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Best way to measure firearm dB levels? Background dB at outdoor range?

Discussion in 'General Gun Discussions' started by wacki, Apr 6, 2012.

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  1. wacki

    wacki Member

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    I'm curious to see if anyone has taken the time to measure dB background levels at a outdoor firing range. You aren't shooting, you are just sitting in your own lane with people shooting in the lanes next to you. Back of the envelope calculations say there's a 6-15 dB drop. Roofs and walls complicate things and increase noise levels.

    Most of the sound measuring devices on amazon have a max of 130 dB which is way too low. I found this PDF on how to measure firearm sound. They used a 1796€ GRAS 40DP pressure microphone. The stats on that microphone certainly measure up to the task:

    Frequency range 6.5 Hz - 140 kKz
    Dynamic range 50 dB - 184 dB re. 20µPa

    So it would appear that measuring firearm dB levels requires a significant monetary commitment with highly specialized equipment. I'm wondering if there's a cheaper way or way I can borrow / lease equipment. Or if there's a alternative equipment.

    As far as I know the only people in the state of Indiana that own this kind of stuff are Universities and possibly hearing clinics.
     
    Last edited: Apr 6, 2012
  2. denton

    denton Member

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    To measure high levels of sound, you just need an ordinary sound meter, maybe even something as simple as the one in an iPhone or a Droid, and an attenuator. In the case of sound, an attenuator can be something as simple as a Mason jar or a thick walled plastic box. Or you can wrap the microphone in multiple layers of foam.

    You do have to figure out how much attenuation you're getting. To do that, measure a steady ambient sound, pop your microphone or whatever into the attenuator, and note the difference. You'll probably need something on the order of 30-40 dB of attenuation.

    A home-made system isn't going to be perfectly flat across the audio spectrum, but it should be decent.

    Traditionally, firearm sound measurements are made 1 meter from the muzzle. Sound energy level falls off by R^2, dB is a logarithmic scale.
     
  3. Owen

    Owen Moderator Emeritus

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    Wacko, don't forget NSWC Crane, which handles the Navy's small arms efforts.
    Denton, will the attenuators help with the rise time? Most sound meters aren't fast enough to catch the peak of the signature, so end up integrating it away.
     
  4. ESP

    ESP Member

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  5. denton

    denton Member

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    Swing needle sound meters are indeed much too slow to capture the peaks.

    The simple sound meters in iPhones and Droid phones are more like audio range oscilloscopes, and most likely have much faster response times. I have the Decibel app installed on me Droid phone, and it registers hand claps at 80-82 dB from a few inches away. With an external mic and an attenuator, I think reasonable estimates of peaks can be made.

    An attenuator is not going to improve whatever the response time is. The hope is that it does not roll off the highs enough to degrade the response time.
     
  6. wacki

    wacki Member

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    Denton,

    I just installed Decibel Pro on my droid. Do you have any suggestions on microphones? I have a Shure SM57 but I'm willing to purchase something better suited to the task. As for attenuators, does it matter much what the materials are? I would think that something that is 20 NRR against a 90 dB noise would not be 20 NRR against 160 dB. But I'm just speculating there...

    EDIT: Attenuation is highly variable along frequencies.

    3M Bull's Eye Shooting Earmuff " For high intensive noise environments".
    SNR=35dB, H=40dB, M=32dB, L=23dB

    I guess frequency is not the same as intensity within the same band. This is obviously a field that I don't know a whole lot about.

    owen,

    Thanks for the NSWC pointer.
     
    Last edited: Apr 7, 2012
  7. DesertFox

    DesertFox Member

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  8. kludge

    kludge Member

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    wacki, I posted that article quite some time ago... that magazine comes to our office every month. The way they set up that test was very good, in my professional opinion.

    It's true that handheld sound meters can't capture the very short SPL peaks, your need a measurement microphone... and many of those top out at 125dB. You'll need a micrphone capable of ~160-170dB. And you'll have to have a way to record the very quick impulses -- a doubt that a simple sound meter app will be able to do any better than a handheld meter in that regard.

    Indoor ranges are indeed louder... in the duration of the sound impulse... because of reflections off hard surfaces... but probably do not register a higher "peak" SPL readings. Because of the reflections in an indoor range the total energy of the sound getting to your ear can be several times higher that shooting out in the open, and do more damage to your hearing.
     
  9. Owen

    Owen Moderator Emeritus

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    the B&K 4495A mics have a 178dB max, IIRC. I just wish the B&K stuff wasn't so expensive.

    Sent from my Transformer TF101 using Tapatalk
     
  10. BCRider

    BCRider Member

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    The swing needle style is fine IF the meter has a peak hold feature.

    The information on making up an acoustic attenuator given by denton above is .... er... "sound advice" :D He mentions that you won't know how it works over the whole frequency range. But if you test for the amount of attenuation at the range by using gunshots then you don't really care about how it attenuates other things. Simply move it away by some distance until the gunshots register within the bounds of the meter. Then fit the attenuator and try a few more shots to see what the attenuation value is for actual gunshot frequencies. Now you can move in close to the gun and get the shooter's exposure level by taking the value and adding the amount of the attenuator to the figure. Or you can stick the meter with attenuator into a stall at a covered range and see what the bystander exposure is.
     
  11. denton

    denton Member

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    I had forgotten about that. You are absolutely right.

    I think that the mic/headphone set you use for the phone function will work just fine, if you have good attenuation ahead of it. This isn't going to be a lab grade measurement, unless you're ready to spend a lot of money.

    I also suspect that putting the mic in a foam padded pistol case and closing the lid would work about as well as anything I can quickly imagine. The hard plastic reflects a lot of the sound, and the foam absorbs and dissipates it. Or just bury the mic in the center of about a cubic foot of foam rubber.

    It's tricky getting good attenuation at low frequencies, since they penetrate better.
     
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