CN109490425B - Passive material low-frequency reflection coefficient measuring method based on Green function reconstruction technology - Google Patents
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Abstract
A passive material low-frequency reflection coefficient measuring method based on Green function reconstruction technology comprises the following steps: 1) obtaining a reflection time reversal term in the multi-dimensional Markov equation by using a time reversal technology; 2) reconstructing a scattering wave Green function from the hydrophone to the surface of the sample; 3) realizing scattered wave focusing in a numerical value mode; 4) and (4) calculating a reflection coefficient measurement value. According to the method, the time reversal technology is used for obtaining the reflection wave time reversal terms in the multi-dimensional Markov equation, then the simplified multi-dimensional Markov equation is substituted to obtain the scattering wave green function from the hydrophone to the surface of the sample, the reconstructed scattering wave green function and the reflection wave time reversal terms corresponding to the position of the hydrophone are convolved, and all array element convolution terms are summed to obtain the focusing of the scattering wave on the surface of the sample; the pressure tank measurement experiment verifies the effectiveness of the invention in the measurement of the reflection coefficient of the low-frequency underwater sound passive material.
Description
Technical Field
The invention relates to a method for measuring a low-frequency reflection coefficient of a large sample of an underwater passive material.
Background
The underwater sound passive material refers to special functional acoustic materials and structures laid on underwater parts, and is an acoustic protection system mainly composed of series products with different acoustic functions, such as an acoustic tile, an acoustic isolation tile, a vibration suppression tile, a decoupling tile, an array silencer and the like. The underwater sound passive material can absorb active detection sound waves and reduce the sound target strength of an underwater structure, and can be used as a material for inhibiting the self radiation noise of the structure.
The strength of the echo signal is closely related to the reflection characteristic of the target, which is one of the important indexes for measuring the sound absorption performance of the underwater sound passive material, and the target strength is used for describing the target sound reflection capacity in engineering.
Compared with sound tube testing, the overall sound absorption performance of an underwater passive material structure can be better reflected by large-scale measurement of the sound-damping pool, but due to the fact that the sound absorption performance of the sound-damping material around the pool under the low-frequency condition is limited, sound waves are obviously multipath propagated, the period of the low-frequency sound waves is large, multipath superposition caused by reverberation is more obvious, and great interference is caused to measurement. The time reversal focusing method utilizes the multipath of the time reversal compensation measurement environment, reduces the interference of the interface reverberation on the direct wave signal, and improves the signal-to-noise ratio of the measurement, thereby improving the measurement precision. However, in actual measurement, in order to obtain the direct wave and the reflected wave required for calculating the reflection coefficient, the hydrophone and the measurement sample need to be kept at a certain distance to ensure the separation of the direct wave and the reflected wave. In particular, in low frequency measurement, the distance between the hydrophone and the measurement sample is much larger than in the high frequency case, because the signal period is long, in order to ensure the separation of the direct wave and the reflected wave. Since the reflected wave of the sample is also affected by the waveguide effect, the received reflected signal contains a large amount of interface reverberation, and is superimposed with the reflected wave of the direct path, which brings great interference to the measurement. Although the time reversal method reduces the interference of the interface reverberation on the incident wave signal, the interference of the interface reverberation on the sample reflected wave under the low-frequency condition cannot be solved, and the measurement performance of the method at the low frequency is seriously influenced.
Disclosure of Invention
In order to overcome the defects of the prior art, the separation of the reflection echo and the direct wave of the underwater sound passive material is realized under the low-frequency condition, the multi-path interference suppression is carried out, and the precision of the measurement result of the time reversal focusing measurement method under the low-frequency condition is further improved. The method is suitable for low frequency bands, effectively reduces measurement errors, improves measurement accuracy, and reduces requirements on experimental equipment, sample size and experimental space. The pressure tank measurement experiment verifies the effectiveness of the invention in the echo reduction measurement of the underwater sound passive material.
The technical scheme adopted by the invention to solve the technical problems is as follows.
A passive material low-frequency reflection coefficient measuring method based on a Green function reconstruction technology comprises the following steps:
1) and (3) obtaining a reflection time reversal term in the multidimensional Markov equation by using a time reversal technology: under the condition of placing a sample, selecting a certain hydrophone on a receiving array as a focusing hydrophone, sequentially transmitting signals by each transmitting transducer and recording the signals received by the focusing hydrophone, and carrying out time reversal on the received signals and synchronously transmitting the signals by the corresponding transmitting transducers, so that the time focusing signals are received by the hydrophones, and intercepting the waveform of the time focusing signals before the focusing time as a reflection wave time reversal term in a multi-dimensional Markov equation;
2) reconstructing the scattered wave green function of the hydrophone to the sample surface: substituting the reflection terms in the step 1) into the simplified multidimensional Markov equation to obtain a scattering wave Green function from the focused hydrophone to the surface of the sample;
3) the scattering wave focusing is realized inversely in numerical value: performing inverse convolution on the scattered wave green's function reconstructed in the step 2) and the reflected wave at the position corresponding to the hydrophone in the step 1), summing convolution results corresponding to each hydrophone after performing the same operation on all hydrophones on the receiving array, realizing the focusing of the scattered wave on the surface of the sample, and recording the focused wave as pr；
4) Calculation of reflectance measurements: using the focused wave obtained by time reversal emission in the step 1) as an incident wave piAnd 3) calculating the obtained focusing wave prCalculating the reflection coefficient as the reflected wave
The technical conception of the invention is as follows: and obtaining a reflection wave time reversal term in the multi-dimensional Markov equation through a time reversal technology, further obtaining a scattering wave Green function from the hydrophone to the surface of the sample through the simplified multi-dimensional Markov equation, then convolving the reconstructed scattering wave Green function and the reflection wave time reversal term of the corresponding hydrophone position, and summing all the array element convolution terms to obtain the focusing of the scattering wave on the surface of the sample, thereby eliminating the interference of interface reverberation on the sample reflection wave and achieving the purpose of suppressing the reverberation.
Compared with the prior passive material reflection coefficient measuring method based on the time reversal focusing technology, the invention has the following technical advantages: the method is characterized in that a multidimensional Markov equation and a time reversal technology are utilized to obtain a scattered wave Green function from a hydrophone to the surface of a sample, and the scattered wave is focused on the surface of the sample through numerical time reversal, so that the purpose of inhibiting the interference of reverberation on the reflected wave of the sample is achieved, and the method is particularly suitable for measuring the reflection coefficient of the low-frequency-band sample.
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FIG. 1 is a reflection time-reversal term for a focused hydrophone taken from a set of experimental data for carrying out the method of the invention.
FIG. 2 is a schematic illustration of a position marker for carrying out the method of the present invention.
FIG. 3 is a schematic diagram of the connection of an experimental system for carrying out the method of the invention.
FIG. 4 is a graph comparing the theoretical value of the reflection coefficient of a 6mm thick steel plate with the test results under the pressure tank condition.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Referring to fig. 1 to 4, a passive material low-frequency reflection coefficient measuring method based on a green function reconstruction technology is used for measuring the reflection coefficient of a finite space underwater sound passive material. The technical scheme of the whole measuring method is as follows:
1) and (3) obtaining a reflection time reversal term of the multidimensional Markov equation by using a time reversal technology:
under the condition of placing a sample, selecting a certain hydrophone on a receiving array as a focusing hydrophone, sequentially transmitting signals by each transmitting transducer, recording the signals received by the focusing hydrophone, time-inverting the received signals and synchronously transmitting the signals by the corresponding transmitting transducers, wherein the time-focusing signals are received by the hydrophones, and as shown in fig. 1, intercepting the waveform of the time-focusing signals before the focusing time as a reflection wave time inverse term in a multi-dimensional Markov equation.
2) Reconstructing the scattered wave green function of the hydrophone to the sample surface:
recording the reflection term when the reflection wave is obtained in the step 1) as R (x, x)R-t), wherein the letters x, x are bold as shown in figure 2RRepresenting spatial xz plane coordinate points, x representing the position of any hydrophone on the receiving array, xRRepresenting a focused hydrophone position; the relationship between the Green's function and the reflection response obtained from the multidimensional Markov equation is
Wherein G (x)F,xRAnd t) represents from xRTo xFThe green's function of (a), including the direct wave component and the scattered wave component, xFShowing the position of the material sample to be tested, as shown in FIG. 2; r (x)RX, t) denotes from x to xRA reflected wave of (c); gd(x,xFAnd t) represents from xFA direct wave green function at x; τ represents time, wherein the integral term is subjected to time integration;
representing the distribution range of the receiving array hydrophones; as shown in fig. 2, the hydrophone array is located in the xz plane, then d2x-dxdz denotes the area integration unit on the plane, here the integrated sum of the reflected wave signals received by each hydrophone.
For engineering implementation, two targeted simplifications can be made to the formula (2): the actual measurement problem only needs to reconstruct the scattering wave Green function; the direct wave Green function from different hydrophones to the tested sample on the receiving array can be replaced by the same direct wave Green function, and the direct wave Green function is the function related to time delay and distance attenuation, thereby calculating
Reconstructing the scattered wave green's function G of a hydrophone onto a sample surfaces(xF,xRT), where L represents the vertical distance of the hydrophone array from the sample surface, Gs(xF,xRAnd-t) is the scattering Green function Gs(xF,xRAnd t) is the time-reversed version.
3) The scattering wave focusing is realized inversely in numerical value:
summing convolution results corresponding to all hydrophones after performing the same operation on all hydrophones of the receiving array by using the scattering wave Green function reconstructed in the step 2) and the reflection wave time inverse term corresponding to the hydrophone position in the step 1), namely calculating
Focusing the scattered wave on the surface of the sample and recording the focused wave as pr。
4) Calculation of reflectance measurements:
using the focusing waveform obtained by time reversal emission in the step 1) as an incident wave piAnd 3) calculating the obtained focusing wave prThe reflection wave is substituted into the formula (1) to calculate the calculated reflection coefficient r.
Examples illustrate that: in order to verify the effectiveness of the invention in the low-frequency measurement of the reflection coefficient of the passive material, test verification in the environment of the pressure silencing water tank is carried out under the laboratory condition. In the experiment, the distance between the ternary transmitting array and a measuring sample is 4.5m, and 3 circular transmitting transducer array elements are uniformly distributed on a circle with the radius of 1.5 m. The test specimens are steel plates with a geometry of 1.1 m.times.1.0 m.times.6 mm and a density of 7.84X 103kg/m3The sound velocity is 5470m/s, and the distance between the 8-element hydrophone array and the surface of the steel plate is about 1 m. The experimental set-up is shown in figure 3. The test was conducted to measure the reflection coefficient of the steel plate at a frequency of 0.5kHz to 4 kHz. As can be seen from FIG. 4, the measurement result is basically consistent with the theoretical value, and the error is less than 1dB, so that the method has effectiveness in the low-frequency measurement of the reflection coefficient of the underwater sound passive material.
The embodiments described in this specification are merely illustrative of implementations of the inventive concept and the scope of the present invention should not be considered limited to the specific forms set forth in the embodiments but rather by the equivalents thereof as may occur to those skilled in the art upon consideration of the present inventive concept.
Claims (4)
1. A passive material low-frequency reflection coefficient measuring method based on Green function reconstruction technology comprises the following steps:
1) and (3) obtaining a reflection time reversal term in the multidimensional Markov equation by using a time reversal technology: under the condition of placing a sample, selecting a certain hydrophone on a receiving array as a focusing hydrophone, sequentially transmitting signals by each transmitting transducer and recording the signals received by the focusing hydrophone, and carrying out time reversal on the received signals and synchronously transmitting the signals by the corresponding transmitting transducers, so that the time focusing signals are received by the hydrophones, and intercepting the waveform of the time focusing signals before the focusing time as a reflection wave time reversal term in a multi-dimensional Markov equation;
2) reconstructing the scattered wave green function of the hydrophone to the sample surface: substituting the reflection terms in the step 1) into the simplified multidimensional Markov equation to obtain a scattering wave Green function from the focused hydrophone to the surface of the sample;
3) the scattering wave focusing is realized inversely in numerical value: performing inverse convolution on the scattered wave green's function reconstructed in the step 2) and the reflected wave at the position corresponding to the hydrophone in the step 1), summing convolution results corresponding to each hydrophone after performing the same operation on all hydrophones on the receiving array, realizing the focusing of the scattered wave on the surface of the sample, and recording the focused wave as pr；
4) Calculation of reflectance measurements: using the focused wave obtained by time reversal emission in the step 1) as an incident wave piAnd 3) calculating the obtained focusing wave prCalculating the reflection coefficient as the reflected wave
2. The method for measuring the low-frequency reflection coefficient of the passive material based on the green function reconstruction technology as claimed in claim 1, wherein: in the step 2), the process of reconstructing the scattering wave green's function from the hydrophone to the surface of the sample is as follows:
recording the reflection term when the reflection wave is obtained in the step 1) as R (x, x)R-t), wherein the bold letters x, xRRepresenting spatial xz plane coordinate points, x representing the position of any hydrophone on the receiving array, xRRepresenting a focused hydrophone position; the relationship between the Green's function and the reflection response obtained from the multidimensional Markov equation is
Wherein G (x)F,xRAnd t) represents from xRTo xFThe green's function of (a), including the direct wave component and the scattered wave component, xFShowing the position of the tested material sample; r (x)RX, t) denotes from x to xRA reflected wave of (c); gd(x,xFAnd t) represents from xFA direct wave green function at x; τ represents time, wherein the integral term is subjected to time integration;
representing the distribution range of the receiving array hydrophones; the hydrophone array is located in the xz plane, then d2x-dxdz denotes the area integration unit on the plane, here the integrated sum of the reflected wave signals received by each hydrophone.
3. The method for measuring the low-frequency reflection coefficient of the passive material based on the green function reconstruction technology as claimed in claim 2, wherein: for the convenience of engineering implementation, two targeted simplifications are made to the formula (2): the actual measurement problem only needs to reconstruct the scattering wave Green function; the direct wave Green function from different hydrophones to the measured material on the receiving array is replaced by a direct wave Green function which is a function of time delay and distance attenuation, thus calculating
Reconstructing the scattered wave green's function G of a hydrophone onto a sample surfaces(xF,xRT), where L represents the vertical distance of the hydrophone array from the sample surface, Gs(xF,xRAnd-t) is the scattering Green function Gs(xF,xRAnd t) is the time-reversed version.
4. The method for measuring the low-frequency reflection coefficient of the passive material based on the green function reconstruction technology as claimed in claim 1, wherein: in the step 3), the process of realizing the scattered wave focusing in a numerical time reversal mode is as follows:
summing convolution results corresponding to all hydrophones after performing the same operation on all hydrophones of the receiving array by using the scattering wave Green function reconstructed in the step 2) and the reflection wave time inverse term corresponding to the hydrophone position in the step 1), namely calculating
The scattered wave is focused on the surface of the sample, and the focused wave is recorded as pr。
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| CN105301114A (en) * | 2015-10-20 | 2016-02-03 | 浙江大学 | Acoustic coating layer insertion loss measurement method based on multi-channel space-time inverse filtering technology |
| CN107102355A (en) * | 2017-04-27 | 2017-08-29 | 吉林大学 | The parallel Marchenko imaging methods of low-frequency reconfiguration |
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| CN102539530A (en) * | 2012-01-12 | 2012-07-04 | 浙江大学 | Method for measuring echo reduction/reflection coefficient of underwater sound passive material based on time reversal focusing |
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