Ultra-thin composite underwater honeycomb-type acoustic metamaterial with broadband sound insulation and high hydrostatic pressure resistance

This research proposes a new type of composite underwater honeycomb-type acoustic metamaterial (AM) plate with the advantages of low-frequency broadband sound insulation and high hydrostatic pressure resistance. The proposed AM is composed of a thin plate (rubber-steel-rubber, RSR) clamped between two layers of honeycomb plate, and the thickness of the whole structure is 20.25 mm. The underwater sound insulation performance and mechanisms of the new type of AM were investigated numerically and experimentally. Wave propagation properties of the proposed AM were modeled using the finite element method (FEM). In addition, modal analysis of the eigenmode shapes were used to investigate the underwater sound transmission loss (STL) performance and local resonance bandgap formation. The results show that the honeycomb-type AM has good underwater sound insulation performance at low frequencies and the STL is more than 20 dB higher than that of homogeneous materials with the same area density. Furthermore, the theoretical STL of the proposed metamaterial was consistent with experimental values, thus verifying the STL performance and validating the theoretical modeling approach for the proposed AM. Moreover, the proposed AM structure can still achieve an average STL of 10 dB in the frequency range [2 kHz, 10 kHz] under the hydrostatic pressure of 3 MPa.