When noisy air flowes through a duct or a pipe, a silencer is required to reduce the noise but allow free air flow. Circular silencers are often used to treat noisy blowers. 

Circular silencers are shaped in the form of a pipe and are built from sound absorbing materials that help reduce airborne noise.

There are two types of circular silencers; with and without cores. Addition a centre core significantly improves the noise reduction effectiveness of the silencer. In special cases you can add another silencer ring around the core and thus make the silencer into a triple layered silencer. Silencer effectiveness is in direct correlation to its diameter and is measured in values of 1D or 2D (silencer length is equal to its diameter, or twice the diameter)