Ultrathin Composite Metasurface for Absorbing Subkilohertz Low-Frequency Underwater Sound

The crucial ability to suppress low-frequency waterborne sound efficiently has far-reaching implications for underwater noise-control engineering. Here, we propose an ultrathin composite metasurface in deep subwavelength thickness based on pentamode metamaterials (PMs) backed by rubber-metal resonators (RMRs). As a demonstration, average absorptance $87.8\mathrm{%}$ at subkilohertz frequencies ranging from 760 to 920 Hz (wavelength $\ensuremath{\lambda}$ from 30.9 to 25.5 times of absorber thickness) is achieved. In the composite system, PM layer functions as an energy converter, which converts incident waterborne sound into mechanical vibrations, suggesting that sound waves could be absorbed only if the converted vibrations are eliminated by the backing RMRs. Beyond as a converter, PM itself also exhibits an additional stringlike resonant mode originating from the fixed-boundary constraints, which gives rise to intense vibrations of RMRs that facilitate energy dissipation. Further investigation demonstrates the unique property of robust high-efficiency absorption for extended frequency region and wide oblique incidence angles. The proposed methodology paves the way for a class of low-frequency underwater absorber design platforms, allowing devices for versatile applications to be envisioned.