I am a little confused.
Numerous sources have said that suppressors bring the sound of gunfire down to about 120 - 140 dB, and not the pffit sound we hear in Hollywood movies. In the video the rounds fired were no louder than those simulated in the movies.
Has anyone here had actual experience with suppressors, or anyone explain the pffit heard in the video. I assume the louder echoing sound in the slow motion is a result of that recording and playback function.
First, it sounds like the electronics has ANS (automatic noise suppression). That's common among cheaper video cameras, but is a feature on mid to high end models, as well. Thus, it's probably not the suppressor attenuating all the sound, but the audio circuitry.
Second, how much a noise suppressor attenuates the noise has more to do with design than with volume. I've seen some large, complicated designs that do little to attenuate the volume, as well as small, well-designed units that do a great job. One of the best "designs" I've seen is a nothing more than an adapter to use a motor oil filter. Whisper quiet.
For any given firearm and load, however, the expansion of gas both preceding the bullet (small) as well as following the bullet (large) has a certain expansion rate, peak, and decline profile, each section with specific times. Taken together, the entire profile can be described as a "note" with "timbre." Designing a tuned exhaust for a particular frequency creates a minimum back pressure across the same period of time as the exhaust gases are exiting the chamber. This creates a lot of noise, more so than if the engine had no header at all.
Designing a suppressor is the same process, but in reverse. It's a "detuned" pipe. The only difference is that instead of a couple thousand explosions in rapid succession, where a previous release of exhaust gases can be used to create a partial vacuum for a succeeding one, there's only one explosion. While that explosion does create a partial vacuum inside the barrel, which does produce a slight but rapidly declining ring, it's not much use to us in quieting the report.
Essentially, you want to minimize both the compression wave ahead of the bullet as well as the propulsion wave behind the bullet.
The first clear suppressor in the video was of an aerodynamically poor design. The compression wave moved out of the bullet's path, around the bullet, and actually got in front of it, dumping some of its force in front of the bullet. That design can both blow the round while destabilizing it, and actually adds to the compression wave ahead of the bullet.
While diamond-shaped baffles can be used in supersonic airflows, that design's implementation wasn't scientific at all. Its inventor tried some different placements until he found one that worked ok. As I said, I've heard far quieter suppressors that used oil filters. You want the shock wave off the diamond baffles to stall in a pit, slowing to sub-Mach velocity while creating serious, energy-robbing turbulence, not blast through multiple streamlined "baffles" only to jet ahead of the round, adding to the compression wave and destabilizing the bullet!
The old design using successive, equally-spaced steel washers works well, provided the outer diameter of the washers is just large enough such that the propulsion wave races outward, reflects off the tube, and races down the next gap to arrive just after the bullet has passed. That space is important, as well, as it's like tuning the bass port of a speaker. Too narrow and it forces most of the gases to continue along behind the bullet. Too wide and it allows too much of the gases to enter the silencer all at once. And one second thought, the correct answer won't be equal spacings between the washers, but rather, variable spacings, with tighter spacings initially to handle the higher pressures closer to the muzzle, and larger spacings further on, to provide greater flow under lower pressure closer to the suppressor's exit point. In this way, one can keep the diameter of the washers the same along the length of the suppressor.
I'd had to dive back into my textbooks for a couple of months just to be able to generate some meaningful equations. It's been a
long time since I fiddled with Prandtl-Meyer expansions.
I am not arguing against legalization of suppressors. Hearing protection is important.
It has nothing to do with crime. There is no law that requires making a loud sound when stabbing, strangling, or clubbing someone.
I agree with you completely. It's absurd to think that law-abiding citizens are somehow more apt to commit a crime if firearms were quieter. Any criminal with a 2-liter polycarbonate or 3 PET soda bottle and a roll of duct tape can turn the sound of a single gunshot into nothing more than a ruler slapping a table. Legalizing suppressors for legitimate use such as self-defense would
not legalize their use in committing crimes.
