Tunable underwater low-frequency sound absorption via locally resonant piezoelectric metamaterials

Acoustic metamaterials with passive dissipative elements have demonstrated excellent underwater low-frequency sound absorption abilities, but still suffer from the narrow bandwidth, fixed absorption frequencies and bulky size. In this research, tunable underwater low-frequency sound absorption of the locally resonant piezoelectric metamaterials (LRPM) is systematically studied. A theoretical model of underwater sound absorption based on the LRPM is established. From the perspective of effective materials, the tunable sound absorption characteristics and perfect absorption mechanism are analyzed. The theoretical results are in good agreement with the numerical ones. It is demonstrated that a thin LRPM layer can achieve perfect sound absorption at targeted low-frequency which can be actively tuned by manipulating the resonant shunt circuit. Furthermore, the negative capacitance (NC) shunt can be introduced to the resonant circuit, which significantly improves the sound absorption bandwidth. By applying causality principle to the proposed LRPM, the causal constraints are discussed, which results in an improvement of causal optimality by NC shunt. This research can provide useful guidance for the realization of efficient and widely adjustable ultra-thin underwater sound absorbers.