Underwater acoustic manipulation using solid metamaterials with broadband anisotropic density

A new type of all-solid metamaterial model with anisotropic density and fluid-like elasticity is proposed for controlling acoustic propagation in an underwater environment. The model consists of a regular hexagonal lattice as the host that defines the overall isotropic stiffness, in which all lattice beams have been sharpened at both ends to significantly diminish the shear resistance. The inclusion structure, which involves epoxy, rubber, and lead material constituents, is designed to attain a large density-anisotropy ratio in the broad frequency range. The wave-control capability of metamaterials is evaluated in terms of underwater acoustic stretching, shifting, and ground cloaking, which are generated by the transformation acoustic method. The decoupling design method was developed for the metamaterial microstructure using band-structure, effective-medium, and modal-field analyses. The acoustic performance of these metamaterial devices was finally verified with full-wave numerical simulations. Our study provides new insight into broadband underwater acoustic manipulation by all-solid anisotropic-density metamaterials.