An angular momentum acoustic communication method across water and air based on acoustic metamaterials

Abstract

The present invention discloses a cross-water-air angular momentum acoustic communication method based on acoustic metamaterials, and constructs a cross-water-air angular momentum acoustic communication system including a transmitting end, an artificial angular momentum demodulation device on the water-air interface, and a receiving end; designs a water-air interface acoustic metamaterial unit with impedance matching and phase regulation functions according to the working frequency; determines the required spatial phase distribution on the water-air interface according to the frequency and orbital angular momentum order of the composite vortex acoustic beam and the receiving position corresponding to each order of vortex at the receiving end, performs spatial discretization and phase discretization on the required continuous phase, and constructs an artificial angular momentum demodulation device by using the water-air interface acoustic metamaterial unit to achieve acoustic transparency between water and air media and information decoding of each channel; the present invention combines frequency multiplexing technology and orbital angular momentum multiplexing technology to realize real-time angular momentum acoustic communication with high capacity, high signal-to-noise ratio and low bit error rate across the water-air interface, and improves the transmission rate of the cross-water-air angular momentum acoustic communication system.

Description

Technical Field

[0001] The present invention belongs to the field of acoustics, and particularly relates to a cross-water-air angular momentum acoustic communication method based on acoustic metamaterials. Background Art

[0002] Realizing acoustic communication across water-air media has always been a classic and important fundamental problem in acoustics, and has important application values in fields such as sonar communication, deep-sea exploration, and ultrasonic medical treatment. However, under normal circumstances, due to the 3600-fold difference in acoustic impedance between water and air, only 0.1% of the acoustic energy can penetrate this nearly absolute reflection boundary, which is equivalent to the transmission loss of a 10 kHz sound wave transmitted 30 km underwater. Therefore, the existence of the water-air interface is a natural barrier that is difficult to overcome in sound wave transmission. The rise of acoustic metamaterials provides a new option for solving this classic acoustic problem. However, the current implementation methods based on acoustic metamaterials and metasurfaces not only cannot reach the upper limit in terms of energy transmission efficiency, but also, since soft materials such as thin films and bubbles are used as the main implementation means, the mechanical strength of the devices is extremely low, and the service life is quite limited. More critically, these methods can only improve the energy transmittance of the water space and do not have the ability to arbitrarily control the propagation phase. This means that problems such as diffraction and dissipation of sound waves during long-distance propagation remain unsolved, and more complex spatial sound field control of the transmitted sound waves cannot be performed.

[0003] Acoustic orbital angular momentum, as a new degree of freedom independent of time and frequency, enables vortex sound beams carrying different orders of orbital angular momentum to form a complete set of orthogonal bases, which can independently carry information and be multiplexed onto the same transmission path for transmission. At the same time, angular momentum multiplexing can also be perfectly compatible with traditional wavelength division multiplexing technology, frequency division multiplexing technology, etc., and has great application potential in high-speed and high-capacity communication. Currently, high-speed acoustic communication based on orbital angular momentum is mainly realized in uniform background media such as air or water, and the problem of angular momentum communication between non-uniform media, especially between water and air, has not been effectively solved. To achieve long-distance and efficient cross-water-air interface acoustic angular momentum communication, an artificial coupling device with both impedance matching and phase modulation functions needs to be constructed to achieve high transmission of vortex sound beams and decomposition and detection of angular momentum modes. Summary of the Invention

[0004] Objective of the Invention: Aiming at the deficiencies of the existing technology, the present invention proposes a cross-water-air angular momentum acoustic communication method based on acoustic metamaterials. First, an acoustic artificial structure based on a rigid substrate-air channel binary composite material is proposed to solve the impedance mismatch and phase modulation problems at the water-air interface. It can not only achieve enhanced transmission of acoustic energy within a certain incident angle range but also arbitrarily modulate the phase of the transmitted acoustic wave. An artificial angular momentum demodulation device is constructed using the acoustic metamaterial at the water-air interface to realize the efficient transmission and real-time demodulation of the composite angular momentum acoustic signal across the water-air interface, constructing a new mechanism for cross-water-air angular momentum acoustic communication. At the same time, combined with the existing frequency multiplexing technology, the transmission rate of the cross-water-air angular momentum acoustic communication system is further improved.

[0005] Technical Solution: A cross-water-air angular momentum acoustic communication method based on acoustic metamaterials provided by the present invention includes the following steps:

[0006] (1) Construct a cross-water-air angular momentum acoustic communication system including a transmitting end, an artificial angular momentum demodulation device on the water-air interface, and a receiving end; the transmitting end emits a coaxial vortex acoustic beam multiplexed with multiple frequencies and multiple angular momenta. Vortex acoustic beams with different frequencies and different orders form independent channels for information transmission; the data stream to be transmitted is sequentially encoded onto the amplitude and phase of the acoustic wave in each channel as the signal source;

[0007] (2) Design an acoustic metamaterial unit at the water-air interface with impedance matching and phase regulation functions according to the working frequency;

[0008] (3) Determine the required spatial phase distribution on the water-air interface according to the frequency and orbital angular momentum order of the composite vortex acoustic beam and the receiving position corresponding to each order of vortex at the receiving end. Discretize the required continuous phase spatially and phase-wise, and construct an artificial angular momentum demodulation device using the acoustic metamaterial unit at the water-air interface to achieve acoustic transparency between the water and air media and decode the information of each channel;

[0009] (4) The multi-frequency multi-mode vortex acoustic beam is transmitted through free space to the water-air interface, is unwound after passing through the artificial angular momentum demodulation device and separated in space, and then is focused on different spatial positions, and the acoustic wave signal carrying the original encoded information is received by the receiving end.

[0010] Further, the transmitting end described in step (1) is located underwater or in the air, and the corresponding receiving end will be located in the air or underwater.

[0011] Further, the acoustic metamaterial unit at the water-air interface in step (2) is a hybrid acoustic structure with an air channel having a three-layer structure constructed in a rigid substrate. By controlling the cross-sectional size of the air slot, the equivalent acoustic impedance of each layer structure is modulated to achieve impedance matching and resonance transmission of sound waves between water and air media. By controlling the length of the folded space in the middle layer, phase modulation of the transmitted sound waves is performed. There are two air channels in the acoustic metamaterial unit at the water-air interface. By optimizing the structural parameters of the two air channels, the adjacent coupling between the channels is eliminated, enabling the acoustic metamaterial unit at the water-air interface to independently control sound waves of two frequencies.

[0012] Further, the required spatial phase distribution in step (3) is as follows:

[0013]

[0014] where M is the number of multiplexed orbital angular momenta, k is the wave vector of the incident sound wave, and l m is the order of the orbital angular momentum carried by the m-th vortex sound beam. is the relative distance from the m-th focus to the artificial angular momentum demodulation device. The spatial coordinates (x m , y m , z m ) of the m-th target focus are also the spatial coordinates of the focus after demodulation of the l m -th order vortex sound beam. θ is the azimuth angle in polar coordinates, and (x, y, z) are the spatial coordinates.

[0015] Further, the artificial angular momentum demodulation device in step (3) is a single-layer planar structure formed by arranging a variety of acoustic metamaterial units at the water-air interface according to the required spatial phase distribution at the water-air interface. The lengths L1 and L2 of the folded spaces in the middle layers of the two air channels of the acoustic metamaterial unit at the water-air interface constituting the artificial angular momentum demodulation device are different.

[0016] Further, the artificial angular momentum demodulation device in step (3) is composed of 12×12 acoustic metamaterial units at the water-air interface, with a size of 240 mm×240 mm and a thickness of 19 mm.

[0017] Further, the receiving end in step (4) is a microphone placed at each focus.

[0018] Further, in step (1), the transmitting end is 0.8 m away from the water surface, and the vertical distance from the receiving end to the artificial angular momentum demodulation device is 0.2 m.

[0019] Advantageous effects: Compared with the prior art, the advantageous effects of the present invention are as follows:

[0020] 1. The present invention constructs a cross - water - air angular - momentum pure - acoustic - wave communication system including the full link of encoding, transmitting, and receiving for the first time without using active complex relay devices and cross - water - air communication technologies for wave - property conversion, breaking through the inherent limitations of existing cross - water - air communication technologies in terms of transmission efficiency, channel capacity, hardware complexity, stability, and real - time performance;

[0021] 2. The present invention proposes a novel water - air interface acoustic metamaterial with impedance - matching and phase - regulation functions, which can arbitrarily modulate the wavefront of transmitted acoustic waves on a sub - wavelength spatial scale, providing an effective guarantee for the practical implementation of constructing a cross - water - air pure - acoustic communication mechanism and filling the gap in this research field of acoustic metamaterials; in addition, by increasing the number of air channels in the metamaterial unit and eliminating the adjacent coupling between different channels through parameter optimization, independent regulation of acoustic waves with different frequencies by the metamaterial unit can be achieved, making the metamaterial unit more suitable for cross - water - air acoustic communication systems with high communication rates;

[0022] 3. Compared with existing large - volume angular - momentum communication systems, the present invention uses water - air interface metamaterial units to construct a structurally compact artificial angular - momentum demodulation device to receive and decode the information carried by the composite vortex acoustic beam. This structure also has significant advantages such as high mechanical strength, strong robustness, and high lifespan compared to traditional soft materials; the artificial angular - momentum demodulation device not only realizes acoustic coupling at the water - air interface but also unfolds and focuses the vortex acoustic beams carrying different orders of orbital angular momentum to different spatial positions. By placing a single microphone at the focal point, efficient reception of the information transmitted through this channel can be achieved. Therefore, a simple and low - cost receiving system is designed without microphone scanning and signal post - processing, achieving a bit - error rate far lower than the forward - error - correction threshold; at the same time, this receiving mechanism also reduces interference between different channels and improves the robustness of the communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of the principle of the present invention;

[0024] Figure 2 is a schematic diagram of the active transducer array and circuit control part required for the experiment of the present invention;

[0025] Figure 3 is a schematic diagram of a water - air interface metamaterial unit with double air channels;

[0026] Figure 4 is the acoustic characteristic of a water - air interface metamaterial unit with specific intermediate - layer lengths (L1 = 33mm, L2 = 31.6m); among them, (a) is the variation of the transmittance of the two air channels with frequency; (b) is the distribution of the transmitted acoustic pressure field of the metamaterial unit when acoustic waves with frequencies of 12kHz and 16kHz are incident respectively;

[0027] Figure 5 The acoustic response of the metamaterial unit varies with the folding lengths L1 and L2 of the intermediate layer between the two air channels. Among them, (a) shows the variation of the transmission coefficient and phase shift of the sound wave at 12 kHz passing through the metamaterial unit with parameters L1 and L2; (b) shows the variation of the transmission coefficient and phase shift of the sound wave at 16 kHz passing through the metamaterial unit with parameters L1 and L2.

[0028] Figure 6 The discrete phase distribution of the artificial angular momentum device designed for the experiment of the present invention. Among them, (a) is the discrete phase distribution that the artificial angular momentum device needs to satisfy under the incidence of sound waves at 12 kHz and 16 kHz; (b) is the phase distribution of the artificial angular momentum demodulation device with frequency multiplexing.

[0029] Figure 7 Schematic diagram of the structure of the artificial angular momentum demodulation device in this example.

[0030] Figure 8 Actual photo of the cross-water-air angular momentum acoustic communication system constructed in this example.

[0031] Figure 9 When channel 1 is open, the spatial field strength distribution obtained by sweeping the field in the target focus F1 region. Among them, (a) is the distribution result of the spatial field strength when there is an artificial angular momentum demodulation device on the water-air interface; (b) is the distribution result of the spatial field strength when there is no artificial angular momentum demodulation device on the water-air interface.

[0032] Figure 10 The sound energy enhancement values measured at two foci with and without the action of the artificial angular momentum demodulation device when different channels are opened. Among them, (a) is the sound energy enhancement values at the two foci when the incident sound wave is 12 kHz; (b) is the sound energy enhancement values at the two foci when the incident sound wave is 16 kHz.

[0033] Figure 11 The complex four-color grayscale image transmitted in the communication system of this example.

[0034] Figure 12 The change of the bit error rate of the transmitted image with the signal-to-noise ratio. Detailed implementation manners

[0035] The present invention will be further described in detail below with reference to the accompanying drawings.

[0036] The present invention proposes a cross-water-air angular momentum acoustic communication method based on acoustic metamaterials, as Figure 1 shown, including the following steps:

[0037] Step 1: Construct a cross - water - air angular momentum acoustic communication system including a transmitting end, an artificial angular momentum demodulation device on the water - air interface, and a receiving end; the transmitting end emits a coaxial vortex acoustic beam multiplexed with multiple frequencies and multiple angular momenta, and the vortex acoustic beams with different frequencies and different orders form independent channels for information transmission; encode the data stream to be transmitted onto the amplitude and phase of the acoustic wave of each channel in sequence as the signal source.

[0038] The transmitting end uses an active underwater acoustic transducer array to synthesize a coaxial vortex acoustic beam multiplexed with frequency and angular momentum, and propagates it through free space to the water - air interface; the artificial angular momentum demodulation device is used to achieve impedance matching at the water - air interface and perform phase modulation on the transmitted acoustic wave, so as to unwind and redirect the incident multiplexed vortex acoustic beam, that is, separate and converge the de - angular - momentum - quantified plane waves to different positions in space, and the vortex acoustic beams with different orders have different focusing positions.

[0039] The transmitting end is located in water and emits a composite vortex acoustic beam multiplexed with N frequencies and M orbital angular momentum modes towards the water - air interface. Each order of vortex acoustic beam can independently carry information for transmission, so a total of NM independent channels are formed. Combining the frequency multiplexing technology with the orbital angular momentum multiplexing technology further improves the transmission rate of the cross - water - air interface acoustic communication system. Note that the transmitting end here can be located in water or air, and here a cross - water - air communication system from water medium to air medium is shown.

[0040] In the present invention, an active transducer array is used as the transmitting device to generate a multi - order frequency - multiplexed vortex acoustic beam. After the composite vortex acoustic beam reaches the water - air interface, it passes through the artificial angular momentum device to break through this natural barrier of the water - air interface and achieve high - efficiency transmission. The artificial angular momentum device on this water - air interface not only solves the serious impedance mismatch problem between water and air, but also can be used as an angular momentum demodulation device to unwind and spatially separate the multiplexed vortex acoustic beam, that is, converge the unwound plane waves to different positions in air according to the order of the vortex acoustic beam. In addition, different local resonance units, that is, air channels of metamaterial units, are designed by hybrid resonance to increase the density of states while suppressing the resonance coupling between channels, so that the metamaterial unit can independently control acoustic waves of multiple frequencies using different air channels. Therefore, the artificial angular momentum demodulation device of the water - air interface composed of the above - mentioned frequency - multiplexed metamaterial units can simultaneously achieve the demodulation and detection of the composite vortex acoustic beam with multiple frequencies and multiple angular momentum modes, further improving the communication capacity of the cross - water - air angular momentum acoustic communication system. The receiving end only needs to place a microphone at each focus to receive each demodulated signal in real - time and in parallel. Due to the existence of the focusing effect, the acoustic energy of the received signal increases, improving its signal - to - noise ratio, which also improves the robustness and accuracy of the communication system.

[0041] In this embodiment, M = 2 and N = 2 are selected, that is, the acoustic beam emitted from the emission surface is a composite vortex acoustic beam with dual frequencies and dual orbital angular momentum modes. The multiplexed orbital angular momentum orders are positive and negative first orders, and the multiplexed frequencies are 12 kHz and 16 kHz.

[0042] Figure 2 It is a schematic diagram of the active transducer array composed of 8 transducers and the control circuit used in this system. The 8 transducers are equally spaced and arranged in 1 circle to form a transducer array with a radius of 95 mm. The field programmable gate array (FPGA) is used to input signals to each transducer and independently control the amplitude and phase of each input signal, so that the array can generate vortex acoustic beams with positive and negative first orders. Among them, transducers No. 1, 3, 5, and 7 are used to generate vortex acoustic beams carrying the first-order orbital angular momentum, and the remaining transducers are used to generate vortex acoustic beams carrying the negative first-order orbital angular momentum.

[0043] In the actual communication process, both the amplitude and phase of the vortex acoustic beam are functions of time. Therefore, the existing amplitude modulation technology and phase modulation technology can be used to load the data stream onto the amplitude and phase of the vortex acoustic beam for information transmission. In this example, binary keying technology is used to encode 4 groups of data streams composed of 0 and 1 onto the amplitude of each order of the vortex acoustic beam, with amplitude 0 representing data 0 and amplitude 1 representing data 1. 8 relays are respectively connected in series to the circuits of 8 underwater acoustic transducers, and a single-chip microcomputer is used to control the opening and closing of each relay respectively, so as to encode 0 and 1 data onto the amplitude of each order of the vortex acoustic beam.

[0044] After the multiplexed vortex acoustic beam passes through the artificial angular momentum demodulation device at the water-air interface, it is unwound and converges to different focal points in space according to the order of the original orbital angular momentum. The amplitude of the acoustic signal received at each focal point is normalized, and 0 and 1 data streams are output according to the set threshold.

[0045] Step 2: Design an acoustic metamaterial unit at the water-air interface with impedance matching and phase regulation functions according to the working frequency.

[0046] The acoustic metamaterial unit at the water-air interface is a hybrid acoustic structure that constructs an air channel with a three-layer structure in a rigid substrate. By controlling the cross-sectional size of the air groove, the equivalent acoustic impedance of each layer of the structure is modulated to achieve impedance matching and resonance transmission of sound waves between water and air media. By controlling the length of the folded space in the middle layer, the phase of the transmitted sound wave is modulated.

[0047] To achieve dual-frequency multiplexing, two air channels are designed in the metamaterial unit. By optimizing the structural parameters of the two channels, the adjacent coupling between the channels is eliminated, so that the metamaterial unit can independently regulate sound waves of two frequencies.

[0048] As Figure 3As shown in the figure, the air-water interface metamaterial unit in this embodiment is an acoustic hybrid structure composed of a rigid substrate (ABS plastic) and two air channels with a three-layer structure. The acoustic impedance of each layer structure can be arbitrarily adjusted by controlling the cross-sectional size of the air grooves. By optimizing the structural parameters of the two air channels, the resonance coupling between the two channels is eliminated, and independent regulation of 12 kHz and 16 kHz sound waves is achieved for air channel 1 and air channel 2 respectively. In each air channel, the a and c layers at both ends are used as matching layers to contact air and water respectively, and efficient acoustic transparency of air and water media is achieved through strong resonance transmission. The middle b layer extends the sound propagation path through a spatial folding method to introduce an adjustable propagation phase delay. The purpose of designing the c layer as an air slit is to effectively increase the radiation area of the c layer while ensuring resonance matching, and suppress the extremely large acoustic resistance added at the outlet by the near-field mode, thereby ensuring nearly 100% transmission of sound energy.

[0049] According to the operating frequency of the communication system in this embodiment, the structural parameters of the metamaterial unit are designed by combining theoretical analysis and numerical simulation. The fixed structural parameters are W = 12 mm, H = 19 mm, h a1 = 5.7 mm, w a = 8.5 mm, w b1 = 2.7 mm, h c = 0.5 mm, h a2 = 2.7 mm, w b2 = 3.5 mm, l c1 = 16.8 mm, l c2 = 13.5 mm. In addition, L1 and L2 are variable parameters of the structure. By manipulating a single structural parameter L1 (L2), full coverage modulation of the sound wave phase within the range of 0 - 2π can be achieved for 12 kHz (16 kHz) sound waves. That is, the optimized structural parameters mentioned above eliminate the adjacent coupling between the two air channels, enabling the two air channels to independently modulate sound waves of different frequencies, and realizing frequency multiplexing of the metamaterial unit. Figure 4 In (a), it shows the variation of the transmittance of the two air channels with the incident sound wave frequency for specific values of L1 and L2 (L1 = 33 mm, L2 = 31.6 mm). Figure 4 In (b), it shows the sound field distribution of sound waves with frequencies of 12 kHz and 16 kHz passing through this metamaterial unit. From Figure 4As can be seen in (a), when the incident acoustic wave frequency is 12 kHz (16 kHz), the transmittance of air channel 1 (2) is much higher than that of air channel 2 (1). This indicates that when a sound wave with a frequency of 12 kHz (16 kHz) is incident on the metamaterial unit, the structure of air channel 1 (2) plays a major role in modulating the sound wave, while the extremely low transmittance of air channel 2 (1) only generates negligible background noise and does not affect the transmission modulation of the sound wave by the metamaterial unit. By adjusting the geometric parameters L1 and L2, the metamaterial unit can achieve high transmittance for incident sound waves of 12 kHz and 16 kHz and full-coverage modulation of the phase within 2π, as Figure 5 shown. Figure 5 In (a), the transmission amplitude and phase of the metamaterial unit for an incident sound wave of 12 kHz vary with the lengths L1 and L2 of the intermediate layer, Figure 5 and in (b), the transmission amplitude and phase of the metamaterial unit for an incident sound wave of 16 kHz vary with the lengths L1 and L2 of the intermediate layer. Figure 5 The results also show that the two air channels in the metamaterial unit can independently modulate incident sound waves of two frequencies. Although under certain combinations of the parameters (L1, L2), the resonance coupling between the two air channels is strong, making it impossible to linearly control the sound wave of a specific frequency by the air channels, this does not affect the high transmittance and full-range phase control of the incident sound wave by the metamaterial unit. Four points are marked in Figure 5 which represent four types of metamaterial units. Unit one, two, three, and four achieve a sound energy transmittance of more than 60% for incident sound waves of 12 kHz and 16 kHz and phase modulation of (0, 0), (0, π), (π, 0), and (π, π) respectively. These four types of metamaterial units will be used as four basic structures to construct the artificial angular momentum demodulation device in this example to achieve demultiplexing and information decoding of the composite vortex sound beam with dual-frequency and dual-angular momentum modes. The reason why the sound wave transmission efficiency of the above-mentioned metamaterial unit is not close to 1 is that the thermoacoustic effect of the structure is considered in the simulation calculation. The existence of the thermoacoustic effect will reduce the transmission coefficient of the sound wave, and the higher the sound wave frequency, the more obvious the thermoacoustic effect of the structure. Nevertheless, the designed metamaterial unit still achieves a sound wave transmission efficiency of nearly 70%. Compared with the natural situation with a bare boundary, the sound field intensity at the same detection point (z = 0.2 m) in the air is increased by 31 dB. This record-breaking enhanced sensitivity is far beyond the reach of any existing technical means.

[0050] Step 3: Determine the required spatial phase distribution on the water-air interface according to the frequency and orbital angular momentum order of the composite vortex sound beam and the receiving positions corresponding to each order of vortex at the receiving end. Discretize the required continuous phase spatially and phase-wise, and use the acoustic metamaterial unit on the water-air interface to construct an artificial angular momentum demodulation device to achieve acoustic transparency between the water and air media and information decoding of each channel.

[0051] The artificial angular momentum demodulation device has the following spatial phase distribution:

[0052]

[0053] where M is the number of multiplexed orbital angular momenta, k is the wave vector of the incident acoustic wave, and l m is the order of the orbital angular momentum carried by the m-th vortex acoustic beam, is the relative distance from the m-th focal point to the artificial angular momentum demodulation device (with spatial coordinates (x, y, z)). The spatial coordinates (x m of the m-th target focal point F m , y m , z m ) are also the spatial coordinates of the focal point after demodulation of the l m -order vortex acoustic beam, and θ is the azimuth angle in polar coordinates.

[0054] Taking the demodulation of positive and negative first-order vortex acoustic beams as the goal, in this embodiment, the cross-sectional size of the artificial angular momentum demodulation device is 240 mm × 240 mm. According to the focal positions F1 and F2 after demodulation of the positive and negative first-order vortex acoustic beams, the ideal phase distributions that the artificial angular momentum device needs to satisfy under the incidence of 12 kHz and 16 kHz acoustic waves can be calculated respectively. After spatially discretizing and phase-discretizing the two continuous phase distributions, the discrete phase distribution shown in (a) of Figure 6 is obtained. The spatial discretization degree is 20 mm × 20 mm, and the phase discretization is 0 and π. By combining the discrete phase distributions that the artificial angular momentum demodulation device needs to satisfy at 12 kHz and 16 kHz, the phase distribution of the frequency-multiplexed artificial angular momentum demodulation device shown in (b) of Figure 6 is obtained. Using four metamaterial units ( Figure 5 the four marked points in Figure 7 ) arranged according to the required phase distribution, the artificial angular momentum demodulation device shown in

[0055] Figure 8 is obtained. If an artificial angular momentum device for demodulating other-order vortex acoustic beams is to be designed, only the arrangement order of the four basic units needs to be changed according to the discretized target phase distribution. This is a real-shot picture of the cross-water-air angular momentum acoustic communication system constructed in this example. In the underwater part, a water tank with a size of 1 m × 1 m × 1 m is constructed. Absorbing rubber sheets are laid around the water tank to simulate an underwater anechoic free-field environment. The transducer array is placed at the center of the bottom of the water tank to emit acoustic waves upward. Then, the multiplexed vortex acoustic beam is demultiplexed and converged to different focal positions after passing through the artificial angular momentum demodulation device on the water-air interface. The acoustic signal at the focal point passes through a quarter-inch air microphone (Brüel& For real-time acquisition and reception of type-4961). In the air part, in order to avoid the superposition interference of reflected sound waves, sound-absorbing sponges are also covered on the four sides and the top area of the box. The artificial angular momentum demodulation device is placed parallel to the water-air interface through a support frame, and the center is collinear with the underwater sound source.

[0056] Step 4: The multi-frequency and multi-mode vortex sound beam is transmitted through free space to the water-air interface, and after passing through the artificial angular momentum demodulation device, it is unwound and separated in space, and then focused on different spatial positions, and the sound wave signal carrying the original encoded information is received by the receiving end.

[0057] The communication system adopting the present invention is tested as follows:

[0058] In this embodiment, a multiplexed vortex sound beam with coaxial transmission of dual-frequency and dual-angular momentum modes forms a total of four independent channels (labeled as Channel 1 to Channel 4) for transmitting data. The positive first-order and negative first-order vortex sound beams at 12 kHz respectively constitute Channel 1 and Channel 2, and the positive first-order and negative first-order vortex sound beams at 16 kHz respectively constitute Channel 3 and Channel 4.

[0059] In order to verify the high transmission of the artificial angular momentum demodulation device to sound waves and its demodulation ability for vortex sound beams, first measure the sound field distribution in a 11.2×11.2 cm 2 square area centered on the target focus in the focal plane when different channels are opened. Here, the scanning field situation when Channel 1 is opened is taken as an example for illustration. Figure 9 In (a), it is the normalized sound intensity distribution result in the focal point F1 area when the artificial angular momentum demodulation device is placed on the water-air interface; Figure 9 In (b), it is the normalized sound intensity distribution result in the focal point F1 area when there is no artificial angular momentum demodulation device on the water-air interface, that is, the bare water-air interface. It can be seen that when the artificial angular momentum demodulation device is introduced on the water-gas interface, the spatial distribution of the transmitted sound field presents a focused sound beam shape, and the energy is mainly concentrated near the target focus. Compared with the natural situation of the bare boundary, the sound field intensity at the detection point is increased by 24 dB. Figure 10 It is the sound energy enhancement values measured at two focal points with and without the artificial angular momentum demodulation device when different channels are opened. Figure 10 In (a), it is the sound energy enhancement generated by the artificial angular momentum demodulation device at two focal points when the incident sound wave frequency is 12 kHz and different channels are opened; Figure 10In Fig. (b), it shows the acoustic energy enhancement generated by the artificial angular momentum demodulation device at two focal points when the incident acoustic wave frequency is 16 kHz and different channels are open. The above results demonstrate the peculiar acoustic performance of the artificial angular momentum demodulation device on the water-air interface. Under its action, the extremely mismatched boundary between water and air becomes almost transparent, and the spatial distribution of the transmitted sound field can be arbitrarily modulated. Based on this, a high-capacity and high-signal-to-noise ratio acoustic angular momentum communication system across the water-air interface is realized for the first time.

[0060] Next, a complex four-color grayscale image was transmitted in real time using this communication system. The image contains 338×460 pixel points, and the pixel value of each pixel point consists of 4-bit data. As Figure 11 shown, Channel 1 transmits the data of the first bit of each pixel point, and so on. The complex image is synchronously transmitted using four channels, and each channel will transmit a data stream of 338×460 bits. The 2ASK keying modulation technique is used to encode the data stream onto the amplitude of the vortex acoustic beam in each channel. The data-bearing signal uses pulse modulation, with a pulse period of 20T0 (T0 = 1.6 ms), and each pulse period contains 1 bit of data. The output image was obtained by combining the data streams decoded from each channel. The measured bit error rate of the image transmission was 0, so the output image was exactly the same as the target image, and the communication system achieved lossless image information transmission. Therefore, the communication system in this example realizes real-time, simple, accurate, and high-channel-capacity cross-water-air angular momentum acoustic communication only using a receiving system with two microphones. To further characterize the performance of the communication system, the signal-to-noise ratio of the received signal was changed by increasing the ambient noise, and the variation of the bit error rate of the transmitted image with the signal-to-noise ratio was measured through experiments, as Figure 12 shown. It can be clearly observed that the bit error rates of the four channels are almost close to 0 when the signal-to-noise ratio is higher than 10 dB. As the signal-to-noise ratio decreases, the channel bit error rate increases. Nevertheless, even when the signal-to-noise ratio is close to 0, the channel bit error rate is still much lower than the forward error correction threshold of 3.8×10 -3 , which indicates that the communication system using the present invention has strong anti-interference ability.

[0061] The above experimental results prove that the present invention can break through the acoustic transmission barrier of the water-air interface, provide high-performance artificial materials with impedance matching and phase regulation functions for acoustic wave regulation across water-air media, and also provide a new research paradigm for acoustic wave transmission and manipulation between non-uniform media. At the same time, the design of this water-air interface metamaterial has high flexibility. By increasing the number of air channels and combining parameter optimization, the frequency multiplexing function of the metamaterial unit is realized, further improving the channel capacity of cross-water-air interface angular momentum acoustic communication.

[0062] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A cross - water - air angular momentum acoustic communication method based on acoustic metamaterials, characterized in that, It includes the following steps: (1) Construct a cross - water - air angular momentum acoustic communication system including a transmitting end, an artificial angular momentum demodulation device on the water - air interface, and a receiving end; The transmitting end emits a coaxial vortex acoustic beam multiplexed with multiple frequencies and multiple angular momenta. Vortex acoustic beams with different frequencies and different orders form independent channels for information transmission; the data stream to be transmitted is sequentially encoded onto the amplitude and phase of each channel acoustic wave as the signal source; (2) Design an acoustic metamaterial unit on the water - air interface with impedance matching and phase - regulation functions according to the working frequency; the acoustic metamaterial unit on the water - air interface is a hybrid acoustic structure that constructs an air channel with a three - layer structure in a rigid substrate. By controlling the cross - sectional size of the air groove, the equivalent acoustic impedance of each layer structure is modulated to achieve impedance matching and resonance transmission of sound waves between water - air media. By controlling the length of the folded space in the middle layer, phase modulation of the transmitted sound wave is achieved; there are two air channels in the acoustic metamaterial unit on the water - air interface. By optimizing the structural parameters of the two air channels, the adjacent coupling between the channels is eliminated, so that the acoustic metamaterial unit on the water - air interface can independently regulate sound waves of two frequencies; (3) Determine the required spatial phase distribution on the water - air interface according to the frequency and orbital angular momentum order of the composite vortex acoustic beam and the receiving position corresponding to each order of vortex at the receiving end. Perform spatial discretization and phase discretization on the required continuous phase, and use the acoustic metamaterial unit on the water - air interface to construct an artificial angular momentum demodulation device to achieve acoustic transparency between water - air media and information decoding of each channel; (4) The multi - frequency and multi - mode vortex acoustic beam is transmitted through free space to the water - air interface, is unwound after passing through the artificial angular momentum demodulation device and separated in space, and then is focused on different spatial positions, and the acoustic wave signal carrying the original encoded information is received by the receiving end.

2. The cross-aqueous and aerial angular momentum acoustic communication method based on acoustic metamaterials according to claim 1, wherein In step (1), the transmitting end is located underwater or in the air, and the corresponding receiving end will be located in the air or underwater.

3. A cross-water-air angular momentum acoustic communication method based on acoustic metamaterials according to claim 1, characterized in that, The required spatial phase distribution in step (3) is: where M is the number of multiplexed orbital angular momenta, k is the wave vector of the incident acoustic wave, and l m is the order of the orbital angular momentum carried by the m-th vortex acoustic beam, is the relative distance from the m-th focal point to the artificial angular momentum demodulation device, and the spatial coordinates (x m , y m , z m ) of the m-th focal point are also the spatial coordinates of the focal point after demodulating the l m -order vortex acoustic beam. θ is the azimuth angle in polar coordinates, and (x, y, z) are spatial coordinates.

4. A cross-water-air angular momentum acoustic communication method based on acoustic metamaterials according to claim 1, characterized in that, The artificial angular momentum demodulation device in step (3) is a single - layer planar structure composed of multiple acoustic metamaterial units on the water - air interface arranged according to the required spatial phase distribution on the water - air interface; the lengths L1 and L2 of the folded spaces in the middle layers of the two air channels of the acoustic metamaterial unit on the water - air interface that constitutes the artificial angular momentum demodulation device are different.

5. A method for cross - water - air angular momentum acoustic communication based on acoustic metamaterials according to claim 1, characterized in that, The artificial angular momentum demodulation device in step (3) is composed of 12×12 acoustic metamaterial units on the water - air interface, with a size of 240 mm×240 mm and a thickness of 19 mm.

6. The cross - water - air angular momentum acoustic communication method based on acoustic metamaterials according to claim 1, characterized in that The receiving end in step (4) is a microphone placed at each focus.

7. The cross-aqueous and aerial angular momentum acoustic communication method based on acoustic metamaterials according to claim 1, characterized in that In step (1), the transmitting end is 0.8 m away from the water surface, and the receiving end is 0.2 m away from the artificial angular momentum demodulation device vertically.

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