An underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection

Abstract

The application belongs to the technical field of acoustic metamaterials, and discloses a water acoustic metasurface structure for low-frequency broadband uniform diffuse reflection, which is an axisymmetric structure and comprises N sub-blocks, the N sub-blocks are sequentially connected to form a strip-shaped body, the strip-shaped body is divided into two sub-strip-shaped bodies from the middle, the sub-blocks adjacent to the geometric centers of the two sub-strip-shaped bodies are cavity structures, the cavity structures are prepared by using weak equivalent materials, and the remaining sub-blocks are prepared by using strong equivalent materials, wherein N is a positive integer greater than 2 and is an even number. The structure simultaneously uses weak equivalent materials and strong equivalent materials, so that the structure can reflect the vertical incident plane acoustic waves with a frequency range of 200Hz-650Hz in the form of uniform diffuse reflection, and the normalized diffusion coefficient is greater than 0.4 in the frequency range.

Description

Technical Field

[0001] This invention belongs to the technical field of acoustic metamaterials, and more specifically, relates to a hydroacoustic metasurface structure for low-frequency broadband uniform diffuse reflection. Background Technology

[0002] Acoustic metasurfaces are artificially designed subwavelength structures that can acquire acoustic properties not found in natural materials, such as anomalous reflection and refraction of sound waves, beam bending and focusing, and wavefront manipulation. They can be divided into airborne acoustic metasurfaces and underwater acoustic metasurfaces depending on their application environment.

[0003] Existing anechoic tiles used for underwater submarine acoustic concealment have limited ability to improve sound absorption at low frequencies. Emerging metamaterials are mostly designed for manipulation at specific frequencies and deflection angles, resulting in narrow bandwidths that cannot handle wideband multi-receiver sonar systems. The main reasons for this are: the high speed of sound in seawater makes low-frequency sound wave manipulation particularly difficult; and the interaction between sound waves and underwater structures (acousto-structure coupling) leads to extremely complex manipulation mechanisms.

[0004] Therefore, overcoming the limitations of the above-mentioned underwater acoustic metasurface design would be of great significance for improving the acoustic stealth of submarines. Summary of the Invention

[0005] To address the above-mentioned deficiencies or improvement needs of existing technologies, this invention provides a hydroacoustic metasurface structure for low-frequency broadband uniform diffuse reflection. The structure employs both weak and strong equivalent materials, enabling it to reflect vertically incident plane acoustic waves in the frequency band of 200Hz to 650Hz in a uniform diffuse reflection manner, resulting in a normalized diffusion coefficient greater than 0.4 in this frequency band.

[0006] To achieve the above objectives, according to one aspect of the present invention, a hydroacoustic metasurface structure for low-frequency broadband uniform diffuse reflection is provided. The hydroacoustic metasurface structure is an axisymmetric structure comprising N sub-blocks, which are sequentially connected to form a strip. The strip is divided into two sub-strips from its middle. The sub-block adjacent to its geometric center in each sub-strip is a cavity structure, which is made of a weakly equivalent material. The remaining sub-blocks are made of a strongly equivalent material. Wherein, N is a positive integer greater than 2 and is an even number.

[0007] Furthermore, N is 20. The 4th, 5th, 6th, 15th, 16th and 17th sub-blocks from left to right are cavity structures made of weakly equivalent materials, while the remaining sub-blocks are made of strongly equivalent materials. That is, the 1st, 2nd, 3rd, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th, 18th, 19th and 20th sub-blocks from left to right are made of strongly equivalent materials.

[0008] Furthermore, the underwater acoustic metasurface structure is used for uniform diffuse reflection modulation of low-frequency broadband acoustic waves, the frequency band of which is 200Hz to 650Hz.

[0009] Furthermore, the length of the underwater acoustic metasurface structure is twice the wavelength at which it can diffusely reflect light, and its height is 0.01 times the wavelength at which it can diffusely reflect light.

[0010] Furthermore, the underwater acoustic metasurface structure is suitable for the manipulation of sound waves with wavelengths of 2.3m to 7.5m, which corresponds to frequencies of f = 200Hz to 650Hz, and the wavelength λ, frequency f, and sound velocity c satisfy the relationship c = λf.

[0011] Furthermore, the equivalent density of the weakly equivalent material is taken as the equivalent density of air, and its equivalent sound velocity is taken as the equivalent sound velocity of air.

[0012] Furthermore, the equivalent density of the strong equivalent material is taken as the equivalent density of seawater, and its equivalent sound velocity is taken as the equivalent sound velocity of seawater.

[0013] Furthermore, the strip-shaped body is backed by a steel backing plate, the thickness of which is 0.1 to 0.4 times the diffuse reflection wavelength of the underwater acoustic metasurface structure.

[0014] Furthermore, the thickness of the steel back plate is 0.2 times the wavelength, i.e., 1.04 m, and the density is 7800 kg / m³. 3 The speed of sound is 6100 m / s.

[0015] In summary, compared with the prior art, the underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection provided by the present invention has the following beneficial effects:

[0016] 1. The underwater acoustic metasurface of the present invention comprises two types of equivalent materials, namely strong equivalent materials and weak equivalent materials. The strong equivalent materials and weak equivalent materials can be realized by using appropriate microstructures and cavities. Overall, it can achieve the diffuse reflection effect of low-frequency broadband sound waves. When laid on the conning tower of a submarine, it can defend against the detection of multi-receiver sonar systems.

[0017] 2. The underwater acoustic metasurface structure of the present invention takes into account the interaction between sound waves and the structure, which is different from the stringent requirements of impedance matching acoustic metamaterials or metasurfaces on material properties, and has design flexibility.

[0018] 3. The underwater acoustic metasurface structure of the present invention is composed of rectangular sub-blocks of the same size. The height of the sub-blocks is on the deep subwavelength scale. The design is simple and thin, and each sub-block can be processed individually, making it easy to splice and combine. Attached Figure Description

[0019] Figure 1 This is a two-dimensional schematic diagram of the underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection provided by the present invention.

[0020] Figure 2 yes Figure 1 The scattered acoustic pressure diagram of the underwater acoustic metasurface structure at 288 Hz was simulated using the finite element method.

[0021] Figure 3 This is a schematic diagram showing the dimensions of the five-mode material microstructure in an embodiment of the present invention;

[0022] Figure 4 This is a diagram showing the scattering sound pressure and specific structural arrangement of a five-mode material microstructure filled at 288Hz according to an embodiment of the present invention.

[0023] Figure 5 This is the far-field sound pressure level pattern at 288Hz according to an embodiment of the present invention;

[0024] Figure 6 yes Figure 1 A three-dimensional structural diagram of a hydroacoustic metasurface structure used for low-frequency broadband uniform diffuse reflection. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Please see Figure 1 and Figure 6 This invention provides a hydroacoustic metasurface structure for low-frequency broadband uniform diffuse reflection. This structure is axisymmetric and comprises N sub-blocks, where N is a positive integer greater than 2 and is even. The N sub-blocks are sequentially connected to form a strip, which is divided into two sub-strips from its center. Within each sub-strip, the sub-block near its geometric center is a cavity structure, fabricated using a weakly equivalent material. The remaining sub-blocks are fabricated using a strongly equivalent material. The strongly equivalent material is implemented using microstructure design, while the weakly equivalent material is implemented using cavities.

[0027] In this embodiment, the structure is used for uniform diffuse reflection control of low-frequency broadband sound waves (200Hz~650Hz); N is 20, and the 4th, 5th, 6th, 15th, 16th and 17th sub-blocks from left to right are cavity structures made of weakly equivalent materials, while the remaining sub-blocks are made of strongly equivalent materials, that is, the 1st, 2nd, 3rd, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th, 18th, 19th and 20th sub-blocks from left to right are made of strongly equivalent materials; the structure can reflect vertically incident plane sound waves in the frequency band of 200Hz~650Hz in the form of uniform diffuse reflection, so that the normalized diffusion coefficient is greater than 0.4 in this frequency band.

[0028] The structure has a length twice the wavelength of its diffuse reflection capability and a height 0.01 times the wavelength of its diffuse reflection capability. This underwater acoustic metasurface structure is suitable for controlling sound waves with wavelengths from 2.3m to 7.5m, corresponding to frequencies f = 200Hz to 650Hz, and a sound velocity in seawater of c = 1500m / s. The wavelength λ, frequency f, and sound velocity c satisfy the relationship c = λf. In this embodiment, the wavelength λ = 5.2m at a frequency f = 288Hz is taken, meaning the width of the underwater acoustic metasurface structure is 10.4m and the height is 52mm. It is then discretized laterally into 20 identical sub-blocks, each with a width of 0.52m.

[0029] The weak equivalent material has the same equivalent density and equivalent sound velocity as air. Specifically, the equivalent density ρ of the weak equivalent material is... S Equivalent to the equivalent density of air, 1.205 kg / m³. 3 Equivalent speed of sound c S This is equal to the equivalent speed of sound in air, 340 m / s. The strong equivalent material has the same equivalent density and equivalent speed of sound as seawater, specifically: the equivalent density ρ of the strong equivalent material... H Equivalent to the equivalent density of seawater 1000 kg / m³ 3 Equivalent speed of sound c H It is equivalent to the speed of sound in seawater, which is 1500 m / s.

[0030] The strip-shaped body is backed by a steel backing plate. The thickness of the steel backing plate can be 0.1 to 0.4 times the wavelength of diffuse reflection of the underwater acoustic metasurface structure. In this embodiment, the thickness of the steel backing plate is 0.2 times the wavelength, i.e., 1.04 m, and the density is 7800 kg / m³. 3 The speed of sound is 6100 m / s.

[0031] The finite element method was used for simulation, with Comsol Multiphysics 5.6 as the simulation software and version. The incident wave was a perpendicularly incident plane wave, and the background medium was seawater with a density of 1000 kg / m³.3 The speed of sound is 1500 m / s. (By...) Figure 2 It can be seen that a diffuse reflection waveform effect is formed near the metasurface.

[0032] To characterize the uniformity of acoustic wave diffuse reflection modulation by metasurface structures, the "normalized diffusion coefficient d" is typically used. n This parameter is used for evaluation, and its calculation formula is as follows:

[0033]

[0034] Among them, physical quantity d act and d ref Let d represent the diffusion coefficient of the actual designed metasurface and the diffusion coefficient of the reference plate, respectively. act The following methods can be used for calculation:

[0035]

[0036] Among them, L i Let M represent the far-field sound pressure level in the i-th direction, where M is the number of directions. In this embodiment, M is 181, and i ranges from 0° to 180° in increments of 1°. The physical quantity d... ref Calculate in a similar manner; the standardized diffusion coefficient d n It is a scalar, typically between 0 and 1, representing the degree of sound energy diffusion. When d n When d = 1, it indicates uniform diffusion in all directions; when d = 1, it indicates uniform diffusion in all directions. n When d = 0, it indicates that diffusion has not been improved compared to the reference plate; it is generally considered that when d = 0, diffusion is not improved. n When the value is greater than 0.4, the metasurface exhibits good diffusion properties.

[0037] In this embodiment, the diffusion coefficient d of the reference plate is... ref It is 0.233. Figure 2 The diffusion coefficient d corresponding to the structure act The value is 0.878, therefore the normalized diffusion coefficient d is calculated. n The value is 0.841, which is obviously greater than 0.4, indicating that it has good diffusion performance.

[0038] Please see Figure 3 In this embodiment, the microstructure unit cell of the five-mode material is a regular hexagon, which can be closely arranged in a plane and consists of 6 mass blocks and 6 connecting rods. The side length of the hexagonal unit cell is 'a', the length of the connecting rod is 'h', the thickness is 't', the base height of the mass block is 'b', and the top height is 'r', for a total of 5 geometric parameters. In this embodiment, a = 10 mm, h = 4 mm, r = 2.4 mm, and the other two geometric parameters are determined as follows.

[0039] The equivalent density ρ of the unit cell eqFrom its volume fraction V f and the density ρ of the substrate material b We obtain by multiplying, where ρ b The density of aluminum used is 2810 kg / m³ 3 Volume fraction V f It can be obtained from geometric relationships. The equivalent sound velocity c of the unit cell eq It can be determined by calculating its dispersion relation curve. The slope of the tangent line to the second branch curve at the high symmetry point in the dispersion curve is taken as c. eq The value of corresponds to the longitudinal wave velocity. Let the equivalent density ρ of the unit cell be... eq Equal to the equivalent density ρ of seawater H And the equivalent sound velocity c of the unit cell eq Equal to the equivalent speed of sound of seawater, c H Two equations can be obtained, from which the other two geometric parameters t and b can be calculated. In this embodiment, t = 0.48 mm and b = 0.56 mm are obtained. Subsequent studies have shown that good diffusion effects can also be achieved when the geometric parameters are within an error of 0.1 mm, that is, when t is between 0.4 mm and 0.5 mm and b is between 0.5 mm and 0.6 mm.

[0040] Please see Figure 4 Calculate its diffusion coefficient d act The normalized diffusion coefficient d is 0.858. n The diffusion efficiency is 0.815, therefore, a good diffusion effect can be achieved even after filling the microstructure. In practice, the five-mode material microstructure is manufactured using wire cutting. Additionally, the weak equivalent material is filled with a cavity and covered with an aluminum plate, the thickness of which is t. h It is half the thickness of the connecting rod, i.e., t h =t / 2 = 0.24mm.

[0041] Please see Figure 5 The results were compared with a reference plate. It can be observed that the undesigned reference plate exhibits three distinct radiation lobes, with the vertical lobe displaying the strongest sound pressure level. Furthermore, the structure curves of the equivalent metasurface structure and the microstructure-filled metasurface essentially overlap, and their normalized diffusion coefficients d0 and d1 are similar. n The error is less than 3.2%, and the diffusion uniformity in all directions is significantly increased compared to the reference plate, resembling a semi-circular shape. The difference between the maximum and minimum sound pressure levels of the metasurface structure filled with microstructures is only 6.61 dB. Further frequency sweep analysis shows that this embodiment exhibits good performance in the 200 Hz to 650 Hz frequency band. n A value of ≥0.4 indicates good diffusion performance in low-frequency broadband environments.

[0042] Please see Figure 6Considering the actual situation, polyurethane waterproof material will be used to seal the front and rear ends of the structure to prevent seawater from seeping in. The properties of polyurethane material are close to those of water, and the thickness is usually 5mm.

[0043] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A hydroacoustic metasurface structure for low-frequency broadband uniform diffuse reflection, characterized in that: The underwater acoustic metasurface structure is an axisymmetric structure comprising N sub-blocks, which are sequentially connected to form a strip. The strip is divided into two sub-strips in the middle. The sub-block near its geometric center in each sub-strip is a cavity structure, which is made of a weakly equivalent material. The remaining sub-blocks are made of a strongly equivalent material. N is a positive integer greater than 2 and is an even number.

2. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in claim 1, characterized in that: N is 20. The 4th, 5th, 6th, 15th, 16th and 17th sub-blocks from left to right are cavity structures made of weakly equivalent materials, while the remaining sub-blocks are made of strongly equivalent materials. That is, the 1st, 2nd, 3rd, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th, 18th, 19th and 20th sub-blocks from left to right are made of strongly equivalent materials.

3. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in claim 1, characterized in that: The underwater acoustic metasurface structure is used for uniform diffuse reflection modulation of low-frequency broadband acoustic waves, the frequency band of which is 200Hz to 650Hz.

4. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in claim 1, characterized in that: The length of the underwater acoustic metasurface structure is twice the wavelength at which it can diffusely reflect light, and its height is 0.01 times the wavelength at which it can diffusely reflect light.

5. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in claim 1, characterized in that: The aforementioned underwater acoustic metasurface structure is suitable for controlling sound waves with wavelengths of 2.3m to 7.5m, which corresponds to frequencies of f = 200Hz to 650Hz. The wavelength λ, frequency f, and sound velocity c satisfy the relationship c = λf.

6. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in any one of claims 1-5, characterized in that: The equivalent density of the weakly equivalent material is taken as the equivalent density of air, and its equivalent sound velocity is taken as the equivalent sound velocity of air.

7. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in any one of claims 1-5, characterized in that: The equivalent density of the strong equivalent material is taken as the equivalent density of seawater, and its equivalent sound velocity is taken as the equivalent sound velocity of seawater.

8. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in any one of claims 1-5, characterized in that: The strip-shaped body is backed by a steel backing plate, the thickness of which is 0.1 to 0.4 times the diffuse reflection wavelength of the underwater acoustic metasurface structure.

9. The underwater acoustic metasurface structure for low-frequency broadband uniform diffuse reflection as described in claim 8, characterized in that: The steel backplate has a thickness of 0.2 times the wavelength, or 1.04 m, and a density of 7800 kg / m³. 3 The speed of sound is 6100 m / s.

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