Taming the bark of an Avenger

[subscriber]

This sounds like a Tesla diode in some ways.

Yes, Marco your design uses the Tesla valve principle; having a split air stream crash into itself to cancel the momentum.  That suggests the streams have roughly equal momentum, or mass x velocity.  The Binelliboym1 design seems to have a fraction of air from the muzzle bypass the baffles to reach the front of the LDC, rather than half the flow.  I am sure that the proportion of bypass is important, either way.

Now the idea of stopping a flowing column of air sounds great, but that increases its bulk pressure.  That high pressure zone then wants to reflow to a low pressure region; thus it reaccelerates.  So, the energy in the air is not gone, except for forcing losses via friction and turbulence.  Now, the energy does not have to be "gone" to be effectively quieter; its delivery over time just needs to be smoothed out.

[subscriber]

Jim,

The Avenger barrel is vaguely free floating at the muzzle, from the in-shroud support.  This designs seems to negatively affect group size and POI stability.  Any LDC that improves muzzle positioning stability should help - independently of air flow effects.  Conversely, ignoring this weakness would tend to have any LDC not generate a possibly expected improvement in grouping.

Buck-Rail seems to use variants on one theme.   The PRod does not include a section view, but from their site for similar products, see these:  https://buck-rail.com/product/benjamin-akela-suppressor/






Before, I have expressed negative opinions on small holes at the rear of the shroud.  I have not seen evidence to change my mind:  My positions is that there isn't enough combined hole area to flow air during such a short duration event, as firing the airgun, to make a useful difference.  Should you increase the number or size of shroud holes enough, then I predict a reduction of blast at the LDC muzzle - at the expense of more noise the the side and rear of the shroud.   If a reduced peak, more omnidirectional sound is what you seek, then the right configuration of shroud holes may provide that. 

If the the shroud had a perforated tube inside, spaced off with a gap between them, where small holes have perhaps a combined 25 to 50% open area, that vent into the space between the inner and out shroud tube; then absolutely, that could reduce the total noise escaping the system.

My groups are, if anything, a little tighter with the in-the-shroud moderator than with a DonnyFL.  But that is probably random variation of the shooter.  I think both are neutral on my accuracy - no positive or negative effect.  The only moderator I noticed an apparent accuracy benefit for is the Buck Rail on my Prod.  I wonder what it's design is like.

Another factor that may or may not be significant is the back of my shroud has small holes.  I added them when I was messing with my own modifications in the shroud since my P35 has holes and works well.  I have not had my hand up around the holes when shooting with the Avenger but I've noticed a puff of air coming out of the P35 shroud.  Could the holes help offset the small volume?

I've also been shooting mostly and possibly totally at 25 yards since installing this moderator.  I can try it at 33 yards easily but that is as far as I can go in my yard. 

Jim

[Mr.P]

This sounds like a Tesla diode in some ways.

Yes, Marco your design uses the Tesla valve principle; having a split air stream crash into itself to cancel the momentum.  That suggests the streams have roughly equal momentum, or mass x velocity.  The Binelliboym1 design seems to have a fraction of air from the muzzle bypass the baffles to reach the front of the LDC, rather than half the flow.  I am sure that the proportion of bypass is important, either way.

Now the idea of stopping a flowing column of air sounds great, but that increases its bulk pressure.  That high pressure zone then wants to reflow to a low pressure region; thus it reaccelerates.  So, the energy in the air is not gone, except for forcing losses via friction and turbulence.  Now, the energy does not have to be "gone" to be effectively quieter; its delivery over time just needs to be smoothed out.

This all sounds right though I doubt there is anything remotely like equal momentum in the two streams in my design unless merely by dumb luck. I imagine the bulk of it goes right up the center but really have no idea! 

[subscriber]

As this is a near magical device, I would be happy to have one drop shipped to Bob, to compare against the recent front runners:  https://www.ebay.com/itm/Price-drop-Air-Venturi-Avenger-177-22-and-25-Cal-Barrel-Stabilizing-Insert/125134087158

Bob, if you are interested, PM me your preferred shipping address.

[subscriber]

I doubt there is anything remotely like equal momentum in the two streams in my design unless merely by dumb luck.

I agree that getting two streams exactly equal and opposite would take some doing.

[TorqueMaster]

Started printing the three newest designs.  The 2-piece with felt printed "ok."  Small details are tricky, especially with PETG, so my printer officially hates you.    It finished looking a bit ugly, but intact.  However...after cleaning up the loose strings I could get to, and cleaning out the pellet path, I broke it trying to gently insert the felt covered cage into the outer shell.  I'll attach a pic where it broke. Just a slight twisting motion was all it took.  I "repaired" it with masking tape, and was able to get the two parts assembled with felt in place.  The area I circled is what broke most of the way around and is now taped over and won't breathe like the rest.  I believe it is intact enough and seems concentric enough to survive testing, so I won't plan to reprint.  I have ideas to strengthen it, but not worth the effort unless the design proves audibly worthy.

The digital sponge seems to be printing ok. 

Looking at the digital felt, all the small details -- it should print ok, but my printer will be grumbling about it.  Seriously though, I do wonder if all the small gaps will stay gaps, or if the melty imprecision will cause some to get closed over.  I will watch those areas when the print starts, and abort if necessary.

Rainy/coldish next few days, probably no testing until late in the week at soonest.

[subscriber]

Thanks, Bob

Very diplomatic and funny:  "my printer officially hates you.  "

Any useful design should ultimately be printable; or else it is just theoretical.  Also, it is useless, if super fragile.

[TorqueMaster]

Agreed.  If the design merits further evaluation, it needs to be more durable.  I consider myself a fair judge of strengths of materials, and I was not overly rough with it, so I suspect the PETG is sub-par.  A am printing plenty hot enough to get good layer adhesion.  It's always been fine with more monolithic prints, though.  I'll be trying a different brand after these prints are done.

[subscriber]

Should you increase the number or size of shroud holes enough, then I predict a reduction of blast at the LDC muzzle - at the expense of more noise the the side and rear of the shroud.   

The above statement is conditional:  It is true, providing the shroud has no "stuffing" material in the rear of the shroud.  Stuffing, such as seen in this video will slow flow and muffle shock waves; acting like the perforated tube in shroud I described above: 

So; rear shroud holes with an empty shroud that are large enough to decrease muzzle blast will increase side and rear noise.  Rear shroud holes that vent through a "lossy" material my reduce muzzle blast, without increasing side and rear noise objectionably.

[JimD]

Thanks for the educated guess as to the guts of my Prod moderator.  I worry about it being fragile but I enjoy it's effect on noise and accuracy.  Looks pretty conventional.

I have not seen a benefit from the small holes at the rear of my shrouds.  My P35 came that way but I tested before and after with my Avenger and saw no benefit.  But that was before I had the printed moderators.  Ideally I'd love to see someone test the same in shroud moderator without the holes to see if they get a similar benefit on noise.  If not and they want to, I mentioned the holes in case they would want to try that.  The holes do not bother me but it's a permanent modification so I would expect some to prefer to just use a different moderator.  I expected to see at least a small benefit from the holes in prior tests since they do leak some air but I guess it may not be easily measurable.

When I was making moderators I saw a clear benefit from size.  Bigger was quieter.  So I understand why an in shroud design should not be quieter.  My only counter point is the space forward of the muzzle in my Avenger isn't dramatically smaller than that of my P35.  I may do some measurements to see better what the size difference is.  The P35 isn't using as much air with it's current 32 fpe tune (Avenger is about 48 fpe) but it is about as quiet as the Avenger with the in shroud moderator.  I've also reviewed the 20 moderator tests and while there is a clear trend to bigger is quieter there are some exceptions.  It also seems possible that it is sort of an anomaly fairly unique to my tune and/or other factors. 

My Avenger didn't have any clipping issues I ever detected when I used the stock end cap leaving the front of the barrel hanging in space.  I've see where others did have issues.  But I like the fact that the DonnyFL adapter and the printed moderators I've used provide a little support to the end of the barrel.  I don't see any benefit of leaving it unsupported.

[subscriber]

Jim,

How close a fit to the shroud ID is the Avenger's shroud support, located about 7" from the barrel muzzle?  Is there an air path to the breech end of the shroud; or does that barrel to shroud support block off the space pretty well? 

I can't tell if there are through vents in that part - bottom of frame, here:    Obviously, if there is no flow path, then the rear of the shroud volume would not help by accepting air.

[Outer_Rondacker]

Just tossing this out. I made a support for the barrel to the shroud with an O-ring. I spun the thingivers internal with the drill press and use a dremel on a fixed arm to cut in a grove for a O-ring. much more snug fit and better support for the barrel. Just tossing it out.

[subscriber]

I made a support for the barrel to the shroud with an O-ring. I spun the thingivers internal with the drill press and use a dremel on a fixed arm to cut in a grove for a O-ring.

Jolly good.

It would be useful to post some pics and some specs, if you have them.

[WhatUPSbox?]

  However...after cleaning up the loose strings I could get to, and cleaning out the pellet path

Other than the pellet path, I would think that for once stringing is a "feature" ... more felt.

[JimD]

I tried a few shots with the in-the-shroud moderator.  I was checking accuracy at 30 meters but also measured sound and velocity.  Results: 945 fps 86.6db; 928.8 fps 87.3db; 930.77 fps 87 db.  The first shot is a little high in velocity, reg may creep up a little during storage (it's set about 2200 psi).  First 3 went into a group about 1/4 center to center.  That is good for me with this gun.  I then shot three into the lake to get gun sound and got 80.6 db, 81.6 db, and 81.1 db.  I then shot another 3 shot group.  The first two went just about where the first three went but the last was low and the group was about 1/2 center to center.  I think I was the difference in the group sizes.  I was shooting FX 25.4 grain pellets.   

[TorqueMaster]

Two topics in my brain...
Last night I was looking at more Android "Sound Level Meter" apps -- there are quite a few -- varying in features and functionality.  I found one that lets me pick weighting, and to some degree "response time."  Fast, Slow, and numeric options from 50ms up to 1000ms.  The question is -- what's best?  Looking at the 90 degree shot recordings, it depends on if you look at the regular "waveform" or the "db waveform" where they can linger as long as 0.400 second....  Anyone using "fast" at 125ms is not really getting the full picture, just a fairly significant portion of it...  200 or 250ms looks about right.  Or just use "Fast" 125ms to stick with an established standard?

Printing the digital felt LDC -- as expected, it's a bit melty and the maze-like structures have thicker walls and thinner passages than designed.  There is an airgap still, but it's pretty small.  14 hour print - I can abort or let it go...
Pic below.

[subscriber]

Other than the pellet path, I would think that for once stringing is a "feature" ... more felt.

Stings in the "felt" can't hurt; providing the strings don't break off and interfere with the pellet path to much.

Bob's image below is a useful indicator of the limits of printing.  The features are a bit thicker and less square than the CAD, but that means they are stronger.

[subscriber]

There is an airgap still, but it's pretty small.  14 hour print - I can abort or let it go...
Pic below.

Just got to read this - late to the party - let it print, was what I would have advised.

[subscriber]

On the topic of slow VS fast meters, my understanding is that there is some averaging going on in the measurement period.  As the meter reports the loudest sound that spans a series of short measurement periods, is it really going to miss a shot sound that spans more than one measurement period?  Surely it captures the peak and ignores the rest?

If there is averaging, then a long period will measure lower, because it contains more of the tapering off of the shot sound.

Or am I misunderstanding how peak sound metering works?

I am not suggesting that only the peak is important, because I think the total sound "volume" or intensity x time matters to our ears.  But, the peak sound is what is compared traditionally for muzzle report.

[TorqueMaster]

Finer printing is possible, with finer nozzles, but what takes forever now would take for ucking ever.  Smaller nozzle, go from .4 to .3, takes 33% longer, then  max recommended layer height gets reduced from .2 to .15 , so another 33%...overall adds 75% to print time!  Great for small stuff...

Left it printing, it looks usable.

I've been under the impression "max" sound level was more representative of what we hear.  "Max" being the weighted (or not) average of the loudest samplings that fit in the sampling period.  I believe it samples very rapidly, for example lets say it's every 25ms, and takes the highest consecutive 5 samples (in a 125 ms "fast" mode) to figure out the average and call that "max."  If the burst is 300ms long, it's reporting the loudest portion of the burst, but not averaging the whole burst.   "Peak" is simply the highest recorded db level of any one of those samples, and doesn't really represent the true "loudness" we perceive.

I think it makes sense for the sampling period to be about the same duration as the stock Avenger sound length.  Any LDC that reduces both sound level and duration, should doubly benefit with a much lower "max" reading.  Whatever sample length is used, it needs to be consistent.  The tools within Audacity can give a relative db reading based on any sample length...I'll be curious to see if those tools, when measuring the same sample length as a stand alone meter or app (set to say "fast" 125ms) show similar numbers.

The recordings (when not clipping) reveal the relative "peak" values, too, so both the meter/app and recording/Audacity can be compared and hopefully show similar numbers...that may be more dependent on the granularity of the samples that each method is using, though...