A 7.62 NATO round is supersonic and would cause a miniature sonic boom even if the muzzle blast from the rifle was muffled 4. Yes, the miniature sonic boom is not as easy to pinpoint as a muzzle blast but does produce a very noticeable noise which can draw attention to a shooter.
Even silencing the muzzle blast to a mere "fut" is next to impossible. Muzzle blast noise can exceed 150 decibels 5 (measured at the shooter's location) and is one of the loudest sounds humans are likely to hear. Silencers, suppressors, or cans as they are sometimes called 6 have to be precision made using very exacting technology to have any hope of quieting such a loud noise.
Considering that the threshold of pain is only 130 dB, we're actually glad Hollywood sound tracks don't accurately reproduce the noise of muzzle blasts. If they did, the only sound action movie fans would hear as they staggered out of the theater would be the ringing in their ears. In Blackhawk Down, the soldier who had an automatic weapon fired near his ears really would have been left temporarily, if not permanently, deafened.
SWAT teams sometimes use silencers, not for stealth, but to insure that they will be able to hear if one of the SWAT team members fires a shot inside the confined space of a room. Discharging an unsilenced firearm in a room can cause temporary deafness. Silencers are also sometimes used in raids on clandestine methamphetamine labs. Discharging a normal firearm produces a muzzle flash which can set off volatile fumes. Silencers act as flash suppressors.
Sound is a form of energy transfer and we could define loudness in terms of the energy per unit of time or power output of the sound, but it wouldn't give the complete picture. Sound waves travel outward like balloons expanding around their source. The sound's power is distributed on the surface of the wave which increases with the square of the distance from its source. In other words, the amount of energy per unit of area in the wave declines rapidly as the wave moves away from the source. About the same wave area contacts a person's ear regardless of how far she or he is away from the source. However, the area contains significantly less energy when the source is far away rather than up close. This explains why the sound is not as loud.
Power per unit of area (called sound intensity) would be a better measure of loudness than just power alone. Sound intensity accounts for the fact that the ear receives less power when the source is far away rather than close. Unfortunately, human perception of loudness is not linear with respect to sound intensity. In other words, doubling the sound intensity does not double the perception of loudness. The perception of loudness is, roughly speaking, logarithmic and is represented somewhat better by the decibel scale as follows:
b = 10 log(I / Io)
where:
b = relative sound intensity in decibels
Io = sound intensity at the threshold of hearing (1 x 10-12 W/m2)
I = sound intensity of the noise (W/m2)
Even the decibel falls short of being a true indicator of perceived loudness. The loudness of a noise also depends on its frequency or pitch. Sound measuring equipment, at least partially, accounts for this fact by using various frequency weighting filters. The dBA scale is the most common of these applications. However, if we assume that a muzzle blast's frequency content is in the general vicinity of optimum hearing and that the blast's frequency content doesn't change with loudness, then the unweighted decibel scale is a reasonable indicator of relative loudness for purposes of discussion.
The logarithmic nature of hearing makes muzzle blasts even harder to silence. Let's see what happens to the relative loudness level if we reduce the sound intensity of a muzzle blast by a factor of two. This means we're removing half of the energy from the sound waves. Using the above equation we get the following:
b = 10 log[I / (2Io)]
= 10 log(I / Io) - 10 log(2)
= b0 - 10 log(2)
= 150 - 3.0
= 147 dB
Cutting sound intensity in half only reduces the relative loudness by merely 3 dB. This would be barely noticeable. A good set of ear plugs typically reduces noise by about 30 dB and so, would reduce a muzzle blast from 150 to 120 dB, still a very loud noise. We estimate that the innocuous "fut" sound made by a movie silencer is roughly 50 dB 7, a whopping noise reduction of 100 dB from the dB level of a muzzle blast! In other words, a silencer has to reduce sound intensity of a muzzle blast by a factor of 1010 to give such a low relative loudness. This can be done with a very well designed and precision made silencer using subsonic ammunition. However, even commercially available silencers are more likely to give a reduction of 30 to 40 dB similar to ear plugs, than the incredible 100 dB reduction frequently portrayed in movies, especially when used on high-powered rifles.
