In certain applications silencers can reduce noise by up to 50% like an Exhaust Muffler on a Car.

Sound can be thought of like the ripples in a lake when you throw a stone into it creating ripples and the ripples travel outward in every direction. If you then took that lake and squeeze it into a canal those ripples would now be channeled to flow through the length of the canal. This is what’s happening when you connect your duct to a fan. All the sound waves from the fan are now being channeled through the duct.

The duct acts as a barrier preventing the sound waves from escaping, but there could be some breakout noise.Going back to our analogy, as the ripples travel through the canal still trying to protrude outward in every direction, some will fall on the shore or walls of the canal creating some noise. In the same way, breakout noise is the sound waves (caused by the airflow and/or fan) trying to escape through the duct.

This is where a fan silencer comes in. Placing a fan silencer  in the airstream will help to absorb some of these sound waves.

When the sound waves enter the silencer, they expand into the acoustic media. The acoustic media is made up of millions of air pockets. When the sound waves impinge on the air pockets it causes the molecules to move and collide with each other dissipating energy in the form of heat. This is what gives you your noise reduction.