Acoustic absorber and sound-absorption method

An acoustic absorber comprising a wall in the form of a ring defining an interior chamber, a duct extending axially in said ring for flow of fluid therethrough, radial openings in said wall connecting said duct and said chamber, said chamber having a natural frequency and acoustic impedance to absorb sound in the fluid in the duct, an annular elastic membrane extending across said ring to divide said chamber into an inner absorber chamber which communicates with said opening and an outer pressurizing chamber which is separated from the absorber chamber by said elastic membrane, means for varying pressure in said outer chamber to vary volume thereof and thereby volume of said inner absorber chamber to adjust the natural frequency and acoustic impedance thereof, and a heating element in said absorber chamber for heating fluid therein also to control the volume of said absorber chamber and vary the natural frequency and impedance thereof.

An acoustic absorber comprising a wall defining an interior chamber, an elastic membrane extending across said wall to divide said interior chamber into an inner absorber chamber and an outer pressurizing chamber which is separated from the absorber chamber by said elastic membrane, a duct extending through said wall for flow of fluid therethrough, means for varying volume of said inner absorber chamber to adjust the natural frequency and acoustic impedance thereof, said means including a heating element in said absorber chamber for heating fluid therein and means for adjusting pressure in said pressurizing chamber.