Acoustic. Vibration Damping
Vibrating surfaces push the air around them as they move up and create a partial vacuum that pulls the air back in as they move down. The effect of this is to create a pressure wave in the air, converting the structure borne vibration into air-borne noise.
Minimising how much a surface is able to vibrate by applying a dampening material to absorb the vibrating motion is known as vibration dampening and is one of the key techniques for noise control.
Vibrating surfaces generate noise like loud speakers.
A dampening material acts like a spring to 'absorb' some of the vibrations. This reduces the noise created by the vibrating surface.