Taming the bark of an Avenger

[soupyyy]

I have this video comparing some of the popular airgun.  It is not the most precise way of measure the noise but they are comparatively to each other at the same condition.  I have since decided to standardize my setup with equal distance between pellet trap and muzzle to 16 feet from sound meter.  My number and result can't be compared to other but I can say is for me anything under 84 dB is backyard friendly for me. 



Subscriber, I will take your suggestion into consideration and try to fit that into my next upgrade.  Thanks.
Hawkeye51, I do have 3 (5/64) hole in the back of the shroud to vent air but have not tried the felt around the barrel.  I will definitely try that this weekend.

One more for your entertainment.

[DMR214]

Here is a sound test of my no mod Liberty .22 in an urban area.  With my cheap sound meter anything under 84 dB reading is backyard friendly to me and at 80-82 dB I can take a shot or two at 2 AM without much concern.  My moderator is 3D printer very similar to the two mentioned above.  Use heavy pellet to reduce noise if you need to.

Just curious,  if I drill holes in the donyfl adapter like the ones soupy pictured. That should help reduce more noise correct. Sometimes I hear the extra length & pressure could add more fps. I don't think that's true in air rifles is it.? So drilling holes into the adapter won't loose any fps since all that is produced in the actual barrel. Do I imagine this correctly?

[subscriber]

The small number of tiny vent holes placed at the rear sides of a shroud, or front sides of a LDC make me smile.  I was told that all the expensive name brands are doing it, therefore "that proves these vents work".

Sure, if you have enough flow area at region of significant air pressure, that result in a reduction in the volume and pressure of the air leaving the shroud or LDC bore-axis "muzzle"; this bore-axis flow reduction will reduce the primary sound that is the focus of traditional measurements; so will seem like a "win". 

Yet, all that so many holes that are large enough to reduce forward blast achieve (by my thinking), is to make the airgun louder towards the sides and somewhat to the rear.  If that has value,  then by all means have at it.  Perhaps having a lower level of sound that is omnidirectional does have value because it won't travel as far, and pinpointing the source might be harder.

On the other hand, directional sound can be just as useful, when you ensure that it is directed away from people upset by such sounds.  As long as there is not significant echo towards them.

Bottom line:  A handful of tiny holes cannot possibly flow enough air to make any difference, for better or worse.  Small holes simply don't flow well because a significant proportion of their area is subject to the boundary layer effect.  This, even when their total combined nominal area is equal to one significant hole, such as the muzzle of a shroud or LDC; and that is nowhere near the case with the vent holes seen on shrouds and most LDCs.  Most of the latter have only a few tiny holes.

The mesh seen at the front sides of some Huggett LDCs might have enough combined flow area to "work".  Work, as in letting out a useful volume of air, while blocking the slap of pressure waves that reach the inside of the mesh.

Is it possible to create destructive interference, using cleverly engineered vent holes, where the same wave forms, at the same intensity, meet 180 degrees out of phase to cancel each other?  Maybe, but the symbolic vent holes actually used by name brand shroud and LDC makers, work by what I call the "Placebo Damingo" effect. 

Don't take my word for it, do your own testing.   With a meter and a few sets of human ears.  Tape over the stock vent holes and see if you can tell a difference.  Drill more holes and test that.  Eventually the "muzzle blast" should be reduced, at the expense of noise to the sides.

If you want to use a multitude of vent holes to make a shroud or LDC quieter on the outside, copy vehicle exhaust systems:  Keep the vent holes inside the device casing. 

[subscriber]

Rick,

The projectile acceleration that occurs inside a silencer is negligible.   If the bore of the device was "tight" enough not to leak air past the projectile, you are inviting baffle strike.  Even if that does not occur, with an overly tight LDC bore the projectile can be "aerodynamically steered" by the sideways or vertical movement of the LDC (related to barrel harmonics). 

The energy gained in an LDC would depend on the average air pressure behind the projectile, less the average pressure in front of it; multiplied by the length of the device.  Consider that the whole point of adding expansion volume and air stripping is to decrease the air pressure behind the projectile, and to slow that air down.  Thus, if an LDC is effective at noise reduction, it makes a poor "barrel extension".  These functions are mutually exclusive.  Showing me a 10  FPS increase with an LDC on, will make me go, "big whoop".  How does that compare to shot to shot variation?

Barrel lengths are often reduced when silencers are added.  If velocity was a primary consideration, longer barrels would be used.    Of course, when a standard caliber is just supersonic, ultra short or heavily vented barrels are often used with integral suppressors; to reduce velocities to perhaps 1050 FPS - thus avoiding the supersonic "crack".

Bottom line; more holes in your DFL adaptor won't change your projectile velocity meaningfully, even if you could measure an arguably statistically significant difference.

[DMR214]

Cool,,, thanks for the clarification Subscriber.. In the long run the thump of a pellet hitting small game is still loud enough to alert even neighbors.

Peter
Your moderator looks great! My question is do you vent the shroud back at the breech end? I've seen some top dollar setups with multiple tiny holes in the shroud & sometimes in the proximal portion of the moderator. I personally put about 4 tiny holes staggered from an inch to 1 1/2 inches around the moderator at the breech end and wrap felt around the barrel under the holes. I drill holes in the shrould support so the air can get there, and use the full length. I have no idea if this helps, but it mentally makes me feel good about things, so I do it.

I too wonder if that helps, but just getting it done will give me that little extra assurance whether it helps or not.

[Tomygun1]

Very timely info in this thread and I am doing my best to keep my pcp’s as quiet as possible.      I’m trying new stuff daily as over 45yrs neighbors have closed in on my rural setting and I do not want to be offensive to others.
I use my Cayden with a TAC 65 rimfire silencer mounted on it for comparison as I know that it made my rimfire rifles and pistols as quiet as you would ever hope for.     Surprisingly, the Cayden, as quiet as it is now, is still slightly louder than my Browning Challenger 22lr rimfire target pistol.      I compare everything else to these results.

My best results come from mods to my 25cal Avenger that really barked and I ain’t done yet, so I wont go into others but focus on this rifle.     
Mods: one small hole drilled in the shroud near the breach for pressure release.      Four wads cut to fit in the shroud with a hole for the bbl to go through cut from Mr. Clean cleaning pads and stacked near the breach to cover the hole in the shroud.      4ought steel wool packed from there up to the centering piece that rides in the shroud over the bbl. (Like the old steel pack car mufflers).      A DonnyFL shroud extender/thread adapter that slides over the actual muzzle in the shroud and extends some 3”to the shroud cap with the 1/2X20 threaded end.      And then fitted with a drilled “fuel filter” on the end.     For full power in this 25cal it is quiet silent.

Im still not satisfied because I believe it can be even better, as the +3” space from the muzzle to the vented end cap is wasted and should be able to further separate the air volume between the shroud and suppressor.      I plan to cut off the muzzle tube on the DonnyFL extender and fill the space with an air splitter over the muzzle and ad an additional air chamber and stacks.      This should really work!     

What are others doing?

Tom

[subscriber]

Tom,

I suspect that your shroud stuffing is usefully effective in preventing pressure waves being reflected off the inside rear end of your Avenger's shroud.  With that much material over the external hole you drilled in the shroud, I would wager that the hole is not flowing very much air to the outside.

To test this hypothesis, would you please drape a layer of tissue paper over the outside of the hole so that the paper is held down by gravity.  Then fire a pellet in a safe direction with the rifle held horizontally in that position.  If the paper moves at all, tape over the hole and shoot a few shots, to compare the sound with the hole open and taped closed. 

Sure, a large enough shroud hole will reduce the sound coming from the bore axis muzzle (shroud or LDC), but will it reduce the total sound of the shot?   

If a bed sheet were large and strong enough, could you use it as a sail on a small boat?  Or would too much air leak through the billion (?) tiny holes between the threads in the weave? 
I suspect that at such low pressure differential, a negligible volume of air flows through such small holes; even with a stiff breeze.  Even if there are a billion of holes. 

The same principle applies to "holy shrouds", in my opinion.  Yes, the holes are larger, but due to the very short event that is the shot cycle (stretched to perhaps 0.1 to 0.5 second in the shroud / LDC), and the rather low pressure differential to the outside of the shroud, very little air flows from the shroud holes.  Until you make the holes large and may; at which point the rear of the airgun becomes noticeably louder...

I would be happy to be proven wrong.

[avator]

Maybe a stupid thought but...
Has anyone considered the effect of a pellet pushing the air out of the barrel ahead of the pellet? Seems there is some level of pressure.
Seems like I read some stuff about adding vacuum or something along those lines.

[subscriber]

The air pressure in front of the pellet depends on the density of the air, the length of the barrel and the average acceleration of the pellet.  As air is not very dense, the air in front of the pellet is much less significant than the air behind the pellet.  The air pressure behind the pellet depends on the supply pressure, plenum and barrel volume, valve duration and transfer port area and efficiency. 

It is the average pressure behind the pellet that predicts its energy and velocity.   But when considering report, it is muzzle pressure and volume that matter.  Muzzle pressure is a much lower value than average air pressure down the barrel; unless a "dump valve" is used.

That said, evacuating the air ahead of the pellet should enable it to reach a higher muzzle velocity.  Here is a "PCP" shooting a baseball at over the speed of sound, with the help of a vacuum pump to remove most of the air in front of the ball (while in the barrel):

[avator]

LOL.... I just knew I could get your motor running this morning.... 

[JimD]

I haven't shot my Avenger much recently because of the noise.  I just looked at some of the data I've taken and will offer it.

First, I think I overstated the unsuppressed noise level mine is making.  Looks like it is more like 105 db at the muzzle and I measured an average of 89.1 at 25 feet.  With my "best" home made suppressor at 25 feet I measured 80.1 db. 

I did a bunch of testing indoors in large part to avoid annoying neighbors.  I am not sure how those results would compare to more important outside measurements.  I tested 2 1/16 inch holes up to 8 3/16 holes indoors.  The holes were in the inside tube of the moderator.  Several times I taped over the holes.  Sometimes adding the holes increased the measured sound but it was pretty much inside the variability of the individual measurements to the average.  I tested 2, 4, and 6 holes outside (did not record size but it was later so they were probably the 3/16 inch ones).  The variability of the measurements went down but the readings for the various number of holes varied by less than 1db on average.  Adding a scratch pad (plastic) inside the moderator did not seem to affect results either.  I ended up just putting lots of holes in the inner aluminum tube.

I made three different moderators.  All had a piece of 5/8 OD x 3/8 ID aluminum tube down the center with holes in it that I discuss above.  I threaded one end of this tube to fit the Donnyfl adapter.  I do not have a metal lathe and had trouble getting the threads aligned well with the center of the tube.  The 9 inch moderator and one made of an old sprinkler clipped.  Only the 4 inch moderator with a PVC drain pipe outer piece did not clip.  But now it has started clipping too.  The 4 inch reduced the measured sound by about 7.5 db.  The 8 inch reduced it by about 12 db and the sprinkler moderator was the best at 13 db.  But all except the 4 inch clipped so they were unusable.

I also bought a Benjamin trail baffle, part NP214-001 and some carbon fiber tube to put it in.  The baffle is plastic and the moderator looks cool.  But it didn't reduce the measured sound at all. 

My most recent testing has been with my Snow Peak P35.  With nothing in it's baffle, I measured 98.2 db.  I think all these measurements were at 15 feet in front of the muzzle, slightly to one side.  With the baffles that it came with, I measured 80.4, 87.1, and 87.8 db.  Aaron Cantrell did a youtube about putting half hair curlers wrapped with felt inside a few of the stock baffles and extending the shroud as necessary.  I tried that.  With two baffle curler combinations, I measured 77.8, 78.4, and 78.9 db.  With 3 I measured 76.3, 76.4, and 73.7 db.  With four I measured 69.4, 70.5, and 67.4 db.  I was surprised it made this sort of difference.  I initially used two curler/baffle combinations because it does not extend the gun length at all and is pretty quiet.  Later I went to 3 which extends it about 1/2 inch.  4 extends the gun length about another inch.  I got some clipping initially but that was because I used fabric instead of felt to wrap the hair curler and it came unwrrapped.  To help not get clipping, I use a straw down the baffle into the end of the barrel as I assemble the baffle and I snug the baffle up a little.  With only two combinations you can't do this, it is a little loose which doesn't seem like a good thing.  The P35 has holes in the shroud away from the muzzle end.  They are small.  Air definitely comes out when you shoot, depending on how I hold the gun I feel it.  The end of the barrel has an aluminum piece that screws onto it that has an O-ring on the outside to seal against the shroud.  The aluminum piece has openings and the first baffle kind of fits into this piece over the openings.  It looks like the airflow into the back of the shroud is not blocked but is obstructed somewhat by this arrangement. 

The P35 shroud works a lot better to reduce noise than the Avenger shroud.  I'd love to try something like the baffle/hair curler modifications to my Avenger shroud but it doesn't seem to have the volume in front of the barrel that the P35 has (must be about 2.5 inches).  The P35 shroud screws into a metal piece that can be slid out along the barrel (it secures with a grub screw).  I don't think the Avenger can be extended that way, can it?  It would also take some baffles which fit the inside of the shroud fairly well.  I don't know that they have to be cup shaped like the P35 uses, however.  I saw a moderator build on you tube (sub 12 channel) where he used flat plate baffles in the moderator (with hair curler/felt sections). 

[JimD]

I looked again and there is some space in the Avenger shroud that might work if I can find or make baffles and a hair curler that fits.  Seems like I've had the shroud off but when I tried to remove it a few minutes ago it did not seem to want to move.  I need to mess with it some more.  But the Avenger shroud is smaller in diameter than the P35 but it's longer too.  I'm sure it could do more than it is doing to reduce the report. 

Another thing to be aware of in the realm of moderators is the position of the ATF.  They  have no jurisdiction over air rifles but they do have responsibility of regulating fire arms and moderators for firearms.  The issue is their belief that air rifle moderators might be used on firearms.  If they were, then you have to apply for a permit and pay a fee and allow searches by the ATF.  Their position (different from a law) is that if an airgun moderator would reduce the report of even one shot from a firearm, then it falls under their jurisdiction.  A potato could meet their definition.  But some people are scared enough they will not use moderators that are not permanently attached to their airgun.  I am not that worried but I did think about it a bit when I was making them.  I used plastic and wood in mine to try and make it clear that it was not suitable for a firearm.  Might be an advantage of a plastic airgun moderator.  But I worry about durability. 

[Tomygun1]

GTA,
To answer your question, I have monitored the air escaping through the one vent hole at the rear of the shroud.     It was quite a blast with only two wads over it so I added two more.      That reduced the air blast and the noise at the rear noticibly.     I am still experimenting but this seems to be effective.
Tom

[JimD]

There is a centering piece inside the shroud that slides over the barrel.  Anybody know if it can move along the barrel?  My initial thought is to find washers that are a reasonable fit in the shroud and will slide over the barrel.  I would not want a tight fit but I want them to take up most of the space.  Then I need hair curlers that are also fairly close that I can wrap with felt to fit.  Then I would slide a curler or half curler down over the barrel inside the shroud, then a washer, then another curler etc. until the space if full.  I will probably add small holes at the end of the shroud like the P35 has too.  If the centering piece moves, then the length of the curlers is not critical.  If not then they need to not be overly long or the end of the shroud (DonnyFL or stock) won't go on.  I am trying to mimic what works in the P35 shroud. 

[soupyyy]

I made some changes to the LDC as suggested by Subscriber and I hope it is what he was suggesting.  I also added some felt at the back end of the shroud to work with the vent holes and that lowered the dB by about 1/2 dB.  With the felt in place and at high power setting I can tell the pitch dampen a little bit. 

With the felt average dB is 79.1.  3 shots at low power, CPHP 14.3 at 2900 PSI, Liberty .22, muzzle is 16' from sound meter.

[subscriber]

Jim,

You provided a lot of detail, yet it is hard to know how the parts in your system interact, without specific details - or images.    If a system that works in other similar configurations does not work in the one you just made, then you should look for causes other than airflow that might account for that:   

For instance, your internal bits may be loose inside the outer tube, and rattle or "slap around" enough on the shot to create more noise than it suppresses.  Certainly, pellets clipping will create a lot of noise that will emanate from the body of your device - like shooting a plastic part and listening to it at short range...

What you seem to have noticed is that there is not just one way of reducing airgun noise.   Conical baffles work to slow down the air leaving the suppressor muzzle.  So do flat baffles, for example.  Felt wrap damps pressure wave reflection.  The main source of sound dictates which aspect needs improvement.  Experimenting helps identify that.  This assumes that your particular device is not generating noise.

The fact that your sound measurements change when you remove parts from your P35 suggests that your measurement strategy works.  Confirming this is rather important.  If you got about the same sound measurements, no matter the shroud changes, I would look closer at noise coming from your indoor trap, and how close you are to it.   

If you measure the sound 5 feet from the muzzle, that should be a lot louder than 10 feet away.  Every time you increase the range by a factor of 1.4 (square root of 2), the sound level should drop by about 3 dB.   If not, something may not right with your meter.   This is in open space.   What will complicate your measurements is shooting in a small room or passage, with sound bouncing off the walls and ceiling.     

Systems respond not only to the deliberate variables, but also to the incidental or unintentional variables.  Look at every aspect that has changed, not just the ones you expect to have an effect on performance.

What is the tune on your Avenger?  That can make a huge difference too.  Are you sure its breech O-ring is installed correctly....

[subscriber]

There is a centering piece inside the shroud that slides over the barrel.  Anybody know if it can move along the barrel?  My initial thought is to find washers that are a reasonable fit in the shroud and will slide over the barrel.  I would not want a tight fit but I want them to take up most of the space.  Then I need hair curlers that are also fairly close that I can wrap with felt to fit.  Then I would slide a curler or half curler down over the barrel inside the shroud, then a washer, then another curler etc. until the space if full.  I will probably add small holes at the end of the shroud like the P35 has too.  If the centering piece moves, then the length of the curlers is not critical.  If not then they need to not be overly long or the end of the shroud (DonnyFL or stock) won't go on.  I am trying to mimic what works in the P35 shroud.

Jim,

I may be confused:   The centering piece you describe sounds like the stock air stripper attached to the front of the barrel.  If it is the air stripper, that cannot be slid down the barrel.  A photo would help clear that up.

Adding baffles that are a close fit in the shroud, in front of the barrel muzzle can help.  Adding them behind the barrel muzzle would choke off the flow into the rear of the shroud, and reduce the effective expansion volume there.  Washers that leave a gap between shroud ID and barrel OD may be useful, although I would prefer very open cell foam, scrubbing pads or wire mesh.  The latter materials can breath, but act as very poor reflectors for pressure waves; and thereby allow air to expand in a controlled manner, but damp sound.

Now, don't be put off because I don't seem bullish towards your plan.  I may have the cat by the tail .  Try it and see.

[Lobo]

There is a centering piece inside the shroud that slides over the barrel.  Anybody know if it can move along the barrel?  My initial thought is to find washers that are a reasonable fit in the shroud and will slide over the barrel.  I would not want a tight fit but I want them to take up most of the space.  Then I need hair curlers that are also fairly close that I can wrap with felt to fit.  Then I would slide a curler or half curler down over the barrel inside the shroud, then a washer, then another curler etc. until the space if full.  I will probably add small holes at the end of the shroud like the P35 has too.  If the centering piece moves, then the length of the curlers is not critical.  If not then they need to not be overly long or the end of the shroud (DonnyFL or stock) won't go on.  I am trying to mimic what works in the P35 shroud.

The centering piece can be slid up and down. There is a recess that the grub screw tightens into at the stock location. With the centering piece being plastic, I dont know if the grub screw with no recess would hold. It very well may, I don't know.

Lobo

[subscriber]

Thanks Lobo,

I was thinking generically of the Marauder and FX type setup.  Hence my request for a picture.  Should have found an actual resource, such as this video: 

  (starts at the point the shroud is removed).   

It shows the barrel to shroud support as something that could be moved from its factory position.

[avator]

The centering piece he is referring to sounds like the depinger in the Marauder.