Interesting Sound Measurements

[JimD]

Sorry, I misunderstood how you would use the sand. 

I am interested in Peter's deep pellet trap idea with respect to sound off to the side.  That is where my neighbors are.  I might even try just taping cardboard around my trap or something and then measuring off to the sides.  It would be nice to have lower noise from the trap in the path of the pellet but even if that were not achieved, less off to the sides would be helpful with respect to negative feedback from the neighborhood. 

I tried putting a small, about 12x12 inch, piece of cardboard close behind my meter.  It was angled and intended to reduce the noise from the trap.  Didn't seem to do anything.  May have been too small. 

I volunteer at my church on Mondays and one of my projects there gave me another material to test.  I was replacing some damaged ceiling tiles in the offices.  I saved a scrap that cannot be used at church due to the damage.  I don't have enough to try it on the sides of the trap but I'm planning to tape it over the cardboard backer and then taping the target to it.  It may do nothing but might help to dampen the impact noise.  The back side seems softer so maybe I'll put that facing the target.  I have not purchased ceiling tiles but I think they are relatively inexpensive and might be another idea for the surround of a deep trap.  They are supposed to reduce noise between floors of a building.

If I don't blast too many holes in it first I might also try it as a baffle behind my meter. 

[WhatUPSbox?]

Jim, I hope those ceiling tiles are not old enough to be from the asbestos acoustic tile days.

You guys inspired me to do some testing today. I tried three different trap configurations.
The first is my standard cardboard trap, filled with clothes, steel plate in the back. The front face is standard issue cereal box cardboard. (first image)
The second is a foam face trap with two layers of foam and clothes behind that (second image)
The third image is the muffler I put together yesterday to check out the sand idea I posted. It consists of two containers separated mostly by sand and some foam, The wood blocks are only at the top for centering. The core wire tube is wrapped in a shop towel (at least I thought she said it could be a shop towel) and some clothes. The third image shows it with the carboard trap clamped to the rear. The hole is about 3".
The shooting distance is 25ft and the soundmeter is 5ft from the target plane. I aimed the meter at the trap since that was what was of interest. Next to the meter you can see a small microphone (iMM-6) processed by a 96KHz USB sound ADC and captured by Audacity S/W (free) on a laptop. This second instrument collected a lot of data but I won't distract this thread.
I used a stock P-17 as my loud gun and a shrouded CP1 as the quiet gun. I was shooting Daisy wadcutters since I guessed those would have the loudest impacts.

The results from the meter (set on Fast, A, Max)
The foam was about 10db lower than the reference cardboard trap. If you put a sheet of printer paper in front of the foam the readings went back up almost to the cardboard levels
The muffler suppressed the cardboard trap by about 12 db for the louder P17 and more so for the CP1. I think the P-17 muzzle noise was limiting the reduction.

These numbers should be taken with a grain of salt. I'm still trying to figure out what the meter readings contain compared to the microphone results.
The foam is a viable trap if you can get it cheap/free. You could stencil/paint the bulls on the surface and the wadcutter pellets cut clean holes.
The muffler is a way to attenuate the noise from the trap front surface. You probably don't need all that I put into it (I had fun building it) in actual use, you could add a couple of LEDs to illuminate the recessed bull.

[subscriber]

Those traps look proper emanation diodes, Stan.

Makes me think of this one:

[subscriber]

How loud is shooting directly at a brick surface?    Somehow I don't think the "tink" is all that loud, providing the pellet frag does not go somewhere it can cause harm.  And, that no one minds the marks on the brick.  So, sacrificial bricks would be required.

Perhaps a bad idea for lots of shots in the same location, as a pockmark could eventually act to direct pellet frag back towards the shooter or other fragile or living things. 

It is over 40 years since I shot an airgun, using a face-brick wall as the ultimate backstop.   From memory; it was not loud at all.   Don't have any bricks around here, and cinderblock is not as robust, although it would probably be as quiet.

[JimD]

Stan's measurement of paper over foam at around the level of cardboard convinced me shooting through acoustic tile will not be quieter.  I don't think these tiles have asbestos but I haven't tested them.  I tried putting a rectangular tube made of cardboard around my meter with the opening pointed to the gun away from the target.  Didn't seem to help. 

I did some measurements at 15 feet away from the trap to better match up with Peter's.  The meter was on a drywall bucket in the yard for these measurements.  Both in line with the gun and at right angles to that line, I measured about 88 db.  That is just about the same as what I measure from my most common placement of the meter, 15 feet in front of the gun but 20 yards from the target.  But in those measurements the meter is also up against a window of my house.  So reflections off the house may negate the greater distance from the trap.  At about 45 degrees to the trap (still 15 feet away)  I measured 82, 75.7, and 73.5 db.  Significantly lower.  I didn't expect that. 

I haven't shot any bricks yet but I have a bunch and probably will.

[subscriber]

Thread title is appropriate indeed.

[JimD]

I went and did a little yard work and then decided to shoot a brick.  I had not shot my P35 today so I used it.  It likes 20.06 grain H&N FTTs and I have tuned it to shoot them at 850 fps or a little more.  Not a very powerful 25 but it works great on squirrels.  I placed three brick stacked on each other at 25 yards from my shooting bench.  I put the sound meter 5 yards from my bench in it's normal spot up against my house.  I measured 87.6 db, 86.3 db, and 86.4 db.  That is 2-3 db less than I measured for pellets hitting the cardboard front of my pellet trap. 

[subscriber]

Thanks Jim,

I suspect that the sound off a brick has a very high pitch to it.  Something that dies off very rapidly with distance?

This, while a carboard box sounds like a drum.

[JimD]

I googled for examples of things that are 85 db.  It came back with heavy traffic when you are in your car, a food blender, a cinema, and a noisy restaurant.  These examples go on for seconds to hours, not a fraction of a second like our pellet impacts.  It is about where it is suggested hearing loss starts.  But for our very brief exposure, I doubt it affects us.  Now my swinging target at over 100db.....

The brick might be higher pitched, I'm not sure.  It sounded different from the cardboard. 

The distinctive cardboard strike is sometimes useful.  I've recently managed to shoot a few small groups at 25 yards, 1/8 inch or so.  It started to make me wonder if I missed or something when I couldn't see another hole through the scope.  But I remembered hearing the pellet strike the cardboard. 

[WhatUPSbox?]

For your hanging target, I wonder if you were to glue a couple of layers of 3/4" plywood to the back side to suppress the ring (and maybe truckbed liner on the front). In the past, I've melted used pellets to fill a pellet tin (to use as a weight in the shop) but it might make a quiet target with pellets hitting the lead face. For some indoor testing I've used a short piece of pipe filled with LDPE bags and an end cap (idea borrowed from Lloyd Sykes). That would make a good hanging target. The bags are very efficient at stopping pellets.

[JimD]

The next thing I want to do with the swinging target is to measure the sound of each of the 8 targets.  I only measured the 2 inch which was over 100 db.  But that is not a very challenging target so I could easily skip it.  My impression is the others are not as noisy but I don't trust my impressions of sound level. 

I appreciate the ideas of how to quiet it down.  I just dug over 10 lbs of pellets out of my old trap and have a little melting furnace so I could cover the face of that plate with lead and see if it helps.  It probably would.  The sound is like a slap.  Kind of a neat sound but loud.  All my guns will move all these plates but none of them spin the 2 inch around.  So lead on it would make the movement even less.  I wonder how long a lead face would last.  I make the swingers out of 1/4 inch rod and washers.  I have to double or triple up the washers so they are thick enough to withstand the impact.  I weld the hole closed on the washer.  But i could leave the hole at least partially open to help the lead stay on.  But it seems like the lead would still get beat up.  But it could be melted down and reused. 

[WhatUPSbox?]

I think the lead would need to be contained in something (pipe cap?), otherwise it might just crack apart.

I looked at some of the microphone/Audacity data for the shots I took the other day. The basic geometry of the test is in the first image with some of the key distances. For each path I show the time estimate based on the speed of sound for most and on the pellet muzzle velocity for the pistol to target distance. I also calculate the difference from the arrival of the muzzle noise. The second image shows the Audacity plots for two of the shots, one facing the target and one facing the pistol. both are for the quiet foam target. For both shots, the first blip on the trace is the muzzle noise and it looks like the target blips show up at about the predicted time. The target facing trace has an extra blip which I'm guessing is the reflection off of the wall (the mic is facing the wall). By changing around the test geometry you can move these around. I don't know what the sampling time of the soundmeter is and this is hard to find online. I show .125 sec for reference since that is the time constant for the fast setting.
My takeaway is that for my configuration, everything takes place in around .060 sec and I imagine all three events (muzzle, wall bounce, and target) are all in the soundmeter measurement.
I think Jim's shoot into the lake 200 yards away is the way to go. Otherwise pointing the mike at what you want to emphasize may help.

[AlanMcD]

That is an outstanding analysis, and it is great to see the trace.  It looks like if the trap were about 75 feet from the target, the sound from the trap would not make it into the trace for the pistol facing  mic.  Of course a faster pellet from a more powerful gun would change the timing, and it most likely would require even greater distances.  Great work!

[JimD]

I have another idea I may set up later today.  It builds on Stan's successful reduction with a sort of muffler in front of the trap.  I will shortly go pick up an order at Home Depot and do some other errands.  While I am out I will buy a couple 16x16 concrete pavers.  These will form the top and bottom of a little enclosure.  The walls and back and maybe a front will be stacked brick.  I plan to just literally stack things up on an old workmate in the yard at 25 yards from my bench.  Without a front I don't expect it to change the sound near my bench where I put the meter.  But I'm interested in what it does off  the sides.  Like at 90 and 45 degrees off to the side of a line between my bench and the target.  I think it may give me a significant reduction.  If so, I might build little structures like this at 25 and at 33 yards from my bench (33 yards is my back fence location).  By stacking bricks in front to create a smaller opening, I may also be able to reduce noise coming back at me, for moderator testing.  But I'm thinking of just ignoring that most of the time for convenience.  I have several hundred recycled bricks waiting on me to find something useful to do with them.  So my cost would just be the pavers and some mortar (and time).  I might even be able to rig up a way to put swinging targets in there although I might need more depth. 

It may not do much, of course.  But it won't cost much to find out. 

[WhatUPSbox?]

Nice idea.
If you close off the front, one of these selfie lights mounted inside the front, running off of a USB powerbank could be a cheap way to light up the target. https://www.amazon.com/s?k=B091T6Z9ZY or even some solar powered garden lights where you separate the panel, leave it on the roof to charge and just cover it so the interior lights come on when it is time to shoot.

The other thought is to use 2 8X16 pavers for the top and leave enough of a gap to slide a target frame down. Kind of like an old slide viewer. That would make target changes easy.
Post some pictures.

[WhatUPSbox?]

Another piece of information is how the ear responds to short sounds. I found the curve below that shows how much louder a short sound needs to be to sound as loud as a longer (or continuous) sound. Below .2 sec (200ms) the hearing starts to drop off and you need more db to sound the same. In the traces above, most of the combined noise takes place in less than .060s (60ms) so our hearing already needs an extra 3 db or so. The actual muzzle blast happens within 10-20 ms so that is an extra 10 db. Who knows how the soundmeter response relates to that.

Having someone off to the side listening is probably the best evaluation

[JimD]

It stopped raining long enough I got some measurements.  Unfortunately I don't see much benefit.  I started measuring with my meter in my normal spot up against the house and 5 yards in front of the gun.  I measured the same sound level there as 5 yards in front of the trap previously.  My readings were 86.7, 87.5, 87.9.  That is slightly lower than the 88+ I last measured but only slightly.  I then measured at 90 degrees to the line of fire and 5 yards from the trap.  I got 85.9, 86.1, and 86.9 db.  I measured the same as in line previously so this is very slightly better but only slightly.  Last I measured at 45 degrees to the line of fire and 5 yards from the trap.  I got 85.9, 86.1, and 86.9 db.  I got a couple lower readings before at 45 degrees but they look odd, these make more sense to me.

Then I tried blocking off all but the top row of bricks with more bricks in the front.  So I had a 1 brick tall slit to shoot through.  I got readings of 78.6, 83.2, 81.2, 77.1, and 82 db.  I shot two extra times because I noticed a brick moved from the impacts and I wondered if that had an effect.  I moved it back and got 77.1.  But then I shot again and got 82 and the brick had not moved.  These are a little better but shooting through a slit is not practical for a normal practice.  These aren't low enough for moderator testing. 

The fact that these are not sealed, not laid in mortar, is undoubtedly raising the sound due to leakage.  But I doubt it is making an impact more than a few db.  If I had gotten results like the slit ones for an open front, I would probably have made traps this way.  But for a couple db it doesn't seem worth the effort.  A longer tunnel would also undoubtedly help but it would also make seeing the target more difficult.  I could put a light in there but I don't really want a really big trap in the back yard. 

I left it set up.  Maybe I'll think of something else.  I haven't shot that accoustic tile yet.  Probably have to at least do that. 

[WhatUPSbox?]

Well, you could stack the closeout bricks on edge and shoot through the holes. 
I think the bare brick interior just reflects the sound. Maybe try lining the interior with a few layers of your acoustic ceiling tiles. Depending on how pliable or crushable the acoustic tile is, you may be able to put it between the layers as a gasket of sorts.

I'm not sure if you have enough room on the 16X16 but I wonder if changing the stack to a chevron type would help break up the flat surface. Or cutting the bricks through the holes to create a shaped edge.

[subscriber]

Thanks for your effort, Jim

[subscriber]

Stan,

You seem to have specialized acoustic knowledge.  Do you think it possible to make a trap that uses active noise cancellation?  Or is that only something that can be applied at the ears of a specific person?

If noise cancellation works by overlaying the inverse of a sound wave in the right phase, then accidentally overlaying waves in the wrong way can make them louder, depending on just where you (or the meter) are positioned.  AKA constructive and destructive superpositioning. 

It would seem obvious that we should avoid constructive superpositioning, but how do we achieve that?  More importantly, how to ensure that this applies to the neighbor across the wall?

If we stand in just the wrong place, might muzzle blast (or reflected report) superimposed on "trap slap" create an unusually high peak that we can hear (or measure) further away?






https://courses.lumenlearning.com/boundless-physics/chapter/interactions-with-sound-waves/

https://www.acs.psu.edu/drussell/demos/superposition/superposition.html