Stepped symmetrical underwater broadband sound absorption metamaterial and preparation method thereof

Abstract

The invention relates to the technical field of underwater noise control, and discloses a stepped symmetric underwater broadband sound absorption metamaterial and a preparation method thereof. The underwater sound absorber is formed by horizontally stacking M sound absorption units, each sound absorption unit is composed of a layer of stainless steel acoustic hard boundary and a layer of Fabry-Perot (FP) resonator composed of a composite material; the composite material is a high-energy-density composite material formed by mixing tungsten powder, polyurethane resin and polyurethane rubber in proportion. The arrangement mode of the FP resonators is perpendicular to the incident direction of incident sound waves, so that low-frequency sound waves are absorbed to the maximum extent. Adjustment of different sound absorption frequencies can be achieved by adjusting the number and the unit size of the sound absorption units. A certain stepped underwater sound absorber designed according to the invention is subjected to theoretical analysis and numerical simulation, so that the quasi-perfect sound absorption effect that the sound absorption coefficient reaches 0.9 from 1kH to 20kHz is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater noise control, and particularly to a stepped symmetric ultra-wideband underwater acoustic absorption metamaterial and a preparation method thereof. Background Art

[0002] Underwater acoustics is a discipline that studies the excitation, transmission, and detection of underwater sound waves, and underwater sound absorption is an important research field among them. In air, the absorption of sound waves usually relies on the resonance of air columns or the resulting thermo-viscous losses. However, due to the specific acoustic impedance and sound velocity of water, the designs for sound wave absorption in air cannot be used underwater. To solve this problem, some underwater sound absorption structures have emerged. Traditional sound absorption structures (such as Helmholtz resonators and cavity resonance structures) rely on a single resonance frequency to achieve sound absorption and cannot effectively increase the underwater sound absorption bandwidth. Due to the high sound velocity in water (about 1500 m / s), traditional sound absorption structures need to match the wavelength size to work effectively, resulting in difficulty in reducing the size of underwater low-frequency sound absorbers. Summary of the Invention

[0003] The object of the present invention is to address the problems in the background art and propose a stepped symmetric ultra-wideband underwater acoustic absorption metamaterial based on a composite material that has a smaller structural thickness in a higher frequency band (1 kHz to 20 kHz) and achieves efficient sound absorption in a relatively wide frequency band.

[0004] The technical solution of the present invention is that in a first aspect of the present invention, a stepped symmetric ultra-wideband underwater acoustic absorption metamaterial is provided, which includes a plurality of sound absorption units. The sound absorption units with different sizes are stacked in the order of length along the X-axis direction to form an underwater acoustic absorption metamaterial; the stacked underwater acoustic absorption metamaterial is in a stepped symmetric shape.

[0005] Each sound absorption unit is composed of a combination of FP resonators formed by one layer of acoustic hard boundary and one layer of composite material; the arrangement of the FP resonators forms a certain angle or even perpendicular to the incident direction of the incident sound wave, which is used to absorb low-frequency sound waves; such a structural design can significantly reduce the thickness of the sound absorption structure; and in the present invention, it is composed of M sound absorption units, each sound absorption unit is composed of a composite material FP resonator (with a thickness smaller than the sound wave wavelength) and a hard sound field structure with the same longitudinal dimension. The sound absorption units are arranged along the X-axis direction, and the arrangement direction is parallel to the incident direction of the sound wave.

[0006] By adjusting the number and size of the sound absorption units, the sound absorption frequency is adjusted.

[0007] A wide-band underwater acoustic absorption metamaterial is formed after combining multiple sound absorption units.

[0008] Furthermore, each sound absorption unit is composed of an FP resonator formed by one layer of acoustic hard boundary relative to water and a composite material with an acoustic impedance similar to that of water.

[0009] Further, the composite material is a high energy density composite material composed of tungsten powder, polyurethane resin and polyurethane rubber in a ratio of 6.287:0.638:0.362.

[0010] Further, the sizes of each sound absorption unit along the X-axis are the same or different; the sizes of each sound absorption unit along the Y-axis are all the same; the sizes of each sound absorption unit along the Z-axis are all different.

[0011] Further, in each sound absorption unit, the size along the Z-axis determines the working frequency of a single unit.

[0012] Further, the hard sound field structure of each unit is connected by two rigid stepped symmetric plates (for overall fixation) to prevent resonance. The connection with the rigid stepped symmetric plates needs to be firm, especially the connection between the FP resonator and the rigid stepped symmetric plates. This unique arrangement enables the hard sound field structure to be regarded as an acoustic hard boundary within the target frequency range, and almost no resonance occurs.

[0013] Further, in each unit, the composite material, the hard sound field structure, adjacent units, the FP resonator and the rigid stepped symmetric plates are in natural contact, and there must be a certain air gap. To prevent water from entering the air gap and prevent coupling between adjacent solids, thereby ensuring the independent resonance of the FP resonator, a waterproof soft glue is used to seal the air layer in the sound absorber, and the thickness of the air layer is about 50 microns.

[0014] The second aspect of the present invention provides a preparation method of a stepped symmetric underwater ultra-wideband sound absorption metamaterial for manufacturing the above-mentioned stepped ultra-wideband underwater sound absorption metamaterial, including the following specific steps:

[0015] S1. Determine the working frequency range of the underwater noise to be absorbed;

[0016] S2. A single sound absorption unit is composed of a composite material FP resonator sound absorption structure and an acoustic hard boundary structure, and its size along the Z-axis is denoted as 2l1, 2l2,..., 2l M , the lateral size of a single FP resonator is denoted as t c , the lateral size of a single hard sound field structure is denoted as t r ;

[0017] S3. Obtain the working frequency of the FP resonator through the 1 / 4 wavelength FP resonance effect, calculate the working frequency of the above-mentioned FP resonator according to the required sound absorption spectrum, and determine the Z-axis size, lateral size of the FP resonator and the lateral size of the acoustic hard boundary;

[0018] S4. Arrange the sound absorption units with determined sizes in the order of length along the X-axis to form a sound absorption array;

[0019] S5. Connect the hard sound field structures of each unit at the top and bottom using two stepped plates to prevent resonance.

[0020] S6. Seal the space between two adjacent solids with waterproof glue to maintain the independent resonance characteristics of the FP resonator.

[0021] Furthermore, by deriving according to the acoustic wave equation, the theoretical formula for the working frequency of a single sound absorption unit is obtained. Since the structure is symmetric about the X-axis in the Z-axis direction, only the upper half on the X-axis is analyzed. Then, the theoretical formula for the working frequency of the upper half of each sound absorption unit is the working frequency f n as a function of the dimensional parameter l n , and the working frequency of FP resonance is:

[0022]

[0023] where v w is the sound speed in water, with a value of 1500 m / s.

[0024] Furthermore, by adjusting the dimensional parameter l n , the working frequency of FP resonance can be changed to achieve the tunability of underwater sound absorption.

[0025] The third aspect of the present invention provides a stepped symmetric type ultra-wideband underwater sound absorber, including the above-mentioned sound absorption units prepared using the above-mentioned method;

[0026] The sound absorption units include 17 different units for underwater sound absorption in the frequency range of 1 kHz to 20 kHz;

[0027] Each sound absorption unit includes an acoustic hard boundary layer and a composite material layer; the acoustic hard boundary layer and the composite material layer are used to form an FP resonator; there is an air layer between the acoustic hard boundary layer and the composite material layer;

[0028] In each sound absorption unit, the air layer is sealed in the sound absorber using waterproof soft glue, and the thickness of the air layer is 50 microns to ensure that each FP resonator maintains independent resonance.

[0029] Compared with the prior art, the present invention has the following beneficial technical effects:

[0030] 1. The structure has a relatively thin dimension in the thickness direction, enabling thin and light underwater sound absorption;

[0031] 2. The constraint optimization of the structure improves its adaptability in different usage scenarios and gradually transforms it towards the actual application field;

[0032] 3. The structure has multiple resonance frequencies, thus achieving broadband underwater sound absorption instead of a single peak;

[0033] 4. The design method provides a calculation formula for the working mode frequency, and the structural parameters can be changed according to the sound absorption spectrum requirements.

[0034] 5. The present invention can achieve broadband sound absorption in any frequency band through size adjustment. For example, using 17 FP resonators, perfect sound absorption from 1 kHz to 20 kHz can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a schematic diagram of the overall structure of the present invention;

[0036] Figure 2 is a cross-sectional view and a schematic diagram of the sound absorption effect of the present invention;

[0037] Figure 3 is a theoretical sound absorption curve diagram of the present invention;

[0038] Figure 4 is a simulated sound absorption curve diagram with 304 stainless steel regarded as a hard boundary;

[0039] Figure 5 is a simulated sound absorption curve considering the acoustic parameters of 304 stainless steel. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] Embodiment 1

[0041] This stepped symmetric absorber can theoretically achieve perfect sound absorption in any broadband, as Figure 1 shown. The specific implementation will be described by taking the underwater perfect sound absorption from 1 kHz to 20 kHz as an example.

[0042] As Figure 1 shown, since each x-z cross-section of this three-dimensional stepped symmetric absorber is the same, and the structure is symmetric about the X-axis on the Z-axis, only half of the upper side of the X-axis of the simplified 2D structure is analyzed, as Figure 2 shown. In the figure, the dark part is the 304 stainless steel acoustic hard boundary, and the light part is the FP resonator composed of composite materials. In this invention, the width t r of the 304 stainless steel acoustic hard boundary is 10 mm, and the thickness of each FP resonator is t c = 5 mm. As Figure 1 shown, there is an air layer with a thickness of 50 microns sealed by waterproof soft glue between every two adjacent solids. Assuming that every two composites and 304 stainless steels with the same height form a unit, the thickness of a unit is t = t r + t c . According to the working frequency f n with respect to the size parameter ln The functional relationship of (n = 1, 2…, M):

[0043]

[0044] To achieve underwater ultra-wideband sound absorption from 1 kHz to 20 kHz, a total of 17 different units are selected, and their heights decrease from largest to smallest as l1 to l 17 They are 69 mm, 59 mm, 5 mm, 42 mm, 35 mm, 29 mm, 24 mm, 19 mm, 14.5 mm, 12.3 mm, 10 mm, 8 mm, 6 mm, 5 mm, 4 mm, 3 mm, 2 mm in sequence.

[0045] The composite material is formed by mixing PU resin, PU rubber and tungsten powder in a ratio of 6.287:0.638:0.362. By appropriately adjusting the ratio of each part of these three raw materials, a composite material with a density 5.5 times that of water density (i.e., ρ c = k·ρ w , where ρ c is the density of the composite material, ρ w = 1 g / cm 3 is the density of water, and k = 5.5), and the sound speed is 1 / 5 times that of the sound speed in water (i.e., where v c is the sound speed in the composite material, v w = 1500 m / s is the sound speed in water) can be obtained.

[0046] One of the purposes of using the composite material is to obtain a higher energy density ε. Assuming that the energy density of the target composite material is k times that of the water energy density, on the premise of keeping the acoustic impedance of the composite material and the underwater acoustic impedance matched, the density and equivalent modulus of the composite material should satisfy the following relationship:

[0047]

[0048] where the bulk modulus of water M w = 2.16×10 9 Pa. Under the above constraints, the acoustic impedance of the composite material is matched with the acoustic impedance of water.

[0049] Through calculation, it can be obtained that this composite material is theoretically impedance-matched with water but has a higher energy density, so the size of the sound absorber can be greatly reduced.

[0050] As Figure 2 shown, the incident plane wave can be written as When the sound wave hits the acoustically hard boundary, a secondary source is formed, which radiates the sound wave energy into the composite FP resonator. In the very thin water layer above the composite material, where the sound wave has not yet reached the composite material and the lateral dimension t of the composite material c is at the sub-wavelength scale, the incident wave equation transmitted to the composite material can be reasonably written as Regarding the sound wave equation as a one-dimensional sound wave, the wave number in the y direction where f is the frequency of the incident wave.

[0051] When the sound wave propagates in the composite material, FP resonance occurs if and only if the unit length l n matches the 1 / 4 wavelength of the sound wave. The analysis is as follows:

[0052] Only considering the longitudinal mode of the composite material, the Green's function of each unit of the composite material, that is, the surface response of the composite material structure, can be calculated as:

[0053]

[0054] where u n is the displacement field on the surface of the FP resonator on the nth composite material, and Z n is the surface impedance of the nth FP resonator. Considering the first 11th order FP resonance of the composite material, G n can be written in the following Lorentz form:

[0055]

[0056] where, is the surface oscillation intensity of the nth composite material FP resonator, is the first-order resonance angular frequency, and β is the dissipation coefficient, which is related to the damping of the composite material and the sound wave frequency.

[0057] According to the above Green's equation, the surface impedance of the nth FP resonator can be calculated

[0058]

[0059] According to the impedance theory, when the imaginary part of the surface impedance is equal to 0, that is, ω = (2m - 1)Ω, the incident sound wave frequency matches the resonance frequency of the nth FP resonator. At this time

[0060]

[0061] the maximum dissipation of the incident sound wave appears, that is, FP resonance occurs.

[0062] If 304 stainless steel is regarded as an acoustically hard boundary, the overall acoustic impedance of the sound absorber of the present invention can be regarded as the parallel connection of 17 FP resonators, which is calculated by the following formula

[0063]

[0064] where γ n is the ratio of the surface volume of the nth FP resonator to the effective volume of the entire sound-absorbing structure. The sound absorption coefficient of this structure is:

[0065]

[0066] where Z w = ρ w ·v w is the acoustic impedance of water. The theoretical calculation results of the present invention are shown by Figure 3 as follows.

[0067] On the other hand, the sound absorption principle of this structure can also be explained by the classical FP resonance theory. When the longitudinal dimension of an FP resonator matches an odd multiple of 1 / 4 acoustic wave wavelength, that is

[0068]

[0069] where λ c is the acoustic wave wavelength in the FP resonator. At this time, standing waves will be formed in the FP resonator, and the nodes of the standing waves are exactly located at the top of the FP resonator, forming a notch effect for the corresponding acoustic waves to achieve the maximum sound absorption effect.

[0070] One of the greatest advantages of the present invention is that the propagation direction of the incident wave is perpendicular to the arrangement direction of the FP resonators. Not only by reasonably utilizing the composite material, the omnidirectional dimensions of various devices used in underwater sound absorption, underwater communication and other fields are reduced, and through structural constraint optimization, the adaptability in different usage scenarios is further improved, which is conducive to the gradual transformation of this ultra-wideband underwater sound absorber to the practical application field.

[0071] The present invention uses COMSOL Multiphysics for 3D finite element modeling and numerical simulation. This modeling and physical field setting are divided into two modules: the pressure acoustics module and the solid mechanics module. Among them, the solid materials of the present invention are all modeled based on the solid mechanics module. Appropriate material parameters are given to the composite material and 304 stainless steel according to the actual situation. The air gap between the composite material and 304 stainless steel is not drawn, and fixed constraints are applied to the bottom and top surfaces of the entire sound absorber, and the damping loss factor is set to 0.26. In pressure acoustics, the domain material is set as water, the background pressure field is used to simulate the sound source, and the reflected sound pressure is measured. For simplicity of consideration, 304 stainless steel is regarded as an acoustic hard boundary.

[0072] By continuously adjusting the length of the FP resonator, 17 different sizes of FP resonators are determined, such as Figure 4As shown, the simulation results can achieve almost perfect sound absorption for sound waves from 1 kHz to 20 kHz.

[0073] However, in the real world, there is still a certain impedance gap between 304 stainless steel and the acoustic hard boundary for water. To make the sound absorption effect of the present invention better reflect the actual situation, the acoustic parameters of 304 stainless steel will be considered in COMSOL Multiphysics for further simulation. The simulation results are as Figure 5 shown.

[0074] From Figure 5 it can be seen that although the sound absorption curve becomes less smooth than before after considering the acoustic parameters of 304 stainless steel, the sound absorption of sound waves still reaches more than 90% in most frequency ranges. It should be noted that Figure 5 the troughs of the sound absorption coefficient that appear are all caused by the resonance of 304 stainless steel. For example, when the frequency is equal to 5 kHz, the rightmost stainless steel ( Figure 2 ) resonates, resulting in a decrease in the sound absorption coefficient. Therefore, if a broadband sound absorption effect is to be achieved, the higher-order FP resonance problem of the composite material needs to be considered.

[0075] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited thereto. Various changes can be made without departing from the spirit of the present invention within the knowledge scope of those skilled in the art.

Claims

1. A stepped symmetrical ultra-wideband underwater sound-absorbing metamaterial, comprising a plurality of sound-absorbing units, characterized in that: The sound absorbing units of different sizes are stacked in length order along the X-axis direction to form an underwater sound absorbing metamaterial; the stacked underwater sound absorbing metamaterial is in a step-symmetric shape; Each sound absorption unit is composed of a layer of acoustic hard boundary and a layer of FP resonator composed of composite materials; the FP resonator is arranged at a certain angle to the incident direction of the incident sound wave to absorb low-frequency sound waves; The sound absorption frequency can be adjusted by adjusting the number and size of the sound absorption units; Multiple sound-absorbing units are combined to form a wide-band underwater sound-absorbing metamaterial.

2. The step-symmetric ultra-wideband underwater sound-absorbing metamaterial according to claim 1, characterized in that: Each sound absorbing unit consists of an acoustic hard boundary relative to water and a FP resonator made of a composite material with a similar acoustic impedance to water.

3. The step-symmetric ultra-wideband underwater sound-absorbing metamaterial according to claim 2, characterized in that: The composite material is a high energy density composite material which is composed of tungsten powder, polyurethane rubber and polyurethane resin mixed in a certain proportion.

4. The step-symmetric ultra-wideband underwater sound absorbing metamaterial according to claim 1 or 3, characterized in that: The size of each sound absorbing unit along the X-axis is the same or different; the size of each sound absorbing unit along the Y-axis remains the same; the size of each sound absorbing unit along the Z-axis is different.

5. The step-symmetric ultra-wideband underwater sound-absorbing metamaterial according to claim 4, characterized in that: In each sound absorbing unit, the dimension of the Z axis determines the operating frequency of the single unit.

6. The step-symmetric ultra-wideband underwater sound-absorbing metamaterial according to claim 1 or 3, characterized in that: The acoustic hard boundaries of each sound-absorbing unit are subject to top and bottom surface constraints to suppress the resonance of the hard sound field structure.

7. A method for preparing a stepped symmetrical underwater ultra-wideband sound absorbing metamaterial, comprising preparing the stepped ultra-wideband underwater sound absorbing metamaterial as claimed in any one of claims 1 to 6, characterized in that: The specific steps include: S1. Determine the operating frequency range of underwater noise that needs to be absorbed; S2. A single sound absorbing unit is composed of a composite material FP resonator sound absorbing structure and an acoustic hard boundary. Its dimensions along the Z axis are 2l1, 2l2, ..., 2l M , the lateral dimension of a single FP resonator is denoted as t c , the lateral size of a single acoustic hard boundary is denoted as t r ; S3, obtaining the operating frequency of the FP resonator through the FP resonance effect of 1 / 4 wavelength, calculating the operating frequency of the FP resonator according to the required sound absorption spectrum, and determining the Z-axis dimension and lateral dimension of the FP resonator and the lateral dimension of the acoustic hard boundary; S4, arranging the sound absorbing units of a determined size in order of length along the X-axis direction to form a sound absorbing array; S5, use two stepped panels to connect the hard sound field structure of each unit at the top and bottom to prevent it from resonating; S6. Use waterproof glue to seal the two adjacent solids to maintain the independent resonance characteristics of the FP resonator.

8. The method for preparing a step-symmetric underwater ultra-wideband sound-absorbing metamaterial according to claim 7, characterized in that: According to the sound wave equation, the theoretical formula of the working frequency of a single sound absorbing unit is derived; since the structure is symmetrical about the X axis on the Z axis, only the upper half of the X axis is analyzed, and the theoretical formula of the working frequency of the upper half of each sound absorbing unit is the working frequency f n About the size parameter l n As a function of, the operating frequency of FP resonance is: Among them, v w is the speed of sound in water, which is 1500m / s.

9. The method for preparing a step-symmetric underwater ultra-wideband sound-absorbing metamaterial according to claim 7, characterized in that: By adjusting the size parameter l n , to change the operating frequency of the FP resonance, and to achieve the tunability of underwater sound absorption.

10. A stepped symmetrical ultra-wideband underwater sound absorber, comprising the sound absorbing unit according to any one of claims 1 to 6, characterized in that: The sound absorbing unit is prepared by the method according to any one of claims 7 to 9; The sound absorption unit includes 17 different units for underwater sound absorption in the frequency range of 1kHz to 20kHz; Each sound absorbing unit includes an acoustically hard boundary layer and a composite material layer; An FP resonator is formed by using an acoustic hard boundary layer and a composite material layer; an air layer is between the acoustic hard boundary layer and the composite material layer; In each sound absorbing unit, a waterproof soft glue is used to seal the air layer in the sound absorber. The thickness of the air layer is 50 microns, so that the FP resonators can maintain independent resonance.

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