Manufacturer of Custom Firearms and Suppressors
P.O. Box 225 Elliston, MT 59728
INTEGRALLY SUPPRESSED PISTOLS
NOTE: Prices quoted do not include Federal Transfer Tax or any applicable state sales tax
MEASURING THE SOUNDS OF SILENCE
WARNING! Determining the effectiveness of a suppressor is one part science, one part art, and one part PR (or BS, depending upon your viewpoint). If math bores you or gives you a headache, then you may want to skip the next few paragraphs. If you are glutton for punishment, or really want to understand some of the voodoo and incantations associated with sound pressure measurements, then read on. For those of you with an advanced appreciation of mathematics and physics, the author must apologize in advance for keeping the following explanations simple and incomplete. Now for the hard part.
The standard unit of measurement for sound pressure level (loudness) is the decibel (dB, or 1/10th of a bel). The term bel, and by extension, the decibel, was created in the 1920s by engineers at the Bell Telephone Laboratories, and named in honor of the father of the telephone, Alexander Graham Bell. Essentially, it was the renaming of a unit of measure previously developed and refined over the years for the purpose of determining the lowest audible reduction in sound the average human ear could detect through a telephone line. The longer a telephone line is, and the more switches the signal passed through, the greater the detectable loss of sound, so the decibel proved to be a useful measurement for determining the efficiency of lines and switches, as well as the power levels required to keep a sound reasonably audible. As a result, decibels are used in other branches of science, most notably electronics, but for our purposes this discussion is limited to its application in acoustics.
The decibel is a representation of the ratio of the intensity of a sound to a stated baseline reference, and therefore can be subjective. That is, whatever sound is used to form the baseline reference also determines the resulting decibel readings. If a whisper forms the baseline reference, the dB reading for a shotgun muzzle blast will be higher than if a Tarzan yell forms the baseline reference. Fortunately, a standard sound pressure level of .0002 microbar (or 20 micropascals if you prefer), is the accepted standard for 0 dB. A "bar" approximates the atmospheric pressure on Earth at sea level, and a microbar is one millionth of a bar. Micropascals are equivalent but much smaller units of measure (one bar equals 100 billion micropascals). Although the decibel has never been standardized by the International Committee for Weights and Measures, it remains in common use by the manufacturers of sound measuring devices. All else being equal, differences in any readings obtained by each device most likely can be attributed to calibration errors or manufacturing quality variances.
Because it is a ratio, the decibel is expressed as a logarithm. You may recall from grade school math that a logarithm is represented as an exponent (the raised number) of another number (called the base), such as in the following example where the base number is 2.
22 = 2×2= 4 23 = 2×2×2 = 8 24 = 2×2×2×2 = 16 25 = 2×2×2×2×2 = 32...........210 = 2×2×2×2×2×2×2×2×2×2 = 1024
In the preceding example, 22 is the logarithm for the value 4, just as 210 is the logarithm for the value 1024. Note the great difference in actual values these two logarithms represent. The last one is not simply five times larger than the first one, it is 512 times larger. So as a logarithm increases arithmetically (2, 3, 4, 5...10), its value grows exponentially (in this example, 4, 8, 16, 32...1024).
If we use a larger base value, such as 10 (which just happens to be the base value of bels and decibels), the differences are far more dramatic.
102 = 10×10 = 100 103 = 10×10×10 = 1000 104 = 10×10×10×10 = 10,000 105 = 10×10×10×10×10 = 100,000...........1010 = 10×10×10×10×10×10×10×10×10×10 = 10,000,000,000
The following table is an example comparing dB levels from 0 to 100 with their equivalent values expressed as power ratios and amplitude ratios (measure of proportional change in sound wave pressure).
Since firearms create sound pressures well above 100 dB, the raw numbers would become quite huge if the dB scale wasn't applied. The threshold for permanent hearing damage is generally considered to be 120 dB (one trillion times greater than 0 dB), so dropping a firearm's report below this number is sometime seen as the Holy Grail of sound suppressors. Of course, it isn't really all that simple. Other factors come into play such as the frequency range involved and the duration of the sound pressure. The latter is of importance because one sound pressure may be perceived as being louder than another simply because it has a greater duration, regardless of the actual dB reading. A 135 dB sound that lasts a microsecond is likely to be perceived as quieter than a 130 dB sound that lasts a half second. Interpreting dB values can be further confusing because, even though a single dB number increase corresponds to a logarithmic increase, at the extreme pressure ranges involved, the human ear is not always able to detect these changes. On average, a firearm's sound pressure must drop 6 dB before the change is detected by the human ear. Even if the subjective human factor is eliminated, test results by "impartial" scientific analysis can be affected by many factors, including test location (indoor range, wooded area, open field), ambient temperature, ammunition selection, type of firearm, barrel length, placement of the microphone, or brand, model, and calibration of the acoustic measuring device. In the final analysis, raw data, testimonials, recorded video, or sound demonstrations are not conclusive evidence of suppressor performance. But short of being there and shooting the suppressor yourself, they may be all you have to go on.