Testing and Evaluation

Unfortunately the criteria and techniques for judging small arms sound suppressors are not standardized nor even validated. The principal, but not the only criterion of silencer quality, is the degree of acoustical attenuation. This requires the definition of some terms and units of measurement.

Muzzle blast is measured in terms of blast overpressure, usually pounds per square inch, or in terms of peak sound pressure level or peak deci-Bels. Although the deciBel scale is nonlinear, we can relate to it when we convert it to sones, a unit of loudness. For instance, a 10 dB increase doubles the loudness in sones; a 10 dB decrease cuts the loudness in half, using the same linear units. If a silencer drops the dB level 10 dBs, it is 50 percent efficient. It has cut the loudness in half. If it drops the dB level 20 dBs, it is 75 percent efficient; three quarters of the loudness is quieted. A 30 dB drop means almost 90 percent efficiency. Put a different way, a 20 dB silencer is twice as effective as a 10 dB silencer and half as effective as a 30 dB silencer.

Note, however, that while a 30 dB suppressor is twice as effective as a 20 dB unit, the improvement in efficiency is really only about 14 percent. This is the sort of statistical manipulation that you must watch out for. When in doubt, check the raw dB data, which are usually absolute.

The way in which the dB level is measured, whether the equipment measures peak SPL or not, the microphone type and location, and even the brand of equipment can have a major effect on this numbers game. Other external conditions such as variations between test guns, weather condi tions, altitude, and temperature can account for small, but significant discrepancies. However, if the same procedure is followed with the same kind of equipment and environment, then a meaningful comparison between two silencers can be obtained. Uniformity and consistency in testing is vitally important.

What kind of equipment are we talking about? The two basic varieties of silencer testing equipment are the oscilloscope and the sound meter. Oscilloscopes are cathode ray tube instruments used to electronically analyze signals. A microphone will convert sound, such as a muzzle blast or a suppressor's acoustical signature, into an electrical signal which will show on the oscilloscope screen as a waveform a picture of the pressure/ time event. This can be measured physically by calibrating the instrument, then reading the screen. This waveform can be photographed from the screen or it can be digitally stored and retrieved for analysis or conversion into hard copy by a chart recorder.

An even more elaborate treatment is obtained by using a digital Fast Fourier Transform real-time high resolution signal analyzer with a digital cassette recorder as a peripheral so that a library of sound signatures can be accumulated and manipulated more extensively by the analyzer.

The advantage to the design lab of these techniques is that much more is learned than just the quantitative peak SPL data. Individual components of the muzzle noise from the suppressor such as blow-by, precursor wave, and projectile noise can be isolated and measured. These can be handled individually in various ways. The mechanical noises testing and evaluation

Muzzle Blast Sound Grenade

area of Deception

confusion area of confusion area of certain location fixed source at various angles from the source.

Silencer Area DeceptionM14 Rifle 2011

T>»e muzzle blast spread of an unsuppressed weapon shows relatively easy target acquisition of sound from a fixed source. The second source is - ght angles to sound wave which locates center or point of origin.

Test firing M14 rifle; note muzzle blast wave. This is used as a base comparison for effect of suppressors on the wave.

of the weapon can also be analyzed in terms of their often large contribution to the suppressed weapon's sound signature. All of this analysis can lead to design improvements, especially if computer derived.

For simple quantitative comparison, the instrument of choice is the precision peak SPL meter with a suitable microphone. The meter of choice is the Bruel & Kjaer 2209 and the almost universal choice for microphone is the B & K 4136 quarter-inch pressure mike. This equipment is portable and battery operated. It is also accurate and very expensive. The 4136 mike can accurately measure peak sound at levels up to 168 dB, which is mandatory because 7.62 NATO weapons and short-barreled 5.56mm weapons push hard at that upper limit. A related meter, the 2210, is even more accurate, being a digital Type 0 NBS rated model, but it cannot be used above 160 dB even with the 4136.

General Radio Company manufactures impulse noise meters also and their equipment is used by some military laboratories. At the higher SPLs the GR meters usually show a lower reading than B & K, while at the suppressed levels the readings are quite compatible. Thus, B & K equipment will give a higher dB drop for an actual performance, if the unsuppresscd weapon in question is a 5.56mm, 7.62mm, or other heavy caliber weapon. Pistols and submachine guns such as the 9mm usually have unsuppresscd peaks in the mid 150s so this is not a problem with them.

If you have read other silencer literature, you might note that some dB levels listed here are higher than those reported in other material. This is because of inconsistency in use of laboratory impulse noise equipment. It seems that a few civilian silencer manufacturers have tested their products with dB meters intended for OSHA type environmental studies, not impulse noise peak SPL measurements. With such equipment, major errors of 30 to 40 dBs are common. Measurements above 140 dB are impossible with such equipment. Even if the meter would read higher, the microphone wouldn't respond accurately.

Let's establish some standards for comparison. Measured at one meter, 90 degrees from the muzzle, a .22 rimfire rifle with normal, high-speed .22 long rifle ammunition has a peak SPL of about 135 dB; subsonic .22 LR, such as Eley Match, has a SPL of 133. A 9mm pistol or submachine gun reads about 155 and a 5.56mm goes 165 from an eighteen to twenty inch barrel, and more from a short barrel such as that of an XM177 or an HK53. Yet a 30 dB suppressor on a 9mm submachine gun gives a reading of 126. while on an Ml6, using ball ammunition, it reads 137. A high-quality, suppressed .22 pistol is in the 110-120 range. Note that these are all peak readings on the A-wcighted scale. When properly written, these would be described as peak SPLs and dB(A).

Now that you are armed with the facts about dB testing, here are the other criteria for judging suppressors. One is muzzle flash. Some silencers kill all flash; some don't. Some suppressors, particularly the baffled variety, will flash intermittently on full auto, whenever the mixture of powder gases and atmospheric oxygen permits. This includes the so-called first round flash. Other suppressors, especially those which involve efficient heat transfer, quench the flame-front of the primary muzzle flash and prevent reignition, so nothing escapes from the muzzle except an occasional stray piece of propellant.

The major point in test and evaluation is to standardize measurements and techniques and use top quality equipment in a uniform fashion. Then the resulting data must be objectively reported in a scientific fashion. Or, as everyone's high school math teacher must have said a few times, "Don't try to compare apples and oranges."

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