Broadband acoustic focusing via binary rectangular cavity/Helmholtz resonator metasurface

In this work, we numerically and experimentally demonstrate that broadband acoustic focusing can be realized using a sub-wavelength binary metasurface. Rectangular cavities and Helmholtz resonators are utilized to construct a coding system, which brings the desired transmittance and phase difference in a wide range of wavelengths. The apparent acoustic focusing is validated in a bandwidth of 0.8f₀-1.6f₀ by experimental measurements, which agrees well with the numerical simulations and offers a degree of freedom to manipulate the focal length actively. This work provides a solution to design a sub-wavelength planar lens with broadband and robustness properties, which may have promising applications in numerous acoustic engineering procedures, including biomedical diagnosis and non-destructive testing.