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2-1-3 piezoelectric composite is advantageous for its high piezoelectricity, mechanical stability and ease of preparation. Applying piezoelectric shunt damping technology in underwater semi-active sound absorption increases the adjustability of sound absorption performance. This paper analyzes the underwater semi-active sound absorption performance of 2-1-3 composite. The theoretical model and finite element model of 2-1-3 composite are established and the sound absorption coefficient of the composite coating is deduced based on transfer matrix and Mason equivalent circuit theory. The effects of the volume fraction of PZT-5H phase μ, the aspect ratio α and the shunt circuit on the sound absorption coefficient are studied and the root causes are analyzed. The results show that μ and α can affect the peak frequency, bandwidth and the range between open-circuit and short-circuit peak frequencies by changing the longitudinal wave velocity, mechanical loss factor and electromechanical conversion coefficient of the composite. Shunt circuits can adjust peak frequency and bandwidth by changing the elastic constant component c33 and mechanical loss factor of the composite. The conclusions of this study have value as reference for the design of underwater semi-active sound-absorbing material in various application scenarios. [ABSTRACT FROM AUTHOR]

The acoustic absorption characteristics of anechoic coatings attached to the surface of underwater vehicles are closely related to their acoustic stealth. Owing to the essential property of local resonance, the narrow sound-absorption band cannot meet the underwater broadband sound absorption requirements. To this end, a multi-mechanism synergistic composite acoustic structure (MMSC−AS) was designed according to the integration of multiple acoustic dissipation mechanisms in this paper. Then, the acoustical calculation model for MMSC−AS was developed by using the graded finite element method (G-FEM), and the feasibility and the correctness of the established acoustical calculation model were verified. The underwater sound absorption behaviors of MMSC−AS were studied, and the optimization of the sound absorption characteristics of the MMSC−AS was also carried out. The results indicated that the calculation accuracy of the G-FEM was better than that of the FEM under the condition of the same mesh elements. Moreover, there were many sound wave regulation mechanisms in the MMSC−AS, and the synergy between the mechanisms enriched the mode of sound acoustic energy dissipation, which could widen the absorption band with effect. This study provides theoretical and technical basis for breaking through the challenge of low-frequency and broadband acoustic structure design of underwater vehicles. [ABSTRACT FROM AUTHOR]

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]

The underwater sound absorption material for submarines should fulfill the demand for remarkable performance in medium-low frequency sound absorption at low thicknesses. Hence, in this paper, the innovative “filled-microperforated plate-like” (F-MPPL) structure was introduced to the hollow microballoons filled underwater sound absorption composite (HBF composite) using the tailor-made 3D spacer fabric. The test results show remarkable mechanical properties with a compression strength and compression modulus of 2256 KPa and 14.6 MPa for the new type of F-MPPL composite, respectively. In addition, the peak sound absorption coefficient of the F-MPPL composite reaches 0.77 at just 7.5 mm thickness based on the resonant sound absorption characteristic introduced by the F-MPPL structure. Moreover, the study investigated the effects of seawater aging on the F-MPPL composite and found that while there was a decrease in compression strength and average sound absorption coefficient, the degradation of the F-MPPL composite was significantly smaller than the HBF composite due to the inhibitory effect of spacer yarns on pore expansion. Finally, this study proposed the method of using compression strength to predict the long-term life of the F-MPPL composite under seawater aging, and a 60% retention rate after 20 months was displayed.

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]

Abstract To address the challenging task of effective sound absorption in the low and broad frequency band for underwater structures, we propose a novel grating‐like anechoic layer by filling rubber blocks and an air backing layer into metallic grating. The metallic gratings are incorporated into the anechoic layer as a skeleton for enhanced viscoelastic dissipation by promoting shear deformation between rubber and metal plates. The introduction of an air backing layer releases the bottom constraint of the rubber, thus intensifying its deformation under acoustic excitation. Based on the homogenization method and the transfer matrix method, a theoretical model is developed to evaluate the sound absorption performance of the proposed anechoic layer, which is validated against finite element simulation results. It is demonstrated that a sound absorption coefficient of the grating‐like anechoic layer of 0.8 can be achieved in the frequency range of 1294–10 000 Hz. Given the importance of sound absorption at varying frequencies, the weighted average method is subsequently used to comprehensively evaluate the performance of the anechoic layer. Then, with structural density taken into consideration, an integrated index is proposed to further evaluate the acoustic properties of the proposed anechoic layer. Finally, the backing conditions and the boundary conditions of finite‐size structures are discussed. The results provide helpful theoretical guidance for designing novel acoustic metamaterials with broadband low‐frequency underwater sound absorption.

Rapid improvements in underwater vehicle technology have led to a significant increase in the demand for underwater sound absorption materials. These materials, unlike their counterparts utilized in air, must have high hydrostatic pressure resistance, corrosion resistance, and other advantageous attributes. This necessitates the development of innovative, composite sound-absorbing materials with multifunctional properties, which presents substantial challenges for researchers. In this comprehensive review, we systematically analyze and categorize the mechanisms governing underwater sound absorption, hydrostatic pressure resistance, and corrosion prevention while considering related research advances. Furthermore, we provide an extensive overview of research advancements, existing challenges, and potential solutions pertaining to multifunctional and integrated underwater sound-absorbing materials. This review aims to serve as a valuable resource for future investigations into the development and optimization of multifunctional integrated underwater sound-absorbing materials, thereby contributing to the advancement of underwater vehicle technology. [ABSTRACT FROM AUTHOR]

Abstract 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.

The acoustic absorption characteristics of anechoic coatings attached to the surface of underwater vehicles are closely related to their acoustic stealth. Owing to the essential property of local resonance, the narrow sound-absorption band cannot meet the underwater broadband sound absorption requirements. To this end, a multi-mechanism synergistic composite acoustic structure (MMSC−AS) was designed according to the integration of multiple acoustic dissipation mechanisms in this paper. Then, the acoustical calculation model for MMSC−AS was developed by using the graded finite element method (G-FEM), and the feasibility and the correctness of the established acoustical calculation model were verified. The underwater sound absorption behaviors of MMSC−AS were studied, and the optimization of the sound absorption characteristics of the MMSC−AS was also carried out. The results indicated that the calculation accuracy of the G-FEM was better than that of the FEM under the condition of the same mesh elements. Moreover, there were many sound wave regulation mechanisms in the MMSC−AS, and the synergy between the mechanisms enriched the mode of sound acoustic energy dissipation, which could widen the absorption band with effect. This study provides theoretical and technical basis for breaking through the challenge of low-frequency and broadband acoustic structure design of underwater vehicles.

Underwater noise pollution, mainly emitted by shipping and ocean infrastructure development of human activities, has produced severe environmental impacts on marine species and seabed habitats. In recent years, a polyurethane-based (PU-based) composite with excellent damping performance has been increasingly utilized as underwater sound absorption material by attaching it to equipment surfaces. As one of the key parameters of damping materials, dynamic mechanical parameters are of vital importance to evaluating the viscoelastic damping property and thus influencing the sound absorption performance. Nevertheless, lots of researchers have not checked thoroughly the relationship and the mechanism of the material dynamic mechanical parameters and its sound absorption performance. In this work, a finite element model was fabricated and verified effectively using acoustic pulse tube tests to investigate the aforementioned issues. The influence of the dynamic mechanical parameters on underwater sound absorption performance was systematically studied with the frequency domain to reveal the mechanism and the relationship between damping properties and the sound absorption of the PU-based composite. The results indicate that the internal friction of the molecular segments and the structure stiffness were the two main contributors of the PU-based composite's consumption of sound energy, and the sound absorption peak and the sound absorption coefficient could be clearly changed by adjusting the dynamic mechanical parameters of the composite. This study will provide helpful guidance to develop the fabrication and engineering applications of the PU-based composite with outstanding underwater sound absorption performance. [ABSTRACT FROM AUTHOR]

• An novel ultrathin meta-coating for low-frequency underwater sound absorption at high hydrostatic pressures based on tree-shaped acoustic-black-hole (ABH). • High and quasi-perfect absorption in multiple and ultra-wide low-frequency bands at a deep subwavelength thickness under hydrostatic pressures up to 4.5 MPa. • Mechanism enabled by the enriched local dynamics of multi-scale ABHs, pressure-resistance of the frame skeleton and the support column, and damping of the matrix. • Experimental validation of the model and sound absorption performance. Acoustic coating is the main technical means to absorb incoming sound from detection sonars. Considering the advance of sonar technology to cope with low frequencies and the underwater working environment, acoustic coatings are required to provide effective broadband low-frequency sound absorption (SA) under hydrostatic pressure. However, current underwater acoustic coating materials have poor low-frequency broadband SA performance, especially under hydrostatic pressure. Herein, we propose an ultrathin meta-coating with tree-shaped acoustic black hole (ABH) units embedded in an elastomer matrix. Each tree-shaped ABH unit consists of multi-scale ABH plates as scatterers, a center-support column as "trunk" and a frame as "skeleton". This design combines the rich dynamics of multi-scale ABH plates, the pressure-resistance characteristics of the frame and the column, and the damping of the matrix, to collectively lead to superior performance. Capitalizing on the rich local and coupled large-amplitude local resonances of the unit, and strong resistance to static-pressure-induced deformation, high and quasi-perfect SA is achieved in multiple and ultra-wide low-frequency bands at a deep subwavelength scale under different hydrostatic pressures. Notably, quasi-perfect SA (>0.92) is achieved at 500 Hz with a meta-coating whose thickness is 2 % of the sound wavelength. The quasi-perfect SA is also demonstrated experimentally within 1200–7500 Hz, which persists till 1.0 MPa. The SA is still above 0.6 from 3–10 kHz at 4.5 MPa. For SA model under hydrostatic pressures, hyperelastic constitutive model and pressurized dynamic mechanical parameters of the matrix were established. Numerical results are then verified by experimental tests under different hydrostatic pressures. With both high local-modal densities and hydrostatic-pressure resistance, this design represents a new type of acoustic metamaterials and promises broad underwater applications. [ABSTRACT FROM AUTHOR]

In this research, the underwater sound absorption performance of porous media saturated with water is investigated through long straight tubes (LSTs) with circular cross-section and sintered fibrous metals (SFMs) containing complex pores, and effective underwater sound absorption capability is demonstrated theoretically, numerically and experimentally. Specifically, the theoretical model for LSTs, verified by direct numerical simulations (DNSs), reveals that much smaller pore diameter and much larger layer thickness of porous material is needed to gain high waterborne sound absorption coefficient than to obtain large airborne sound absorption coefficient. Besides, to examine the sound absorption capacity of realistic porous materials, SFMs backed with a finite cavity are experimentally measured under 7 different hydrostatic pressures in a water-filled tube, and numerically characterized via multi-physics couplings. Insensitivity of effective sound absorption capability to high pressure is established, which stems from the different underwater wave energy consumption mechanism (i.e. viscous-thermal effect) of porous materials from conventional rubber kind of underwater sound absorption materials. Physically, porous materials possess great potential in developing the next generation of high-performance underwater sound absorption materials. • Effective underwater sound absorption of water-saturated porous media is shown. • Viscous-thermal dissipation effect is insensitive to high hydrostatic pressure. • A porous material sample is tested in the water-filled tube for sound absorption. [ABSTRACT FROM AUTHOR]

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• Broadband waterborne absorptive metamaterial with hydrostatic pressure resistance is proposed. • Acousto-electric analogy is adopted to determine the equivalent parameters for transfer matrix method. • Oscillators of helical configurations demonstrate strong hydrostatic pressure resistance. • Experimental results confirm the superior performance of the configurations at 3 MPa. Underwater absorptive metamaterials are rapidly developed duo local resonance mechanisms, but they still suffer from the disadvantages of poor hydrostatic resistance. The poor acoustic absorption performance at high hydrostatic pressure mainly attributes to the decreased damping coefficient and increased modulus of substrate at high hydrostatic pressure. Acoustic metastructures with helical oscillators are proposed and optimized with simulating annealing algorithm, where the helical oscillators are beneficial for easing the stress in the polymer substrate and resulting in good acoustic absorption abilities. Four acoustic absorptive metastructures without any oscillators (pure polymer substrate), with cylindrical oscillators, with helical oscillators, with both cylindrical and helical oscillators are proposed and experimentally verified in this study. All three metastructures with oscillators demonstrate excellent acoustic absorption coefficient with the absence of the hydrostatic pressure, while the metastructures with helical oscillators exhibit better hydrostatic pressure resistance compared with the metastructures with cylindrical oscillators. Besides, the coupling of helical oscillators and helical oscillators enhanced the acoustic performance furtherly. It is revealed that the acoustic metamaterials with a combination of cylindrical oscillator and helical oscillator exhibit superior performance under hydrostatic pressure up to 3 MPa in the frequency range of 0.5kHz∼10 kHz compared with the others, demonstrating a new avenue for underwater acoustic metamaterials application. [Display omitted] [ABSTRACT FROM AUTHOR]

Rapid improvements in underwater vehicle technology have led to a significant increase in the demand for underwater sound absorption materials. These materials, unlike their counterparts utilized in air, must have high hydrostatic pressure resistance, corrosion resistance, and other advantageous attributes. This necessitates the development of innovative, composite sound-absorbing materials with multifunctional properties, which presents substantial challenges for researchers. In this comprehensive review, we systematically analyze and categorize the mechanisms governing underwater sound absorption, hydrostatic pressure resistance, and corrosion prevention while considering related research advances. Furthermore, we provide an extensive overview of research advancements, existing challenges, and potential solutions pertaining to multifunctional and integrated underwater sound-absorbing materials. This review aims to serve as a valuable resource for future investigations into the development and optimization of multifunctional integrated underwater sound-absorbing materials, thereby contributing to the advancement of underwater vehicle technology.

In this work, the acoustic absorption performance of polyurethane (PU) elastomer consisted of different molecular masses of polypropylene glycol (PPG), and a PU composite sample was made by adding multi-wall carbon nanotubes (MWCNT) was studied. Using the acoustic pulse tube device and spectrum of dynamic mechanical thermal analysis, modulus, glass transition temperature (Tg), damping, and acoustic absorption properties of PU with different components were analyzed. The sound absorption results in the frequency range of 2–18 kHz showed that the PU_2 (PPG 2000, TDI) had good acoustic absorption properties. The average acoustic absorption coefficient of PU_2 was 0.8, and the maximum acoustic absorption coefficient was 0.95. The PU_3 (PPG 1000, TDI + 0.1 mass% MWCNT) had lower acoustic absorption properties than PU_2, while the sample had a lower acoustic reflection coefficient compared to the samples PU_1 (PPG 1000, TDI) and PU_3. The damping properties showed that, with an increase in polyether molecular mass and also with the addition of MWCNT, the maximum value of tan(δ) was increased, while the glass transition temperature Tg decreased and increased compared to PU_1, respectively. The PU samples' hardness showed that the increase in molecular mass increases the soft segment and decreases the hardness, while the PU_3 hardness is superior to PU_2. [ABSTRACT FROM AUTHOR]

Underwater sound absorption materials with broadband and resistance to high pressure are crucial for noise control in the ocean. Herein a new method is presented to synergistically improve material's resistance to high pressure with broadband by hybridizing carbon nanotubes (CNTs) with graphene nanoplatelets (GNPs) in polydimethylsiloxane (PDMS) matrix. The micro‐structure shows the geometrical synergy between the nanoplatelets (GNPs) and nanotubes (MWCNT‐COOH). Besides, GNPs also form junctions with air micro‐voids. The underwater acoustic tests reveal that, compared with equivalent nanocomposites containing only CNTs or GNPs, the hybridized materials show significantly better sound absorption performance with a high hydrostatic pressure increased from 0.4 MPa to 1.0 MPa at the frequency range of 1500 Hz to 7000 Hz. Therefore, the hybridization of one‐dimensional (1D) and two‐dimensional (2D) nano‐carbon materials provides a new route for increasing the underwater sound absorption performance at high hydrostatic pressure. [ABSTRACT FROM AUTHOR]

Underwater noise pollution, mainly emitted by shipping and ocean infrastructure development of human activities, has produced severe environmental impacts on marine species and seabed habitats. In recent years, a polyurethane-based (PU-based) composite with excellent damping performance has been increasingly utilized as underwater sound absorption material by attaching it to equipment surfaces. As one of the key parameters of damping materials, dynamic mechanical parameters are of vital importance to evaluating the viscoelastic damping property and thus influencing the sound absorption performance. Nevertheless, lots of researchers have not checked thoroughly the relationship and the mechanism of the material dynamic mechanical parameters and its sound absorption performance. In this work, a finite element model was fabricated and verified effectively using acoustic pulse tube tests to investigate the aforementioned issues. The influence of the dynamic mechanical parameters on underwater sound absorption performance was systematically studied with the frequency domain to reveal the mechanism and the relationship between damping properties and the sound absorption of the PU-based composite. The results indicate that the internal friction of the molecular segments and the structure stiffness were the two main contributors of the PU-based composite’s consumption of sound energy, and the sound absorption peak and the sound absorption coefficient could be clearly changed by adjusting the dynamic mechanical parameters of the composite. This study will provide helpful guidance to develop the fabrication and engineering applications of the PU-based composite with outstanding underwater sound absorption performance.

[Display omitted] • A novel force-chain metastructure is proposed to resist hydrostatic pressure. • The effects of particle size and filling rate on the performance of metastructure are described in detail to guide the design of the metastructure. • It is verified that the metastructure has a wide range of engineering applications by 3D model. With the increase of water depth, the degradation of the performance of acoustic coating in underwater equipment has long plagued researchers. In this paper, a viscoelastic underwater sound-absorbing metastructure that can withstand 3 MPa underwater pressure is formed by embedding particulate matter in viscoelastic materials. Taking the two-dimensional structure as the research object, the factors affecting the force-chain properties are analyzed using simulations. It is important that the effectiveness of the filling of particulate matter in the 3D model is verified, which provides a possibility for the design and manufacture of water pressure insensitive underwater sound-absorbing materials. [ABSTRACT FROM AUTHOR]

Underwater acoustic wave absorption and control play an important role in underwater applications. Various types of underwater acoustic metamaterials have been proposed in recent years with the vigorous development of acoustic metamaterials. Compared with airborne sound, underwater sound waves have a longer wavelength and much smaller propagation loss, making them more difficult to control. In addition, given that the acoustic impedance of water is much greater than that of air, numerous conventional materials and structures are not suited to underwater use. In this paper, we propose a composite structure based on an excellent broadband low-frequency sound absorber of air using aluminum mixed with rubber. Our composite structure possesses broadband low-frequency (

Greater demands for underwater sound absorption materials have been growing due to the concern about underwater noise control in water. Among the range of existing materials, polymer-based materials are increasingly being utilized as underwater sound absorption materials. In this paper, different kinds of polymer-based materials for underwater sound absorption with regards to key factors associated with sound absorption properties, measurements, applications, and mechanisms are reviewed and summarized. Commonly used polymers for underwater sound comprise, in general, interpenetrating polymer networks (IPN), polymer foams, and gradient polymers. To further improve underwater sound absorption performance, different types of inclusions that are introduced into the polymer matrix to transform the polymers as underwater sound absorption materials via air voids, solid inclusions, nanofillers, and phononic crystals are discussed. Challenges for further development of better polymer-based acoustic materials to meet requirements of current and future underwater applications are also presented.

• 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]

Abstract Although the conventional locally resonant acoustic metamaterials (LRAMs) have shown better sound absorption performance in the low frequency band, the effective frequency range of the sound absorption performance is quite narrow relatively. To overcome this drawback, a novel type of multilayered locally resonant acoustic metamaterials (M-LRAMs) is proposed in this paper. The sound absorption performance of the proposed LRAMs can be broadened and enhanced by coupled resonance with embedding multilayered locally resonant scatterers into the matrix structure. The finite element method (FEM) is employed to investigate the band gap characteristics and the sound absorption performance of the proposed M-LRAMs. The coupled resonance effects of the scatterers on the band gap characteristics and the sound absorption performance of acoustic metamaterials are studied as well. Moreover, the effects of the physical parameters of matrix, the thickness of each layer, the number of locally resonant scatterers and the materials of the scatterers on sound absorption performance are investigated. Results indicate that coupled resonance can broaden band gaps and engender multiple band gaps, and the M-LRAMs have many advantages in underwater sound absorption over the conventional LRAMs due to the enhanced coupled resonance effects. [ABSTRACT FROM AUTHOR]

Abstract The finite element method (FEM) is commonly used to analyze the underwater acoustic performances of rubbery coatings due to its flexibility to model scatterers of complex geometric dimensions. However, its calculation efficiency is always denounced as a drawback for its application on optimization design where a large number of repeated calculations should be fulfilled. In this paper, the accuracy of an axisymmetric simplified model, to model the unit cell as circular cross section, is systematically discussed by respectively comparing the absorption coefficients with those calculated by the square and hexagonal models under different punching rates of the cavities. Results show that the axisymmetric model can better describe the sound absorption of the coating with a hexagonal arrangement; however, it may cause comparative large deviations to describe the results of the coating with a square arrangement under not only large punching rates but also small punching rates. This phenomenon can be related to the different boundary differences between two arrangements of cavities and can be relieved when the rubber material possesses a large loss factor. Then, a shape optimization of the embedded cavity using the axisymmetric simplified model and a genetic algorithm has been done to achieve optimal sound absorption in 1–20 kHz. Compared with the coating with isochoric cylindrical cavities, the optimized coating can achieve better sound absorption in low frequencies due to the larger pore-aperture radius of the optimized cavity than that of the isochoric cylindrical cavity. The frequency-dependent parameters of the rubber material play an important role for the broadband high absorption of the optimized coating. [ABSTRACT FROM AUTHOR]

The underwater sound absorption material for submarines should fulfill the demand for remarkable performance in medium-low frequency sound absorption at low thicknesses. Hence, in this paper, the innovative "filled-microperforated plate-like" (F-MPPL) structure was introduced to the hollow microballoons filled underwater sound absorption composite (HBF composite) using the tailor-made 3D spacer fabric. The test results show remarkable mechanical properties with a compression strength and compression modulus of 2256 KPa and 14.6 MPa for the new type of F-MPPL composite, respectively. In addition, the peak sound absorption coefficient of the F-MPPL composite reaches 0.77 at just 7.5 mm thickness based on the resonant sound absorption characteristic introduced by the F-MPPL structure. Moreover, the study investigated the effects of seawater aging on the F-MPPL composite and found that while there was a decrease in compression strength and average sound absorption coefficient, the degradation of the F-MPPL composite was significantly smaller than the HBF composite due to the inhibitory effect of spacer yarns on pore expansion. Finally, this study proposed the method of using compression strength to predict the long-term life of the F-MPPL composite under seawater aging, and a 60% retention rate after 20 months was displayed. [Display omitted] • The filled-microperforated plate-like (F-MPPL) structure endows material with resonant sound absorption characteristics. • The spacer yarns limits PU foaming and prevents pores expanding during seawater aging. • The F-MPPL composite exhibits lower porosity and water absorption, resulting in a reduced corrosion by seawater. • The compression strength retention rate of the F-MPPL composite remains 60% after 20 months of aging. [ABSTRACT FROM AUTHOR]

Rubbery coatings embedded with air cavities are commonly used on underwater structures to reduce reflection of incoming sound waves. In this paper, the relationships between Poisson's and modulus loss factors of rubbery materials are theoretically derived, the different effects of the tiny Poisson's loss factor on characterizing the loss factors of shear and longitudinal moduli are revealed. Given complex Young's modulus and dynamic Poisson's ratio, it is found that the shear loss factor has almost invisible variation with the Poisson's loss factor and is very close to the loss factor of Young's modulus, while the longitudinal loss factor almost linearly decreases with the increase of Poisson's loss factor. Then, a finite element (FE) model is used to investigate the effect of the tiny Poisson's loss factor, which is generally neglected in some FE models, on the underwater sound absorption of rubbery coatings. Results show that the tiny Poisson's loss factor has a significant effect on the sound absorption of homogeneous coatings within the concerned frequency range, while it has both frequency- and structure-dependent influence on the sound absorption of inhomogeneous coatings with embedded air cavities. Given the material parameters and cavity dimensions, more obvious effect can be observed for the rubbery coating with a larger lattice constant and/or a thicker cover layer. [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]

A rubber layer embedded with air-filled cavities is often used as a coating of underwater structures for acoustic absorption. Previous studies have reported experimental results of the significant effect of hydrostatic pressure on acoustic properties of the acoustic coatings, but theoretical methods and physical mechanisms of the hydrostatic pressure effect have not been studied sufficiently. In this work, a linearization theory is proposed for representation of the acoustic scattering process of the coating with a large static predeformation caused by the hydrostatic pressure. A constitutive model with hyperelasticity and viscoelasticity is employed for characterizing the dynamic behavior of the rubber with a static predeformation. The corresponding numerical method is established by the procedure of finite element analysis and validated by experimental results from acoustic tests with a water-filled pressure tube system. The fields of deformation and incremental constitutive tensor are analyzed in detail for providing a comprehensive understanding of the physical mechanisms of the effects of hydrostatic pressure. As a consequence, the results indicate that not only the deformation of the cavities but also the prestress and predeformation induced change of the value of incremental constitutive tensor affect the acoustic absorption of the coating. • General theory for acoustic behavior of rubber coating under hydrostatic pressure. • FEM-based method for acoustic scattering of coating under hydrostatic pressure. • A constitutive model including hyperelasticity and viscoelasticity for the rubber. • Acoustic tests by a water-filled pressure tube system for validating the method. • The effects of deformation and constitutive tensor on sound absorption. [ABSTRACT FROM AUTHOR]

• The underwater sound absorption coefficient and the sound transmission loss (STL) of elastic MPPs are calculated by considering interaction between water and vibration of the elastic MPPs. • The predictions match well with the FEM results. • The underwater sound absorption coefficient and the STL are basically peaky in frequency domain. • The peak frequencies of the sound absorption and the STL remain nearly unchanged for different values of the cavity depth. In this study, the underwater sound absorption and insulation performance capabilities of elastic micro-perforated plates (MPPs) in circular impedance tubes are investigated. The underwater sound absorption coefficient and the sound transmission loss (STL) of elastic MPPs are calculated by considering interaction between water and vibration of the elastic MPPs. As the frequency increases, higher order terms in addition to the plane wave solution are needed for convergence of the prediction. A comparison of the predictions from this study and the results obtained from the finite element method (FEM) shows good agreement. The fluid–structure interaction significantly affects the vibration of the MPP and the sound absorption characteristics are dominated by the vibration of the MPP, which results in sharp peaks in the sound absorption coefficient. For a low-frequency range, the characteristics of the STL are examined using a plane wave solution. The STL in water shows peaks, while that in air exhibits a wide bandwidth. For both underwater sound absorption and sound insulation, the peak frequencies remain nearly unchanged for different values of the cavity depth, and as the micro-perforation ratio increases, the peak frequencies increase. [ABSTRACT FROM AUTHOR]

Greater demands for underwater sound absorption materials have been growing due to the concern about underwater noise control in water. Among the range of existing materials, polymer-based materials are increasingly being utilized as underwater sound absorption materials. In this paper, different kinds of polymer-based materials for underwater sound absorption with regards to key factors associated with sound absorption properties, measurements, applications, and mechanisms are reviewed and summarized. Commonly used polymers for underwater sound comprise, in general, interpenetrating polymer networks (IPN), polymer foams, and gradient polymers. To further improve underwater sound absorption performance, different types of inclusions that are introduced into the polymer matrix to transform the polymers as underwater sound absorption materials via air voids, solid inclusions, nanofillers, and phononic crystals are discussed. Challenges for further development of better polymer-based acoustic materials to meet requirements of current and future underwater applications are also presented. • Key factors and tests of polymer-based underwater sound absorption materials. • Different polymers with or without inclusions for underwater sound absorption. • Mechanisms behind the underwater sound absorption. • Challenges for further improving polymer-based underwater water sound absorption materials. [ABSTRACT FROM AUTHOR]

• A water-filled impedance tube is design and set up with good accuracy. • High sound absorption coefficient is achieved under normal pressure. • Sound absorption mechanism under both normal and high pressures is analysed. Nanotechnology has been widely used for developing sound absorption materials in air, but studies for underwater applications remain limited. In this study, carboxyl functionalized multi-walled carbon nanotubes (MWCNT-COOH) were added into polydimethylsiloxane (PDMS) with dispersant to enhance the underwater acoustic properties. A water-filled impedance tube was built to determine the underwater sound absorption coefficient in the low frequency range from 1500 Hz to 7000 Hz and under variable hydrostatic pressures. Results demonstrate that pure PDMS is nearly acoustically transparent. With the addition of MWCNT-COOH and under normal pressure (0 MPa), the sound absorption coefficient was marginally improved to 0.3. The coefficient can nonetheless be further increased to above 0.75 with the introduction of both surfactant and MWCNT-COOH. It could be postulated that the trapped micro-voids and uniformly dispersed MWCNT-COOH contributed to the improvement when the pressure is at 0 MPa. Effects of MWCNT-COOH concentrations in a range of 0.1 wt% to 2 wt% were investigated. It was ascertained that the concentrations have only a marginal effect on the sound absorption performance. Tests were also conducted at variable hydrostatic pressure above 0 MPa. Results revealed that underwater sound absorption performance decreased with an increase in the hydrostatic pressure in a range of 0.5 MPa to 1.5 MPa. The drop in performance due the increase of hydrostatic pressure supported the claim that the presence of micro-voids within the material matrix structure contributed to the sound absorption. [ABSTRACT FROM AUTHOR]