Broadband low-frequency sound absorption of underwater metamaterial coverings combining slit-helical structures and stepped plates.

An underwater acoustic metamaterial coating-comprising a slit, helical water cavity, stepped metal plate, and rubber-is proposed for broadband low-frequency sound absorption. Finite-element method analysis reveals that the slit facilitates acoustic wave entry, exciting local resonances and promoting mode conversion to enhance shear energy dissipation. The helical water cavity reduces structural stiffness, facilitating low-frequency resonance and amplifying the mode conversion effect within the rubber damping layer. Simultaneously, the stepped metal plate optimizes impedance matching and enhances frictional energy consumption at the rubber-metal interface. This plate promotes wave-mode conversion and thereby enhances viscoelastic energy dissipation in the surrounding rubber, contributing to improved high-frequency absorption performance. Structural optimization using a deep neural network and particle swarm optimization demonstrates that effective absorption is achievable at 80 Hz with a 0.5 loss factor. The average absorption coefficient reaches 0.94 across the 10 Hz to 10 kHz range. This study offers a robust approach for designing high-performance underwater acoustic coating structures through synergistic multi-structural mechanisms.