Abstract:Acoustic metasurfaces are a kind of artificial structure with carefully designed microstructures and spatial order. They can exhibit exotic wave behaviors beyond the nature. So they are attracting more and more attentions by researchers from different disciplinary fields. However, most existing studies on metasurfaces focus on single medium such as air or water. Sound manipulation through trans media is rarely investigated. In this paper, we focus the modulation of sound waves through water-air interface by using the combined metasurfaces. Firstly, genetic algorithm is used to design airbone metasurface with high transmission and wavefront control capability. Then, the combined metasurfaces are designed by integrating a discrete metasurface with unitary amplitude through water-air interface to achieve the wavefront manipulation of acoustic waves with high transmission. The impact of the coupling spacing between combined metasurfaces on wave manipulation is also discussed. Finally, samples of the combined metasurfaces are fabricated, and experimental measurement of the acoustic focusing is conducted. The results indicate that abnormal transmission and focusing of acoustic waves through water-air interface can be realized by combining the two independently designed metasurfaces. Coupling spacing of the combined metasurfaces significantly affects the sound modulation through water-air interafce. When the coupling spacing is small, although the combined metasurface can realize the manipulation of the acoustic waves through water-air interface, the manipulation performance is relatively poor. With the increase of the coupling spacing, the manipulation performance of the combined metasurface on the trans interface improves rapidly and then becomes stable. The acoustic focusing of sound across water-air interface is observed by employing the combined metsaurfaces in the experiment. And the influence of the coupling spacing on acoustic focusing is also obtained. Experimental results are generally in good agreements with the simulations. This research provides numerical and experimental foundations for the design of novel acoustic devices through water-air interface.