What is the purpose of sound attenuators in duct work?

Duct silencers operate by restricting the airflow through a system of baffles exposing as much of the air stream as possible to sound-absorptive filler materials and/or resonant cavities. As a result, they have "self-noise" characteristics, meaning that they generate noise themselves due to turbulence in the airflow through their internal baffles. Silencers should be located far enough upstream of any acoustically sensitive space to ensure that the noise they generate is adequately attenuated before it reaches the occupied room.

Another frequent misuse of sound attenuators involves their placement in the duct system. If a silencer is located within a mechanical room, noise may enter the system through the sheet metal duct after the silencer. If a silencer is located away from the mechanical room walls, noise may escape the system through the sheet metal duct before it is attenuated by the silencer. Ideally, a sound attenuator should be located within or immediately adjacent to the mechanical room's walls.