Low-frequency broadband acoustic transmission through air–water interface using sonic black hole-based metasurface

Acoustic transmission across the air–water interface is limited by the extreme impedance mismatch between two media. Although recent acoustic metamaterials have enabled cross-interface transmission, their performance is typically restricted to high frequencies or narrow bandwidths. To address this challenge, we propose a sonic black hole (SBH) metasurface for compact, low-frequency, and broadband acoustic transmission across the air–water interface. A transfer-matrix method and finite-element simulations are developed to analyze the transmission characteristics and underlying mechanism. The results show that the metasurface substantially enhances transmission and reduces the transmission loss of the bare interface over a broad low-frequency band. In particular, a pronounced transmission band is predicted near 870 Hz, while the thickness is only 0.0418 m, corresponding to λ/9.34 in air. An effective-parameter inversion is introduced to reveal continuous impedance variation and strong terminal pressure amplification through slow-wave compression. Thermoviscous analysis indicates that dissipation remains well controlled. Experiments validate this concept, showing a maximum transmitted-level enhancement of 18.7 dB at 1070 Hz and significant transmission from 549 to 1319 Hz. These results demonstrate that SBH metasurface provides a feasible route for compact, low-frequency, and broadband acoustic air–water transmission, with potential applications in cross-medium communication, sensing, and energy transfer.