Why implement a lining inside a metallic casing, a metallic screen, a partition or an enclosure made of plasterboard or with wood sidings with a sound-absorbing material to improve their acoustic performance?

Absorbent material can increase the sound insulation of a plate on which it is applied (on noise source side).

With its absorption property, the absorbent limits the increase of sound pressure level on noise source side that would occur if the support alone (metal housing, plasterboard, wood plate) was interposed.

With its surface density (if it is not negligible), the absorbent material increases - all other things being equal - the sound reduction index (limits the sound transmission factor) of a thin plate at frequencies for wich the mass law is applicable.