Experimental investigation of underwater locally multi-resonant metamaterials under high hydrostatic pressure for low frequency sound absorption

This research develops an underwater acoustic metamaterial, which can serve to absorb broadband low-frequency underwater sound under high hydrostatic pressure ranging from 200 to 2000 Hz. Every unit cell includes 12 harmonic oscillators as multi-scatterers in the proposed acoustic metamaterial, which induces lots of local resonance modes for absorbing sound energy in the investigated frequency range. The sound absorption band can be greatly widened in the low frequency range due to strong coupling and multiple scattering effects among harmonic oscillators. The effectiveness of the prototype is experimentally demonstrated in a standing wave tube under different hydrostatic pressure conditions. The effects of high hydrostatic pressure on local multi-resonance mechanism are investigated experimentally. Experimental results show that the average sound absorption coefficient of the proposed acoustic metamaterial achieves 0.78 in the frequency range [600 Hz, 2000 Hz] under 0.5 MPa. Furthermore, the proposed acoustic metamaterial can achieve better sound absorption performance than the pure viscoelastic polymer under high hydrostatic pressure.