Discrete metasurface for extreme sound transmission through water-air interface

This study proposes a discrete metasurface for extreme sound transmission based on the impedance matching theory. The aim is to overcome the extreme mismatch of acoustic impedance at the water-air interface. By employing topology optimization, discrete unit cells with different aspect ratios are inversely designed with unitary sound transmission. The unit cell of continuous metasurface is also obtained for comparison. After analyzing the wide-angle performance of discrete unit cells, the enhancement of sound transmission of the samples at the water-air interface is experimentally measured. It is evident that the discrete metasurface significantly enhances sound transmission compared to the bare water-air interface. And the transmission amplitude is relatively larger than that of the continuous sample. Experimental results are generally agreement with numerical ones when the viscosity of the sample is considered. Furthermore, the resonate frequency shifts observed during the experiment are discussed. They can be primarily attributed to the random immersion of unit cells for discrete metasurface and the bending of continuous metasurface, respectively. The present work will greatly promote the practical application of acoustic metasurfaces in transmedia communication and sensing.