Lightweight underwater acoustic metasurface with water-air-Fano-resonance unit cells

Conventional underwater acoustic metasurfaces commonly rely on high-density solid materials to provide the impedance contrast required for wavefront manipulation, resulting in relatively high structural mass and solid–fluid vibroacoustic coupling. Here, we propose a lightweight underwater acoustic metasurface based on a water–air Fano resonance (WAFR) mechanism, in which an embedded air cavity forms an acoustically soft boundary for reflection-phase control. A multiphase-coupled analytical model combining effective-medium theory and the transfer-matrix method is developed to elucidate the underlying phase-modulation mechanism. At the design frequency of 37.5 kHz, the WAFR unit cell achieves complete 0−2π reflection-phase coverage, while its wet equivalent density is approximately 1089 kg/m³, about 86% lower than that of stainless steel. Numerical simulations and experiments further demonstrate on-axis, off-axis, and multipoint acoustic focusing, with a measured focal-spot full width at half maximum of 0.63λ. These results provide a lightweight approach to underwater acoustic wavefront manipulation with reduced reliance on high-density solid resonators.