Acoustic meta-atom with maximum Willis coupling

Acoustic metamaterials can produce unusual sound behavior, which is useful for tasks like steering and focusing sound, matching acoustic impedance, and improving absorption or isolation. The abstract highlights Willis coupling as an extra parameter that can boost metamaterial efficiency, analogous to bianisotropy in electromagnetic systems. While theory suggests an upper limit for Willis coupling in an acoustic meta-atom, the work asks whether that limit is practically reachable. The authors introduce a meta-atom design that closely approaches the theoretical maximum Willis coupling. They report two-dimensional experiments to measure strong Willis coupling, supported by numerical calculations. They also state that the geometry can be modeled analytically, allowing the Willis coupling strength and its peak frequency to be controlled, and that the design is simpler and less prone to thermo-viscous losses, which they argue will help future high-efficiency acoustic device design.