Thin metasurface emulating an airborne acoustically soft boundary

Acoustically soft boundaries are of substantial potential in acoustical designs relevant particularly to directional radiation of sound waves. Here an alternative strategy is proposed for designing a much thin metasurface capable of emulating an airborne soft boundary, by utilizing the unusual resonance that relies on excitation of evanescent waves on subwavelength scale. The metasurface is structured by a two-dimensional periodic array of oblate cavities that are open on the same plane to air ambience through apertures of negligible thickness. An analytical theory is developed on the reflection of plane and spherical sound waves incident on the proposed metasurface, and the trade-off between the thickness of the metasurface and working bandwidth are analyzed for adapting the metasurface to different design requirements. As a proof-of-concept demonstration, an airborne metasurface is designed and fabricated in our laboratory, with its thickness roughly 3 times thinner than the thinnest ever reported. The theoretical predictions are well confirmed both by the full-wave numerical simulations and the experimental measurements.