An acoustic lens implemented using a curved waveguide

Abstract

The application makes in-depth research on the field of sound wave front regulation in three-dimensional cases. A special acoustic lens is constructed by using a metasurface, so that the required target sound field is obtained, and the sound focusing, sound splitting and sound invisibility phenomena are realized. The research on acoustic lens is a challenging and innovative field. In the past, the research on acoustic lens was hindered by many factors such as too large material structure, complex transmission wave phase regulation and the like. The application designs a high-transmittance curved waveguide structure, and the length of the straight pipe at the neck of the curved waveguide can be adjusted to realize accurate control of the phase of the transmitted wave, and a complex internal structure is not needed. At the same time, the phase shift in the whole 2pi range can be realized by 8 basic units. Based on this, according to the theoretical calculation results, the 8 basic units are freely combined to construct a specific metasurface, so that effects such as sound focusing, acoustic lens, invisibility cloak and the like can be realized.

Description

TECHNICAL FIELD

[0001] The application utilizes a bending waveguide to realize an acoustic lens with acoustic focusing, acoustic splitting and acoustic cloaking functions, and belongs to the field of acoustic metamaterials. BACKGROUND

[0002] Acoustic wave transmission is an important research topic in many scientific and engineering fields, and the control and phase adjustment of the acoustic wave transmission path is one of the key problems. In acoustic research, traditional methods generally need to rely on complex structures and internal devices to adjust the transmission characteristics of acoustic waves, such as using periodically arranged structures, the size characteristics of which are usually on the same order of magnitude or several times the length of the wavelength of the acoustic wave. However, this design method has the disadvantages of large loss, narrow bandwidth and complex production, which makes it difficult for traditional acoustic wave control devices to be used in modern multifunctional and multi-scene environments. Considering these defects, it is particularly important to develop a thin acoustic wave control structure. Meanwhile, the acoustic wave loss in the lens is also a problem, especially in the high frequency range, and controlling the loss to improve the efficiency of the lens is a difficulty. As a kind of artificial material fundamentally different from traditional materials, acoustic metasurface is usually composed of periodically arranged subwavelength structures. On this basis, people have full freedom to design various unique structures, thereby obtaining diversified material properties and realizing many abnormal acoustic wave transmission phenomena, such as abnormal reflection / refraction, acoustic focusing, acoustic cloaking, etc. In addition, the development of acoustic lenses with excellent performance has potential application value in the fields of acoustic imaging, medical ultrasonic imaging and acoustic communication. The core of using acoustic metasurface to realize efficient control of transmitted acoustic field information lies in the control of the phase and amplitude of the transmitted wave. However, due to the limitations of existing acoustic materials, it is still challenging to achieve 2π range phase control. At the same time, even if the phase adjustable range can cover the full phase, efficient transmission is also a difficulty. At present, the phase control of acoustic waves can be realized by using a curled maze structure or a Helmholtz resonator or a membrane type structure. However, the amplitude of the transmitted wave cannot be maintained in a relatively high range, and the existing acoustic wave loss may cause the phenomenon to be not obvious, reducing the application scenarios of the metasurface.

[0003] Based on this, a bending waveguide unit with a subwavelength thickness is designed to realize full-range control of the phase of the transmitted wave. Based on this, an acoustic lens based on acoustic metasurface is constructed, which can control the three-dimensional wave front of the transmitted acoustic wave, and then realize the functions of acoustic focusing, acoustic splitting and acoustic cloaking of the transmitted acoustic field. SUMMARY

[0004] The application proposes a curved waveguide structure with high transmittance, which can realize precise control of the phase of the transmitted wave by adjusting the length of the straight pipe of the curved waveguide neck, and does not require complex internal structure. At the same time, by constructing 8 basic units, the phase shift in the entire 2π range can be realized. Based on this, according to the theoretical calculation results, 8 basic units are freely combined to construct a specific metasurface, which can realize an acoustic lens with acoustic focusing, acoustic splitting and acoustic stealth functions.

[0005] The key technology adopted by the application to solve its technical problems is:

[0006] (1) Design of acoustic lens

[0007] Based on the theory of transformation acoustics, increasing the length of the intermediate "path" can make the transmission path of the sound wave longer, thereby changing its equivalent sound speed, i.e. changing the phase of the transmitted sound wave, without changing the position of the entrance and exit. Compared with the traditional curl structure, the structure proposed in this paper does not require complex internal structure, it is composed of two pairs of straight pipes and three circular arcs, the whole structure is distributed in axial symmetry, one pair of straight pipes of the waveguide is the entrance and exit, and the length of the straight pipe of the neck of the curved waveguide can be adjusted to control the phase of the transmitted sound wave, which significantly simplifies the complexity of the structure and improves the transmittance of the sound wave.

[0008] (2) Realization of focusing and splitting functions of sound waves based on generalized Snell's law

[0009] When the sound wave passes through the curved waveguide, the curved waveguide surface will refract the sound wave, and change the propagation path of the sound wave according to the generalized Snell's law. The design of the curved waveguide can be used to realize the focusing and splitting of the sound wave, especially in a multi-channel system, by designing waveguide structures with different curvatures and refraction angles, the sound wave can be focused and split to different target positions.

[0010] (3) Use of metasurface technology to realize conversion of plane wave to cylindrical wave

[0011] The metasurface can realize the conversion of the incident sound wave, converting the plane wave into a cylindrical wave or other forms of sound wave. This conversion can disperse or focus the sound wave energy in space, thereby changing the propagation path of the sound wave, avoiding the concentration of the sound wave on the object, and thereby realizing the "acoustic stealth" of the object.

[0012] The beneficial effects of the application are:

[0013] The application designs an acoustic lens based on a curved waveguide structure, which realizes precise control of the phase of the transmitted wave by adjusting the length of the straight pipe of the waveguide neck. Compared with the traditional method, the design greatly simplifies the internal structure and does not require complex units or structures. Only by adjusting the length of the straight pipe can the entire 2π phase range (phase from 0 to 2π) be covered, improving the convenience and flexibility of the structure.

[0014] The present application realizes high transmittance and stable transmittance amplitude by designing an optimized curved waveguide structure. This means that during the operation of the lens, the acoustic wave does not reduce the energy due to structural loss, thereby ensuring the effective transmission and energy concentration of the acoustic wave, which is crucial for the acoustic transmission system in practical applications.

[0015] By combining different basic units, various acoustic phenomena and applications can be realized, including acoustic focusing, acoustic cloaking and acoustic splitting. These three effects have wide application prospects in the fields of acoustic imaging, ultrasonic treatment, acoustic communication, acoustic cloaking, etc. For example, the designed acoustic cloaking lens can "hide" objects in the acoustic environment, making them difficult to be detected; the acoustic focusing lens can be applied to efficient acoustic imaging and medical imaging. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Schematic diagram of front view and geometric dimensions of curved waveguide structure

[0017] Figure 2 (a) Schematic diagram of three-dimensional curved waveguide structure, the stretching length in z-axis direction is d; (b) Transmitted wave phase and transmittance change with neck straight tube h. The solid line represents the change of the transmitted phase, and the dashed line represents the change of the transmittance. Black dots represent the selected 8 basic units; (c) Transmitted sound pressure field of 8 basic units at 8000Hz frequency (top view)

[0018] Figure 3 (a) Schematic diagram of acoustic focusing principle based on metasurface; (b) Distribution of phase change provided by metasurface along z-axis, where the red solid line represents the phase distribution at each position after discretization; (c) Acoustic energy field diagram of acoustic focusing simulation, the inset is the front view of the metasurface; (d) Normalized sound pressure amplitude on the central axis

[0019] Figure 4 (a) Approximate phase distribution of Airy beam; (b) and (d) are sound pressure field intensity diagrams in the absence and presence of obstacles, respectively; (c) Sound pressure field intensity at the dashed line in (b) and (d), solid and dashed lines represent the absence and presence of obstacles, respectively

[0020] Figure 5 (a) Conceptual diagram of acoustic focusing lens; (b) Simulation sound pressure field diagram of focusing lens with cylindrical wave incidence

[0021] Figure 6(a) schematic of the principle of a surface-based acoustic splitting lens; (b) the phase change required for acoustic splitting, where the star marks represent the phase distribution at each location after splitting; (c) simulated acoustic pressure field of acoustic splitting, inset is the front view of the metasurface; (d) normalized acoustic pressure amplitude at a distance of 100 mm from the metasurface

[0022] Figure 7 (a) schematic of the principle of an acoustic "invisibility cloak" based on an acoustic focusing lens; (b) and (c) are the acoustic intensity maps in the presence and absence of the "invisibility cloak"

[0023] Figure 8 (a) schematic of the principle of an "invisibility cloak" based on acoustic beam splitting; (b) simulated acoustic pressure field in the presence of a triangular cloaked region; (c) simulated acoustic pressure field in the absence of the metasurface DETAILED DESCRIPTION

[0024] The preferred embodiments of the acoustic lens realized by the curved waveguide according to the present application are described in detail below with reference to the accompanying drawings. Figures 1 to 8 The specific embodiments of the acoustic lens design based on the curved waveguide are described below. In order to make the features and advantages of the present patent more obvious and easy to understand, examples are given below and described in detail as follows:

[0025] Figure 1 is a schematic diagram of a single curved waveguide structure in two dimensions, which is composed of two pairs of straight pipes and three circular arcs, and the entire structure is distributed in axial symmetry. One pair of straight pipes of the waveguide is the entrance and exit, and the neck straight pipe is used to control the phase of the transmitted wave. The length of the straight pipe of the entrance and exit is a = 25 mm, and the width is 2d = 6 mm. The radius of the circular arc is r = a / 2-2d, and the angle θ = π / 4.

[0026] Figure 2 (a) shows a schematic diagram of the structure of a three-dimensional curved waveguide, and the width of the entire structure on the z-axis is d. After the sound wave is incident from the entrance, the length of the neck straight pipe is adjusted to change the time for the sound wave to reach the exit, thereby realizing accurate adjustment of the phase of the transmitted sound wave. Figure 2 The solid line in (b) is the change of the phase of the transmitted wave of the basic unit with the length h of the straight pipe obtained by finite element simulation when the frequency of the incident wave is 8000 Hz. It can be observed from the straight line graph that the curved waveguide can realize a full phase range distribution of 0 to 2π with the change of the length h of the straight pipe. In order to simplify subsequent research, one phase period (0~2π) is discretized into 8 parts (0, π / 4, π / 2, 3π / 4, π, 5π / 4, 3π / 2 and 7π / 4), which respectively correspond to Figure 2 The solid dots in (b) are 20.4 mm, 17.9 mm, 15.2 mm, 12.5 mm, 9.8 mm, 7.2 mm, 4.4 mm and 1.8 mm. At the same time Figure 2(b) The dashed line is the transmission amplitude. It can be seen that the amplitude of the transmitted wave is not affected by the length of the straight tube at the neck of the curved waveguide, i.e. perfect transmission of the sound wave can be achieved. As Figure 2 (c) shows the transmission sound field intensity of the eight basic structures under the same incident conditions. It can be seen from the figure that the phase of the transmitted wave changes in the range of 2π after passing through different structures, which proves that the curved waveguide structure has excellent phase control ability.

[0027] Figure 3 (a) illustrates the concept of converting a plane wave into a cylindrical wave. When a plane wave is incident vertically on the metasurface, focusing can occur at a certain point in the transmission region. The acoustic energy at the focal point is significantly higher than at other positions in the transmission region. Assuming that the incident wave is vertically incident along the x-axis from below, all transmitted waves pass through the focal point. In order to focus the sound wave at the focal point, the phase distribution of the metasurface should satisfy:

[0028]

[0029] Take the focal point S(z0, x0) = (0, 170 mm) as an example, Figure 3 (b) plots the phase distribution of the metasurface along the z-axis according to equation (1). The black solid line in the figure is the phase distribution at each position after discretization, based on which the metasurface structure is constructed. The front view of the metasurface is shown in Figure 3 (c) as shown in the insert. At this time, the focal length f = 0. The transmitted sound field energy after the plane wave is incident is shown in Figure 3 (c) as shown, it can be found that the transmitted wave energy is focused at a certain point. In order to more intuitively illustrate the focusing effect, the sound pressure amplitude distribution along the axial direction of the focal point is measured. In Figure 3 (d) it can be seen that there is a clear peak at 168 mm, which is basically consistent with the theoretically designed focal point (0, 170 mm).

[0030] Figure 4 (a) is a metasurface that can generate Airy beams by reasonable arrangement. A perfect Airy beam is generated at 8000 Hz, as shown in Figure 4 (b). The ideal Airy beam should have self-healing and self-bending properties. The self-bending property can be clearly verified from the sound pressure field intensity diagram. Next, five obstacle balls with a radius of 15 mm are placed uniformly at a distance of 50 mm from the metasurface, and the sound pressure field intensity diagram is as follows Figure 4(d) shows that although the transmission of sound waves is affected to some extent, the transmitted wave has self-healing ability and generates high-quality Airy beams. To better illustrate the self-healing of the generated Airy beams, the sound pressure intensity at the same position (dashed line) is shown in Figure 4 (c). As can be seen in the figure, with the obstacle, only the peak value is reduced, and the sound pressure field distribution is basically unchanged. It shows that the obstacle only reduces the transmission intensity of the transmitted wave, which proves the self-healing of the Airy beam.

[0031] Figure 5 (a) is a schematic diagram of an acoustic focusing lens. A point sound source is emitted from point S and focused on point F after passing through the two-part combined metasurface. Figure 5 (b) shows the sound pressure field after the cylindrical wave is incident. It can be seen that there is a clear focal point after passing through the metasurface.

[0032] Figure 6 (a) gives the schematic diagram of an acoustic splitter. When a plane wave is incident vertically, the transmitted wave is split into two beams with an angle of about 45°. Figure 6 (b) shows the phase change required to achieve acoustic splitting calculated theoretically. The black mark in the figure is the phase unit adopted in this paper. The sound pressure amplitude at a frequency of 8000 Hz is shown in Figure 6 (c) shows that the plane wave is "split" into two sound waves, which is basically consistent with the theoretical splitting direction. To better prove the splitting of the transmitted wave, the sound pressure amplitude at a distance of 100 mm from the metasurface is measured, and the Figure 6 (d) shows that there are two obvious wave peaks in the sound pressure amplitude, which proves the feasibility of acoustic splitting.

[0033] Figure 7 (a) is a conceptual schematic diagram of an acoustic invisibility cloak. When a plane wave is incident, the sound wave forms an acoustic focus after passing through the lower metasurface, and continues to propagate to the upper metasurface as a plane wave. In the middle region, due to the focusing property of the sound wave, there is no sound energy in a pair of triangular regions. Therefore, if an object that needs to hide information is placed in these two triangular regions, the "invisibility cloak" effect can be achieved. When the sound wave passes through, the object inside the "cloak" does not interfere with the transmission of the sound wave. At the same time, the quality of the focusing effect also directly affects the size of the "invisibility cloak". Figure 7 (b) gives the acoustic "invisibility cloak" model. To better illustrate the "invisibility" effect, a pair of symmetric triangular regions are added in the simulation as structures that need to be "invisible", and the outer boundary is added with a hard boundary constraint. The sound pressure field is shown in Figure 7 (c). From Figure 7 (b) and Figure 7(c) can be seen that the distribution of sound pressure field is basically the same, which proves the rationality of the sound "cloak of invisibility".

[0034] Figure 8 (a) is a schematic diagram of the "cloak of invisibility" based on sound beam splitting. Figure 8 (b) is the amplitude diagram of the sound pressure field with "invisibility" area. As a comparison, as shown in Figure 8 (c) is shown, after placing a triangle with hard boundary in the transmission area, while removing the metasurface, when the plane wave is incident, it will be "forced to cut" into two parts, and there will be no sound splitting transmission field that divides into two complete transmission waves. Thus, the superiority of the designed sound splitting invisibility cloak is proved, and in actual situations, the "invisibility cloak" needed can be freely designed by combining the two cases of sound focusing and sound splitting.

Claims

1. An acoustic lens implemented using a curved waveguide, characterized by: Comprising the following steps: S1: Design a set of curved waveguide units, each unit consists of two pairs of straight pipes and three circular arcs, the whole structure is axisymmetrically distributed, the inlet and outlet straight pipes of the waveguide are 25mm long and 6mm wide, the neck straight pipe length is adjustable, the circular arc radius is 6.5mm, the radian angle θ = π / 4, by adjusting the length of the neck straight pipe, the transmission of sound wave phase is accurately controlled, Discretize a phase period 0~2π into 8 parts, i.e. 0, π / 4, π / 2, 3π / 4, π, 5π / 4, 3π / 2 and 7π / 4, respectively corresponding to the discrete values of the neck straight pipe length 20.4mm, 17.9mm, 15.2mm, 12.5mm, 9.8mm, 7.2mm, 4.4mm and 1.8mm, under the same incident conditions, by changing the length of the curved waveguide neck straight pipe, the transmission of sound wave phase changes in the range of 2π, the amplitude of the transmitted sound wave remains close to 1, realizing high transmittance and wide phase control range, S3: Form an acoustic metasurface by combining multiple curved waveguide units to realize the functions of acoustic focusing, acoustic splitting and acoustic stealth; S2: the foregoing discrete bending waveguide units are combined according to a specific spatial arrangement to construct a metasurface structure, wavefront manipulation of incident sound waves is realized, and the relationship between the transmission sound wave radiation angle θ re and the incident angle θ i and the phase gradient satisfies the following formula: by adjusting the length distribution of the straight pipe of the neck of the bending waveguide, the phase gradient of the metasurface along the x-axis is realized, so as to accurately control the transmission sound wave radiation direction, (1) Acoustic focusing: a planar wave to cylindrical wave acoustic focusing lens is designed by curved waveguide metasurface, when the planar wave is incident to the metasurface, the transmitted sound wave converges at the focal point, the acoustic energy is significantly enhanced at this point, showing excellent focusing effect, (2) Acoustic splitting: by adjusting the phase gradient of the metasurface, the incident sound wave is split into two sound waves with propagation direction about 45° after transmission through the metasurface, the acoustic pressure field at 8000Hz frequency shows two obvious wave peaks, verifying the feasibility of acoustic splitting function, (3) Acoustic stealth: by adjusting the phase distribution of the metasurface, a sound-free region can be constructed around the focal point, and the object placed in this region will not interact with the sound wave, realizing the effect of "stealth cloak". The working frequency of the curved waveguide metasurface is 8000Hz, i.e. the frequency of the incident sound wave remains at 8000Hz.

2. The acoustic lens implemented with a curved waveguide according to claim 1, wherein: The thickness of the curved waveguide structure is the same as the width of the waveguide, d = 3mm, which extends along the z-axis direction in three-dimensional space to form a three-dimensional curved waveguide metasurface.

3. The acoustic lens implemented with a curved waveguide of claim 1, wherein: The curved waveguide structure is made of metal or polymer material, the inner wall of the waveguide is a rigid boundary to ensure the lossless propagation of sound waves in the waveguide, realizing nearly 100% transmittance.

4. The acoustic lens implemented with a curved waveguide of claim 1, wherein: The acoustic focusing lens and the acoustic splitting lens are composed of 98 curved waveguide structure units, and the acoustic stealth lens is composed of 200 curved waveguide structure units, and the metasurface units are distributed according to the specific phase requirements.

5. The acoustic lens implemented with a curved waveguide of claim 1, wherein: ​

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