Broadband nonreciprocal acoustic scattering using a loudspeaker with asymmetric feedback

Abstract : This paper describes an acoustic system enabling to achieve asymmetric transmission over a broad frequency range. The basic elements of the system are a moving coil loudspeaker placed along the axis of a duct, and a microphone flushed-mounted next to the front side of the loudspeaker. The loudspeaker acts both as a mass-spring oscillator through which incident waves are reflected/transmitted, and as a sound re-emitter controlled by the microphone through a feedback loop. It is shown that the reciprocity of the resulting two-port is broken, and that it can be used to design a broadband asymmetric wave transmitter. The experimental results show that there exists a frequency band ranging from about 300 Hz up to 1500 Hz where an almost perfect transmission in the forward direction is observed whereas a weak (i.e., of about 10%) transmission is achieved in the reverse direction. Owing to the broadbandness of this efficient and compact nonreciprocal scatterer, it is also shown through its response to incident pulses that it can be used as a sound trap.