Abstract: The invention relates to sound-absorbing devices, assemblies and systems and corresponding methods for altering the reverberation time of a room, specifically although not necessarily exclusively at low frequencies. A sound-absorbing device according to the invention comprises basically a body containing one or more cavities (4), where at least a portion of the outer surface of the body is in contact with said sound field S and where said body is inflatable/extendable and collapsible/compressible during the supply of a gas to or the removal of the gas from said at least one cavity (4), respectively, whereby the absorption coefficient (a) and/or the resonance frequency of said body can be varied, thus determining the absorption coefficient and/or the frequency region in which maximum absorption will take place.