Active impedance control of a loudspeaker and its parallel combination with porous materials for broadband sound absorption

In this paper, a new hybrid passive-active sound absorption method is investigated, in which porous materials and a loudspeaker are parallelly combined so that sound absorption over the full frequency band could be expected. The target impedance of the loudspeaker's diaphragm is theoretically derived under this parallel structure, and a novel active impedance control method is further proposed. In the proposed control method, dual microphones installed on the surface of the composite structure as well as in the back cavity of the loudspeaker are used to pick up the sound pressure and the velocity of the loudspeaker's diaphragm, respectively. Modal expansion analyses are performed theoretically to find the optimal microphone placement on the structure surface. An error signal is then established with an auxiliary filter corresponding to the target impedance as well as an extra delay. With the help of this extra delay, a stable and causal design result of the auxiliary filter could be obtained by solving a weighted least square problem. Finally, the impedance control of the loudspeaker's diaphragm is realized by minimizing the energy of the established error signal with the filtered-x least mean square algorithm. Both simulations and experiments are carried out based on a standing wave tube, through which it has been verified that the proposed active impedance control method can effectively enhance the low-frequency sound absorption performance of porous materials. The proposed composite structure with a thickness of 80 mm can exhibit a sound absorption coefficient greater than 0.9 for all the frequencies above 20 Hz.