Underwater metastructure with broadband sound absorption capability in low-frequency range above 20 Hz

We present an underwater metastructure with excellent sound absorption effect below 50 Hz. The periodic metastructure unit consists of a conical cavity, rubber matrix, and two metal disks. FEM results show that, in the range of 20-300 Hz, the proposed metastructure demonstrates the excellent sound absorption within 279 Hz bandwidth when the reference absorption coefficient is considered to be 0.5. Displacement vibration diagrams illustrate the addition of two layers of metal disks break the propagation law of acoustic wave in rubber matrix. An anti-phase motion of the rubber matrix emerges due to the presence of the metal disks, then consumes the energy of incident acoustic waves. The geometric parameters of lattice constant a, the thickness h1, and the height d1 of the upper metal disk are positively correlated with the sound absorption coefficient, while the upper radius r1 and the height hair of the conical cavity are negatively correlated with the sound absorption coefficient. The novel design presented in this study could have the potential applications in the realization of an acoustic underwater anechoic layer.