Using a thin actuator as secondary source for hybrid passive/active absorption in an impedance tube

This paper describes the practical implementation of a piezoelectric actuator as secondary source for hybrid passive/active broadband sound absorption in a standing-wave tube. This actuator consists of a thin circular aluminium plate driven by a piezoelectric patch and glued to a flexible rubber support. The resulting device has been mounted in a thin metallic ring that fits perfectly to the tube diameter. Passive absorption is afforded by either a porous layer or a microperforated panel, backed by an air gap. Active absorption is accomplished by releasing the sound pressure at a microphone behind the material, using either a loudspeaker or the actuator as secondary source. Results of broadband sound absorption reveal the feasibility of the piezoactuator. Compared to the loudspeaker, this actuator allows to greatly reduce the whole thickness of the hybrid absorber.