Underwater thin-layer low-bandwidth sound-absorbing superstructure

Abstract

A thin-layer, low-bandwidth sound-absorbing superstructure is disclosed. The superstructure includes multiple sound-absorbing units arranged in space to form a folded water cavity array with a length gradient. Each sound-absorbing unit includes a perforated cover plate, a sealed bottom plate, and a wall plate disposed between the perforated cover plate and the sealed bottom plate. The wall plate, the perforated cover plate, and the sealed bottom plate together form a water cavity. The inner surface of the side wall and the inner surface of the bottom of the water cavity are covered with a low-modulus flexible material with a modulus range of 0.2-30 MPa. The perforated cover plate has through holes for guiding sound waves into the water cavity. The overall thickness of the sound-absorbing superstructure does not exceed 50 mm.

Description

Technical Field

[0001] This invention relates to the field of underwater sound absorption technology, and in particular to an underwater thin-layer low-bandwidth sound-absorbing superstructure. Background Technology

[0002] Underwater acoustic materials have shown great potential for engineering applications in underwater communication, sonar detection, marine biological research, and marine engineering. In recent years, underwater sound-absorbing materials have attracted widespread attention. However, traditional single homogeneous sound-absorbing materials have poor absorption rates for low-frequency sound waves, requiring a thickness exceeding one-quarter of the wavelength corresponding to the lowest operating frequency to achieve low-frequency sound absorption. This necessitates very thick sound-absorbing materials. However, under strict size and space constraints, it is impossible to configure heavy sound-absorbing structures. Therefore, overcoming the contradiction between low-frequency sound absorption and the thinning of sound-absorbing structures is crucial.

[0003] Acoustic metastructures are artificial composite structures possessing extraordinary acoustic properties not found in many conventional natural materials. They break with traditional theories of sound-absorbing materials, enabling the control of large-wavelength sound waves at a small scale. From the perspective of sound absorption mechanisms, underwater sound-absorbing metastructures can currently be mainly divided into two categories: locally resonant and non-resonant. Locally resonant metastructures utilize the principle of local resonance to achieve low-frequency resonant sound absorption. Based on different structural design schemes, they can be further divided into three types. The first type often embeds locally resonant scatterers such as metal oscillators, elastic thin plates, and cavity composite metal oscillators within a viscoelastic matrix, utilizing the resonance and waveform conversion of the local oscillators, as well as the viscoelastic damping of the matrix material, to dissipate sound energy. The second type is acoustic metastructures based on bubble resonance mechanisms, constructed from bubbles immersed in a soft elastic matrix, utilizing the resonant characteristics of the bubbles to effectively control sound waves. The third type is a composite sound-absorbing structure based on a resonant cavity mixed with viscoelastic materials, utilizing the resonance of the cavity composite viscoelastic material or achieving impedance matching to achieve effective sound absorption. Nonlocal resonant sound-absorbing structures include exponentially gradient metamaterial structures, acoustic metasurfaces, and other novel nonresonant underwater acoustic metastructures. Currently reported local resonant sound-absorbing metastructures exhibit some sound absorption performance in the low-frequency range, but due to the longer duration of low-frequency sound waves in water and the lower viscous loss, the thickness of underwater sound-absorbing structures remains relatively large. Nonresonant structures, on the other hand, demonstrate excellent sound absorption performance in the mid-to-high frequency range, but their low-frequency sound absorption performance still needs improvement. Among the reported metastructures, cavity-type structures have relatively small thicknesses, promising to absorb low-frequency noise within a limited space. However, compared to the water medium, the coupling between sound waves and solid structures is limited except for the resonant frequency, resulting in lower losses in cavity-type structures. Even if the operating frequency band can be reduced to the required range, the sound absorption amplitude cannot meet the expected requirements.

[0004] The information disclosed in the background section is only intended to enhance the understanding of the background of the present invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] To address the shortcomings or defects of the existing technology, an underwater thin-layer low-bandwidth sound-absorbing superstructure is provided, which is thinner and has a wider bandwidth.

[0006] The objective of this invention is achieved through the following technical solutions.

[0007] An underwater thin-layer low-bandwidth sound-absorbing superstructure includes multiple sound-absorbing units arranged in space to form a folded water cavity array with a length gradient. Each sound-absorbing unit includes a perforated cover plate, a sealed bottom plate, and a wall plate disposed between the perforated cover plate and the sealed bottom plate. The wall plate, the perforated cover plate, and the sealed bottom plate together form a water cavity. The inner surface of the side wall and the inner surface of the bottom of the water cavity are covered with a low-modulus flexible material with a modulus range of 0.2-30 MPa. The perforated cover plate has through holes for guiding sound waves into the water cavity. The overall thickness of the sound-absorbing superstructure does not exceed 50 mm.

[0008] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the diameter of the through-hole is 3–12 mm.

[0009] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, each sound-absorbing unit has a through hole at each end of its perforated cover plate along the length of the water cavity.

[0010] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the thickness of the low-modulus flexible material does not exceed 5 mm.

[0011] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the sound wave propagation velocity within the low-modulus flexible material is... ,in, For complex bulk modulus, , and These represent Young's modulus, Poisson's ratio, and loss factor of the flexible material, respectively. This refers to the density of flexible materials.

[0012] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the real part of the sound wave propagation velocity within the low-modulus flexible material is 150–200 m / s.

[0013] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the folded water cavity array is spatially arranged through fan-shaped, spiral, or stacked and rolled-up methods.

[0014] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the average sound absorption coefficient of the sound-absorbing superstructure in the 1–4000 Hz frequency band is not less than 0.76, and the average sound absorption coefficient in the 1–1000 Hz low-frequency band is not less than 0.6.

[0015] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, a low-modulus flexible material is prepared by casting, and the low-modulus flexible material is bonded to the inner surface of the sidewall and the inner surface of the bottom of the water cavity using an adhesive.

[0016] In the aforementioned underwater thin-layer low-bandwidth sound-absorbing superstructure, the sound-absorbing superstructure is a symmetrical periodic structure.

[0017] Compared with existing technologies, the beneficial effects of this invention are as follows: The sound-absorbing structure of this invention has a thickness of only 46mm and features deep subwavelength characteristics, achieving an average sound absorption coefficient of 0.6 in the 1-1000Hz frequency band and 0.76 in the 1-4000Hz frequency band; the sound absorption performance can be adjusted by changing the parameters of the low-modulus flexible material and the aluminum alloy structure parameters; this sound-absorbing structure is easy to mass-produce, meeting the requirements of compact structure and lightweight design; it solves the contradiction between low-frequency sound absorption and structural thinness by using a spatial folding design of the cavity, transferring the thickness required for low-frequency sound absorption to the length direction, demonstrating a lightweight sound-absorbing structure design scheme. By adding a low-modulus flexible material to the inner wall of the cavity structure, the sound velocity is significantly reduced while the sound energy is positioned at the interface of two media with significantly different impedances. Since this sound absorption enhancement method does not require increasing the overall external size of the original sound absorber or changing the structural parameters, and can achieve effective enhancement over a wide frequency range, this method is applicable to various cavity structures, and the fabrication process is simple, giving it advantages in space-constrained applications.

[0018] The description provided is merely an overview of the technical solution of this invention. In order to make the technical means of this invention clearer and more understandable, so that those skilled in the art can implement it according to the contents of the specification, and to make the described and other objects, features and advantages of this invention more obvious and understandable, specific embodiments of this invention are described below. Attached Figure Description

[0019] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0020] In the attached diagram:

[0021] Figure 1 Schematic diagram of the sound-absorbing unit design;

[0022] Figure 2 This is a schematic diagram of the structural parameters of the sound-absorbing unit;

[0023] Figure 3 This is a schematic diagram of a broadband sound-absorbing structure;

[0024] Figure 4 A schematic diagram comparing the theoretical and simulated sound absorption coefficients of broadband sound-absorbing structures;

[0025] Figure 5 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures with different perforation diameters;

[0026] Figure 6 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures with different water cavity thicknesses;

[0027] Figure 7 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures with different thicknesses of low-modulus flexible material applied to the sidewalls.

[0028] Figure 8 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures with different thicknesses of low-modulus flexible material laid on the bottom.

[0029] Figure 9 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures made of flexible materials with different Young's moduli.

[0030] Figure 10 A schematic diagram comparing the sound absorption coefficients of sound-absorbing structures with flexible materials of different damping factors.

[0031] Figure 11 Image of sample preparation for sound-absorbing superstructure;

[0032] Figure 12 A schematic diagram showing the comparison between simulation and experimental results of the sound-absorbing structure;

[0033] Figure 13 This is a schematic diagram comparing the sound absorption coefficients of a certain unit under different flexible material parameters.

[0034] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0035] Specific embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0036] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0037] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments, and the accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0038] To better understand, such as Figures 1 to 13 As shown, an underwater thin-layer low-bandwidth sound-absorbing superstructure includes multiple sound-absorbing units arranged in space to form a folded water cavity array with a length gradient. Each sound-absorbing unit includes a perforated cover plate 1, a sealed bottom plate 4, and a wall plate 3 disposed between the perforated cover plate 1 and the sealed bottom plate 4. The wall plate 3, the perforated cover plate 1, and the sealed bottom plate 4 together form a water cavity. The inner surface of the side wall and the inner surface of the bottom of the water cavity are covered with a low-modulus flexible material 2 with a modulus range of 0.2-30 MPa. The perforated cover plate 1 has through holes for guiding sound waves into the water cavity. The overall thickness of the sound-absorbing superstructure does not exceed 50 mm.

[0039] Table 1. Parameters of Flexible Materials

[0040]

[0041] Figure 13 The graph compares the sound absorption coefficients of a unit under different flexible material parameters. The results show that the first-order sound absorption peak is the same for all four sets of material parameters, revealing that the selection of flexible materials is not singular.

[0042] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the diameter of the through-hole is 3–12 mm.

[0043] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, each sound-absorbing unit has a through hole at each end of the perforated cover plate 1 along the length of the water cavity.

[0044] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the thickness of the low-modulus flexible material 2 does not exceed 5 mm.

[0045] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the sound wave propagation velocity within the low-modulus flexible material 2 is... ,in, For complex bulk modulus, , and These represent Young's modulus, Poisson's ratio, and loss factor of the flexible material, respectively. This refers to the density of flexible materials.

[0046] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the real part of the sound wave propagation velocity within the low-modulus flexible material 2 is 150–200 m / s.

[0047] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the folded water cavity array is spatially arranged in a fan-shaped, spiral, or stacked manner.

[0048] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the average sound absorption coefficient of the sound-absorbing superstructure is not less than 0.76 in the 1–4000 Hz frequency band, and the average sound absorption coefficient is not less than 0.6 in the 1–1000 Hz low-frequency band.

[0049] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, a low-modulus flexible material 2 is prepared by casting, and the low-modulus flexible material 2 is connected to the inner surface of the side wall and the inner surface of the bottom of the water cavity by an adhesive.

[0050] In a preferred embodiment of the underwater thin-layer low-bandwidth sound-absorbing superstructure, the sound-absorbing superstructure is a symmetrical periodic structure.

[0051] In one embodiment, an underwater thin-layer low-bandwidth sound-absorbing superstructure utilizes a spatial folding design to transfer the large thickness dimension required for low-frequency absorption to an extremely thin planar layer. Furthermore, by arranging a low-modulus flexible material 2 on the rigid walls inside the superstructure's cavity, replacing the aluminum alloy walls of the original sound-absorbing structure with a flexible material interface, the low-frequency sound absorption performance of the structure is significantly improved. 1) Determine the size of the sound-absorbing units based on the spatial dimensions, and arrange the sound-absorbing units in the given installation space through spatial folding; 2) Select the proportion and thickness of the low-modulus flexible material 2 according to the designed sound absorption frequency band, and prepare the flexible material; 3) Apply the prepared flexible material to the cavity walls and bottom to construct the underwater thin-layer sound-absorbing structure.

[0052] like Figure 1 As shown, considering that most underwater acoustic testing equipment is circular, the sound-absorbing unit is designed as a fan-shaped structure to facilitate subsequent sample testing. To clearly demonstrate the sound-absorbing unit, Figure 2 The water cavity and part of the low-modulus flexible material are omitted on the right side of the middle structure. The width of the water cavity... It is 24 mm thick. The diameter of the small hole is 40 mm. The thickness of the perforated cover plate 1 is 6 mm. 1 mm thick, sealed base plate 4 mm thick The wall panel 3 of the spacer water chamber unit is 2 mm thick. The distance is 3 mm. This refers to the distance from the center O of the sector unit to the outermost wall of the sound-absorbing unit. The central angle corresponding to the unit is 100 mm. The central angle corresponding to the distance from the small hole to the front end of the unit solid is 100°. The angle is 7°, and the thickness of the flexible material on the sidewalls and bottom is [missing information]. All are 3 mm. To achieve broadband sound absorption, such as... Figure 3 As shown, using the concept of multi-unit parallel coupling, a group of cavities with length gradients are spatially folded and arranged. The lengths of the cavities, from smallest to largest, are 88 mm, 105 mm, 119 mm, 131 mm, 132 mm, 142 mm, 163 mm, 183 mm, and 203 mm.

[0053] In the finite element software COMSOL Multiphysics 5.6, a structural finite element simulation model was established using the acoustic-structure coupling module, with the water area designated as the pressure acoustics module. The metal frame and flexible material were designated as solid mechanics modules, and the boundary between the water and solid domains was designated as the acoustic-solid interaction boundary. The metal frame was made of aluminum alloy, with a mass density, Young's modulus, and Poisson's ratio of [missing values]. = , = Pa and =0.28; the mass density, Young's modulus, and Poisson's ratio of the flexible material are respectively... = , = Pa and =0.49, damping factor is 0.5. In the calculation model, one end of the aperture is connected to the waveguide, and a plane wave radiation boundary is set at the front end of the waveguide. The incident sound pressure amplitude is a plane wave of 1 Pa. The other walls are set as hard boundaries of the sound field. The acoustic dissipation characteristics of the aperture and the internal region of the resonant cavity are approximated by using the acoustics of the narrow region. Since the bottom of the structure is fixed in the underwater acoustic pipe in the actual test, the bottom of the metal frame is set as a fixed constraint in the simulation calculation.

[0054] Figure 4 The simulated and theoretical sound absorption coefficient curves for this structure in the 10-2000 Hz frequency range are shown. The structure exhibits a sound absorption coefficient exceeding 0.5 in the 130-1610 Hz band, exceeding 0.8 in the 230-410 Hz band, and an average sound absorption coefficient of 0.59 in the 10-2000 Hz band. This demonstrates the structure's excellent low-frequency broadband sound absorption performance. To investigate the influence of different structural parameters on sound absorption performance, the pore size, water cavity thickness, and flexible material parameters were compared, and the resulting curves are shown below. Figures 5 to 10 As shown, it was found that the parameters of flexible materials have a greater impact on sound absorption performance than traditional structural parameters such as pore size, with the characteristics of flexible materials having the most significant effect. The Young's modulus of flexible materials mainly determines the sound absorption frequency band, while the damping factor mainly determines the sound absorption amplitude.

[0055] To verify the sound absorption performance of the structure, an underwater broadband sound-absorbing superstructure sample was prepared. An aluminum alloy frame, base plate, and perforated cover plate 1 were fabricated using machining. A low-modulus flexible material 2 was prepared using a casting method. The material selected was Sorta-Clear18 from the Smooth-on series, which consists of two components, A and B, with a curing ratio of 10:1. The prepared material was fixed to the side walls and base plate of the aluminum alloy frame structure to assemble the sound-absorbing structure, as shown below. Figure 11 As shown. In order to enhance the coupling characteristics between the units, each sound-absorbing unit adopts a double-hole design. Figure 12The sound absorption coefficient of the structure is 0.76 in the 1-4000Hz range, with a maximum amplitude of 0.89. In the low-frequency range of 1-1000Hz, the average sound absorption coefficient reaches 0.6. The half-absorption band of the structure is 312-4000Hz, exceeding three octaves. It is worth noting that the boundary conditions were changed from fixed constraints to steel-air backing conditions, with a steel plate thickness of 10mm. The figure also shows that the finite element simulation calculations and experimental results have good consistency. However, during the fabrication of the flexible material, errors such as material inhomogeneity and loose adhesion to the wall surface inevitably occur, which differ from the boundary conditions in the simulation, resulting in slight differences in the frequency of the absorption peaks, although the overall trend of the curves is the same. Secondly, although a vacuum degassing machine was used beforehand, some air bubbles were still generated inside the flexible material during the forming process. Furthermore, in the actual fabrication process, the material is connected to the solid wall surface by adhesives, both of which increase the damping of the structure to some extent, leading to the experimental results being better than the simulation results.

[0056] The sound-absorbing superstructure exhibits a sound absorption coefficient exceeding 0.5 in the 312-4000Hz frequency band. Through the multi-unit parallel coupling design of the superstructure units, the operating frequency band of the superstructure is further broadened. Based on the aforementioned characteristics of the underwater thin-layer low-bandwidth sound-absorbing superstructure provided by this invention, a broadband sound absorption effect can be achieved in the mid-to-low frequency band with a relatively thin-layer structure. This type of structure can be compressed into a compact space through folding, rolling, etc., firstly reducing the dimension in the thickness direction of the sound-absorbing structure. Furthermore, by introducing a low-modulus flexible material 2, the low-frequency sound absorption performance of the structure is improved. In summary, compared with existing sound-absorbing structures, this underwater sound-absorbing structure design method is thinner, has a wider bandwidth, and exhibits superior low-frequency sound absorption performance, and is easy to mass-produce, providing a new technical approach for underwater sound-absorbing structure design.

[0057] Furthermore, this invention "folds" the effective acoustic length to a planar dimension by using a length gradient design and a compact fan-shaped / curved arrangement of the traditionally long water cavity that needs to extend along the thickness direction (corresponding to the order of low-frequency wavelengths). This design compresses the overall thickness of the structure to ≤50 mm (depth subwavelength, such as 46 mm), which is far less than 1 / 30 of the wavelength of 1 kHz sound wave in water (about 1.5 m), breaking through the traditional limitation of "1 / 4 wavelength thickness". At the same time, the gradient water cavity array forms multi-frequency resonant units (such as a length sequence of 88–203 mm), and achieves continuous coverage of the resonance peak through inter-cavity acoustic coupling, extending the narrowband response of a single cavity to an ultrawideband sound absorption of 1–4000 Hz. The core function of applying a low-modulus flexible material (Young's modulus 0.2–30 MPa, thickness ≤5mm) to the rigid walls of the water cavity is as follows: sound velocity regulation and interface localization: the real part of the sound velocity within the material is precisely controlled at 150–200 m / s (far lower than 1500 m / s in water), forming a significant acoustic impedance mismatch interface, forcing the incident sound wave energy to be strongly localized on the surface of the flexible material, extending the interaction path between the sound wave and the material; the low sound velocity induces the conversion of shear wave and bending wave modes at the acoustic-solid coupling interface, combined with the material's high loss factor (e.g., η_r=0.5), efficiently converting acoustic vibration energy into heat energy dissipation; Young's modulus dominates the resonant frequency shift (regulating the sound absorption frequency band), and the damping factor directly increases the sound absorption amplitude (e.g., the average coefficient in the low-frequency band reaches 0.6 in the experiment); after replacing the aluminum alloy rigid wall, the high reflection problem at the water-solid interface is completely eliminated, solving the industry pain point of "weak coupling and low loss" in cavity-type structures. The perforated cover plate employs a 3–12 mm aperture and a dual-hole layout at both ends of the unit. This reduces the acoustic impedance entering the cavity (avoiding the blockage effect of small holes) and promotes lateral flow of sound within the gradient water cavity, enhancing the acoustic field coupling of multiple units, further smoothing the sound absorption curve, and suppressing frequency band gaps. These technical methods form an innovative closed loop integrating "geometric topology, material properties, and acoustic field control": spatial folding solves the physical constraint of "thinness," flexible interfaces address the efficiency bottleneck of "absorption," and gradient coupling solves the requirement of "wide" frequency band. Ultimately, a breakthrough performance of an average sound absorption coefficient of 0.6 in the 1–1000 Hz low-frequency range and 0.76 in the 1–4000 Hz full-frequency range is achieved within a 46 mm ultra-thin structure, with adjustable parameters and a simple manufacturing process.

[0058] The basic principles of this application have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in this application are merely examples and not limitations, and should not be considered as essential features of each embodiment of this application. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the application to the necessity of employing the aforementioned specific details for implementation.

[0059] The above description has been given for purposes of illustration and description. Furthermore, this description is not intended to limit the embodiments of this application to the forms disclosed herein. Although numerous exemplary aspects and embodiments have been discussed above, those skilled in the art will recognize certain variations, modifications, alterations, additions, and sub-combinations thereof.

Claims

1. An underwater thin-layer low-bandwidth sound-absorbing superstructure, characterized in that, It includes multiple sound-absorbing units arranged in space to form a folded water cavity array with a length gradient; each sound-absorbing unit includes a perforated cover plate, a sealed bottom plate, and a wall plate disposed between the perforated cover plate and the sealed bottom plate. The wall plate, the perforated cover plate, and the sealed bottom plate together form a water cavity. The inner surface of the side wall and the inner surface of the bottom of the water cavity are covered with a low-modulus flexible material with a modulus range of 0.2-30 MPa. The perforated cover plate has through holes for guiding sound waves into the water cavity. The overall thickness of the sound-absorbing superstructure does not exceed 50 mm.

2. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, Preferably, the diameter of the through hole is 3–12 mm.

3. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, Each sound-absorbing unit has a through hole at each end of its perforated cover plate along the length of the water cavity.

4. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, The thickness of the low-modulus flexible material does not exceed 5 mm.

5. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, Sound wave propagation velocity in low modulus flexible materials ,in, For complex bulk modulus, , and These represent Young's modulus, Poisson's ratio, and loss factor of the flexible material, respectively. This refers to the density of flexible materials.

6. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, The real part of the acoustic wave propagation velocity within the low-modulus flexible material is 150–200 m / s.

7. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, The folded water cavity array is spatially arranged in a fan-shaped, spiral, or stacked manner.

8. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, The sound-absorbing superstructure has an average sound absorption coefficient of not less than 0.76 in the 1–4000 Hz frequency band and an average sound absorption coefficient of not less than 0.6 in the 1–1000 Hz low frequency band.

9. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, A low-modulus flexible material was prepared by casting, and the low-modulus flexible material was bonded to the inner surface of the sidewall and the inner surface of the bottom of the water cavity using an adhesive.

10. The underwater thin-layer low-bandwidth sound-absorbing superstructure as described in claim 1, characterized in that, The sound-absorbing superstructure is a symmetrical periodic structure.

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