Maximum Willis Coupling in Acoustic Scatterers

TL;DR

This paper establishes fundamental bounds on Willis coupling in acoustic scatterers, demonstrating that maximally bianisotropic inclusions can be designed to achieve highly efficient acoustic metasurfaces for sound manipulation.

Contribution

It derives bounds on Willis response, shows they can be maximized in designed scatterers, and proposes a systematic approach for implementing highly bianisotropic acoustic metasurfaces.

Findings

Maximal Willis coupling can be achieved in designed scatterers.

Maximally bianisotropic inclusions enable efficient sound bending metasurfaces.

Theoretical bounds guide the design of advanced acoustic materials.

Abstract

Willis coupling in acoustic materials defines the cross-coupling between strain and velocity, analogous to bianisotropic phenomena in electromagnetics. While these effects have been garnering significant attention in recent years, to date their effects have been considered mostly perturbative. Here, we derive general bounds on the Willis response of acoustic scatterers, show that they can become dominant in suitably designed scatterers, and outline a systematic venue for the realistic implementation of maximally bianisotropic inclusions. We then employ these inclusions to realize acoustic metasurfaces for sound bending with unitary efficiency.