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

摘要

In this paper, a new hybrid passive-active sound absorption method is investigated, in whichporous materials and a loudspeaker are parallelly combined so that sound absorption over the fullfrequency band could be expected. The target impedance of the loudspeaker’s diaphragm istheoretically derived under this parallel structure, and a novel active impedance control methodis further proposed. In the proposed control method, dual microphones installed on the surface ofthe composite structure as well as in the back cavity of the loudspeaker are used to pick up thesound pressure and the velocity of the loudspeaker’s diaphragm, respectively. Modal expansionanalyses are performed theoretically to find the optimal microphone placement on the structuresurface. An error signal is then established with an auxiliary filter corresponding to the targetimpedance as well as an extra delay. With the help of this extra delay, a stable and causal designresult 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 theestablished error signal with the filtered-x least mean square algorithm. Both simulations andexperiments are carried out based on a standing wave tube, through which it has been verifiedthat the proposed active impedance control method can effectively enhance the low-frequencysound absorption performance of porous materials. The proposed composite structure with athickness of 80 mm can exhibit a sound absorption coefficient greater than 0.9 for all the frequenciesabove 20 Hz.