Realisation of broadband two-dimensional nonreciprocal acoustics using an active acoustic metasurface

Nonreciprocal acoustic devices have been shown to be able to control incident waves propagating in one direction whilst allowing incident waves propagating in the opposite direction to be transmitted without modification. Nonreciprocal sound transmission, typically, has been achieved by introducing nonlinearities or directional biasing through fluid motion or spatiotemporal modulation of resonant cavities. However, the spatial arrangement of these approaches creates preferential characteristics in one direction such that the direction of the nonreciprocal behaviour is fixed and, thus, they are not straightforwardly reconfigurable. To address this issue, it has previously been revealed that feedforward wave-based active controllers can be employed to drive a single subwavelength active unit cell to achieve broadband nonreciprocal sound transmission or absorption in a one-dimensional linear acoustic system. Extending this concept, this paper investigates how the feedforward wave-based active controller can be used to drive an array of subwavelength active unit cells forming a metasurface to achieve broadband nonreciprocal sound absorption over a two-dimensional plane. Through simulation and experimental studies, this paper shows that active wave-based absorption control systems can achieve broadband nonreciprocal sound absorption when the incident waves are generated by normally and obliquely positioned primary sources.