A water–air interface acoustic transmission metasurface based on a single-layer spherical-shell array

Extremely low acoustic transmission across the water–air interface is a critical bottleneck that limits the development of underwater acoustic sensing and communication, and acoustic metasurfaces provide an effective route to enhance interfacial transmission. However, many existing metasurface designs are structurally complex, and a clear physical interpretation is still lacking for how anomalous high-transmission states can arise in low-complexity water–air transmission metasurfaces. Here, we design and fabricate a metasurface consisting of a single-layer array of thin spherical shells. By exploiting the resonant scattering of the shells, the proposed metasurface achieves an average transmission gain of ∼9.0 dB over 3700–4300 Hz and exhibits pronounced transmission enhancement with a peak gain of 27.3 dB at 3729 Hz. Through eigenmode correlation analysis, combined with examinations of the near-interface acoustic pressure distribution, the shell vibration-velocity distribution, and the cross-interface phase difference, we demonstrate that the anomalous transmission enhancement at 3729 Hz cannot be attributed to a pure single-eigenmode resonance. Instead, it is associated with an off-eigenmode coupled response, primarily influenced by the neighboring eigenmode, which reorganizes the near-interface radiation field into a more coherent form and substantially reduces the phase mismatch across the water–air interface. Water-tank experiments, in comparison with numerical simulations, show consistent overall trends over 3000–8000 Hz, validating the proposed metasurface with improved acoustic transmission performance. This work provides a physically clearer interpretation of anomalous transmission enhancement in a simple shell-array metasurface and suggests a practical route toward low-complexity water–air acoustic transmission metasurfaces.