Interesting Sound Measurements

[subscriber]

The carboard face on a cloth trap makes about same noise as the same carboard in front of a rubber mulch trap.   While that carboard slap can be loud, if the carboard is not full of holes already, it acts as a noise shield for whatever is inside the trap.  Use that principle to shied the trap noise and keep most of it from reaching your ears; and especially your meter.  Use two layers of cardboard with an inch gap between them; for example.

Floppy carboard makes a different noise to well braced cardboard.  My point is to make your trap quieter, rather than prescribe how that must be done. 

To state the obvious; your sound meter does not know the difference between the muzzle blast and the projectile slapping the trap.  The meter simply captures the peak sound pressure.  Even when you can discern the delay between the two sounds.  The meter cannot.  The problem is when we pay a lot for a meter, we assume the measurements of muzzle blast are trustworthy (because that is what we intend to capture), even when our ears tell us that the trap is the loudest sound.

[soupyyy]

Here are some number to entertain you.  Some t-shirt material and stainless steel scrubber makes for excellent pellet stopper at a reasonable fpe.  Sound meter is about 1.5' in front and 1' to the side.   

Not scientific in any way.

Just some cloth backing to the stainless steel scrubber - 80.8. 82.3
1" foam in front of scrubber - 87.0
1 layer duck tape - 100.3
standard 20lb paper - 93.0
cardboard Amazon box - 98.0
Costco paper towel folded 8 layers - 92.0
Lemon leave - 93.0

I am shooting the Nova Liberty .22 CPHP 14.3 at low power around 750 fps with an LDC and in my typical setup the meter would be 16' in between the quiet pellet trap and the muzzle and it register 78 dB.

[JimD]

I've been working on other things and neglecting my shooting.  But I plan to try substituting a layer of denim from an old pair of jeans for the cardboard layer on my pellet trap.  I want to see how much that lets the shreaded rubber bulge out and how much it reduces noise.  Your data suggests it will help.  If it doesn't bulge too much I might even use it.  Nothing magic about cardboard but it works and is cheap.  But pellets hitting it make a fair bit of noise. 

[subscriber]

Thanks for sharing your measurements, Peter

As usual, your test is sensible, and your data useful and well documented.

It seems that cardboard and paper are even louder than I imagined, even when placed in front of a "quiet" trap.   Intuitively, I knew that the cardboard was forming a drum-skin of sorts.   Even that is rather loud, if it is close to the meter.  Thus the meter should be further away from the trap when one wants to measure muzzle blast - as you normally do.  Every instance where you double the distance from the meter, the sound pressure should drop by a factor of four - assuming no hard surfaces nearby to reflect sound, and confuse the measurement.

As you normally have the meter midway between muzzle and trap, there is still the possibility that a loud trap may register louder than muzzle when shooting a quiet airgun.  I am not suggesting your historical data is flawed, but it occurs to me that if you could move the trap further away while keeping your standard muzzle to meter distance, confidence in your data would improve further.  Yes, I know that your yard is limited in size, so this comment may have to remain theoretical.

One way to remove my urge to move your trap further away would be for you to measure the muzzle blast the same way you measure your trap sound - as you described above.  We assume that the muzzle is still much louder than the trap at 1.5 foot in front of the muzzle and 1 foot to the side.  If you confirm that, then there is no doubt that by placing the meter in the middle, as is your method, you have never captured the trap as the peak sound over the muzzle.

It is easy for me to write a busy test plan for someone else to carry out, but if you should shoot the same Nova Liberty as you just did and measure that from 1.5 x 1 foot from the front of muzzle, perhaps you could do the same for the benchmark of quiet, the HW30 also.  That would help calibrate the human ear.

[soupyyy]

My swinging, energy absorbing quiet pellet trap.  This time with the sound meter 1.5' in front and 1' to the side of the muzzle.
About 30 feet from muzzle to quiet pellet trap.

HW 30 .177, CPHP 7.9
95.6, 94.7, 94.8, average 95.0 dB

Nova Liberty .22 with LDC, CPHP 14.3
Low setting - 94.6, 94.2
High setting - 99.1, 98.6

* Nova Liberty .22 with LDC, shoot CPHP 14.3 at low power around 750 fps and in my typical setup the meter would be 16' in between the quiet pellet trap and the muzzle and it register 78 dB

[JimD]

I initially noticed the "target is louder than the gun" effect working on a home made moderator.  I was shooting into a large oak tree 25 yards away from the muzzle with the meter no more than 5 yards away.  Nothing I did made the gun quieter and then I thought about shooting into the ground or lake further away. 

My interest in the trap sound level is for avoiding attracting interest with my backyard shooting.  I feel like the guns are quiet enough at 85 db or so or less.  I need the trap to be quieter for it to make any difference my favorite gun is under 80 db. 

[subscriber]

Peter,

I think your sound measurements and methods for airguns are of the most sensible and useful posted on this forum.   You have gone on to provide data in response to the question about how loud your trap is.

I wanted to satisfy myself that placing your sound meter at 15 feet from both the trap and muzzle is not so close to the trap, that the trap might still be the loudest sound captured from a moderated airgun.  So, I did a quick "regression study".  It is based on your dB data for the 1.5 x 1 foot distance between your trap and meter.

Then, following the inverse square law for sound radiation, I attempted to calculate at what distance your 95 and 100 db trap sounds would drop to, or just below the values for muzzle blast you captured at 15 feet.    In other words, I am asking if you are far enough from the trap so that the sound you capture is definitely muzzle blast and not trap noise.

Because you measured your trap noise from 1.5 feet in front of the trap, and 1 foot to the side (to avoiding shooting your meter), the effective distance between meter and trap adds complexity and uncertainty to my calculation.  So, I used three distance as the baseline for your dB measurement:  2.5, 1.5 and 1 foot respectively:

The first table in the attachment below starts at 2.5 feet (30") from the trap because that is the diagonal distance from the center of the trap.  The second table, 1.5 feet.  Now, perhaps the sound radiates from the nearest outside surface of the trap, rather than its center - suggesting the effective distance is closer to 1 foot.


Results:

If the effective distance to the trap was 2.5 feet, then the same trap sounds measured at 15 feet would be about 79 and 84 dB for the HW30 and Nova Liberty respectively.

If the effective distance to the trap was 1.5 feet, then the same trap sounds measured at 15 feet would be about 75 and 80 dB for the HW30 and Nova Liberty respectively.

If the effective distance to the trap was 1 foot, then the same trap sounds measured at 15 feet would be about 71 and 76 dB for the HW30 and Nova Liberty respectively.


Conclusion:

1. As the lowest experimental LDCs on your Liberty has produced peak sound values of 77 dB at 15 feet, the effective distance between your latest trap sounds and meter was closer to 1 foot.  And, you are right on the threshold where the trap and muzzle are equally loud to the meter, when that is placed equidistant between muzzle and trap at 15 feet.  So, you may not be able to develop an LDC quieter than 77 db at 15 feet for your Liberty, because that is what the trap is producing at 15 feet with that airgun, pellet and tune.

2. Whereas, the 86 dB value captured at 15 feet from the bare muzzle of your HW30 means that the trap noise can be ignored at 15 feet.  This assumes that trap noise is constant, which I doubt.  It depends on the state of the trap and how many holes there are in the carboard around the location of any given pellet impact.




Suggestion:

Don't change anything that would render your previous sound data obsolete.  Keep the 15 foot muzzle to meter distance you have been using, with the one foot side offset.  As the placement of your trap cannot be easily extended to much further, leave it where it is.  As your trap is already "soft", trying to improve on that is likely to be difficult and time consuming. 

A much simple confidence booster in keeping trap noise off your meter, is to shield the meter from trap noise by means of a carboard sheet attached to dowels, pushed into the ground.  Place the cardboard so that the point of impact on the trap does not have line of sight "visibility" to the meter.  In fact, none of the trap box should have line of sight to the meter.  See simple sketch of that from a top view, below. 

I suggest that the cardboard be placed midway between the meter and the trap, so that sound does not reflect off the cardboard front face and add to the sound it "sees" from the muzzle.  If the carboard were a shipping blanket then there would be no reflected sound to be concerned about.  If plywood, then that might reflect sound more strongly than cardboard...

[AlanMcD]

That's some great analysis there . . . .

That leads to an idea that would be easy to try, and I will do it next time I do any sound testing, but others can as well:

Given that the trap has a flat surface that faces the shooter (as we all seem to do it), the sound most likely radiates out from the trap with a heavy directional bias - the peak sound pressure level is almost certainly radiated out perpendicular to the cardboard face of the trap.  With that in mind, the idea is that if we rotate the trap about 45 degrees to one side, we should be able to reduce the sound pressure level that is radiated back down the line to the shooter and the sound meter.  Then if we add the cardboard sound blocker in front of the target trap like in Subscriber's illustration, still perpendicular to the direction of the shot, we should really be cutting down the sound level heading back to the sound meter.

Of course all this is just about getting a better reading on the sound level of the gun - pellet impact sounds are still a very real world factor.   If we do that on our shooting ranges, we are simply redirecting the sound to a different place.  But if that new place is facing less sensitive neighbors, or neighbors that are much farther away from the trap, then it is a good thing out there too.

[subscriber]

Good point, Alan; about using directionality to help reduce the trap noise "pointing" towards the meter.

While such shenanigans seem not "real world", it will help prevent someone from giving up in disgust when their expensive LDC does not produce the expected low reading that they are convinced they can hear.  Ditto for evaluating different LDC that all seems to produce about the same benefit, based on measurements.  Whether these LDCs are bought, or under development; professional or commercial (vegetable, animal or mineral; from G+S  ).

[JimD]

I had not seen these comments about trapping the sound from the trap until just now but I did some more measurements this morning.  My way of avoiding the impact masking the gun noise is just to shoot into a lake behind my trap.  I try to make the impact close to 200 yards away.  Still it seems like the noise sometimes comes back.  Maybe I need a barrier in front of the meter. 

I shot each of my three guns at least 3 times.  The averages were:

AV Avenger, 25 caliber, with DonnyFL Tanto : 84.93 db

Prod (22 caliber tuned to 16-17 fpe) : 85.5 db

Prod with Buck Rail silencer : 76 db - note this is totally different from what I measured last time but my trap was making me think the silencer does not do much - it definitely does.  Roughly a 10 db reduction is big and this moderator is not expensive nor does it require an adapter.

P35 (25 caliber tuned to about 32 fpe) : 79.7 db

P35 into trap with cardboard face : 88.3 db

P35 into trap with cloth (jeans) face : 88.4 db

So I learned the Buck Rail silencer works and that using a cloth face instead of cardboard doesn't make a difference.  All my guns are quieter than the noise of their projectile hitting the trap.  My main interest is in keeping the peace with my neighbors.  I don't see any obvious way to quiet the trap but maybe I'll think of something or one of you will have an idea.  My neighbors are not complaining so I guess I'll just keep shooting.  I dug over 10 lbs of pellets out of my trap today. 

[subscriber]

Thanks for the info, Jim

Where was the meter placed for these measurements?

I don't think impact noise from 200 yards away is going to bother your meter...

[JimD]

I put the meter about 5 yards in front of the gun and a little to the side (on a window ledge).  It was thus about 20 yards from the trap for the measurements - so the trap impact is significantly louder than the guns.  15 yards further from the meter and still noiser. 

[Scott_Tx]

I was testing some 3d printed add-ons the other day and was shooting over my meter and had a baffle strike and... shot my meter

[soupyyy]

The new and improved quiet pellet trap for testing LDC.  The main pellet trap is enclosed inside a box and surround by foams.  There is a 7" space in front of the stainless scrubber surround by foam.  Some interesting number with the different backing behind the sound meter.

Liberty .22 with LDC, CPHP 14.3, start 2900 PSI down to ~2750 PSI, power setting Low-750 fps.  Sunday 3/20/22 9 AM quiet outside.
This is the order of test.

Brick backing - 81.8, 82.2, 80.8
Brick with foam - 80.5, 79.3, 80.1
No backing - 79.6, 79.2, 78.5 (surprised it is lower)
Wood 3/4" MDF - 80.5, 80.2, 80.4

[subscriber]

Interesting, Peter

So, sound from the muzzle that would have gone into "space" is reflected off the meter shield, onto the meter, making the reading higher?

A few days ago when I suggested the meter shield, the possibility of reflected sound occurred to me, but you have vindicated my prediction.  That is why I suggested the screen be midway between the meter and the trap.  Even so, the wavy foam should have been OK near the meter; but it was not.

Another forum member suggested the shield be angled so that sound hitting it would be reflected away from the meter.  That should work for both muzzle and trap noise; although wavy foam is omnidirectional (and up close; a problem).

Your deep trap with the small hole should help reduce the total sound leaving the trap.  Although what is able to leave would be rather directional.

Thank you for doing this work, and replacing speculation with actual data.  I think a lot of people who would be happy with a given LDC, have been disappointed with their meter reading, because they were unaware of how sound behaves.  But, since they trust the meter more than their ears, they end up less than happy with their LDC.  You are providing information to help calibrate their measurements; and expectations.

As an aside, I suspect that sound measurement and perception work a little like earthquake measurement:   The Richter scale is not just about peak amplitude.  Duration and total damage potential also factor into it.  We think that the sound meter simply reports the peak sound value, and that is it.  I get the impression that sound exposure is cumulative, and that lots of sound, from more than one side, and / or for a longer duration can sound (and perhaps measure) louder.  Perhaps two sound wave from the same source can meet to create a higher peak.  The point is that measuring sound is not quite as simple as it seems it should be.

If my suggestion sound like nonsense, consider the muzzle blast of a .223 Remington rifle with a 16" barrel.  Compare that with a 12 gauge bird shot load from a 28" barrel.  Both are very loud close up.  Yet the muzzle pressure of the smaller caliber is over half the peak chamber pressure of the larger caliber.  The larger caliber noise "makes up for this" by projecting the sound from a much larger area "speaker" (muzzle).  Now, we won't confuse the two calibers, but to state that one is much louder than the other would be a misstatement.  One has a sharper sound, while the other has a "larger" sound.

What has the above got to do with airgun LDC sound measurement?  Sound reflected off hard surfaces makes even lowly airgun report seem "larger" and more significant.  To the ear, and apparently, to a meter.  Not to mention that any sound that has a significant echo screams "gunshot" to anyone within earshot.

[WhatUPSbox?]

Peter, Interesting results. I think anything firm that you put near the meter will just reflect the muzzle noise back to the meter.
It would be interesting to compare your new trap to the basic cardboard target hanging below it. Maybe put the meter 5 ft off to the right (if you can), even with the trap, and compare the two.
What is the close-out panel on the trap made of?

Here are a couple of links on soundproofing. I'm thinking in the direction of the box in a box approach with sand as the absorbing material. Now to see what scrap materials I have on hand.

Great work.
https://www.acoustiblok.co.uk/soundproofing-materials/
https://soundproofcentral.com/does-sand-absorb-sound/

[subscriber]

The plan-view image below is my perception of how loud airgun muzzle blast sounds, with my ear placed relative to the (virtual) muzzle.  To keep one aspect constant; the inner circle has a radius of one meter from the muzzle.   

The muzzle is pointing up in the image.  Red arrows indicate ISO standard hearing safe measurement meter placement.

Doubling of the line length in my image equals a doubling of perceived loudness.  Something that would measure about 10 dB louder on a meter.

The outer profile in my image is demarcated by the line lengths that represent my perception of how loud the relative sound is, if I were to place my ear on the one meter radius, about someone firing an airgun.  No, I have not placed my head in front of the muzzle, so there is a lot of speculation represented here.

The point is that muzzle blast is very directional; such that the apparent sound level increases sharply as your ear moves forward of perpendicular with the muzzle (if a muzzle brake is used on a firearm, the loudness to the side and rear do not drop off as sharply, compared to a bare muzzle). 

Conversely, the further back from perpendicular with the muzzle at one meter the more rapidly apparent sound level drops off.  Standing directly behind the shooter even at one meter from the muzzle is usually not loud at all with typical airguns.  In that case, their body acts as a shield, in addition to the directionality aspect I am trying to "quantify".

Now, if we are shooting at our trap near a wall towards a neighbor's property, they are in fact subject to near maximum aspect of muzzle blast - less that screened off by the wall (a significant reduction).  I mention this, because Peter's placement of his meter at one foot off bore axis is in fact trying (and succeeding) at capturing this.  If we place the meter at 45 degrees off from the muzzle so it is just as far to the side as it is from the muzzle, the reading will be much lower than with the meter nearly in front of the muzzle.  Unless that is where the ears are, that must not be offended such meter placement is a nice way of fooling ourselves that our airguns are less disturbing than they may actually be.

That said, I think that capturing reading all the way around the muzzle at some distance greater than the one meter I show in my image would be extremely useful data to have mapped out.  I predict that house and property walls nearby will cause the shape of such a sound map to be distorted from the smooth semi-circular curve I suggest it is at one meter from the muzzle.   Even having that degree of sound reflection objectively quantified would be  very valuable.  So, I am not going to "specify" the radius, but would suggest 15 feet so that it attaches neatly to Peter's existing data.

[JimD]

There is a little reflected sound data in my initial data set.  My normal shooting position is from the side porch of my house putting the house about 3 feet to the right of the gun for the first 8 yards of so if the pellet's flight.  So I should see amplification.  But I did not seem to.  My other shooting position was from my back porch with no side wall closer than 5 or 6 yards away although the wall of the house was behind me.  Maybe that is why the readings were about the same.  If so, a wall to the side of the gun or a wall behind the gun has about the same effect on the readings I'm getting.  When I measured from my back porch the meter was out in the yard with nothing around it about 15 feet in front of the gun.  When I shoot from my side porch the meter is on a window sill 15 feet in front of the gun.  I expected to see lower readings from the back porch but did not.

With respect to using sand to absorb the impact, I don't think it will make a difference if the target is still supported by cardboard.  I tried cloth instead of rubber mulch and the readings were the same.  I also tried cloth over the rubber mulch to support the target and got about the same readings.  The cloth also did not support the rubber mulch well.  I wonder if cardboard will support sand.  If you had a big pile of sand so you did not have to support it and then suspended just a paper target in front of it, that would be interesting.  I have not found it to be much quieter shooting into the yard, however. 

So far my only successful way to keep the impact noise out of the measurement is to move the impact very far from the meter - like a couple hundred yards.  Even if I shoot into water 30 yards or so from the meter it makes significant noise.   I realize not everybody had the ability to safely shoot a couple hundred yards, however. 

[soupyyy]

Stan,

Here is the number for the same Amazon cardboard box with the new pellet trap box.  It is definitely much better at containing the noise going out of the box.  I keep the same distance so the number can be compared.  I did put the front cover back on.

Swinging pellet trap - 98.0
New pellet trap - 88.7, 87.1

Here are some sound and dB number for some common items around the house.

[WhatUPSbox?]

The sand suggestion was not to shoot into it as a backstop but to use it for deadening the walls of a sound trap in front of the target similar to what Peter is doing.