Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled.

声学超材料是具有奇异声学特性的结构，在声束转向，聚焦，阻抗匹配，吸收和隔离方面具有广阔的应用前景。最近的工作表明，可以通过控制一个称为Willis耦合的附加参数来提高许多声学超材料的效率，该参数类似于电磁超材料中的各向异性。从理论上已经证明了被动声学亚原子中威利斯耦合的幅度具有上限，但是尚未通过实验研究达到此上限的可行性。在这里，我们介绍一种具有Willis耦合的准原子，该原子非常接近此理论极限，与以前报道的结构相比，它更简单且更不易产生热粘性损失。在数值计算的支持下，我们执行二维实验以测量强威利斯耦合。我们的超原子几何结构易于分析解析，从而可以轻松控制威利斯耦合的强度及其峰值频率。