Old police photo of a captured 38 Special with a Silencer.
Explaining the Science of Noise from a Gun:
When gunpowder in a cartridge or shell is ignited, it creates a high-pressure pulse of hot gas with so much pressure (on the order of three thousand pounds per square inch) that the bullet is forced down the barrel of the gun at enormous speed along with a very loud sound. It’s like uncorking a tightly sealed champagne bottle. You can’t avoid the popping sound, or, in the case of gun, the muzzle blast.
The more powerful the gun and ammo are, the more chance the bullet will travel at supersonic speed and produce that loud ballistic crack. A high-powered, supersonic bullet can’t be completely silenced because it has literally created a tiny sonic boom as it travels from the barrel.
To illustrate these phenomena, forensic researchers used a special imaging camera. The man in the photo below has discharged a .44 Magnum revolver. Two spherical shock waves are seen. One is a bright flash and cloud of gunpowder combustion centered at the gun’s muzzle (the muzzle blast); the other is centered near the cylinder and envelops the hands of the shooter (around the body of the gun and chamber/cylinder). The supersonic bullet is visible at the far left. This kind of split-split-second photography helps forensics experts understand the transfer of gunpowder traces to the hands when firing a gun. It also allows us to “see” the muzzle flash, the gases exploding out, and the flight of the bullet, all of which aren’t ordinarily visible to the naked eye.
Silencers/suppressors generally screw on to the end of the barrel. Inside, there are baffles to absorb some of the hot gases and powder. There’s a lot more room inside the device compared to the very tight barrel of the gun, so the silencer has 20 or 30 times more room for the pressurized gas to expand into. The silencer decreases pressure from the hot gas, and if enough of the propellant gases are bled away, the bullet can be slowed to less than supersonic speed. When the bullet finally exits through the hole in the silencer, the pressure being uncorked is considerably lower, and the sound of the gun firing is much softer. If the shooter in the picture above had used a silencer/suppressor, the photo would be much different.
So a high-quality silencer may remove most of the muzzle blast and perhaps all of the ballistic cracking sound, but it won’t be completely silent. The best example I have been told is that if you use a pin to pop a balloon, it makes a loud noise. But if you untie the end of the balloon and let the air out in a slower rush, you can minimize the noise. That is the basic idea behind how a silencer works.