High-efficiency broadband underwater acoustic metamaterial based on viscoelastic shear dissipation and quasi-Helmholtz resonance

We propose a high-efficiency broadband underwater acoustic metamaterial (HEB-UAM) with a unique grating-like structure made of wider rigid supports and damping layers. The design incorporates an internally embedded quasi-Helmholtz resonator configuration within the rigid support structure. We developed a theoretical model for the HEB-UAM absorption coefficient by combining slit absorption theory with the complex viscosity model and using the transfer matrix method (TMM). The theoretical predictions exhibit excellent agreement with the finite element method (FEM) simulation results. The insertion of rigid supports significantly enhances the shear deformation within the damping layers, thereby substantially improving the viscous shear dissipation of acoustic energy. The wider support helps offset the poor broadband absorption of low-impedance materials. The rigid support contains a quasi-Helmholtz resonator with a narrow slit, a water cavity, a damping layer, and an air cavity arranged in sequence. This carefully optimised configuration maximises the degrees of freedom for the cavity damping layer, resulting in significantly enhanced low-frequency acoustic energy absorption. Parametric studies elucidated the absorption characteristics of each component. Through particle swarm optimization (PSO) of unit cell parameters, the optimized structure achieves efficient sound absorption($\alpha >0.8$) in the deep subwavelength regime($\lambda /326$ at 92 Hz). Notably, it maintains a high average absorption coefficient of 0.94 within the 0–10000 Hz frequency range. Furthermore, we discuss feasible approaches for lightweight design, loss factor reduction, and reduction in structural thickness. These findings highlight the strong potential of HEB-UAM for practical underwater sound absorption applications.