Hiram P. Maxim patented the first truly successful sound suppressor for firearms in March 1908. The physics of sound remains the same today as at the beginning of the 20th century (and before there was anyone on the planet to actually hear sound wave, for that matter). Elastic (longitudinal or compressional) waves through a gas are, of course, responsible for sounds that reach the ear.

The sound produced by a firearm is principally composed of three components. They are: (1) the muzzle blast created by shock waves generated from the sudden expansion of hot propellant gases as they encounter the atmosphere at the muzzle end of a gun barrel; (2) the miniature sonic boom, or "crack," produced by a bullet traveling downrange at a velocity above the speed of sound (1,087.5 fps at 32 Degrees Fahrenheit at sea level - the speed of sound through an ideal gas varies directly as the square root of the absolute temperature, and inversely as the square root of the atomic weight. However, since the medium is air, which has an almost constant molecular weight, that factor in the equation is really not relevant. Further, neither humidity nor altitude affect the speed of sound); and (3) in the case of a semiautomatic or full-auto weapon, there is the noise of the action itself (i.e., the reciprocating slide or bolt group). If the projectile travels downrange at subsonic velocity, it will produce no sonic boom as it passes stationary objects. There is little that can be done about the sound produced by the firearm's reciprocating parts, although some have installed slide locks on sound-suppressed pistols, with only moderate success and, in effect, turning the handgun into a single-shot firearm.

It is the muzzle blast to which all sound suppressors, successful or not, address themselves. They do so by use of a single formula from physics known as the general gas law. Applicable to all ideal gases, the equation states that pressure equals temperature multiplied by a constant divided by volume. As muzzle blast is a consequence of relatively high-pressure gases exiting the barrel, reduction of this pressure immediately before exit, by either increasing the volume or decreasing the temperature (cooling), or both, will reduce the sound. Modern, relatively small, dry-type sound suppressors sometimes exhibit other phenomena. First, they often generate extreme turbulence that delays exit of the propellant gases. Secondly, some designs now use gas energy to generate a high frequency sound in that portion of the audio spectrum where the human ear is not very sensitive. These so-called "frequency shift units" make the suppressor sound more quiet, although this is not usually reflected by a sound meter reading.

Sound suppressors are usually evaluated and compared by a logarithmic ratio called the decibel (dB). The logarithmic nature of the dB is important to keep in mind, as 3 dB is a factor of 2, 10 dB is a factor of 10, 20 dB is a factor of 100 and 30 dB a factor of 1,000. This is in the absolute sound pressure levels as measured in pressure units (usually Pascales, where the 0 dB reference level is 20 microPascales, i.e., the threshold of human hearing). For comparison purposes, quiet conversation is about 55 dB, a handclap 65 dB, a jackhammer about 120 dB, and an M16 rifle 165 dB.

Sound levels also diminish as the observer goes further from the sound source. The sound level drops according to the inverse square law (i.e., the sound decreases with the square of the distance from the source). A sound suppressor that is perceived to be fairly load within the confines of a small room, may not even be heard by an observer when fired from a distance of 25 feet outdoors, and from behind cover and concealment.

(Reprinted with permission of the author.)