Testing Method

 

Overview

We are testing in accordance with MIL-STD-1474D using calibrated military-approved equipment and provide these or unweighted results. Measurement equipment includes calibrated Brel & Kjr model 2209 Impulse Precision Sound Level Meter with 20 μs rise time using GRAS pressure measurement microphone model 40BP (Bruel & Kjaer model 4136 equivalent) while positioned one meter left and level with muzzle, oriented upward, and 1.6 meters above dirt/grass. Digital samples are taken from AC output signal path by M-Audio Solo at 10.4 μs intervals (96,000 per second) and 24 bit resolution (16.7 million possible steps). Recording hardware rise-time has been estimated at (using the 0.35/BW formula) 8.75 microseconds while the microphone is estimated  to have a rise-time of 5.0 microseconds. Total system calibration during each use with Brel & Kjr model 4220 Pistonphone. Sound analysis is linear to better correlate with perception. Linear measurement results in higher levels than A weighing and this data should not be compared to MIL-STD data. Temperature and humidity measurements by Kestrel meters. Silencers will be examined for evidence of artificial environments. If present, this will be noted unless the medium can be removed.

 

 

Disclaimer:

While this data is captured in good faith and without bias under controlled circumstances, it is not always comparable to other environments or test equipment. An effort was made to have the data be reproducible and correlate with what a human observer would agree with, but your results may vary. This data must be interpreted with knowledge of how environmental factors such as temperature, humidity, and location effect the results. We do try to compare related products in the same session to make this data as comparable as possible. Please ask the manufacture for their specific test protocol. Do they use 10 or more shot averages or just take the best shot of the day? Ask them.

Individual suppressors may vary depending on age, condition, manufacturing tolerances, and cleanliness. Finally, there is more to a silencer that sound levels. Durability, weight, accuracy, length, attachment method, customer support, etc. all matter so please don't make a decision based on these numbers alone.

 

How we test:  

This document describes the current military-standard testing method. To read about the previous method last used on May 6th, 2005, see here.

 

Each session is calibrated with a 124.0 dB SPL tone from a recently calibrated Brel & Kjr pistonphone.

 

We use a Brel & Kjr 2209 Impulse Precision Sound Level Meter.

 

We use a 70Khz bandwidth 1/4 inch pressure microphone similar to this one.

 

We use a shooting fixture to ensure that the propagation distance is ~1 meter for each shot and 1.6 meters above the ground.

 

Previously an auxiliary microphone was used. Now all computer data is sampled through the sound level meter's 1/4 inch pressure mic.

 

Computer Analysis is supplemental, and processed at 96,000 samples per second and 40Khz bandwidth. A cable from the 2209's AC output drives the 1/4 inch input.

 

 

Microphone calibration and specifications.

Meter calibration.

Pistonphone calibration.

 

We record 10 shot averages of each silencer. First round-pop is computed as the difference between the dB SPL of the first shot and the average of the remaining 9 shots. I do not trust the first round pop value because the first shot is not an average of many shots -- so don't consider the pop numbers as very trustworthy. All firearms, including semi-autos, are measured from the left. For wet cans we use one capful of water, unless otherwise specified.

Finally, these numbers are provided in both MIL-STD-1474D (which are A-weighted) and 'unweighted. Unweighted is not the same as the 'A' weighted results most manufactures publish. We believe that unweighted, while not perfect, makes more sense for measuring human perception of gun shot noise as A-weighting was created to roughly compensate for human hearing at the 40 dB SPL level, not the 120-140 dB SPL level we are interested in. For a more complete explanation please read Sound Testing Standards.

Also note that indicated dB SPL can fluctuate depending on test conditions. Don't get fixated on 0.1 or 0.5 dB differences as they are not significant. If one silencer is within 2 dB of another, then they are nearly equal in peak sound and one should look to other characteristics to decide which unit you prefer.

Contents are Copyright 2005 by Robert Silvers and may not be reproduced without permission.

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