Water-Saturated Porous Sound Absorbing Metamaterial Resistant to High Hydrostatic Pressures and Low Temperatures

This study proposes a stable underwater sound absorbing metamaterial that performs reliably under high hydrostatic pressures and low temperatures. The metamaterial is fabricated using a coplanar spiral geometry to coil water-saturated porous metal materials, which theoretically ensures strong underwater sound absorption in the subwavelength regime (≈λ/25.62 at 1380 Hz). The water-permeable structure and the exceptional mechanical properties (e.g., high modulus) of the porous metal matrix allowed the metamaterial to withstand high hydrostatic pressure with negligible influence on sound absorption. Furthermore, water temperature has a minimal effect on the intrinsic viscous dissipation mechanism (e.g., absorption peak amplitude and frequency reductions did not exceed 7.7% between 25 °C and 4 °C). Parametric analyzes reveal that the acoustic absorption performance of the proposed metamaterial can be maintained in different application environments through unit tuning or multiunit coupling. Finally, an integrated sample is experimentally measured to validate the theoretical and numerical methods. The sample exhibits satisfactory sound absorption at 1.8 kHz under low water temperature (11.2 °C) and a high hydrostatic pressure (4.5 MPa). This study provides a foundation for designing highly stable underwater acoustic metamaterials.