Patent ID: 7905323

Claim:
Sound-absorbing device which is placed in a sound field of open air for absorbing acoustic energy from the open air in said sound field at least in a predetermined low-frequency region, the device comprising: a body containing one or more cavities, each said cavity including an active outer surface in free and movable contact with the open air of said sound field and additionally being free and movable for absorption of acoustic energy from the open air of said sound field, and having a volume which is movable in use at least by movement of said active outer surface in the open air of the sound field between states where the volume is one of a) inflated and collapsed or b) extended and compressed, by a variation in a gas pressure therein, in order to change one of an absorption coefficient α or a resonance frequency of said body between a very high value and a very low value substantially lower than the very high value; and a means for actively varying the gas pressure in said one or more cavities in use in order to actively vary at least one of the absorption coefficient α or the resonance frequency of said body between the very high value and the very low values substantially lower than the very high value.