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Digital Sound Level Meters Setup and Testing

After years of collecting various moderators, I was just curious and wanted to start testing them to know for myself and also to report my findings on here. I've used sound level meters on cellphone apps but from what I gather, they're not too accurate. I just ordered this Extech 407730 for about $100 from Amazon and waiting for it to arrive. 

Screenshot_20220516-135736_Chrome.1652673918.jpg


I'm not too sure if it's the best in that price range. Does anyone have experience with this model? I saw some other sound level meters on Amazon for around $20 to $40 but I didn't want to be disappointed in the quality. For those doing your own testing, at what distance do you place the mic from the moderator and where do you place them? On the side, in front? Do you test them in an open field away from any hard structures to avoid sound amplification? Is there a standard we should adhere to or just as long as all the testing done is consistent? What are your thoughts?
 
Hello - this will be my first post here but I'm a noise and vibration engineer by trade and have 20+ years of experience with various sound level meters so I can answer your questions. (which are good questions by the way)

1) What distance? Totally up to you. SAE standards for measuring motorcycle exhaust usually position the meter at 45 degree angle off the tailpipe at a distance of 0.5m so I'd probably start there. 

2) Open field would be ideal. If you're close to a structure you'll get reflections that can influence your measurement but the most important part is that you are consistent in your measurement location and setup.

The other things that are important are the weighting curve selected (dB, dBA, dBC) and the time response of the meter. Looks like you can only measure dBA or dBC with this meter. dBA has a frequency filter applied so the measured sound pressure represents what the human ear perceives (rolls off low and high frequencies) and dBC is more linear. Up to you to decide which one you want to use but since you are doing relative comparisons, it's not super critical.

For time weighting, you are going to want to use Fast for sure. Think of this as averaging the sound level over a certain period of time. Fast response has a 0.125 second window and Slow is a 1.0 second window. Ideally, you'd use a sound level meter with Impulse response which is a 0.035 second window.

Looking forward to your results! Hope this info helps.
 
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@akomarek

First off, welcome to AGN and thank you for your reply. Sorry, I just saw this and didn't realize I got a response. I've received my sound level meter and took a few measurements already. I had it set up on a tripod at the height of my shooting bench at exactly 5 feet away, 180° from the side of the moderator. I'll try your suggestion at .5m away and at 45° from the end of the moderator. 

I'm doing all my testing at home which is concrete so I definitely do get a lot of reflections but it is convenient for me. I wish I had a bigger yard to avoid them but like you said, consistency in the testing is key. I'm curious what the powder burner silencer industry and airgun moderator industries such as DonnyFL, Neil Clague, Huggett, etc use as standards. Also, just out of curiosity, what sound level meter do you use? +1 to you for your response. 
 
For most of our measurements we use 1/2" microphones from either Bruel & Kjaer or PCB, signal conditioning is built in to our data acquisition systems which are LMS (now Siemens) Test.Lab SCADAS portable acquisition systems that run their Test.Lab software packages. Usually we're taking 16-40 channels of data so we only use the handheld meters for things that are "quick and dirty" or very simple sound certification measurements. For quick sound level measurements I would grab either a Larson Davis or Bruel & Kjaer sound level meter depending on if I wanted to record the data or not (our B&K 2250 meter had built-in acquisition). 

Since you're just doing relative comparisons between silencers, the most important thing is consistency so if you're on concrete, it won't really matter as long as you don't measure one silencer on concrete and compare to another on grass. Regarding the 0.5m at 45 degrees, I frequently took measurements at this location, both in the sound chamber and outside on the test track so I made an aluminum triangle/template that made positioning the microphone a quick and repeatable task.


 
@theLBC

That's quite a collection you have there and your testing has given me some ideas. It will be awhile until I get my results done but I'll be sure to post a separate thread. Thank you.

just to clarify, i didn't perform the tests and just linked to the thread on that forum.

i did buy a $60 meter and performed my own tests on a mod40 and his numbers lined up pretty well with my results.

my particular rifle is a leshiy 2, and the sound from the action ends up being the limiting factor for that gun (85db - but 81db with a towel over the action).

I would expect my actual numbers to be higher because i tested indoors with walls nearby.

bare muzzle - 109db
stock shroud - 100db
mod40 w/1 section/endcap - 95db
mod40 w/2 sections/endcap - 90db
mod40 w/3 sections/endcap - 86db
mod40 w/4 sections/endcap - 85db

bonus
mod40 w/3 sections and endcap w/paper plate and towel over the magazine area - 81db
mod40 w/3 sections and endcap with meter located where your head is looking through the scope - 96db.
 

ChRiSiS

Member
May 6, 2020
1,049
124
CT, United States
    I used the NIOSH app on an iPad Pro, using an external microphone (Blue Yeti) for my tests after looking at the various models available on Amazon. Reading about the NIOSH app and its ability to use the external microphone led me to conclude that it would be at least as good as the less expensive and non-calibrated models availed from Amazon. Since i really am not trying to relate my measurements against the measurements done by others, but really only against my own other measurements I figured I didn’t need specific, certified, external calibration. An example of some of my results from a previous post are here:
    =======================================================================
    So, I actually did a test last month on a couple of different moderators that I happened to have on hand. I used the NIOSH app on an IPad Pro. For a microphone I used a Blue Yeti mic attached to the IPad. The mic was positioned 12” to the right of the muzzle (the same position for both guns and all the moderators). All the measurements were taken indoors at one location, at the same general time, in sequential order. This way I had a consistency in placement, timeframe, and, hopefully, in measurement.

    A couple key points:
    First, the measurement system was not calibrated, so no one can say that these are definitive measurements of the loudness or effectiveness of the gun/moderator combination.
    Second, given that there was consistency across the measurement they are comparable with each other (but not with any other measurements).
    Third, the key measurements from the app are the Max. number, as well as the LA weighted equivalent (LAeg). The C weighted Peak (LC Peak) numbers are all similar, and are of too short a duration to be particularly relevant here, especially as it is below 140dBC. Above that level impulse noise may be present and this can be very damaging to hearing. This would be very relevant for Firearms, and for sure, if your airgun is that loud you could have a serious problem.

    Anyway, this is my data

    FX Impact MK2 .25

    No Mod

    LAeq 95.1
    Max. 111.3dB
    LC Peak. 124 dB
    TWA. 59.2 (10s)

    DFL Tanto
    LAeq 91.5
    Max. 108.7 dB
    LC Peak. 123.2 dB
    TWA. 56.2 dB (10s)

    DFL Tanto (2)
    LAeq 92.8
    Max. 109.2 dB
    LC Peak. 123.5 dB
    TWA. 57.3 dB (8s)

    DFL Tatsu
    LAeq 94.1
    Max. 109.9 dB
    LC Peak. 123.6 dB
    TWA. 58.0 dB (7s)

    FX Hollow
    LAeq 94.4
    Max. 110.2 dB
    LC Peak. 123.6 dB
    TWA. 57.7 dB (7s)


    FX by DFL
    LAeq 88.9
    Max. 108.4 dB
    LC Peak. 123.4 dB
    TWA. 56.2 dB (9s)

    Hugget Belita
    LAeq 94.3
    Max. 109.5 dB
    LC Peak. 123.4 dB
    TWA. 57.3 dB (6s)


    FX Wildcat MK3 BT Compact.25

    No Moderator

    LAeq 95.8
    Max. 111.1 dB
    LC Peak. 123.8 dB
    TWA. 59.0dB (6s)

    0dB
    LAeq 92.8
    Max. 109.9 dB
    LC Peak. 123.8 dB
    TWA. 56.8dB (8s)

    DFL Tatsu
    LAeq 92.0
    Max. 108.9 dB
    LC Peak. 123.6 dB
    TWA. 55.3dB (8s)”


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