A method for constructing a single-source sonar device based on an acoustic metasurface

Abstract

The application discloses a single-source sonar device construction method based on an acoustic metasurface, point source approximation is performed on a linear array sound source with a frequency gradient, a deflection angle of a superimposed sound beam is derived under a far field condition, and the deflection angle changes with time, an automatic scanning sound beam is generated, scanning speed of the sound beam can be freely adjusted by adjusting the frequency gradient is analyzed and determined, parameters of the metasurface single-source sonar device are determined, length of a frequency space separation area and a continuous phase distribution required by the metasurface single-source sonar for introducing the frequency gradient are calculated, and the continuous phase distribution is converted into a binary amplitude distribution through a discretization operation, a kind of amplitude coding digital acoustic structure is designed as a practical implementation means of the metasurface single-source sonar, a sound source with a comb-shaped frequency spectrum is constructed, and the metasurface single-source sonar device is built. The application introduces the new sound beam control freedom of the frequency gradient, significantly reduces the size of the sonar transmitting device, and significantly reduces the energy consumption and complexity of the sonar transmitting system.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of acoustics, and particularly relates to a single-source sonar device construction method based on an acoustic super surface. BACKGROUND

[0002] For a long time, high-speed, high-efficiency and wide-angle beam spatial scanning has important application value in the fields of ultrasonic imaging, underwater target positioning and sonar communication. The current spatial dynamic scanning of the beam is mainly realized through mechanical movement of the transmitting device or a multi-channel active phased array sonar. On the one hand, the mechanical movement of the transmitting device inevitably introduces noise, and the scanning rate is limited by the speed of the mechanical movement; on the other hand, the active phased array sonar is generally composed of multiple independently addressable transducers, and the dynamic scanning of the beam is realized by regulating the driving signal of each transducer. However, the complex control system, large device size, high cost and high energy consumption seriously hinder the miniaturization and integration process of the sonar device. In recent years, acoustic metamaterials and super surfaces with abnormal acoustic properties provide a new degree of freedom for precisely manipulating sound waves in subwavelength spatial scales, realizing various special sound wave manipulations that are difficult or impossible to achieve by conventional methods. Therefore, applying the cutting-edge technology of acoustic super surfaces to the design of new sonar devices can not only break through the performance short board of the existing technology, but also retain the respective advantages of the existing technology, which is expected to provide a new solution for the miniaturization, simplification and integration of sonar transmitting devices.

[0003] The current conventional sonar transmitting system is generally based on active phased array technology, which uses a large number of individually addressable transducer units to form a transmitting array to realize the regulation of the beam directivity. However, the active phased array sonar not only has high cost and complex system, but also has large device size. Especially for ultrasonic frequency band transducers, the unit size is generally on the order of wavelength, so according to the beam forming theory, the deflection direction of the beam is extremely limited, which not only limits the detection range of the sonar, but also affects the integration and miniaturization of the sonar equipment. Although passive acoustic super surfaces can arbitrarily manipulate sound waves in subwavelength spatial scales, the acoustic function of such artificial structures is relatively single, and once assembled, the acoustic performance cannot be changed. Active acoustic super surfaces have the advantages of reconfigurability, real-time adjustment and compact structure, but the current active acoustic super surfaces have inherent dependence on external flow field, energy consumption and multi-channel active driving, which makes it difficult to be directly applied to integrated sonar transmitting systems. In summary, whether using the existing active phased array sonar technology or the cutting-edge super surface technology to realize the spatial scanning of the beam, multiple independent sound sources and multi-channel signal driving systems are indispensable. How to construct a new type of sonar device that only needs a single sound source and does not need time and space modulation is still a great challenge. SUMMARY

[0004] The application provides a single-source sonar device construction method based on an acoustic metasurface.

[0005] The application provides a single-source sonar device construction method based on an acoustic metasurface.

[0006] (1) Point source approximation is performed on a linear array sound source with a frequency gradient, and a deflection angle of a superimposed sound beam is derived under far field conditions to change with time, so that an automatic scanning sound beam is generated, and it is analyzed and determined that the scanning speed of the sound beam can be freely adjusted by adjusting the frequency gradient;

[0007] (2) The parameters of the metasurface single-source sonar device are determined, including the center frequency, frequency interval, frequency component number, incident angle of the sound wave and the interval of the focal points of the sound beams of different frequencies on the target focal plane of the synthetic comb-shaped frequency source;

[0008] (3) The relationship among the frequency gradient on the target focal plane, the center frequency of the sound source, the incident angle and the focal length is derived under the condition of paraxial approximation according to the wave equation of the sound wave, the parameters of the metasurface single-source sonar device determined in step (2) are combined, and the length of the frequency space separation region, that is, the distance from the metasurface single-source sonar to the focal plane where the focal points of the frequency components are located, is calculated;

[0009] (4) The continuous phase distribution required by the metasurface single-source sonar for introducing the frequency gradient is calculated, and the discrete operation is performed to convert the binary amplitude distribution;

[0010] (5) According to the binary amplitude distribution obtained in step (4), a digital acoustic structure with amplitude coding is designed as an actual implementation means of the metasurface single-source sonar;

[0011] (6) The metasurface single-source sonar is actually processed by using three-dimensional additive printing or numerical control machine tool processing, the sound source with a comb-shaped frequency spectrum is constructed according to the parameters in step (2), and the metasurface single-source sonar device is built.

[0012] Further, the step (1) is implemented as follows:

[0013] Each frequency focal point in the focal plane satisfies the point source approximation, 2N+1 frequency components with the same interval Δω constitute the incident linear momentum, and each basic unit of the frequency gradient metasurface has the following harmonic form:

[0014]

[0015] where a n and ω n = ω0+ nΔω (n = 0, ±1, ±2,..., ±N) are the amplitude and angular frequency of each frequency component, ω0is the center frequency, Δω is the frequency interval, n represents the serial number of the frequency component, and t represents time; thus, the superimposed acoustic field formed by the 2N+1 acoustic foci is represented as:

[0016]

[0017] where r n = (nd, 0) is the spatial position of the nth focus, and G(r-r n ) is the Green function of a point source in free space; under the far-field condition, G(r-r n ) is further written as Thus, the superimposed acoustic field is further represented as:

[0018]

[0019] where c is the sound speed, k0= ω0c is the wave vector in free space, and α represents the azimuth angle of the observation point; the summation term in the formula represents the envelope shape of the spatio-temporal superimposed acoustic beam, and t is replaced by t-r / c at a fixed distance r, and the relationship between the acoustic beam deflection angle α and time t is further derived as:

[0020]

[0021] The acoustic beam deflection angle α is in a proportional relationship with time t, that is, the deflection angle of the acoustic beam changes continuously with time, and thus the outgoing acoustic beam continuously scans around the center position; in addition, the scanning speed of the acoustic beam is proportional to the frequency gradient; the scanning speed of the acoustic beam can be freely adjusted by adjusting the frequency gradient.

[0022] Further, the super-surface single-source sonar device of step (2) comprises a sound source, a super-surface sonar, a frequency-space separation zone, and a beam scanning zone, and the entire device is placed in a two-dimensional waveguide system constructed by two parallel acrylic plates, and sound-absorbing cotton is applied to the boundary of the waveguide to simulate a sound-eliminating environment.

[0023] Further, the length of the frequency-space separation zone of step (3) is:

[0024]

[0025] where f is the length of the frequency-space separation zone, d is the distance between the foci of different frequencies on the target focal plane, θ is the oblique incidence angle, ω0is the center frequency, and Δω is the frequency interval.

[0026] Further, the step (4) is implemented as follows:

[0027] The new degree of freedom of frequency gradient is introduced to separate the frequency components of the incident frequency comb in space and focus them at the corresponding target focal points respectively; the continuous phase distribution of the metasurface single-source sonar should satisfy:

[0028]

[0029] Further, the step (5) is implemented as follows:

[0030] The continuous phase distribution of the above formula is replaced by a simple binary amplitude distribution to obtain the following binary amplitude coding sequence:

[0031]

[0032] The transmission amplitude is controlled by 0 and 1 by whether the rigid thin plate is perforated or not.

[0033] Advantages: Compared with the prior art, the advantages of the present application are: the present application introduces a new manipulation degree of freedom of frequency gradient by using a single-layer amplitude-coded acoustic metasurface, and provides a compact, single-source and mechanically moving-free and active phased array-free design method for spatial scanning sonar, which benefits from the subwavelength scale of the acoustic metasurface, and the single-source sonar device constructed by the present application has important advantages such as compactness and high spatial resolution; the present application breaks the inherent dependence on active control systems and multi-channel driving in the prior art, and can generate a dynamically scanning acoustic beam by a time-space static metasurface single-source sonar, which significantly reduces the energy consumption and system complexity of the sonar transmitting device; in addition, the present application can also be effectively combined with the conventional phase gradient control method, which has important significance for further improving the spatial resolution and signal-to-noise ratio of the metasurface single-source sonar. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a schematic diagram of the principle of the present application;

[0035] Figure 2 is a schematic diagram of the envelope evolution of the time-space interference acoustic beam when the frequency gradient is Δω / d;

[0036] Figure 3 is a photograph of the metasurface single-source sonar transmitting device in a two-dimensional system with air as the background medium;

[0037] Figure 4 is a continuous phase distribution of the metasurface single-source sonar and a binary amplitude coding sequence after discretization operation;

[0038] Figure 5 is a photograph of the perforated structure of the metasurface single-source sonar designed in the present application;

[0039] Figure 6 Simulation and measurement results of the sound intensity spatial distribution on the focal plane for the individual excitation of the comb frequency components of the sound source;

[0040] Figure 7 Theoretical calculation results of the sound intensity distribution of the sound field in the scanning area at five different time points in a period of time;

[0041] Figure 8 Simulation and measurement results of the time-domain signals generated by the metasurface monopulse sonar at several specific positions P1-P5 and PA-PE in the scanning area;

[0042] Figure 9 Simulation and measurement results of the time delay of the signals between adjacent measurement points in several specific positions P1-P5 and PA-PE in the scanning area. DETAILED DESCRIPTION

[0043] The application will be further described in detail below with reference to the accompanying drawings.

[0044] The application provides a construction method of a metasurface-based monopulse sonar device, derives the continuous phase distribution of the metasurface monopulse sonar involved, designs a single-layer amplitude-coded digital acoustic structure, redirects and focuses the comb frequency components of the incident synthetic sound source, converts them into spatially separated multi-focus sound beams according to the different frequencies, and thus introduces the new degree of freedom of frequency gradient on the focal plane. The time-space superimposed sound beam generated behind the focal plane can automatically steer over time without additional active control, time-space modulation or mechanical movement. In the air acoustic background with a working frequency of 36 kHz, the 0, 1 coding of the transmitted sound amplitude is realized by whether the rigid thin plate is perforated or not. On this basis, according to the discrete amplitude distribution, the metasurface monopulse sonar is constructed by perforating the rigid thin plate at specific positions. The numerical simulation results and experimental results both show that the designed compact metasurface monopulse sonar device can effectively separate the incident sound waves of different frequencies and focus them on the respective corresponding focal points with high precision, and can generate high-speed and wide-angle continuous scanning sound beams in the scanning area. In addition, the application can be effectively combined with the traditional phase gradient regulation method, and by adding different initial phase information to each frequency component, the spatial form of the scanning sound beam can be reshaped, so that the spatial resolution and signal-to-noise ratio of the metasurface monopulse sonar can be further improved. The specific process is as follows:

[0045] As Figure 1Fig. 1 shows a schematic diagram of the principle of the automatic steering of the sound beam by the metasurface monostatic sonar. When the one-way linear momentum with comb-like spectrum is incident on the metasurface sonar, the wave-structure interaction will convert the incident plane waves of different frequencies into a plurality of spatially separated focused sound beams, whose focal points are arranged in the focal plane at equal intervals, and a frequency gradient Δω / d is introduced, Δω is the frequency interval of the incident frequency comb, and d is the spatial interval of the focal points of each frequency in the focal plane.

[0046] Figure 2 Fig. 2 shows a schematic diagram of the envelope of the time-space superimposed sound beam evolving with time when the frequency gradient is Δω / d. Assuming that each frequency focal point in the focal plane satisfies the point source approximation, 2N+1 frequency components with the same interval Δω constitute the incident linear momentum, so each basic unit of the frequency gradient metasurface has the following harmonic form:

[0047]

[0048] where a n and ω n = ω0+nΔω (n = 0, ±1, ±2,..., ±N) are the amplitude and angular frequency of each frequency component, ω0 is the center frequency, Δω is the frequency interval, n represents the serial number of the frequency component, and t represents time. Thus, the superimposed sound field formed by 2N+1 sound focal points is represented as:

[0049]

[0050] where r n =(nd, 0) is the spatial position of the nth focal point, and G(r-r n ) is the Green function of the point sound source in free space. Under the far-field condition (r » Nd), G(r-r n ) in the two-dimensional system can be further written as Therefore, the superimposed sound field can also be further represented as:

[0051]

[0052] where c is the sound speed, k0= ω0c is the wave vector in free space, and α represents the azimuth angle of the observation point. The summation term in the formula represents the envelope form of the time-space superimposed sound beam, and by replacing t with t-r / c at a fixed distance r, the relationship between the sound beam deflection angle α and time can be further derived as:

[0053]

[0054] The above formula shows that the sound beam deflection angle a is proportional to the time t, that is, the deflection angle of the sound beam changes continuously with time, so the outgoing sound beam continuously scans around the center position. In addition, the scanning speed of the sound beam is proportional to the frequency gradient. By using the universality of this mechanism, the scanning speed of the sound beam can be freely adjusted by adjusting the frequency gradient.

[0055] Now taking the hyper-surface single-source sonar device under the system of air sound two-dimensional waveguide as an example. As shown in Figure 3 The actual photo of the device is shown, including a sound source, a hyper-surface sonar, a frequency space separation zone and a sound beam scanning zone. The entire device is placed in a two-dimensional waveguide system constructed by two parallel acrylic plates. Absorptive cotton is laid on the boundary of the waveguide to simulate a sound-eliminating environment. The center frequency of the incident frequency comb is set to 36 kHz, with 21 frequency components with a frequency interval of 360 Hz. The oblique incidence angle θ is set to 45°. The focal point spacing d of different frequencies on the target focal plane is set to λ0 / 2, where λ0is the wavelength corresponding to the center frequency of the sound source. According to the above system parameters, the focal length f can be calculated by the relationship between the frequency gradient and the center frequency, the frequency interval and the focal length:

[0056]

[0057]

[0058] where f is the length of the frequency space separation zone, d is the focal point spacing of different frequencies on the target focal plane, and θ is the oblique incidence angle. In this embodiment, the focal length f is 0.67 m, that is, the length of the frequency space separation zone.

[0059] In order to introduce the new degree of freedom of frequency gradient, it is necessary to separate the frequency components of the incident composite frequency comb in space and focus them on the corresponding target focal points respectively. Physically, this requires the hyper-surface sonar to simultaneously redirect and focus the incident composite sound beam, so that the focal point array on the focal plane has the same frequency interval and spatial interval. Therefore, it can be deduced that the continuous phase distribution of the hyper-surface single-source sonar should satisfy:

[0060]

[0061] In order to simplify the design of the device, the present application uses a simple binary amplitude distribution to replace the continuous phase distribution of the above formula, so that the following binary amplitude coding sequence can be obtained:

[0062]

[0063] As shown in Figure 4 The continuous phase distribution of the hyper-surface single-source sonar and the binary amplitude coding sequence after discretization operation are shown. The transmission amplitude is controlled by whether the rigid thin plate is perforated or not. As shown inFigure 5 The actual photo of the open hole structure is shown, preferably, the thickness t of the rigid sheet is 1mm, the length L is 40 lambda 0, and the height h is 1cm.

[0064] Figure 6 The simulation results and experimental measurement results of the sound intensity spatial distribution on the focal plane when all frequency components are excited separately are shown.

[0065] Figure 7 The theoretical calculation results of the sound intensity distribution of the sound field in the scanning area at five different time points in a period of time are shown. Figure 3 The time-domain signals generated by the metasurface monopulse sonar at 10 specific positions P1-P5 and PA-PE are measured, as shown in Figure 8 The simulation and experimental results of each point are shown respectively. Figure 9 The time delay of the signal between adjacent measurement points, the simulation and measurement results show good consistency, which verifies the effectiveness of the designed metasurface monopulse sonar to generate an automatically steering sound beam with a scanning frequency of up to 360Hz.

[0066] It should be noted that although the present application only constructs a two-dimensional sonar transmitting device in an air sound environment, the theoretical mechanism of the present application is universal and can be applied to all fluid environments and can be further extended to a three-dimensional system to achieve more complex space-time control of sound waves.

[0067] The monopulse sonar device based on acoustic metasurface of the present application does not need an active control system of an active phased array or a space-time modulation method of an active metasurface, but uses a passive and static single-layer acoustic metasurface to realize the spatial separation and focusing of the synthetic comb-shaped frequency components of the sound waves emitted by a single sound source, thereby introducing a new degree of freedom of the spatial gradient of frequency on the focal plane. Further, the space-time superposition of different frequency components after the focal plane will cause the wave front to continuously rotate in the angle direction with time, and thus a high-speed, high-efficiency, wide-angle continuous scanning sound beam can be generated. In addition, the frequency gradient control method in the present application can be further effectively combined with the traditional phase gradient control method, thereby realizing arbitrary manipulation of the spatial form of the scanning sound beam, and is expected to improve the spatial resolution and signal-to-noise ratio of the metasurface monopulse sonar.

Claims

1. A method for constructing a single-source sonar device based on an acoustic metasurface, characterized in that, Comprise the following steps: (1) Point source approximation is made to the linear array sound source with frequency gradient, and the deflection angle of the superimposed sound beam is derived under the far field condition to produce automatic scanning sound beam, and it is analyzed and determined that the scanning speed of the sound beam can be freely adjusted by adjusting the frequency gradient; (2) The parameters of the super surface single source sonar device including the center frequency, frequency interval, frequency component number, incident angle of the sound wave and the spacing of the focal points of different frequency sound beams on the target focal plane of the synthetic comb frequency source are determined; (3) The relationship between the frequency gradient on the target focal plane and the center frequency, incident angle and focal length of the sound source is derived under the condition of near axis approximation according to the wave equation of the sound wave, the length of the frequency space separation region is calculated, that is, the distance from the super surface single source sonar to the focal plane where the focal points of each frequency component are located, in combination with the parameters of the super surface single source sonar device determined in step (2); (4) The continuous phase distribution required by the super surface single source sonar for introducing the frequency gradient is calculated, and is converted into a binary amplitude distribution through discretization operation; (5) According to the binary amplitude distribution obtained in step (4), a digital acoustic structure of amplitude coding is designed as the actual implementation means of the super surface single source sonar; (6) The super surface single source sonar is actually processed by using three-dimensional additive printing or numerical control machine tool processing, the sound source with comb frequency spectrum is constructed according to the parameters in step (2), and the super surface single source sonar device is built.

2. The method according to claim 1, wherein, The implementation process of step (1) is as follows: Each frequency focal point in the focal plane satisfies the point source approximation, 2N+1 frequency components with the same interval Δω constitute the incident linear momentum, and each basic unit of the frequency gradient super surface has the following harmonic form: where a n and ω n = ω0+ nΔω (n = 0, ±1, ±2,..., ±N) are the amplitude and angular frequency of each frequency component, ω0is the center frequency, Δω is the frequency interval, n represents the serial number of the frequency component, and t represents time; thus, the superimposed acoustic field formed by the 2N+1 acoustic focal points is represented as: where r n = (nd,0) is the spatial position of the nth focal point, d is the focal point spacing for different frequencies on the target focal plane, G(r-r n ) is the Green's function of a point acoustic source in free space; under the far-field condition, G(r-r n ) is further written as Therefore, the superimposed acoustic field is further represented as: Wherein, c is the sound speed, k0=ω0 / c is the wave vector in free space, and α represents the azimuth angle of the observation point; The sum term in the formula represents the envelope form of the time-space superimposed sound beam, t is used to replace t-r / c at a fixed distance r, and the relationship between the sound beam deflection angle α and time t is further derived as: The sound beam deflection angle α is proportional to the time t, that is, the deflection angle of the sound beam changes continuously with the change of time, so the outgoing sound beam will continuously scan around the center position; In addition, the scanning speed of the sound beam is proportional to the frequency gradient; The scanning speed of the sound beam can be freely adjusted by adjusting the frequency gradient.

3. The method according to claim 1, wherein, The super surface single source sonar device in step (2) comprises a sound source, a super surface sonar, a frequency space separation zone and a sound beam scanning zone, and the whole device is placed in a two-dimensional waveguide system constructed by two parallel acrylic plates, and sound absorbing cotton is laid on the boundary of the waveguide to simulate a sound absorbing environment.

4. The method according to claim 1, wherein, The length of the frequency space separation region in step (3) is: Wherein, f is the length of the frequency space separation region, d is the focal point spacing of different frequencies on the target focal plane, θ is the oblique incidence angle, ω0 is the center frequency, and Δω is the frequency interval.

5. The method for constructing a single-source sonar device based on an acoustic metasurface according to claim 1, characterized in that: The implementation process of step (4) is as follows: The new degree of freedom of introducing the frequency gradient separates the frequency components of the incident synthetic frequency comb in space and focuses them on the corresponding target focal points respectively; The continuous phase distribution of the super surface single source sonar should satisfy: Where f is the length of the frequency space separation region, θ is the angle of oblique incidence, and k0 is the wave vector in free space.

6. The method according to claim 1, wherein, The step (5) is implemented as follows: Based on the simple binary amplitude distribution instead of the continuity phase distribution of the above formula, the following binary amplitude encoding sequence is obtained: wherein, is the continuous phase distribution of the metasurface monostatic sonar; the transmission amplitude is controlled by 0 or 1 by whether the rigid thin plate is perforated or not.

Images