Acoustic absorption refers to a material, structure or object absorbing sound energy when sound waves collide with it, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted. The energy transformed into heat is said to have been 'lost'.

     When sound from a loudspeaker collides with the walls of a room part of the sound's energy is reflected and part is absorbed into the walls. As the waves travel through the wall they deform the material thereof (just like they deformed the air before). This deformation has mechanical losses which convert part of the sound energy into heat throughacoustic attenuation, mostly due to the wall's viscosity. The same attenuating mechanics apply for the air and any other medium through which sound travels.

     The fraction of sound absorbed is governed by the acoustic impedances of both media and is a function of frequency and the incident angle.

     Size and shape can influence the sound wave's behavior if they interact with its wavelength,giving rise to wave phenomena such as standing waves and diffraction.

     Acoustic absorption is of particular interest in soundproofing. Soundproofing aims to absorb as much sound energy (often in particular frequencies) as possible converting it into heat or transmitting it away from a certain location.

     In general soft, pliable, porous materials like cloths serve as good acoustic insulators absorbing most sound. Whereas dense, hard, impenetrable materials like metals reflect most.

     How well a room absorbs sound is quantified by the effective absorption area of the walls, also named total absorption area. This is calculated using its dimensions and theabsorption coefficients of the walls.

     The total absorption is expressed in Sabin and is useful in for instance determining the reverberation time of auditoria. Absorption coefficients can be measured using a reverberation room, which is the opposite of an anechoic chamber.