  The mismatch of acoustic impedance at water-air interface can lead a low
transmitted sound energy. In this paper, we propose a discrete metasurface for
extreme sound transmission based on the impedance matching theory. By employing
topology optimization, discrete unit cells with different aspect ratios are
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, samples of both discrete and continuous
metasurfaces are fabricated. Sound transmission enhancement of discrete
metasurface is clearly measured compared to the bare water-air interface. And
the amplitude is relatively larger than that of the continuous sample.
Experimental results are in general agreement with numerical ones when
viscosity of the sample is considered. Furthermore, the frequency shifts
between experiment and simulation are attributed to the random immersion of
unit cells for discrete metasurface and the bending of continuous metasurface,
respectively. The present work suggests an alternative way for improving the
efficiency of water-air acoustic communication.
