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The acoustic impedance mismatch between water and air limits the acoustic communication through the water–air interface. Here, based on the resonances of bubbles, we propose a metasurface consisting of bubbles sealed with membranes for perfect sound transmission at the water–air interface. We have experimentally achieved sound transmission enhancement of over 20 dB at 380–510 Hz, with the thickness of the metasurface about 1/180 times air wavelength. By flexibly adjusting the immersion depth, the metasurface is able to operate at multiple frequencies. The effects of viscosity and geometry of bubbles are discussed simulatively. The present work is relevant to the practical design of advanced acoustic devices for transmedia communication. [ABSTRACT FROM AUTHOR]

Extreme impedance mismatch causes sound insulation at water–air interfaces, limiting numerous cross‐media applications such as ocean‐air wireless acoustic communication. Although quarter‐wave impedance transformers can improve transmission, they are not readily available for acoustics and are restricted by the fixed phase shift at full transmission. Here, this limitation is broken through impedance‐matched hybrid metasurfaces assisted by topology optimization. Sound transmission enhancement and phase modulation across the water–air interface are achieved independently. Compared to the bare water–air interface, it is experimentally observed that the average transmitted amplitude through an impedance‐matched metasurface at the peak frequency is enhanced by ≈25.9 dB, close to the limit of the perfect transmission 30 dB. And nearly 42 dB amplitude enhancement is measured by the hybrid metasurfaces with axial focusing function. Various customized vortex beams are experimentally realized to promote applications in ocean‐air communication. The physical mechanisms of sound transmission enhancement for broadband and wide‐angle incidences are revealed. The proposed concept has potential applications in efficient transmission and free communication across dissimilar media. [ABSTRACT FROM AUTHOR]

• Discrete metasurface is proposed for water-air sound enhancement. • Excellent wide-angle transmission is observed for discrete units. • Influences of random immersion and bending effects are discussed. 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. [Display omitted] [ABSTRACT FROM AUTHOR]