Underwater broadband sound absorption metamaterial based on Helmholtz resonant cavity and design method thereof

Abstract

The invention provides an underwater broadband sound absorption metamaterial based on a Helmholtz resonant cavity and a design method thereof. The sound absorption metamaterial comprises a frame and a cover plate. A resonant cavity is arranged in the frame, one end of the frame is open, and the inner wall of the frame is coated with a damping material coating. The cover plate is connected to the open end of the frame to seal the frame, the cover plate is provided with a plurality of parallel necks vertically extending into the resonant cavity from the cover plate, the necks are in a hollow cylindrical shape, and sound waves enter from one end of each neck and enter the resonant cavity from the other end of each neck. The metamaterial is simple in structure; the sound absorption peak value at the frequency point below 1000Hz can reach more than 0.99, and the thickness is only 1 / 125-1 / 45 of the corresponding perfect sound absorption wavelength; in the range of 0-2000Hz, the sound absorption coefficient can reach more than 0.5, the effective sound absorption bandwidth can reach 127.9%, and the thickness of the metamaterial is within 1 / 45 of the wavelength, so that the metamaterial is an underwater low-frequency perfect sound absorption metamaterial with the deep sub-wavelength scale, has excellent low-frequency sound absorption performance, is wide in sound absorption frequency band, and can meet the low-frequency sound absorption requirement by being small in size.

Description

Technical Field

[0001] The present invention belongs to the technical field of radiated noise control of underwater equipment propulsion systems, and specifically relates to an underwater broadband sound-absorbing metamaterial based on a Helmholtz resonance cavity and a design method thereof, and more specifically relates to a multi-neck parallel underwater broadband sound-absorbing metamaterial based on a Helmholtz resonance cavity structure and a design method thereof. Background Art

[0002] When the underwater equipment propulsion system is running, mechanical noise and hydrodynamic noise will be generated, among which there is a large amount of low-frequency noise. Low-frequency noise is more difficult to control than high-frequency noise due to its long wavelength, low attenuation rate and long propagation distance. At the same time, traditional hydroacoustic materials, such as metal foam, rubber and polyurethane, have poor low-frequency sound absorption capabilities. Therefore, new low-frequency and wide-band underwater sound-absorbing structures or materials are constantly being developed to attenuate the low-frequency radiation noise of underwater equipment propulsion systems.

[0003] Acoustic metamaterials are artificially designed unit structures with sub-wavelength thickness, which can achieve properties that ordinary materials in nature do not have, such as negative equivalent mass density, negative elastic modulus, double negative properties, negative refractive index, near-zero refractive index, etc. In recent years, there are many types of underwater acoustic metamaterials, which can be basically divided into: resonance type, local resonance type, metal foam, gradient index type, chiral type, etc. For underwater acoustic environments, resonant underwater acoustic metamaterials, such as single-neck sound-absorbing metamaterials based on the Helmholtz resonant cavity structure, can use the friction resistance and viscous damping generated by the vibration friction of the sound propagation medium to cause sound energy loss, but there is a problem of narrow working bandwidth in the sound absorption frequency band; local resonance underwater acoustic metamaterials can use the low-frequency band gap generated by the local resonance principle to achieve low-frequency resonant sound absorption, but there is a general problem of narrow low-frequency working bandwidth; if metal foams are to effectively absorb low-frequency sound waves, they need to have a thickness equivalent to the wavelength, that is, they cannot take into account both low-frequency and thin characteristics; and gradient index type and chiral type metamaterials have problems such as overly complex structures and difficult processing and manufacturing. Summary of the invention

[0004] The purpose of the present invention is to solve the problems existing in the prior art of poor low-frequency sound absorption performance, narrow effective sound absorption frequency band, complex structure and the need for larger volume of materials to meet the low-frequency sound absorption requirements of underwater acoustic environment acoustic materials, and to provide an underwater broadband sound absorption metamaterial based on a Helmholtz resonance cavity and a design method thereof.

[0005] To achieve the above purpose, the technical solution provided by the present invention is:

[0006] An underwater broadband sound-absorbing metamaterial based on a Helmholtz resonant cavity, comprising a frame and a cover plate connected to each other;

[0007] The frame is provided with a resonance cavity inside and is open at one end, and the inner wall of the frame is coated with a damping material;

[0008] The cover plate is connected to the open end of the frame to close the frame. The cover plate is provided with a plurality of parallel necks extending vertically from the cover plate into the resonance cavity. The necks are in the shape of hollow cylinders. Sound waves enter from one end of each neck and enter the resonance cavity from the other end of the neck.

[0009] Furthermore, the frame is in the shape of a cuboid, a cylinder or a hexagonal prism.

[0010] Furthermore, the resonance cavity is cylindrical in shape.

[0011] Further, the damping material coating is made of rubber.

[0012] Furthermore, the cover plate and the neck are an integrated structure.

[0013] Further, the cover plate is connected to the frame by gluing or welding.

[0014] Further, the damping material coating is connected to the frame by gluing.

[0015] A design method of the above-mentioned underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity comprises the following steps:

[0016] Step 1, determine the calculation method of the sound absorption coefficient of the sound absorbing metamaterial:

[0017] α=1-R 2 =1-(Z s -1) / (Z s +1) 2

[0018] In the formula, α represents the sound absorption coefficient of the metamaterial, R represents the reflection coefficient of the metamaterial, and Z s represents the surface acoustic impedance of the metamaterial and is calculated by:

[0019] Z s =ΔZ = Δ(Z h +Z c ) / Z0

[0020] Where Z is the surface acoustic impedance of the sound-absorbing metamaterial, Δ is the area correction factor used to describe the effect of the frame wall thickness on the acoustic impedance of the metamaterial, Z0 is the acoustic impedance of the medium, and Z h is the acoustic impedance of the neck, Z c is the total acoustic impedance of the frame, which are calculated as follows:

[0021]

[0022]

[0023] In the formula, the variables with subscript i represent the structural parameters of the corresponding neck. is the panel opening ratio, S h and S c are the cross-sectional areas of the neck and frame, l and d are the length and diameter of the corresponding neck, J0(X) and J1(X) are the 0th and 1st order first-order Bessel functions, respectively. is the ratio of the radius of the corresponding neck to the thickness of the viscous boundary layer, j is an imaginary unit, η0 is the dynamic viscosity of water, ρ0 is the density of water, ω = 2πf is the angular frequency, and f is the frequency;

[0024]

[0025] In the formula, Z r and Z w are the acoustic impedance of the damping material coating and the acoustic impedance of the resonant cavity, respectively, expressed as:

[0026]

[0027] Where V r is the volume of the damping material coating, c0 is the speed of the sound wave, V w is the volume of the resonance cavity, ρ r and c r are the density of damping and the longitudinal wave velocity, c r The following formula is used for calculation:

[0028]

[0029] In the formula, is the complex bulk modulus:

[0030]

[0031] In the formula, K r is the bulk modulus, K r =[E r (1-ν r )] / [(1+ν r )(1-2ν r )], tanδ, E r and ν r are the loss factor, Young's modulus and Poisson's ratio of the damping material, tanδ=E r " / E r ', E r 'With E r "represent the storage modulus and loss modulus of the material respectively;

[0032] Step 2, determining the noise reduction frequency band range;

[0033] Step 3, performing finite element modeling on the sound absorbing metamaterial, deducing the structural parameters of the sound absorbing metamaterial according to the determined sound absorption coefficient calculation method and the noise reduction frequency band range, and obtaining a metamaterial with a sound absorption coefficient of 1 at the corresponding sound absorption frequency, including the following sub-steps:

[0034] Step 3.1, giving the shape and structural parameters of the sound-absorbing metamaterial, including the length, width, height and wall thickness of the frame, and the thickness of the cover plate;

[0035] Step 3.2, fixing the parameters of the damping material coating, and deducing the structural parameters of multiple necks according to the required sound absorption frequency, including the length, diameter and number of each neck;

[0036] Step 3.3, fixing the parameters of the neck, and deducing the parameters of the damping material coating according to the required sound absorption frequency, including the loss factor, Young's modulus, Poisson's ratio and thickness of the damping material coating;

[0037] Step 3.4, based on the required sound absorption frequency, combine steps 3.2 and 3.3 to deduce the internal structural parameters of the metamaterial, including the parameters of the damping material coating and the parameters of the neck.

[0038] Furthermore, the acoustic impedance of the neck is calculated as follows:

[0039]

[0040] The advantages of the present invention are:

[0041] 1. The underwater broadband sound-absorbing metamaterial based on the Helmholtz resonant cavity of the present invention and its design method, wherein the sound-absorbing metamaterial includes a frame and a cover plate, the frame is provided with a resonant cavity and the inner wall is covered with a damping material coating, and multiple parallel necks on the cover plate extend into the resonant cavity, and the sound waves enter the resonant cavity through the necks, and the sound absorption frequency and sound absorption frequency band of the metamaterial can be efficiently designed by the design method. The metamaterial of the present invention has a simple structure; and the sound absorption peak value at the frequency point below 1000Hz can reach more than 0.99, achieving perfect sound absorption, and the thickness is only 1 / 125 to 1 / 45 of the corresponding perfect sound absorption wavelength; in addition, within a certain range of 0 to 2000Hz, the sound absorption coefficient can reach more than 0.5, the effective sound absorption (sound absorption coefficient exceeds 0.5) bandwidth can reach 127.9%, and the thickness of the metamaterial is within 1 / 45 of the wavelength. Therefore, the metamaterial of the present invention is a deep sub-wavelength scale underwater low-frequency perfect sound absorption metamaterial, which has excellent low-frequency sound absorption performance, a wide sound absorption frequency band, and a small volume can meet the low-frequency sound absorption requirements.

[0042] 2. The metamaterial of the present invention has multiple adjustable parameters and variables. In the present invention, the cavity cross-sectional area, cavity height, diameter, length and number of parallel necks, damping material coating thickness and damping material coating mechanical properties (Young's modulus, loss factor, etc.) are all adjustable parameters, which can be reasonably selected and adjusted according to specific usage scenarios, such as the requirements for load-bearing performance or the requirements for acoustic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and / or other features and advantages of the present invention will become more easily understood through the following description with reference to the accompanying drawings, which are not drawn to scale and some features are exaggerated or reduced to show details of specific components. In the accompanying drawings:

[0044] Figure 1 is a stereoscopic diagram of the underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity of the present invention;

[0045] Figure 2 is an exploded stereogram of the underwater broadband sound-absorbing metamaterial based on the Helmholtz resonance cavity of the present invention;

[0046] Figure 3 is a cross-sectional stereoscopic diagram of the underwater broadband sound-absorbing metamaterial based on the Helmholtz resonance cavity of the present invention;

[0047] Figure 4 It is a cross-sectional stereoscopic diagram of a single-neck sound-absorbing metamaterial based on a Helmholtz resonance cavity structure in the prior art;

[0048] Figure 5 is a comparison diagram of the sound absorption frequency bands of Examples 1 and 2 of the present invention and a comparative example of an existing single-neck sound absorption metamaterial when the neck diameter and length are the same;

[0049] Figure 6 It is a comparison diagram of the sound absorption frequency bands of Examples 3 and 4 of the present invention and the comparative example of the existing single-neck sound absorption metamaterial under the same perfect sound absorption frequency point.

[0050] In the figure: 1-cover plate; 2-neck; 3-damping material coating; 4-frame; 5-resonance cavity. DETAILED DESCRIPTION

[0051] The present invention will be described in detail below with reference to the accompanying drawings by means of exemplary embodiments of the present invention. It should be noted that the following detailed description of the present invention is only for the purpose of illustration, and is not intended to limit the present invention.

[0052] The present invention provides an underwater broadband sound-absorbing metamaterial based on a Helmholtz resonance cavity and a design method for designing the sound-absorbing metamaterial. The sound-absorbing metamaterial can have excellent broadband sound-absorbing ability while achieving good low-frequency underwater sound-absorbing performance, thereby reducing the weight of the structure and ensuring the structural bearing performance.

[0053] Now, firstly, the multi-neck parallel underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity structure provided by the present invention is described in detail.

[0054] Reference Figures 1 to 3 As an exemplary embodiment of the present invention, a multi-neck parallel underwater broadband sound absorbing metamaterial based on a Helmholtz resonance cavity structure includes a frame 4 and a cover plate 1 connected to each other.

[0055] The frame 4 is made of steel. The structure has good pressure resistance and is a multifunctional structure with good load-bearing performance and light weight. The frame 4 is provided with a resonance cavity 5 inside and is open at one end. When the sound-absorbing metamaterial is in use, the resonance cavity 5 contains water. The external structure of the frame 4 may be in the shape of a cuboid, a cylinder or a hexagonal prism, etc., and the present invention is not particularly limited thereto, and the wall thickness of the frame 4 may be different at the bottom and the side. The resonance cavity 5 in the frame 4 may be in the shape of a cylinder, a cuboid, etc. In an exemplary embodiment, the external shape of the frame 4 is a cuboid with a square cross-section, and the resonance cavity 5 is cylindrical.

[0056] The inner wall of the frame 4 is covered with a damping material coating 3, which is optionally connected to the frame 4 by gluing. The damping material coating 3 may be made of rubber.

[0057] The cover plate 1 is connected to the open end of the frame 4, preferably connected to the frame 4 by gluing or welding to close the frame 4. The shape of the cover plate 1 may correspond to the outer shape of the frame 4. The cover plate 1 is provided with a plurality of parallel necks 2 extending vertically therefrom into the resonance cavity 5, and the geometric dimensions of the plurality of necks 2 may be the same or different, but in particular the same. The plurality of necks 2 may be arranged in parallel in several rows, and are exemplarily shown in the figure as three rows with three necks in each row, but this is only an example, and the number of necks may be adjusted according to the sound absorption frequency band and the sound absorption frequency, and 2, 3, 4, etc. may be arranged. The cover plate 1 and the neck 2 may be provided as an integral structure, and the neck 2 is in the shape of a hollow cylinder, and the sound wave enters from one end of each neck 2 and enters the resonance cavity 5 from the other end of the neck 2.

[0058] The present invention relates to the processing and manufacturing of the structure, which can be quickly obtained by mechanical processing and metal 3D printing, and the connection method of the structural parts is simple.

[0059] Next, the design method of the multi-neck parallel underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity structure provided by the present invention is described in detail.

[0060] The design method as an exemplary embodiment of the present invention comprises the following steps:

[0061] Step S1, determining a calculation method for the sound absorption coefficient of the sound absorbing metamaterial;

[0062] Step S2, determining the noise reduction frequency band range;

[0063] Step S3, finite element modeling is performed on the sound absorbing metamaterial, and structural parameters of the sound absorbing metamaterial are deduced according to the determined sound absorption coefficient calculation method and noise reduction frequency band range to obtain a metamaterial with a sound absorption coefficient of 1 at the corresponding sound absorption frequency.

[0064] In step S1, in order to simplify the analysis, it is assumed that the backing of the metamaterial is rigid. In this case, the sound absorption coefficient of the metamaterial can be expressed as:

[0065] α=1-|R 2 =1-|(Z s -1) / (Z s +1) 2 (1)

[0066] Where R represents the reflection coefficient of the metamaterial, Z s represents the surface acoustic impedance of the metamaterial. In order to determine Z s , it is necessary to calculate the acoustic impedance of the frame 4 and the embedded neck 2, wherein the frame 4 is composed of a damping material coating 3 and a resonant cavity 5. According to the electroacoustic analogy method, the acoustic impedance of the damping material coating 3 and the resonant cavity 5 which is a water cavity can be regarded as parallel, the acoustic impedances of the multiple embedded necks 2 can be regarded as parallel, and the acoustic impedances of the frame 4 and the multiple necks 2 can be regarded as series.

[0067] According to the shape and structure of the multi-neck parallel underwater sound-absorbing metamaterial, the surface acoustic impedance Z can be obtained: s Definition form:

[0068] Z s =ΔZ = Δ(Z h +Z c ) / Z0 (2)

[0069] Where Z is the surface acoustic impedance of the sound-absorbing metamaterial, Δ is the area correction factor used to describe the effect of wall thickness on the acoustic impedance of the material, which can be expressed as the ratio of the cross-sectional area of ​​the metamaterial to the cross-sectional area of ​​the resonant cavity; Z c is the total acoustic impedance of frame 4, Z h is the acoustic impedance of the embedded neck 2, and Z0 is the acoustic impedance of the medium. s Substituting into equation (1), we can obtain the sound absorption coefficient of the metamaterial in different frequency bands.

[0070] Then determine Z in formula (2) c With Z h Define the form.

[0071] First determine Z cDefinition: According to the acoustic-electric analogy method mentioned above, the total acoustic impedance Z of the resonant cavity is c It can be expressed as:

[0072]

[0073] Where Z r and Z w are the acoustic impedances of the damping coating 3 and the resonant cavity 5, respectively, which can be expressed as:

[0074]

[0075] Where j is the imaginary unit; V r is the volume of the damping coating 3; ρ r and c r are the density of damping and the velocity of longitudinal wave respectively; ω=2πf is the angular frequency, f is the frequency; ρ0 and c0 are the density of water and the velocity of sound wave respectively, V w is the volume of the resonance cavity 5 excluding the embedded neck.

[0076] For damping materials, the longitudinal wave velocity c r It is related to its constitutive relation, which is specifically expressed as:

[0077]

[0078] in is the complex bulk modulus:

[0079]

[0080] Where K r is the bulk modulus, K r =[E r (1-ν r )] / [(1+ν r )(1-2ν r )]; tanδ, E r and ν r They are the loss factor, Young's modulus and Poisson's ratio of the damping material respectively. The loss factor tanδ is given by tanδ=E r " / E r 'Definition, E r 'With E r " respectively represent the storage modulus and loss modulus of the material, E r =E r 'tanδ. It is worth noting that under certain circumstances, Young's modulus E r Storage modulus E r 'Can express each other.

[0081] In fact, the Young's modulus E of the damping material rThe loss factor tanδ is usually related to the frequency f. However, for the narrow frequency band of the resonant cavity structure, in order to simplify the calculation and analysis, it can be assumed that the loss factor is independent of the frequency. The total acoustic impedance of the resonant cavity can be obtained by the above formula, and then it can be brought into formula (2) to derive the surface acoustic impedance of the metamaterial.

[0082] Then confirm Z h In the definition form, the acoustic impedance of the embedded neck 2 in the Helmholtz resonant cavity structure can be expressed as:

[0083]

[0084] The variables with a subscript i in the formula represent the structural parameters of a corresponding embedded neck 2; is the panel opening ratio; S h and S c are the cross-sectional areas of the embedded neck 2 and the frame 4, respectively; l and d are the length and diameter of the embedded neck 2, respectively, the diameter referring to the inner diameter of the neck through which the sound waves can pass; J n (X) is a first-order Bessel function of order n, where n is 0 or 1; is the ratio of the radius of the embedded neck 2 to the thickness of the viscous boundary layer, η0 is the dynamic viscosity of water, and the last two terms in equation (8) are theoretical correction terms. If the diameter and length of the multiple embedded necks are the same, equation (9) can be simplified to equation (10), where n represents the number of necks.

[0085]

[0086] By calculating with the above formula, the acoustic impedance of the embedded neck can be obtained, and then it can be brought into formula (2) to derive the surface acoustic impedance of the metamaterial.

[0087] Step S3 is used to determine the structural dimensions of the sound absorbing metamaterial, including the shape structural parameters and internal structural parameters of the metamaterial. The shape structural parameters of the material can be given according to the application conditions of the sound absorbing metamaterial, and the structural parameters of the sound absorbing metamaterial can be deduced according to the required noise reduction frequency band range and the above-mentioned sound absorption coefficient calculation method. Specifically, step S3.1: the shape structural parameters of the sound absorbing metamaterial are given, including the length, width, height and wall thickness of the frame, and the thickness of the cover plate. For example, the length, width and height of the entire material structure (such as the length, width a and height H) and the wall thickness (such as the side wall thickness t1 and the bottom wall thickness t2) can be directly given;

[0088] Step S3 also includes step S3.2: fixing the parameters of the damping material coating 3, and deducing the structural parameters of the metamaterial parallel multi-embedded neck 2 according to the required sound absorption frequency.

[0089] From the above formula, we can know that the surface acoustic impedance and the loss factor, Young's modulus, Poisson's ratio and thickness t3 of the damping material coating 3 (the thickness information is contained in the parameter V r In this step, after the loss factor, Young's modulus, and Poisson's ratio of the damping material coating 3 are fixed within a reasonable range and its thickness t3 is fixed, the structural parameters of the metamaterial parallel multi-embedded neck 2 are deduced according to the required sound absorption frequency: length l, diameter d, and number n, so that the sound absorption coefficient near the required sound absorption frequency point reaches or approaches 1, and can provide broadband sound absorption capability at the same time.

[0090] Step S3 also includes step S3.3: fixing the parameters of the parallel multi-embedded neck 2, and deducing the parameters of the metamaterial damping material coating 3 according to the required sound absorption frequency.

[0091] It can be seen from the above calculation formula that the surface acoustic impedance that characterizes the acoustic performance of the metamaterial is related to the length l, diameter d and number n of the parallel multi-embedded neck 2. In this step, the structural parameters l, d and n of the neck 2 are fixed, and the loss factor, Young's modulus, Poisson's ratio and thickness of the damping material coating 3 are deduced according to the required sound absorption frequency, so that the sound absorption coefficient reaches or approaches 1 near the required sound absorption frequency point, and can provide broadband sound absorption capability at the same time.

[0092] Step S3 also includes step S3.4: deducing the internal structural parameters of the metamaterial according to the required sound absorption frequency, including the above parameters of the damping material coating and the above parameters of the neck.

[0093] From steps 3.2 and 3.3, it can be obtained that, according to the required sound absorption frequency, there is a set of structural parameters for the parallel multi-embedded neck 2 and a set of parameters for the damping material coating 3. Taking into account the conclusions of steps 3.2 and 3.3, the internal structural parameters of the metamaterial are deduced for the required sound absorption frequency.

[0094] The multi-neck parallel underwater sound-absorbing metamaterial based on the Helmholtz resonance cavity structure obtained according to the described steps has a simple structure; and the sound absorption peak value at the frequency point below 1000 Hz can reach more than 0.99, achieving perfect sound absorption, and the thickness is only 1 / 125 to 1 / 45 of the corresponding perfect sound absorption wavelength; in addition, the sound absorption coefficient can reach more than 0.5 within a certain range of 0 to 2000 Hz, the effective sound absorption (sound absorption coefficient exceeds 0.5) bandwidth can reach 127.9%, and the metamaterial thickness is within 1 / 45 of the wavelength. Therefore, the metamaterial of the present invention is a deep subwavelength scale underwater low-frequency perfect sound-absorbing metamaterial, which has excellent low-frequency sound absorption performance, a wide sound absorption bandwidth, and a small volume can meet the low-frequency sound absorption requirements. In addition, the cavity cross-sectional area, cavity height, diameter, length and number of parallel necks, damping material coating thickness and mechanical properties of damping material coating (Young's modulus, loss factor, etc.) in the present invention are all adjustable parameters, which can be reasonably selected and adjusted according to specific usage scenarios, such as requirements for load-bearing performance or requirements for acoustic performance.

[0095] Now, an underwater broadband sound absorbing metamaterial based on a Helmholtz resonance cavity provided by the present invention is compared with an existing single-neck sound absorbing metamaterial based on a Helmholtz resonance cavity structure by way of examples.

[0096] In an example, the metamaterial structure of the present invention is as follows Figure 3 As shown, in Example 1 of the present invention, the cross-sectional area of ​​the sound-absorbing metamaterial is a rectangle with an outer size of a×a=20mm×20mm, a height of H=30mm, a circle with an inner diameter of 18mm, and an inner height of the cavity of 26mm; the thickness of the cover plate and the bottom wall of the frame are both t2=2mm, and the thickness of the side wall of the frame is t1=2mm; the diameter of the parallel multi-embedded neck is d=2mm, the length of the parallel multi-embedded neck is l=15mm, and the number of parallel multi-embedded necks in parallel is n=4; the thickness of the damping material coating is t3=2mm.

[0097] In Example 2 of the present invention, the cross-sectional area of ​​the resonance cavity is a rectangle with an outer size of a×a=20mm×20mm, a height of H=30mm, a circle with an inner diameter of 18mm, and an internal height of the cavity of 26mm; the thickness of the cover plate and the thickness of the bottom wall of the frame are both t2=2mm, and the thickness of the side wall of the frame is t1=2mm; the diameter of the parallel multi-embedded neck is d=2mm, the length of the parallel multi-embedded neck is l=15mm, and the number of parallel multi-embedded necks in parallel is n=9; the thickness of the damping material coating is t3=2mm.

[0098] In Example 3 of the present invention, the cross-sectional area of ​​the resonance cavity is a rectangle with an outer size of a×a=20mm×20mm, a height of H=30mm, a circle with an inner diameter of 18mm, and an internal height of the cavity of 26mm; the thickness of the cover plate and the thickness of the bottom wall of the frame are both t2=2mm, and the thickness of the side wall of the frame is t1=2mm; the diameter of the parallel multi-embedded neck is d=12mm, the length of the parallel multi-embedded neck is l=10mm, and the number of parallel multi-embedded necks in parallel is n=3; the thickness of the damping material coating is t3=4.5mm.

[0099] In Example 4 of the present invention, the cross-sectional area of ​​the resonance cavity is a rectangle with an outer size of a×a=20mm×20mm, a height of H=30mm, a circle with an inner diameter of 18mm, and an internal height of the cavity of 26mm; the thickness of the cover plate and the thickness of the bottom wall of the frame are both t2=2mm, and the thickness of the side wall of the frame is t1=2mm; the diameter of the parallel multi-embedded neck is d=0.8mm, the length of the parallel multi-embedded neck is l=13mm, and the number of parallel multi-embedded necks in parallel is n=9; the thickness of the damping material coating is t3=5mm.

[0100] In the comparative example, Figure 4 As shown, the cross-sectional area of ​​the resonance cavity is a rectangle with an outer size of a×a=20mm×20mm, a circle with an inner diameter of 18mm, and an inner height of the cavity of 26mm; the thickness of the cover plate is 2mm; the diameter of a single neck is 2mm, and the length of the neck is 15mm; the thickness of the damping material coating is 2mm.

[0101] See also Figure 4 The sound absorption coefficient of comparative example 1 at 260Hz-535Hz is greater than 0.5, the effective sound absorption bandwidth is 69.6%, and perfect sound absorption is achieved at 395Hz, with a peak value of the sound absorption coefficient of 0.881.

[0102] In Example 1, the sound absorption coefficient at 385 Hz to 1200 Hz is greater than 0.998, the effective sound absorption bandwidth is 105.8%, and perfect sound absorption is achieved at 770 Hz, with a peak value of the sound absorption coefficient of 0.998, achieving perfect absorption of low-frequency and wide-band sound waves. At this time, the thickness of the structure, that is, the height H mentioned above, is only 30 mm, which is 1 / 64 of the corresponding perfect sound absorption wavelength. Therefore, the metamaterial structure of the present invention is a deep subwavelength scale low-frequency perfect sound absorption metamaterial, and the sound absorption requirements can be achieved with a small volume.

[0103] In Example 2, the sound absorption coefficient at 460 Hz to 1880 Hz is greater than 0.5, the effective sound absorption bandwidth is 127.9%, and perfect sound absorption is achieved at 1110 Hz, with a peak sound absorption coefficient of 0.968, achieving perfect absorption of low-frequency sound waves. At this time, the thickness of the structure is only 30 mm, which is 1 / 45 of the corresponding perfect sound absorption wavelength. Therefore, the metamaterial structure of the present invention is a deep subwavelength scale low-frequency perfect sound absorption metamaterial, and the sound absorption requirements can be achieved in a small volume.

[0104] Reference Figure 5 The sound absorption coefficient of comparative example 1 at 260Hz-535Hz is greater than 0.5, the effective sound absorption bandwidth is 69.6%, and perfect sound absorption is achieved at 395Hz, with a peak value of the sound absorption coefficient of 0.881.

[0105] In Example 3, the sound absorption coefficient at 215Hz-595Hz is greater than 0.5, the effective sound absorption bandwidth is 96.2%, and perfect sound absorption is achieved at 395Hz, with a peak sound absorption coefficient of 0.972, achieving perfect absorption of low-frequency sound waves. At this time, the structure thickness is only 30mm, which is 1 / 125 of the corresponding perfect sound absorption wavelength. Therefore, the metamaterial structure of the present invention is a deep subwavelength scale low-frequency perfect sound absorption metamaterial, and the sound absorption requirements can be achieved in a small volume.

[0106] In Example 4, the sound absorption coefficient at 210 Hz to 615 Hz is greater than 0.5, the effective sound absorption bandwidth is 102.5%, and perfect sound absorption is achieved at 395 Hz. The peak value of the sound absorption coefficient is 0.972, achieving perfect absorption of low-frequency sound waves. At this time, the thickness of the structure is only 30 mm, which is 1 / 125 of the corresponding perfect sound absorption wavelength. Therefore, the metamaterial structure of the present invention is a deep subwavelength scale low-frequency perfect sound absorption metamaterial, and the sound absorption requirements can be achieved in a small volume.

[0107] It can be seen from the sound absorption coefficient curve that the present invention can achieve excellent low-frequency sound absorption performance within a certain frequency range, and the acoustic performance can be adjusted by designing different structural parameters.

[0108] Finally, it should be noted that the features mentioned and / or shown in the above description of the exemplary embodiments of the present invention may be combined in the same or similar manner into one or more other embodiments, combined with the features in other embodiments or substituted for the corresponding features in other implementations. The technical solutions obtained by these combinations or substitutions shall also be deemed to be included in the protection scope of the present invention.

Claims

1. An underwater broadband sound-absorbing metamaterial based on a Helmholtz resonant cavity, characterized in that: including a frame and a cover plate connected to each other; The frame is provided with a resonance cavity inside and is open at one end, and the inner wall of the frame is coated with a damping material coating; The cover plate is connected to the open end of the frame to close the frame. The cover plate is provided with a plurality of parallel necks extending vertically from the cover plate into the resonance cavity. The necks are in the shape of hollow cylinders. Sound waves enter from one end of each neck and enter into the resonance cavity from the other end of the neck.

2. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1, characterized in that: The frame is in the shape of a cuboid, a cylinder or a hexagonal prism.

3. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1 or 2, characterized in that: The resonance cavity is cylindrical in shape.

4. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1 or 2, characterized in that: The damping material coating is made of rubber.

5. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1 or 2, characterized in that: The cover plate and the neck are an integral structure.

6. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1 or 2, characterized in that: The cover plate is connected to the frame by gluing or welding.

7. The underwater broadband sound absorbing metamaterial based on the Helmholtz resonance cavity according to claim 1 or 2, characterized in that: The damping material coating is attached to the frame by gluing.

8. A design method for underwater broadband sound absorbing metamaterial based on a Helmholtz resonance cavity according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step 1, determine the calculation method of the sound absorption coefficient of the sound absorbing metamaterial: α=1-|R 2 =1-|(From s -1) / (From s +1) 2 In the formula, α represents the sound absorption coefficient of the metamaterial, R represents the reflection coefficient of the metamaterial, and Z s represents the surface acoustic impedance of the metamaterial and is calculated by: WITH s =ΔZ=Δ(Z h +Z c ) / Z0 Where Z is the surface acoustic impedance of the sound-absorbing metamaterial, Δ is the area correction factor used to describe the effect of the frame wall thickness on the acoustic impedance of the metamaterial, Z0 is the acoustic impedance of the medium, and Z h is the acoustic impedance of the neck, Z c is the total acoustic impedance of the frame, which are calculated as follows: In the formula, the variables with subscript i represent the structural parameters of the corresponding neck. is the panel opening ratio, S h and S c are the cross-sectional areas of the neck and frame, l and d are the length and diameter of the corresponding neck, J0(X) and J1(X) are the 0th and 1st order first-order Bessel functions, respectively. is the ratio of the radius of the corresponding neck to the thickness of the viscous boundary layer, j is an imaginary unit, η0 is the dynamic viscosity of water, ρ0 is the density of water, ω = 2πf is the angular frequency, and f is the frequency; In the formula, Z r and Z w are the acoustic impedance of the damping material coating and the acoustic impedance of the resonant cavity, respectively, expressed as: Where V r is the volume of the damping material coating, c0 is the speed of the sound wave, V w is the volume of the resonance cavity, ρ r and c r are the density of damping and the longitudinal wave velocity, c r The following formula is used for calculation: In the formula, is the complex bulk modulus: In the formula, K r is the bulk modulus, K r =[E r (1-ν r )] / [(1+ν r )(1-2ν r )], tanδ, E r and ν r are the loss factor, Young's modulus and Poisson's ratio of the damping material, tanδ=E r " / E r ', E r 'With E r "represent the storage modulus and loss modulus of the material respectively; Step 2, determining the noise reduction frequency band range; Step 3, performing finite element modeling on the sound absorbing metamaterial, deducing the structural parameters of the sound absorbing metamaterial according to the determined sound absorption coefficient calculation method and the noise reduction frequency band range, and obtaining a metamaterial with a sound absorption coefficient of 1 at the corresponding sound absorption frequency, including the following sub-steps: Step 3.1, giving the shape and structural parameters of the sound-absorbing metamaterial, including the length, width, height and wall thickness of the frame, and the thickness of the cover plate; Step 3.2, fixing the parameters of the damping material coating, and deducing the structural parameters of multiple necks according to the required sound absorption frequency, including the length, diameter and number of each neck; Step 3.3, fix the parameters of the neck, and deduce the parameters of the damping material coating according to the required sound absorption frequency, including the loss factor, Young's modulus, Poisson's ratio and thickness of the damping material coating; Step 3.4, based on the required sound absorption frequency, combine steps 3.2 and 3.3 to deduce the internal structural parameters of the metamaterial, including the parameters of the damping material coating and the parameters of the neck.

9. The design method of underwater broadband sound absorbing metamaterial based on Helmholtz resonance cavity according to claim 8, characterized in that: The acoustic impedance of the neck is calculated as follows:

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