# Acoustic meta-atom with maximum Willis coupling

## TL;DR

This paper introduces a simple acoustic meta-atom that nearly reaches the theoretical maximum Willis coupling, demonstrated through experiments and calculations, promising improved efficiency in acoustic device design.

## Contribution

The study presents a new, easily fabricated meta-atom design that approaches the theoretical limit of Willis coupling, enhancing acoustic metamaterial performance.

## Key findings

- Achieves near-maximum Willis coupling experimentally
- Supports analytical modeling for design control
- Facilitates high-efficiency acoustic device development

## Abstract

Acoustic metamaterials are structures with exotic acoustic properties, having 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 an 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. Together with its ease of fabrication, this will facilitate the design of future high efficiency acoustic devices.