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.

Observation of non-reciprocal harmonic conversion in real sounds

“…More generally, large reciprocity breaking is required for unidirectional and perfectly controlled signal delivery when signal back-reflection is not desired. In acoustics, different strategies have led to large reciprocity breaking, realizing diverse non-reciprocal devices such as rectifiers 3 – 5 , switches 6 , 7 , diodes and isolators 7 – 12 , circulators 13 , topological insulators 14 , 15 , and metamaterials with asymmetric functionalities 16 – 18 . Amongst all the developed approaches for eluding reciprocity, taking advantage of non-linear effects is arguably the most straightforward, requiring merely an appropriate integration of non-linearity with spatial asymmetry 1 .…”