Multiplexing acoustical holographic imaging device and method based on mixed porous Helmholtz resonator metasurface

Abstract

The invention relates to the technical field of acoustic imaging, in particular to a multiplexing acoustic holographic imaging device and method based on a mixed porous Helmholtz resonator metasurface. The metasurface antenna comprises an emission source, a receiving end and a metasurface, the emission source is a planar acoustic wave transducer and is used for emitting planar acoustic waves under working frequency; the receiving end is a microphone probe and is used for receiving sound signals in a sound field and carrying out sound field scanning and imaging; the metasurface is an array composed of a plurality of unit structures. Each unit structure is composed of four subunits which respectively correspond to four working frequencies; each subunit is a Helmholtz resonator filled with a porous medium; the emission source emits plane sound waves, stable sound field amplitude distribution is formed within a specific range of a metasurface reflection field through metasurface regular reflection, and the distribution corresponds to an acoustic holographic image. Compared with a traditional reflective multiplexing acoustical holography metasurface, the structure of the acoustic holography metasurface has the advantages that the manufacturing difficulty is remarkably reduced, and a complex optimizing process is also avoided through decoupling modulation of reflected sound waves.

Description

Technical Field

[0001] The present invention relates to the technical field of acoustic imaging, and particularly to a multiplexed acoustic holographic imaging device and method based on a hybrid porous Helmholtz resonator metasurface. Background Art

[0002] In recent years, acoustic holography technology has received great attention because it can project the sound field onto arbitrary shapes and positions by using acoustic metamaterials or metasurfaces. The physical quantity of the acoustic holographic image is the sound wave amplitude (or intensity), which represents the spatial distribution of sound energy and can be applied to sound field reconstruction or sound information transmission, etc. The multiplexing technology can greatly increase the amount of information transmitted by the acoustic holographic image. The traditional acoustic holographic multiplexing technology uses a side-branch Helmholtz resonator metasurface, where the Helmholtz cavity is embedded in the side wall of the metasurface unit. The space is extremely narrow, 3D printing cannot remove the support and the manufacturing accuracy cannot be satisfied, which limits its manufacturability and practicality. Moreover, its amplitude-phase modulation of sound waves does not have a decoupling characteristic, and an optimization algorithm needs to be used to find a suitable parameter combination when selecting geometric parameters. Therefore, designing a multiplexed acoustic holographic metasurface that can achieve amplitude-phase decoupled modulation and is easy to manufacture can greatly increase the design freedom of the metasurface and open up new ways for multifunctional compact acoustic devices. Summary of the Invention

[0003] The object of the present invention is to propose a multiplexed acoustic holographic imaging device and method based on a hybrid porous Helmholtz resonator metasurface for the problems existing in the background art. This communication technology can greatly broaden the sound transmission capacity, reduce the manufacturing difficulty of the metasurface, and has practical application prospects. The Helmholtz resonator metasurface filled with porous media provides a large-capacity multiplexed sound information transmission framework, which has a guiding role for various acoustic applications including underwater sound transmission.

[0004] The technical solution of the present invention, in the first aspect of the present invention, provides a multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface, including a transmitting source, a receiving end, and one metasurface;

[0005] The transmitting source is a planar acoustic wave transducer for transmitting a planar acoustic wave at the operating frequency;

[0006] The receiving end is a microphone probe for receiving the acoustic signal in the sound field and performing sound field scanning and imaging;

[0007] Wherein, the metasurface is an array composed of a plurality of unit structures;

[0008] The size and volume of each unit structure are the same; each unit structure is composed of four sub-units, and the four sub-units are distributed in a grid pattern, corresponding to four working frequencies respectively; each sub-unit is a Helmholtz resonator filled with porous medium inside, and the resonance frequency is the working frequency;

[0009] The emission source emits plane sound waves, which are specularly reflected by the metasurface. Within a specific range of the reflected field of the metasurface, a stable sound field amplitude distribution is formed, and the distribution corresponds to the acoustic holographic image;

[0010] The metasurface reflects the plane waves emitted by the emission source, forming sound fields at different working distances on the reflection side of the metasurface at different working frequencies. The amplitude distribution diagram of the sound field corresponds to the pre-designed hologram;

[0011] The Helmholtz resonator of the sub-unit is divided into three parts, including the neck tube (slit), the cavity, and the porous medium (melamine sound-absorbing sponge);

[0012] The Helmholtz resonator of the sub-unit has the ability to independently modulate the amplitude and phase of the reflected sound waves, that is, one parameter corresponds to one physical quantity without mutual interference.

[0013] In the Helmholtz resonator of the sub-unit, the depth of the cavity corresponds to the working frequency, the width of the slit modulates the phase of the reflected sound wave, and the thickness of the porous medium modulates the reflected amplitude.

[0014] Preferably, the specific geometric parameters of the metasurface unit are determined by the elements in the amplitude matrix and phase matrix of the reflected sound wave source. The matrix dimension is the same as the metasurface unit dimension, and each unit corresponds to a set of elements.

[0015] The second aspect of the present invention provides a multiplexed acoustic holographic imaging method based on a metasurface of hybrid porous Helmholtz resonators, which is applied to the above imaging device, and includes the following specific steps:

[0016] S1. Predesign the image information to be transmitted, and different image information is transmitted at different working frequencies;

[0017] S2. Calculate the grayscale image matrix corresponding to the transmitted image;

[0018] S3. Determine the amplitude matrix and phase matrix of the reflected sound wave source of the metasurface;

[0019] S4. Find the geometric parameters corresponding to the same values as the amplitude matrix and phase matrix of the reflected wave source in the amplitude-phase space corresponding to the metasurface unit, and design the metasurface accordingly.

[0020] S5. Arrange microphones at the working positions corresponding to different frequencies in the reflected field for swept-field imaging.

[0021] Preferably, there is time-reversal symmetry between the pre-designed transmitted image information and the amplitude-phase matrix of the reflection wave source. Each unit of the metasurface can be regarded as a reflection wave source, and the reflection wave source can be considered as a point source of acoustic spherical waves. The wave function of the acoustic spherical wave can be expressed as:

[0022]

[0023] where p represents the sound pressure, A represents the amplitude, represents the phase, r represents the propagation distance of the spherical wave, k is the acoustic wave number, and i is the imaginary unit; the holographic image is the superposition of these spherical waves.

[0024] Preferably, the metasurface can arbitrarily modulate the amplitude and phase of the reflected acoustic wave. The acoustic impedance of the metasurface unit can be expressed by the method of acoustic-electric analogy:

[0025] Z n = Z Sn + Z Cn

[0026] where n represents the number of the Helmholtz resonator in the metasurface unit, Z sn represents the acoustic impedance of the Helmholtz resonator, and Z cn represents the mutual impedance between different Helmholtz resonators.

[0027] Since different Helmholtz resonators are in parallel form, the total acoustic impedance of the metasurface unit can be expressed as:

[0028]

[0029] Correspondingly, the amplitude and phase of the reflected acoustic wave can be expressed as:

[0030] A = |(Z - ρ0c0 / a) / (Z + ρ0c0 / a)|

[0031] φ = arg[(Z - ρ0c0 / a) / (Z + ρ0c0 / a)]

[0032] where ρ0 and c0 are the density and sound speed of air respectively, and a is the side length of the cross-section of the metasurface unit.

[0033] Preferably, different operating frequencies correspond to different imaging positions in the reflection field. The higher the frequency, the shorter the wavelength and the shorter the imaging distance.

[0034] Compared with the prior art, the present invention has the following beneficial technical effects:

[0035] The present invention realizes precise manipulation of reflected sound waves at different operating frequencies through an array of Helmholtz cavities filled with porous media. The cross-section of each Helmholtz cavity is square and has the same cross-sectional area, and the depth corresponds to different operating frequencies. The neck tube of the Helmholtz cavity is a slit with the same depth, and the width serves as a phase modulation parameter for the reflected sound wave. The porous medium inside the cavity is a melamine sponge, and the thickness of the sponge serves as an amplitude modulation parameter for the reflected sound wave. The entire array consists of several units, and each unit contains 4 Helmholtz cavities with different parameters, arranged in a checkerboard pattern, corresponding to 4 operating frequencies.

[0036] The metasurface array in the present invention can decouple and modulate the amplitude and phase of the reflected sound wave. According to the time-reversal symmetry and the superposition principle of sound waves, the reflected sound field modulated by the metasurface can exhibit a pre-designed amplitude-phase distribution. Compared with the traditional reflective multiplexed acoustic holographic metasurface, the structure of the present invention significantly reduces the manufacturing difficulty, and the decoupled modulation of the reflected sound wave also avoids a complex optimization process. The present invention realizes high-communication-capacity acoustic information transmission. It is particularly suitable for applications in electromagnetic-sensitive and difficult-to-propagate electromagnetic wave scenarios. Description of the Drawings

[0037] Figure 1 Schematic diagram of the metasurface unit structure of the present invention;

[0038] Figure 2 Schematic diagram of the metasurface multiplexed acoustic holographic imaging of the present invention;

[0039] Figure 3 Schematic diagram of the equivalent two-dimensional structure of the metasurface unit in Embodiment 1 of the present invention;

[0040] Figure 4 Schematic diagram of the amplitude-phase modulation ability of the metasurface unit to reflect sound waves in Embodiment 1 of the present invention;

[0041] Figure 5 Schematic diagram of the imaging method and results of the metasurface unit in Embodiment 2 of the present invention;

[0042] Figure 6 Flowchart of the imaging method of the metasurface unit in the embodiment of the present invention. Detailed Description of the Invention

[0043] Embodiment 1

[0044] This embodiment provides a multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface, including a transmitter, a receiver, and 1 metasurface;

[0045] The transmitter is a planar acoustic wave transducer for transmitting planar acoustic waves at the operating frequency;

[0046] The receiving end is a microphone probe, which is used to receive the sound signal in the sound field and perform sound field scanning and imaging;

[0047] Among them, the metasurface is an array composed of multiple unit structures;

[0048] The size and volume of each unit structure are the same; each unit structure is composed of four sub-units, and the four sub-units are distributed in a tic-tac-toe pattern, corresponding to four working frequencies respectively; each sub-unit is a Helmholtz resonator filled with porous medium inside, and the resonance frequency is the working frequency;

[0049] The emission source emits a plane wave, which is specularly reflected by the metasurface. In a specific range of the reflection field of the metasurface, a stable sound field amplitude distribution is formed, and its distribution corresponds to the acoustic holographic image; the metasurface reflects the plane wave emitted by the emission source, and sound fields at different working distances on the reflection side of the metasurface are formed at different working frequencies, and the amplitude distribution map of the sound field corresponds to the pre-designed hologram; the Helmholtz resonator of the sub-unit is divided into three parts, including the neck tube (slit), the cavity, and the porous medium (melamine sound-absorbing sponge); the Helmholtz resonator of the sub-unit has the ability to independently modulate the amplitude and phase of the reflected sound wave, that is, one parameter corresponds to one physical quantity and they do not interfere with each other. In the Helmholtz resonator of the sub-unit, the depth of the cavity corresponds to the working frequency, the width of the slit modulates the phase of the reflected sound wave, and the thickness of the porous medium modulates the reflected amplitude. The specific geometric parameters of the metasurface unit are determined by the elements in the amplitude matrix and phase matrix of the reflected sound wave source, the matrix dimension is the same as the metasurface unit dimension, and each unit corresponds to a group of elements.

[0050] The following uses a specific case to introduce the solution of the present invention in detail:

[0051] In this embodiment, as Figure 1 shown is the three-dimensional structure of the metasurface unit in the embodiment of the present invention, and the four sub-units correspond to the resonance (working) frequencies of 1180 Hz, 1490 Hz, 2340 Hz, and 3480 Hz respectively. As Figure 3 shown, it is its two-dimensional equivalent structure, where the geometric parameters are a = 3 cm ( Figure 1 in), b = 1.2 cm, t = 0.2 cm, h1 = 5 cm, h2 = 3.6 cm, h3 = 1.8 cm, h4 = 0.9 cm. w is the parameter for modulating the phase of the reflected sound wave, and d is the parameter for modulating the amplitude of the reflected sound wave.

[0052] The finite element simulation of the metasurface unit is carried out by a commercial simulation software, and the metasurface unit sample is tested by an acoustic impedance tube test system. The simulation results and experimental results are as Figure 4 shown. Figure 4 (a-d) shows that at different working frequencies, the reflection phase response is only related to the neck width w of the corresponding sub-unit,Figure 4 (e-h) indicate that at different working frequencies, the reflection amplitude response is only related to the thickness d of the porous medium of the corresponding subunit, that is, the reflection amplitude-phase response has a decoupling characteristic.

[0053] Example 2

[0054] To illustrate the imaging mechanism of the technology involved, as Figure 2 shown, the schematic diagram of the metasurface multiplexed acoustic holographic imaging of the present invention. In this embodiment, as Figure 5 shown, the metasurface has a total of 160×160 units, and the corresponding area is about 480 cm×480 cm. The letters ABCD are the imaging patterns at the working frequencies of 1180 Hz, 1490 Hz, 2340 Hz, and 3480 Hz respectively, and the corresponding acoustic wave wavelengths are 0.1 m, 0.15 m, 0.23 m, and 0.29 m respectively. Therefore, the sound field distribution at the working distance of about 16 times the wavelength from the metasurface is as Figure 5 shown.

[0055] Example 3

[0056] This embodiment provides a multiplexed acoustic holographic imaging method based on a hybrid porous Helmholtz resonator metasurface, which is applied to the imaging device in Embodiment 1, and includes the following specific steps:

[0057] S1. Predesign the image information to be transmitted, and different image information is transmitted at different working frequencies;

[0058] S2. Calculate the grayscale image matrix corresponding to the transmitted image;

[0059] S3. Determine the amplitude matrix and phase matrix of the reflection acoustic wave source of the metasurface;

[0060] S4. Find the geometric parameters corresponding to the same values of the reflection wave source amplitude matrix and phase matrix in the amplitude-phase space corresponding to the metasurface unit, and design the metasurface accordingly.

[0061] S5. Arrange microphones at the working positions corresponding to different frequencies in the reflection field for swept-field imaging.

[0062] In this embodiment, there is a time-reversal symmetry between the predesigned transmitted image information and the reflection wave source amplitude-phase matrix. Each unit of the metasurface can be regarded as a reflection wave source, and the reflection wave source can be considered as a point source of a spherical acoustic wave; the wave function of the spherical acoustic wave can be expressed as:

[0063]

[0064] In the formula, p represents the sound pressure, A represents the amplitude, \(\varphi\) represents the phase, \(r\) represents the propagation distance of the spherical wave, \(k\) is the acoustic wave number, and \(i\) is the imaginary unit; the holographic image is the superposition of these spherical waves.

[0065] In this embodiment, the metasurface can arbitrarily modulate the amplitude and phase of the reflected acoustic wave, and the acoustic impedance of the metasurface unit can be expressed by the method of acoustic - electrical analogy:

[0066] Z n = Z Sn + Z Cn

[0067] where \(n\) represents the number of the Helmholtz resonators in the metasurface unit, \(Z\) sn represents the acoustic impedance of the Helmholtz resonator, and \(Z\) cn represents the mutual impedance between different Helmholtz resonators.

[0068] Since different Helmholtz resonators are in parallel form, the total acoustic impedance of the metasurface unit can be expressed as:

[0069]

[0070] Correspondingly, the amplitude and phase of the reflected acoustic wave can be expressed as:

[0071] \(A = |(Z - \rho_0c_0 / a) / (Z + \rho_0c_0 / a)|\)

[0072] \(\varphi = arg[(Z - \rho_0c_0 / a) / (Z + \rho_0c_0 / a)]\)

[0073] where \(\rho_0\) and \(c_0\) are the density and sound speed of air respectively, and \(a\) is the side length of the cross - section of the metasurface unit.

[0074] In this embodiment, different operating frequencies correspond to different imaging positions in the reflection field. The higher the frequency, the shorter the wavelength and the shorter the imaging distance.

[0075] The above - mentioned embodiments of the present invention have been described in detail in conjunction with the accompanying drawings. However, the present invention is not limited to this. Various changes can be made without departing from the spirit of the present invention within the knowledge scope of those skilled in the relevant technical field.

Claims

1. A multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface, characterized in that It includes a transmitting source, a receiving end, and a metasurface; The transmitting source is a planar acoustic wave transducer; The receiving end is a microphone probe, which is used to receive acoustic signals in the sound field and perform sound field scanning and imaging; Among them, the metasurface is an array composed of multiple unit structures; The size and volume of each unit structure are the same; each unit structure is composed of four sub-units, corresponding to four working frequencies respectively; each sub-unit is a Helmholtz resonator filled with porous medium inside; The transmitting source emits a planar acoustic wave, which is specularly reflected by the metasurface. In a specific range of the reflection field of the metasurface, a stable sound field amplitude distribution is formed, and its distribution corresponds to a sound holographic image.

2. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, wherein The Helmholtz resonator consists of a neck tube, a cavity, and a porous medium located inside; The neck tube is slit-shaped, and its cross-section is rectangular; the depths of all neck tubes are the same, the lengths of the rectangular cross-sections are the same, and the widths are different; The cavity is a cuboid, and the cross-section of the cuboid is square; the cross-sectional dimensions of all cavities are the same, and the cavity depths are different; The porous medium is melamine sponge, which is filled at the bottom of the cavity. The filling amount of the porous medium inside each cavity in the unit is different, and it is measured by its thickness.

3. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, wherein It is only necessary to emit a planar acoustic wave corresponding to the working frequency, and there is no need to have directivity.

4. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, characterized in that, The resonance frequency of the Helmholtz resonator corresponds to the working frequency of the metasurface, and the resonance frequency is related to the cavity depth.

5. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, wherein The width of the neck tube of the Helmholtz resonator independently controls the phase of the reflected acoustic wave.

6. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, wherein The thickness of the porous medium inside the cavity of the Helmholtz resonator independently controls the amplitude of the reflected acoustic wave.

7. The multiplexed acoustic holographic imaging device based on a hybrid porous Helmholtz resonator metasurface according to claim 1, characterized in that, The receiving end microphone probe collects relevant acoustic signals and performs holographic drawing through the corresponding code algorithm.

8. A multiplexed acoustic holographic imaging method based on a hybrid porous Helmholtz resonator metasurface, which uses the acoustic holographic imaging device according to any one of claims 1-7 for imaging, characterized in that, It includes the following specific steps: S1. Predesign the image information to be transmitted, and matrixize the grayscale image value matrix of the image; S2. Determine the working frequency, and adjust the transmitting source to the working frequency range to emit a planar acoustic wave; S3. Calculate the amplitude and phase of the reflected wave of each unit of the metasurface according to the grayscale image matrix; S4. Design the metasurface according to the parameter space of the Helmholtz resonator in the unit and the modulation range of the reflected wave; S5. Since the reflected wavelengths corresponding to frequency multiplexing are different, the receiving end microphone needs to be placed within different ranges from the metasurface to collect sound signals.

9. A multiplexed acoustic holographic imaging method based on a hybrid porous Helmholtz resonator metasurface according to claim 8, characterized in that, There is time-reversal symmetry between the predesigned transmission image information and the reflection parameter matrix of the metasurface. Each unit in the metasurface can be regarded as a point source of a spherical acoustic wave; the wave function of the spherical acoustic wave can be expressed as: where p represents the sound pressure, A represents the amplitude, represents the phase, r represents the propagation distance of the spherical wave, k is the acoustic wave number, and i is the imaginary unit; the holographic image is the superposition of these spherical waves, the amplitude channel is the amplitude of the sound wave after superposition, and the phase channel is the phase of the sound wave after superposition.

10. A multiplexed acoustic holographic imaging method based on a hybrid porous Helmholtz resonator metasurface according to claim 8, characterized in that The metasurface can arbitrarily modulate the amplitude and phase of the reflected acoustic wave, and the amplitude and phase of the reflected acoustic wave have a decoupling characteristic. One parameter corresponds to one physical quantity and they do not interfere with each other.

Images