Double-gradient design method of sound bunching emission lens and sound bunching emission lens

Abstract

The invention discloses a double-gradient design method for a sound bunching emission lens and the sound bunching emission lens, and the method comprises the steps: constructing a cavity type circular ring structure unit based on a Helmholtz resonator, adjusting the geometric parameters of the cavity type circular ring structure unit, and enabling the height of the cavity type circular ring structure unit to be in linear gradient distribution from the center to the outside along the radial direction; adjusting the structure parameters of the cavity type circular ring structure units according to a design frequency band, so that the refractive index of each cavity type circular ring structure unit in the design frequency band is in linear gradient distribution along the radial direction; constructing a model structure of the sound bunching emission lens by using the dual linear gradient characteristics of the height and the refractive index; the sound field distribution and directivity of the sound bunching emission lens are verified through simulation calculation, and a physical model is manufactured for experimental testing.

Description

Technical Field

[0001] This invention relates to the field of acoustic device technology, and in particular to a dual-gradient design method for acoustic focusing and emitting lenses, and an acoustic focusing and emitting lens. Background Technology

[0002] Since electromagnetic waves cannot propagate long distances in water, sound waves have become the only carrier capable of transmitting information and energy over long distances in the ocean. Underwater acoustic technology has thus become crucial in fields such as underwater communication and navigation, aquaculture, marine resources, marine geology and geomorphology, and military weaponry. The effective operation of underwater acoustic communication equipment relies on high-quality acoustic emission capabilities, which also play a vital role in wave physics. Directional acoustic beams can effectively reduce reverberation, increase detection range, and improve anti-interference capabilities; therefore, improving the acoustic emission directivity of underwater acoustic transducers is of great significance for target detection and long-distance communication.

[0003] To obtain highly directional acoustic waves, early researchers generally employed acoustic arrays and baffle techniques to achieve directional beams in underwater acoustic transmission systems. For example, this involved modulating a design signal and then using a high sound pressure level transducer array to achieve long-distance directional propagation of the sound wave. However, due to the large number of array elements and their dense spacing, the coupling between elements becomes significant. Furthermore, the interactions between the array elements and the shell, as well as between the elements and the baffle, cannot be ignored, posing challenges to the design of array-based transducer directional transmission structures. Modal superposition technology has also been used to achieve directional transmission in underwater acoustic transducers, but the piezoelectric vibrators in the transducer structure have bending modes and shear stress within the material, which can cause mechanical damage in high-power modes. Additionally, the drive signal needs to be adjusted in conjunction with phase parameters, making its application inconvenient.

[0004] In recent years, acoustic metamaterials have demonstrated unique physical properties not found in natural materials, such as negative equivalent mass, negative equivalent modulus, and anisotropic mass. Therefore, an increasing number of scholars are keen to study the superior properties of acoustic metamaterials and apply acoustic metasurfaces to the field of acoustic lenses, combined with underwater transducers, to achieve directional acoustic emission. An acoustic lens is an artificial acoustic structure that can bend the surface of sound waves passing through it, creating a focusing effect on the sound beam. Acoustic lens structures are generally composed of basic units with gradient geometric parameters arranged according to a certain pattern, with periodic arrangements being the most common. Besides the arrangement, the scattering characteristics of the units are a crucial parameter affecting the working effect of the acoustic lens. According to the generalized Snell's law, the angles of reflected, refracted, and diffracted waves can be artificially manipulated by metasurfaces with acoustic phase gradients. The use of acoustic metasurfaces with transverse gradient refractive indices or gradient velocities has been proven to flexibly manipulate the wavefront. Previous research on the construction of acoustic metasurfaces with gradient refractive indices has focused on spatially folded units, such as labyrinthine and interdigitated types, which successfully exhibit a gradient distribution of lateral relative refractive index and, by appropriately selecting the degree of folding, delay the acoustic wave phase, thereby achieving manipulation of the acoustic wavefront. However, since the gradient refractive index is determined by the degree or size of folding of the metasurface units, these metasurfaces have complex structures that are not convenient for application. Furthermore, directional transmission or self-collimating beam focusing based on phase modulation requires adjustment of the wavefront phase. However, this method of achieving highly directional emission through wavefront phase modulation is not suitable for engineering applications because once the deflection angle needs to be changed, the entire structure of the acoustic metalens needs to be adjusted.

[0005] In acoustic reflection metalenses, existing technologies have made some progress. For example, the article "A dual-gradient underwater meta-auricle for broadband sound signal enhancement" published by Ma Fuyin et al. in 2023 and the patent "A dual-gradient acoustic focusing artificial auricle" published in 2022 proposed a dual-gradient artificial auricle that can greatly enhance underwater sound wave signals over a wide frequency range. This article discloses a dual-gradient mechanism of wavefront reflection phase modulation and geometric concave reflection, providing important reference for the development of acoustic reflection metalenses. However, this article does not address a dual-gradient design method applicable to acoustic emission lenses, and suffers from problems such as applicability only to reflection focusing lenses, lack of universality in the design method, poor lens adaptability, and limited working environment.

[0006] The information disclosed in the background section is only for enhancing the understanding of the background of this invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0007] This invention provides a dual-gradient design method for acoustic focusing emission lenses and an acoustic focusing emission lens. By constructing a cavity-type circular ring structure unit based on a Helmholtz resonator with linear gradient height characteristics, and adjusting the structural parameters of the cavity-type circular ring structure unit to achieve a linear gradient distribution of refractive index from the inside to the outside, the linear gradient design of height and refractive index are realized respectively, achieving dual-gradient characteristics, and designing an acoustic focusing emission lens to achieve the purpose of highly directional focused emission.

[0008] A dual-gradient design method for acoustic focusing emission lenses includes,

[0009] A cavity-type circular ring structure unit based on a Helmholtz resonator is constructed, and the geometric parameters of the cavity-type circular ring structure unit are adjusted so that the height of the cavity-type circular ring structure unit is linearly gradient distributed radially from the center outward.

[0010] The structural parameters of the cavity-type ring structure unit are adjusted according to the design frequency band so that the refractive index of each cavity-type ring structure unit has a linear gradient distribution along the radial direction within the design frequency band.

[0011] A structural model of an acoustic focusing and emission lens is constructed by utilizing the dual linear gradient characteristics of height and refractive index.

[0012] The sound field distribution and directivity of the acoustic focusing emission lens were verified by simulation calculation, and a physical model was made for experimental testing to obtain a dual-gradient emission lens with broadband acoustic focusing capability.

[0013] In the method described, the cavity-type annular structure unit is a single resonant cavity structure, including a cavity, an opening channel, and a wall, wherein the gradient distribution of refractive index is achieved by adjusting the cavity width or cavity height.

[0014] In the method described, the operating frequency band shift is achieved by adjusting the concave radius and structural parameters of the cavity-type annular structure unit.

[0015] In the method described, the hollow annular structural unit is made of metal material, and multiple hollow annular structural units are arranged with a common center.

[0016] In the method described, the refractive index is obtained by calculation. ,in It is the refractive index at the center of the lens (r=0); The gradient coefficient is calculated using the following formula. In the formula, H is half the height of the cavity. It is the refractive index at the edge of the lens, which is changed by a. i The value of H iThe value is adjusted to achieve the desired transmittance.

[0017] In the method described above, the simulation uses the finite element method to establish an acoustic propagation model that simulates the propagation process of sound waves in a lens in two-dimensional axisymmetric or three-dimensional space.

[0018] An acoustic beam-focusing emission lens, which is made according to the method described above.

[0019] In the aforementioned acoustic focusing emission lens, a tapered connecting waveguide is installed.

[0020] In the aforementioned acoustic focusing emission lens, the overall size of the acoustic focusing emission lens is smaller than the operating wavelength.

[0021] In the aforementioned acoustic focusing emission lens, the acoustic focusing emission lens achieves a beamwidth of less than 40° with a range of -3dB within the design frequency range, and has broadband acoustic focusing performance in the frequency band from 4kHz to 6kHz.

[0022] Compared with existing technologies, this invention has the following advantages: It employs a dual gradient adjustment mechanism combining the linear gradient distribution of height and the linear gradient distribution of refractive index within a cavity-type annular structural unit. This significantly reduces the beamwidth by -3dB while simultaneously improving the lens's acoustic focusing capability. The lens structure is simple, facilitating rapid mass production, and its overall size is smaller than the operating wavelength, meeting the requirements for compact structure and lightweight design. The lens can be used for focused acoustic signal transmission in both gaseous media such as air and liquid media such as water, demonstrating broad application prospects. The introduction of a tapered connecting waveguide allows the underwater acoustic lens, designed according to the frequency band, to be adapted to underwater acoustic transducers of different diameters. By adjusting structural parameters such as the concave radius and metasurface units, the operating frequency band can be flexibly shifted. By utilizing the dual gradient adjustment mechanism of linear gradient refractive index and linear gradient height within a cavity-type annular structural unit, an acoustic focusing emission lens is constructed to achieve highly directional focused emission. This invention can operate in gaseous media such as air as well as liquid media such as water. It features a simple structure, with overall dimensions comparable to the operating wavelength, meeting the requirements for compact design and lightweight construction, and thus has broad application prospects. This invention can be widely used in acoustic target detection, underwater communication and navigation, and other fields, possessing significant engineering application value. Attached Figure Description

[0023] 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.

[0024] In the attached diagram:

[0025] Figure 1 This is a structural diagram of a single unit;

[0026] Figure 2 A schematic diagram showing the refractive index calculation results for a single element;

[0027] Figure 3 A two-dimensional axisymmetric view of a focusing acoustic lens composed of six units with dual gradients in height and refractive index arranged radially;

[0028] Figure 4 A schematic diagram of a three-dimensional beam-emitting acoustic lens consisting of six units with dual gradients in height and refractive index arranged radially.

[0029] Figure 5 A schematic diagram of a two-dimensional axisymmetric computational model and simulation setup for an underwater acoustic focusing lens composed of six units with dual gradients in height and refractive index arranged radially in a non-free field;

[0030] Figure 6 A schematic diagram of the calculated sound field distribution near the design frequency;

[0031] Figure 7 To create a sound field distribution cloud map near the design frequency;

[0032] Figure 8 The sound pressure level diagrams are shown for different frequencies within a beamwidth of 40° at 2m from the lens exit end.

[0033] Figure 9 This is a sound field distribution cloud map at high frequencies;

[0034] Figure 10 Photographs and measurement diagrams of the fabricated underwater acoustic focusing emission lens test sample;

[0035] Figure 11 This is a schematic diagram of the acoustic signal test results for a focused emission lens.

[0036] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0037] 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 may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0038] 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.

[0039] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0040] like Figures 1 to 11 As shown, a dual-gradient design method for an acoustic focusing emission lens includes,

[0041] A cavity-type circular ring structure unit based on a Helmholtz resonator is constructed, and the geometric height of the cavity-type circular ring structure unit is adjusted so that the height of the cavity-type circular ring structure unit is linearly gradient distributed radially from the center outward.

[0042] The structural parameters (such as opening size and cavity width) of the cavity-type ring structure unit are adjusted according to the design frequency band (such as 4k-6kHz) so that the refractive index of each cavity-type ring structure unit has a linear gradient distribution along the radial direction within the design frequency band.

[0043] Utilizing the dual linear gradient characteristics of height and refractive index, a structural model of the acoustic focusing emission lens is constructed for simulation calculations and serves as a printed model of the experimental sample.

[0044] The sound field distribution and directivity of the acoustic focusing emission lens were verified by simulation calculation, and a physical sample was made for experimental testing of acoustic focusing, thus realizing a dual-gradient emission lens with broadband acoustic focusing capability.

[0045] In a preferred embodiment of the method, the cavity-type annular structure unit is a single resonant cavity structure, including a cavity, an opening channel, and a wall, wherein the gradient distribution of refractive index is achieved by adjusting the cavity width or cavity height.

[0046] In a preferred embodiment of the method, the operating frequency band shift is achieved by adjusting the concave radius and structural parameters of the cavity-type annular structure unit.

[0047] In a preferred embodiment of the method, the hollow annular structural unit is made of metal material, and multiple hollow annular structural units are arranged with a common center.

[0048] In a preferred embodiment of the method, the refractive index is calculated. ,in is the refractive index at the center of the lens (r=0), and sech is the hyperbolic secant function; The gradient coefficient is calculated using the following formula. In the formula, H is half the height of the cavity. It is the refractive index at the edge of the lens, which is changed by a. i The value of H i The value is adjusted to achieve the desired transmittance.

[0049] In a preferred embodiment of the method, the simulation employs the finite element method to establish an acoustic propagation model that simulates the propagation process of sound waves in a lens in two-dimensional axisymmetric or three-dimensional space.

[0050] An acoustic beam-focusing emission lens, which is made according to the method described above.

[0051] In a preferred embodiment of the acoustic beam-focusing emission lens, the acoustic beam-focusing emission lens is equipped with a tapered connecting waveguide.

[0052] In a preferred embodiment of the acoustic focusing emission lens, the overall size of the acoustic focusing emission lens is smaller than the operating wavelength.

[0053] In a preferred embodiment of the acoustic focusing emission lens, the acoustic focusing emission lens achieves a -3dB beamwidth of less than 40° within the design frequency range and has broadband acoustic focusing performance in the 4kHz to 6kHz frequency band.

[0054] In one embodiment, the dual-gradient design method for an acoustic focusing emission lens includes the following steps:

[0055] (1) Construct a cavity-type circular ring structure unit based on a Helmholtz resonator, and adjust the unit parameters to achieve a linear gradient distribution of the unit height from the inside to the outside;

[0056] (2) Adjust the structural parameters of the cavity ring structure unit according to the design frequency band so that the refractive index of each cavity ring structure unit presents a linear gradient distribution;

[0057] (3) By utilizing the dual linear gradient characteristics of unit height and refractive index, a high directivity beamforming of sound waves is achieved, and a basic model structure of a sound beamforming superlens is constructed.

[0058] (4) The acoustic focusing capability of the acoustic focusing emission lens was calculated through simulation, and a design model was fabricated for experimental verification. The constructed cavity-type circular ring structure unit based on the Helmholtz resonator has a linear gradient distribution characteristic from the inside to the outside in terms of height. The refractive index of each cavity-type circular ring structure unit also has a linear gradient distribution characteristic within the design frequency band. The cavity-type circular ring structure unit is a resonant cavity structure based on the Helmholtz resonator. The designed acoustic focusing emission lens is installed on a tapered connecting waveguide, which is beneficial for focusing the emitted sound waves. The underwater acoustic lens designed using this method can be adapted to underwater acoustic transducers of different sizes. The emission lens designed using this method can achieve a highly directional broadband acoustic focusing emission effect in liquid media, represented by water.

[0059] As an implementation example, water is chosen as the working medium, and the unit material is aluminum alloy. The unit cross-section used in this implementation example is attached. Figure 1 As shown, this is a single resonant cavity structure based on a Helmholtz resonator with an internal opening. To ensure that sound waves can interact with the resonant cavity, a sound propagation channel is left on the opening side of the resonant cavity. The width of the resonant cavity is a. i The cavity height is H i The wall thickness is w, the opening height is h, and the width of the overall unit is b. Here, the subscript i is an integer; a smaller value indicates a closer proximity to the metasurface center. By adjusting the parameters of the resonant cavity, the refractive index of each unit can be adjusted to obtain the desired refractive index. The theoretical formula for calculating the refractive index can be obtained through calculation. ,in It is the refractive index at the center of the lens (r=0); The gradient coefficient can be calculated using the following formula. In the formula, H is half the height of the cavity. This refers to the refractive index at the lens edge. Keeping other dimensional parameters constant, by changing a... i The value of H iAll values ​​can be used to adjust the transmittance. Different resonant cavity parameters result in different resonant frequencies. To obtain a linear gradient refractive index at the center operating frequency of 5kHz, we used 5kHz as the resonant frequency of the resonant cavity and used the theoretical formula for the resonant frequency to deduce reasonable resonant cavity parameters. To facilitate manufacturing and reduce the weight of the lens, we first determined the thickness of the resonant cavity to be w = 3.2mm and the opening width of the resonant cavity to be d = 2mm. Given a set of structural parameters (a1 = 23 mm; a2 = 21 mm; a3 = 20 mm; a4 = 19 mm; a5 = 18 mm; a6 = 17 mm; H1 = 100 mm; H2 = 106 mm; H3 = 110 mm; H4 = 115 mm; H5 = 117 mm; H6 = 118 mm; w = 3.2 mm; h = 2 mm; b = 28 mm), a two-dimensional axisymmetric simulation model was established using the acoustic-structure interaction module in the commercial multiphysics coupled finite element analysis software COMSOL. The unit structure is made of aluminum alloy with an elastic modulus, density, and Poisson's ratio of 72 GPa and 2800 kg / m³, respectively. 3 And 0.28. The sound propagation medium is water, with a sound velocity of 1500 m / s and a density of 1000 kg / m³. 3 The elements are placed in a two-dimensional waveguide, and plane wave radiation is applied to the upper and lower boundaries of the waveguide. The lower boundary is used as the incident end, the incident wave frequency range is 4500Hz-5500Hz, and the sound pressure amplitude of the incident plane wave is 1Pa. The refractive index calculation results of the 6 elements are as follows: Figure 2 As shown, at 5000Hz, the refractive indices of the six elements are different, and the refractive index of each element decreases in a continuous gradient as the cavity height H increases. Therefore, arbitrary manipulation of the refractive index can be achieved through this type of element.

[0060] In principle, the more elements there are, the smoother the refractive index gradient distribution, thus enabling fine-tuning of the wavefront. However, numerical calculations revealed that as the resonant cavity width *a* in each element decreases, the increase in the resonant cavity height *H* slows down to maintain the gradient decrease in the equivalent refractive index of the element. Otherwise, the refractive index of the element would increase at the resonant frequency. Therefore, the number of elements is limited. We designed five, six, and eight elements. Since the calculations and physical analyses are the same for all three, we mainly analyze the construction of a six-element underwater acoustic focusing lens with linear gradient refractive index and linear gradient height. For ease of description, the six elements are designated as 1#, 2#, 3#, 4#, 5#, and 6#, with corresponding cavity heights of H1 = 100 mm, H2 = 106 mm, H3 = 110 mm, H4 = 115 mm, H5 = 117 mm, and H6 = 118 mm, respectively. Because the radial dimensions of the sound source and the metasurface are different, we designed a tapered waveguide as a connecting structure based on the plane wave propagation law. The superlens structure composed of the concave metasurface and the connecting waveguide is shown below. Figure 3 As shown. It is worth mentioning that the lens has a centrally symmetrical structure, so only the asymmetrical part is shown here. In addition, in order to increase the transmitted sound energy, we added an auxiliary channel at the end of the lens. Although this channel increases the radial dimension of the lens, it also enriches the lens's adjustment methods. For example, the transmitted sound energy can be increased by changing the radial width of the channel within the operating frequency band.

[0061] By arranging these six units from the inside out in a height gradient and placing them in a tapered connecting waveguide, the following is obtained: Figure 4 The diagram shows an underwater acoustic emission lens structure. The heights of these units are approximately distributed according to a parabolic gradient, with unit #1 having the lowest height. The height of each unit gradually increases with increasing radial distance. The refractive index of each unit is approximately distributed according to a linear gradient, with units #1, #2, #3, #4, #5, and #6 having approximate refractive indices of 1.235, 1.216, 1.197, 1.179, 1.141, and 1.121, respectively. This results in a dual-gradient focusing emission lens structure, where both the geometric height and refractive index of the metasurface units follow a linear gradient distribution. By... Figure 3 Rotating the two-dimensional axisymmetric structure in [the image] 360 degrees around its central axis yields, as shown below: Figure 4 The three-dimensional structure is shown. To obtain the focusing emission effect of this dual-gradient underwater acoustic lens, a finite element full-wave simulation model was established, as follows: Figure 5As shown, a Perfectly Matched Layer (PML) is added around the model to reduce the impact of reflected sound waves. Furthermore, the lens model is placed in water to closely approximate actual operating conditions. Through simulation calculations, the focused emission sound field at the design frequency of 5kHz can be obtained (sound pressure level is used uniformly in this invention), as shown... Figure 6 As shown in the figure, the sound waves exhibit a good focused emission effect after passing through the transmitting lens. Furthermore, we calculated the sound field distribution of the underwater acoustic lens at other frequencies within the 4kHz-6kHz frequency band. Figure 7 As shown, we selected six representative sound field distributions. From the sound field distributions, it can be seen that the underwater acoustic lens designed in the wide frequency band of 4k-6kHz can achieve a good beam-focusing emission effect.

[0062] To determine the focused emission directivity of the designed lens and its -3dB beamwidth, this invention uses the center point of the transmitting end face as a reference in the calculated acoustic field. A measurement baseline with a radius of 2m is selected, and two points are chosen on this baseline. One of these points lies on the axis of the transmitting lens, and the arc formed by the two points corresponds to an angle of 20°, which corresponds to a conical region with a cone angle of 40° in three-dimensional space. This invention characterizes the directivity of the transmitting lens by judging the sound pressure level based on a given beamwidth of 40°. Through numerical calculation, the sound pressure level corresponding to a beamwidth of 40° can be obtained in a wide frequency range of 1k-17kHz, such as... Figure 8 As shown in the figure, the sound pressure level exceeds 3dB in the 4k-6kHz frequency band, meaning the beamwidth at -3dB is less than 40°. It is noteworthy that the sound pressure level generally increases with frequency. However, the sound pressure level is lower near 11.5kHz and 13.5kHz. This is due to the significant coupling effect between the unit resonance and the tapered waveguide and the water medium, resulting in the sound field being localized within and near the lens. The sound field distribution near higher frequencies is shown in the figure. Figure 9 As shown, it can be seen that as the frequency increases, the -3dB beamwidth of the underwater acoustic lens becomes narrower, making it suitable for long-distance propagation and communication.

[0063] To verify the focusing effect of the designed underwater acoustic focusing lens, an aluminum alloy underwater acoustic lens sample was fabricated using traditional milling machining methods, such as... Figure 10 (See right). The test was conducted in an anechoic tank. A cylindrical transducer sound source was placed at the end of the transmitting lens, emitting a linear frequency modulated signal with a frequency range of 4000Hz to 6000Hz. A hydrophone was then placed 2m away from the transmitting end of the lens to collect the sound signal, as shown in the attached diagram. Figure 10 As shown on the left, the acoustic signal near the transmitting lens was acquired by changing the position of the hydrophone, FFT transformation was performed, and a radiation pattern was plotted, as shown below. Figure 11The diagram shows (only the measured radiation patterns at two frequencies, 4400Hz and 6000Hz, are given here; the results for other frequencies are similar). It can be seen from the diagram that when the transducer frequency is 4.4kHz and 6kHz, the -3dB beamwidth of the underwater acoustic lens is 30°-40°, which is consistent with the simulation results.

[0064] This invention utilizes the Helmholtz resonator principle to design a cavity-type circular ring structure unit. This unit possesses acoustic resonance characteristics, and its acoustic response can be adjusted by modifying its geometric parameters (such as cavity width and height). This enables efficient manipulation of incident sound waves, enhancing the lens's ability to control the phase and amplitude of sound waves through resonance. Compared to traditional complex metamaterial structures, this structure is simple and easy to fabricate, making it suitable for large-scale applications. Multiple cavity-type circular ring structure units are arranged sequentially from the center outwards, with the height of each unit increasing linearly. The height variation creates a time delay difference in the sound wave propagation path, resulting in a spatially continuously varying wavefront deflection effect, which helps improve the directionality and focusing ability of the emitted sound beam. Simultaneously, by adjusting the structural parameters of each unit (such as cavity width and opening size), its equivalent refractive index exhibits a linear gradient distribution within the target frequency band. The dual gradient distribution of height and refractive index works synergistically in the sound wave propagation process, resulting in stronger wavefront control capabilities. The dual-gradient design significantly enhances the precise control over the direction of sound wave propagation. Compared to a single-gradient design, it can substantially reduce the -3dB beamwidth, improve acoustic focusing performance, and achieve a narrower main lobe and lower side lobes. A tapered connecting waveguide is placed between the lens and the transducer to achieve effective coupling of acoustic energy. This improves acoustic energy transmission efficiency and allows the lens to be adapted to transducers of different diameters, enhancing its versatility and engineering practicality. By adjusting parameters such as the concave radius and cavity size, the resonant frequency of the overall structure can be changed. This allows for flexible adjustment of the lens's operating frequency band, adapting to the frequency requirements of different application scenarios and improving the lens's multifunctionality and applicability. The designed lens structure is compact, with an overall size much smaller than the wavelength of the sound wave corresponding to the operating frequency. It meets the design requirements of lightweight and miniaturization, facilitating integration into various underwater devices or portable systems, and has good engineering application prospects. The lens structure can achieve good acoustic signal focusing and transmission in both gaseous media such as air and liquid media such as water. This expands the lens's application scenarios, making it suitable for general acoustic systems in the air as well as specialized fields such as underwater communication and detection, thus enhancing the technology's versatility. Within the 4kHz to 6kHz frequency range, the lens achieves excellent beam focusing and directivity enhancement, ensuring stable performance over a wide frequency range and improving robustness and adaptability in practical applications. The unit structure is simple, using conventional materials such as aluminum alloy and processed through mature techniques like milling, reducing manufacturing costs and process complexity, which facilitates industrial mass production and widespread application.

[0065] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A dual-gradient design method for acoustic focusing emission lenses, characterized in that, It includes, A cavity-type circular ring structure unit based on a Helmholtz resonator is constructed, and the geometric parameters of the cavity-type circular ring structure unit are adjusted so that the height of the cavity-type circular ring structure unit is linearly gradient distributed radially from the center outward. The structural parameters of the cavity-type ring structure unit are adjusted according to the design frequency band so that the refractive index of each cavity-type ring structure unit has a linear gradient distribution along the radial direction within the design frequency band. A structural model of an acoustic focusing and emission lens is constructed by utilizing the dual linear gradient characteristics of height and refractive index. The sound field distribution and directivity of the acoustic focusing emission lens were verified by simulation calculation, and a physical model was made for experimental testing to obtain a dual-gradient emission lens with broadband acoustic focusing capability.

2. The method according to claim 1, characterized in that, Preferably, the cavity-type annular structure unit is a single resonant cavity structure, including a cavity, an opening channel, and a wall, wherein the gradient distribution of refractive index is achieved by adjusting the cavity width or cavity height.

3. The method according to claim 1, characterized in that, The operating frequency band shift can be achieved by adjusting the concave radius and structural parameters of the cavity-type annular structure unit.

4. The method according to claim 1, characterized in that, The hollow ring structure unit is made of metal material, and multiple hollow ring structure units are arranged with the same center.

5. The method according to claim 1, characterized in that, The refractive index is obtained through calculation. ,in is the refractive index at the center of the lens (r=0), and sech is the hyperbolic secant function; The gradient coefficient is calculated using the following formula. In the formula, H is half the height of the cavity. It is the refractive index at the edge of the lens, which is changed by a. i The value of H i The value is adjusted to achieve the desired transmittance.

6. The method according to claim 1, characterized in that, The simulation uses the finite element method to establish an acoustic propagation model that simulates the propagation process of sound waves in a lens in two-dimensional axisymmetric or three-dimensional space.

7. A sound focusing and emitting lens, characterized in that, It is made by the method according to any one of claims 1-6.

8. The acoustic focusing and emitting lens according to claim 7, characterized in that, The acoustic focusing emission lens is equipped with a tapered connecting waveguide.

9. The acoustic focusing and emitting lens according to claim 7, characterized in that, The overall size of the acoustic focusing emission lens is smaller than the operating wavelength.

10. The acoustic focusing and emitting lens according to claim 7, characterized in that, The acoustic focusing emission lens achieves a beamwidth of less than 40° with a range of -3dB within the design frequency range and has broadband acoustic focusing performance in the 4kHz to 6kHz frequency band.

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