  and evaluating this LDC version.  Thin open cell foam sheet can be used in place of the felt.

The felt spans the 12 longitudinal "beams", such that the straight sections in between would act as very poorly sprung "trampolines", and thus absorb sound that would otherwise be reflected.


The STL for the LDC and outer tube can be downloaded from this link for 7 days:
https://we.tl/t-z0lpeDf9c0

The M24 x 1 threads are the latest version that you printed and seemed to like.

The space between the 12 spans of blue lines (images below) is where the felt sheet (cut to appropriate size) would be placed.

[subscriber]

Improved version of the above:  I increased the depth of the felt "trampolines" by 1 mm per flat span.  Not much, but just enough so that they will be able to "give" just a little when thumped by air.

Wetransfer download link:  https://we.tl/t-OLwzVfkf4A

[Austringer]

All I can say is, what an awesome team! GTA is such an incredible forum for teams. I don't have an avenger... yet. Thanks for all the work you guys do.
Troy

[WhatUPSbox?]

I'm not sure how useful the Peak measurement is. As I understand it, it is the amplitude of the largest sound spike (as opposed to an RMS averaging over some time period). In looking at some airgun (P17) sounds that I recorded using Audacity software (free), the peak spike was less than 6 samples at 96Khz or 60 micro-sec in duration (see image). Everything I've found so far says that the human ear can't respond that fast. Also at those durations the microphone response and soundcard become question marks. Using the NOISH SW in FAST, MAX mode may be worth trying. I don't have an answer to your point of how a 10 ms event is averaged in that mode.

To reduce some of the non LDC sound contributions, maybe wrapping the gun is a heavy beach towel may help.

Even though I don't have an Avenger, I appreciate you printing all of these variations and collecting the data.

[subscriber]

Thanks, Stan

As a general statement, I think people can be too hung up over instrument readings and computer analysis; rather than trust their own eyes, or ears.  The more sophisticated and expensive, the more prone people are to trust machines over their own senses.   

If the average ear hears "A" as louder than "B", while the meter reads the opposite, I'll go with the average ear.  A possible explanation for the apparent conflict is that the meter is reading frequencies inaudible to humans; or as you suggest, too short a duration for the human ear to register.

This is why I am thankful to Bob for including his impressions.  Rather than throw away meter readings, keeping them may help complete a bigger picture.  So, I am deliberately slow to argue that given readings are "bad" in some way.  The reason being, that I don't have money on using a meter to prove one device is worth more than another; as commercial manufacturers often have to do.   There, they can't say their product sounds quieter to them than another; without being accused of bias.  So, they rely completely on standard instruments and test methods. 

The only thing that matters to me, more than relative loudness, is how objectionable or "ear-catching" a sound is.   If it is less attention grabbing, then I don't mind if it measures 3 or 6 dB louder than another sound.  Some sounds die out faster with distance than others.  That also matters.  In fact, I care more about how a system sounds at 10 meters (and 10 meter increments) than the standard 1 meter.  This is because I am concerned about the potential to annoy people, rather than damage their hearing; or my own.

All that said, I realize that what is important to me is not the same for every airgunner, that has an interest in this topic.  At least one person on the forum stated that he did not want an LDC that emits high ultrasound, as that would scare away his prey on follow-up shots.  Rather than argue that the sound of airgun projectiles landing near his quarry would probably scare them away, I'll take his word for it.

[JimD]

Hobby Lobby has really cheap plastic felt that I use in my P35 shroud.  Seems to work well.  I think it was 33 cents a piece when I last bought it.

Gaffers tape sticks really well but does not leave residue when you take it off. 

[subscriber]

Bob requested a fully threaded detachable version of the "Felt Plume" LDC. 

The images below show that, working with the in-shroud full thread air stripper design from a few days ago.  Combined image compares this to the one-piece design.  Outer tube casing part is kept from that posted above.

The STL for just the Felt Plume LDC with full male threads can be downloaded from here:
https://we.tl/t-zb2YfbiEH5

The felt sheet is not shown, but fits between outer casing and inner framework structure - rolled around the latter, just enough to provide an easy but snug fit.  One layer of felt 1 mm thick should fit easily.  2 layers may be too snug; so one layer with some overlap should work.

[TorqueMaster]

I'm the type that would like to have meter readings to backup what my ears are hearing.  I totally agree that the peak, tone, and duration, etc.  all affect what we PERCEIVE as loudest, regardless of what the meter says.  I have apps that will do a spectrogram, but the resolution is fuzzy -- it does record in roughly 20ms slices though...  I took screenshots of them, and I see slight differences.  Some of the events take as long as 400ms, which seems too long.  Looking at WhatUPSBox?'s audio clip, it should be more like 8ms.  I see slight differences -- mainly the low end (from say 4k down) was attenuated with the LDCs installed vs stock or no cap.  Actually attenuated is the wrong word.  Abbreviated -- shorter duration.  Loudness is indicated with color and the data is there, but difficult to see differences.  I can attach them later if of interest.  Fair warning I wasn't too good about recording which is which -- I think I know the order they were taken but not positive.

On the sound level meter readings -- the "max" readings -- I am fairly convinced the short duration of the noise, combined with the long window of the meter, dilutes the results.  Further complicating it is I am outdoors, with nature sounds and wind, all variably affecting the level of the background noise that makes up the majority of what the meter is averaging....

Wrapping the shroud and or tube (tightly) with multiple layers of tape is tricky -- there's roughly only 1mm of space between them.  I could "hourglass" around them together...  Further complicated is trying to compare with and without the wrapping quickly enough where I may notice a difference.  Swapping between plume and felt-plume exterior LDCs will be very quick...  I'll try to get the newest one printed soon.

@JimD -- Thumbs up on Hobby Lobby felt -- 16 cents - 50% off right now lol.

EDIT - saw a couple things I meant to address -- It would be pretty easy/quick to wrap the shroud/tube in a thick moving blanket if that would address the ringing issue suspected?

I had the iPad NIOSH set on fast, and the max numbers were similar to the Android app Max numbers -- i.e. not showing much in the way of db change.    ENDEDIT

[subscriber]

For perspective, 4 kHz is not "low frequency";  although that may be a matter of opinion.
As such, how loud an LDC is below that probably matters a lot more, than above that:





There may be a lot going on in a shot sound capture at 10+ kHz, but it does not travel as far as low frequencies, nor does it penetrate structures very well.  Hence why the thump-thump of loud disco music is all you hear in your hotel room when you are trying to sleep, while others party by the pool at midnight.


10 kHz may be annoying, but run the video below and leave the room to see how fast the sound drops to nothing:





Here is 20 to 20k Hz:

[subscriber]

I'm the type that would like to have meter readings to backup what my ears are hearing.  I totally agree that the peak, tone, and duration, etc.  all affect what we PERCEIVE as loudest, regardless of what the meter says. 

No arguments there.   

My point is that the desire to boil down a comparison to a single number that a device spits out is fraught with pitfalls.  Much better to overlay the amplitude over a wide spectrum when comparing LDCs.  Then weight the parts of the spectrum that seem louder to humans.   Which is what Stan was promoting, if I may be permitted to define that.

To me, it seems there are several bands in the above frequency sweep that seem louder (or more annoying), suggesting (for me), that suppressing those bands would be a good thing, even if the whole spectrum is louder somewhere else. 

What I don't know is if we find similar frequencies equally annoying; or if it depends on individual anatomy.

[subscriber]

It would be pretty easy/quick to wrap the shroud/tube in a thick moving blanket if that would address the ringing issue suspected?

I like the idea.  Simple and easily reversible.  Thanks to Stan for suggesting it.

A blanket should block tube ringing sound from escaping as loudly, rather than damp it out.  For damping it out, coupling or least direct contact over a significant area is required.

The outer LDC plastic tube could also stand to be wrapped in adhesive tape.  Or even felt wrapped on the outside with tape around it.  But this should be an individual increment to see what difference it makes - if and when you have time.

[subscriber]

Adhesive tape is a cheap substitute of this product:
Lower video volume before starting it....
https://m.media-amazon.com/images/S/vse-vms-transcoding-artifact-us-east-1-prod/v2/3255bca3-1dd6-53ab-bf75-a1b586df7ed6/ShortForm-Generic-480p-16-9-1409173089793-rpcbe5.mp4

[subscriber]

Bob,

I have written a lot about sound testing.  In all that, I hope you saw this:

This is why I am thankful to Bob for including his impressions.  Rather than throw away meter readings, keeping them should help complete a bigger picture.  So, I am deliberately slow to argue that given readings are "bad" in some way. 

To be clear, I am not expecting you to do a full spectrum analysis.  I would like you to do what you did before.  Anything more would be a bonus.

When I talk about the folly of comparing single peak sound readings, I am addressing the full audience.  Especially in the USA.   Where 0 - 60 numbers are given far too much importance; that don't tell you anything about an engine's low to mid-range torque, or how well all the gear ratios have been chosen.

[TorqueMaster]

When I get to retesting I can try the blanket around the whole "system", and try the external part wrapped and unwrapped, no problems there.

I've been looking at the spectrogram apps and screenshots I took. They seem to imply the sampling rate is about 17ms.  "Spect1" does a labeled 6 second sweep, and the "Spect2" an unlabeled ~20 second sweep.  One can zoom in on these images to the pixel level to see the detail.  Spect2 also shows the instantaneous reading in yellow (background noise when I paused the sweep) and the max of the (presumably) test shot in red.  BOTH apps will let me choose the range of frequencies displayed while sampling, if there is a band of particular interest.  Spect2 will also allow me to zoom in on a band of  sound levels as well, for instance from -80 to -20, allowing to ignore the lower background noise if desired.

Sample images attached.  The recorded shots are pretty obvious amongst the rest of the background noise and other sounds.  I labeled them from memory -- so don't shoot me if future images (if any are deemed worthwhile) look significantly different.

[subscriber]

Now you are cooking, Bob!

[subscriber]

I made some improvements to the above "Felt Plume" LDC: 
 
Added "T" sections to stiffen the open "ribcage" structure.
Fixed some overhangs that might have been too large to print.
Added some material where the centering cone meets the large mounting thread.
Added ribs to the outer casing inside, to help keep the soft felt "trampoline" afloat - not jammed immovably against the wall over too large an area.

STL files for the updated LDC and outer casing can be downloaded from here for 7 days: 
 https://we.tl/t-S6UZPj8ISs

[Back_Roads]

 I am not sure if it is possible, but could the inside of the outer tube be made in a honeycomb pattern, it would offer strength and also I have seen evidence that it acts as a sound suppressant, I guess bee hives could get noisy

[subscriber]

Sure, James; any texture of a reasonable shape is possible.

In the case of this tube, the idea is for a rolled felt sheet to be partially unsupported so it makes a poor reflector, with chamber partitions around the circumference to prevent blowby.

[subscriber]

Is this close to what you had in mind, James? 

Perhaps finer; but this illustrates the point.