Gun silencers - What they do and how they do it.
Why does a gun make noise?
To propel a projectile from a handgun, rifle, or basically
any other firearm, a highly explosive material is used. In theory,
the more pressure you can put behind a bullet, the faster it will fly.
If you have read my explosion write-up, you know that the sound
from explosions is caused by the rapid expansion of gases. This rapid expansion
is what pushes the bullet ahead, so logic should leade us to believe that
there isn't such a thing as a silent gun
In theory, if you set of a very very small explosion in a gun
barrel that was closed (in other words, in a very strong piece of pipe), you
wouldn't hear the explosion too well. Why? Because the shock wave from the
expanding gas never reaches the air outside. Although the pressure inside
the barrel might be very high - if the pressure isn't enough to break the
barrel - you don't hear a thing.
The thing is - trying to fire a bullet from a closed pipe is rather
useless (we want the bullet to fly, right).
Think about a balloon. If you pop the balloon (letting all
the pressure out at the same time), you get a bang. If you were to untie the
balloon, and let the air out, you would hear it, but chances of anyone in
the next room to hear it is rather limited.
The theory behind a silencer then, is to "let the
pressure out of the gun gradually".
How a muffler works
Inside your car, you have an engine. Inside this engine, there
can be anything from a few hundred to a few thousands explosions per minute
(when the engine is running, obviously). Hence internal combustion engine.
If you ever have driven around in a car without a muffler, you know how much
noise that makes.
A muffler works like this: The gases that come out of your engine
after the gasoline has been mixed with oxygen and ignited, have very high
pressure. This pressure is a good thing, because that's what makes your engine
turn and your car go forward. Anyway - the pressure goes into your exhaust
system. This system goes into your exhaust muffler. The muffler is, essentially,
a form of buffer. Put crudely, your exhaust system is a pipe leading to a chamber,
with a pipe leading from that chamber out in open air.
Inside this chamber, the high-pressured gas has the possibility
to expland, causing a loud sound. But because the chamber is contained, the
sound can't go anywhere - The pressurecontinues out of your tailpipe, and that
was the story of that explosion. Although no cars are soundless, there
is definitely a difference between a car and a gunshot.
Now for the fun part - how a silencer works
The pressure behind a bullet is immense - about 2 500 PSI is not
unusual. When this amount of pressure is released into the air, you hear a loud
When you affix a silencer to your gun, the exact same thing
happens as with the muffler in your car. Inside the silencer, there is a
series of chambers that lets the pressure expand, before being led out of the
barrel, drastically decreasing the sound.
To make a gun soundless, though, there is one more factor than
the pressure from the actual explosion - namely the speed of the bullet.
If the bullet goes faster than the speed of sound, there will be a sonic
boom. To avoid this, the bullet will have to travel slower than the speed of
sound (about 760 mph / 1200 kph). Because of this requirement, silenced
weapons have to use subsonic ammunition to be truly silenced. Because a
"normal" bullet can happily fly along at 900 mph, the subsonic requirement
causes the guns to be less effective.
killermonkeys' wu on this topic is factually more correct than mine - I fell victim of over-simplifying, thereby adding factual errors to this node.