Sound moderator - a device for reducing the sound level of a shot, muffler, acoustic filter - gas-dynamic muzzle device, used in firearms and pneumatic weapons, whose task is to cool the powder gases and reduce their pressure, and thus noise suppression, occurring when fired. 
Firearms have 4 sound source:
- The flash of gunpowder is the main source of sound of a firearm when fired., can be suppressed using a silencer.
- Overcoming the sound barrier with a bullet - a silencer is not able to remove this sound, however, it can be eliminated by using special cartridges with a subsonic initial ball velocity.
- Weapon mechanism operation.
- Bullet impact on target.
Function sound moderator consists of prolonging the flow of gases, coming out of the barrel into the atmosphere. Additional shot suppression is achieved through the use of subsonic ammunition (ie. projectiles projectiles with a speed less than 340 m/s). The speed of sound in air depends on temperature. Therefore, weapons use ammunition with a speed of the order of 290-310 m/s. Some gun manufacturers use suppressors integrated with the barrel, having gas holes, deflectors and damping materials over a considerable length.
