The measuring unit of sound suppressor suppression ability is decibel, dB(1/10th Bell), named after the inventor of the telephone, Alexander Graham Bell). It is a measuring unit which is used to measure sound or sound pressure level(SPL).
According to the definition SPL=20 lg (p/ p0), p being the measured pressure and p0 being the pressure level of the human hearing range threshold, 20μPa. From this definition we can deduce that the hearing threshold is 0 db. Normal conversation is 56 dB, a lawnmover 85 dB, a jet plane 135 dB. Firearm discharge sound is peak impulse noise where the highest peak is usually measured.
The threshold for hearing damage for peak impulse noise is c. 140 dB. In front of a artillery piece the SPL is over 185 dB which can lead into death when the lung cells are ruptured, not to mention hearing damage. The SPL for an 5.56 mm calibre M4 carbine is approximately 165 dB, when measured 1 meter left of the muzzle. When measuring SPL, several filters are used to remove for example the parts deemed unnecessary.
The most common filters are the A-filter which strives to describe the characteristics of human hearing and where the low (under 20Hz), inaudible to human hearing sounds are suppressed and the C-filter, where the filtering of the lower sounds is lesser. There is not a very large variable between A or C-filters when measuring gunshot noise.
The major problem in measuring sound suppressor suppression abilities is that the whole range of measuring instruments have to be suitable, must to have fast enough risetime, according to the American MIL-STD- 1474D it has to be under 20μs, to be able to react to a suppressed firearm shot, which has been established to be shorter in duration compared to an unsuppressed shot.
The measurements themselves are taken with the microphone 1 meter side from the muzzle of the weapon and at a height of 1.6 meters, directed 90 degrees upward to the trajectory of the bullet.
Without the suppressor, a sufficient statistical certainty is usually accomplished with five shots, with the suppressor attached an average of ten shots and additionally taking notice what is the SPL of the first shot, if there is any First Round Pop (FRP). This FRP is the result of the expanding gases and the oxygen inside of the suppressor converging.
It is typical for SPL measurements that the environment characteristics have an effect in the results so the measurements should be taken in an open land area without reflecting surfaces.
The best comparable results are achieved when the measurements of different suppressors are made at the same event or at least according to the standard procedure; if a manufacturer does not specify their measurement types or their instruments, one should be cautious about the results.