A-C
Absorption
The properties of a material composition to convert sound energy into heat thereby reducing the amount of sound energy that can be reflected.
Acoustical
The properties of a material to absorb or reflect sound (adjective) acoustically, (adverb).

Sound is a form of energy that travels as a wave – when that wave hits a barrier as in a sheet of drywall, that barrier will vibrate and cause the air on the other side to vibrate, thus transferring the sound through the wall. There is no one-shot soundproofing technique that will stop noise in its tracks; rather we try to employ a combination of solutions to achieve solid performance.