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Underwater sound absorption materials with a stable performance at various hydrostatic pressures are important for marine applications. However, most studies about underwater sound absorption materials only focused on the performance at atmospheric hydrostatic pressure, while ignoring the influence of various hydrostatic pressures. Aiming to improve the underwater sound absorption stability of a metamaterial at various hydrostatic pressures, different structures and a Nelder–Mead algorithm with an acoustic-structure fully coupled finite element method (FEM) model are developed to optimize the structure of the metamaterial at various hydrostatic pressures. In this numerical modeling, the metamaterial is a PDMS matrix embedded with periodic cylinders. Firstly, the effect of hydrostatic pressure on the metamaterial is evaluated in the frequency range [0, 8 kHz]. Secondly, different cases are designed to improve the underwater sound absorption stability at various hydrostatic pressures, including different cylinder radii, different distances between the air cylinder and the steel backing, and different void shapes. Then two layers of air and/or steel cylinders are introduced to further improve sound absorption performance under various hydrostatic pressures. The results indicate that PDMS with two layers of air cylinders have the optimal sound absorption stability performance under various hydrostatic pressures, which can be attributed to the top layer of air cylinders absorbing the main deformation. Lastly, the optimization algorithm significantly improves the sound absorption performance of the metamaterials at various hydrostatic pressures. This combination of an optimistic algorithm and FEM can guide the design of underwater sound absorption metamaterials at various hydrostatic pressures. [ABSTRACT FROM AUTHOR]

Based on the recent development of underwater sound absorption coatings, this paper analyzes the evolution of underwater sound absorption coatings from two aspects: traditional structures and metamaterials. The traditional structures are divided into three types: cavity structures, composite structures, and impedance‐matching structures. First, the development history of traditional structures, from cavities to hybrid structures, is presented, and the optimization and recent improvements in sound absorption fundamentals are explained. For different types of structures, the sound absorption effects of their respective structures are presented. At the same time, recent development progress of underwater coating metamaterials is summarized, and the sound absorption fundamentals and effects of local and nonlocal resonance types are compared. In addition, typical underwater sound absorption metamaterials are described in detail, and the application prospect of the acoustic bubble structure in practice is summarized. Moreover, this paper compares and discusses various structural types of underwater sound absorption coatings and points out the shortcomings of recent related research. Finally, it points out current open issues in the field and suggests future development directions of underwater sound absorption coatings, defining the possible development focus of underwater sound absorption coatings. [ABSTRACT FROM AUTHOR]

• A novel type of acoustic metamaterials with multi-resonators is proposed. • Coupled resonance is applied to improve acoustic absorption performance of LRSMs. • The effect of steel plate on sound absorption performance of LRSMs is studied. Generally, the better sound absorption characteristics can be achieved in the low frequency band by embedding resonance units into the conventional anechoic coating. However, in this way, the frequency range of effective sound absorption is relatively narrow, which limits the application of anechoic coating in engineering. In this paper, in order to optimize this sound absorption characteristics, a new type of underwater sound absorption metamaterials with multi-resonator are proposed based on the coupled resonance mechanism. The underwater sound absorption characteristics of the proposed metamaterials with steel plate backing are investigated by finite element method (FEM). The effect of coupled resonance between the resonators on the sound absorption characteristics of metamaterials is investigated. In addition, the effect of the geometric structure of steel plate backing on the sound absorption characteristics is studied as well. Numerical results show that the proposed metamaterials have many advantages over the conventional anechoic coating in underwater sound absorption due to the coupled resonance effect. Moreover, the steel plate backing has a significant effect on the sound absorption characteristics of the proposed metamaterials, especially, in the low frequency range. [ABSTRACT FROM AUTHOR]

• An underwater metamaterial combining aluminum foam and polyurea. • Sound absorption performance comes from the change in the sound propagation path and waveform transformation. • Large and small conical scatterers significantly improve the sound absorption of the polyurea. • Water-filled impedance and anechoic tank prove effectiveness of the underwater metamaterial in this paper. We propose an underwater metamaterial combining aluminum foam and polyurea. Three samples with different aluminum foam scatterers were prepared and tested for sound absorption coefficient in the water-filled impedance and anechoic tank. The results show that the large and small conical scatterers significantly improve the sound absorption of the polyurea. This improvement in sound absorption comes from the change in the sound propagation path and waveform transformation. The design of the proposed underwater metamaterial and relevant experimental results presented in this paper have potential guiding values in developing underwater sound-absorbing functional materials. [ABSTRACT FROM AUTHOR]

• The thin LRPM layer can achieve perfect absorption at targeted low-frequency • The absorption performance can be tuned by manipulating the resonant circuit • Negative capacitance in the resonant circuit can improve the absorption bandwidth 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. [ABSTRACT FROM AUTHOR]

In this work, a hybrid metamaterial is proposed as a new type of underwater absorber, which is composed of carefully designed periodically arranged units. Each unit consists of a perforated panel, a water chamber and a coiled water channel coated with rubber on the channel walls. The coiled channel is designed to reduce the thickness of the structure. The introduction of the rubber coating changes the boundary condition of the coiled channel from rigid to non-rigid, resulting in dissipative slow-sound propagation in the coiled channel. The slow-sound propagation leads to the decrease of resonant frequency, and its inherent dissipation can consume sound energy, resulting in perfect low-frequency sound absorption at subwavelength thickness. Theoretical and numerical results show that the proposed hybrid metamaterial exhibits more than 99% sound energy absorption at 155 Hz, and the thickness of the structure is only 1/456 of the wavelength of sound wave. This work paves the way for the design of ultrathin underwater low-frequency absorbers. • An acoustic metamaterial for underwater sound absorption is proposed. • The finite element model successfully verifies the theoretical model. • The metamaterial has good underwater sound absorption at low frequencies. • This work is helpful to design ultra-thin low-frequency underwater absorbers. [ABSTRACT FROM AUTHOR]