A method for calibrating low-frequency sensitivity of hydrophone based on time-domain scaling and averaging

Abstract

The present invention discloses a method for calibrating the low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing, which relates to the technical field of low-frequency sensitivity calibration of a hydrophone and solves the problem of inaccurate measurement of the amplitude of direct sound waves due to the superposition of boundary sound reflections of an experimental water pool and a measuring device in hydrophone calibration. The method comprises the following steps: placing a reflective sound-transmitting film in a water body, placing a transducer and the film, and laser focusing; reproducing the underwater sound pressure value based on a laser interferometer; when the low-frequency measurement data shows a superposition of reflected sound and direct sound, selecting adjacent frequency points to carry out multiple measurements. The present invention accurately determines the arrival time of direct sound by emitting high-frequency sound pulses, thereby aligning the starting points of waveform data of different frequencies; when carrying out low-frequency measurements, not only the data of the examined frequency is collected, but a narrow frequency band near the examined frequency is selected, and multi-frequency point data is collected in sequence and subsequently processed by an algorithm.

Description

Technical Field

[0001] The invention relates to the technical field of hydrophone low-frequency sensitivity calibration, and in particular to a hydrophone low-frequency sensitivity calibration method based on time domain scaling and averaging processing. Background Art

[0002] As an underwater acoustic sensing unit, the hydrophone converts acoustic signals into electrical signals and is a basic component of many hydroacoustic devices. The accuracy of the hydrophone's sound pressure sensitivity is directly related to the accuracy of hydroacoustic research and the performance indicators of hydroacoustic equipment. Therefore, relevant research institutions at home and abroad attach great importance to the metrological calibration of hydrophone sensitivity to improve ocean detection capabilities. The calibration methods for hydrophone sensitivity include free field and closed cavity. Among them, the free field calibration method is widely used because it is closer to the use environment of the hydrophone and can reflect the directional characteristics of the hydrophone.

[0003] Free-field calibration methods mainly include comparison method, reciprocity method and laser interferometry. The comparison method compares the signal amplitudes measured by the hydrophone to be tested and the standard hydrophone, and uses the sensitivity of the standard hydrophone to calculate the sensitivity of the hydrophone to be tested; the reciprocity method uses the principle of electroacoustic reciprocity, and measures by rotating the positions of the transmitting transducer, reciprocity transducer and hydrophone, and calculates the hydrophone sensitivity in combination with the reciprocity constant; the laser interferometry method measures the Doppler frequency shift corresponding to the vibration of the water particle to reproduce the sound pressure value, and then places the hydrophone to be tested at the same point to read the voltage amplitude to achieve calibration. Among them, the reciprocity method and laser interferometry are primary calibration methods. Compared with the reciprocity method, the laser interferometry method has the advantages of not relying on the standard hydrophone and having low measurement uncertainty.

[0004] The space of laboratory pools is often limited. Incomplete anechoic and water surface reflections make it impossible to completely avoid reflected sound waves, and the time window for free-field calibration (the arrival delay difference between the first path reflected sound wave and the direct sound wave) is limited. For laser interferometry measurements, the experimental device is usually built in a small pool of 2 to 3 m for the convenience of optical path construction and adjustment. The measurement is particularly affected by the reflection of the pool boundary and the film frame. As the operating frequency decreases, the impact of reflected sound waves becomes more serious. It is difficult for the system to output a steady-state direct sound waveform with a sufficient number of cycles, so the direct sound wave amplitude estimation result is unreliable. The above factors limit the low-frequency band calibration capability of hydrophones based on laser interferometry and cannot meet the needs of actual applications.

[0005] In order to break through the low-frequency limitation of hydrophone calibration and improve the applicability of the measurement device, relevant scholars have conducted many studies, designed corresponding measurement schemes and combined signal processing methods to eliminate the influence of reflected sound and direct sound aliasing on the measurement. The main methods include spatial averaging and complex moving weighted average. The former carries out multiple measurements and averages at different positions in the pool while keeping the relative position of the transducer and the hydrophone unchanged, so that the reflected sound components are randomized and canceled out; the latter averages the multi-frequency point responses in a narrow band near the frequency point under investigation by transmitting a continuous linear frequency modulation signal, so that the contributions of the reflected sound waves cancel each other out.

[0006] In the above-mentioned low-frequency extension scheme for hydrophone calibration, the spatial averaging method involves multiple position adjustments of the mechanical structure, which has low measurement efficiency, and the optical window position is fixed and the size is limited, so the selection range of the optical measurement point is narrow. Therefore, the spatial averaging method is not suitable for laser interferometry calibration devices; the complex moving weighted average method uses continuous signals for measurement and calibration, which has high measurement efficiency, but the linear frequency modulation signal is a non-steady-state excitation, and factors such as improper window function setting and frequency domain energy leakage can easily lead to low calibration accuracy at each frequency point, which does not meet existing needs. In response to this, we propose a hydrophone low-frequency sensitivity calibration method based on time domain scaling and averaging. Summary of the invention

[0007] The purpose of the present invention is to provide a method for calibrating the low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing, so as to overcome the difficulties faced by the spatial averaging method and the complex moving weighted averaging method introduced in the above background technology, and solve the problem that the superposition of acoustic reflections from a water pool and a measuring device affects the measurement of the direct sound amplitude.

[0008] To achieve the above object, the present invention provides the following technical solution: a method for calibrating low-frequency sensitivity of a hydrophone based on time domain scaling average processing, comprising the following steps:

[0009] Step 1: Perform acoustic pressure reproduction based on laser interference: Place the transducer and film and align the laser. The transducer emits a high-frequency pulse signal to stimulate water vibration. The film follows the movement of water particles. The laser heterodyne interference system measures the film vibration and outputs the vibration velocity waveform through the demodulation system. The vibration velocity amplitude is read from the vibration velocity waveform. , calculate the sound pressure amplitude at the measurement point , then take out the film frame, place the hydrophone to be tested at the same point in the water body, read the voltage amplitude, and divide the two to get the sound pressure sensitivity of the hydrophone;

[0010] Step 2: For the low frequency band, collect the particle velocity waveform demodulated by laser and the voltage waveform output by the hydrophone;

[0011] Step 3: Use the time domain scaling average method and set the frequency to , the number of cycles is Measure and collect waveforms of low-frequency transmission signals;

[0012] Step 4: Measure the arrival time of the direct sound using a high-frequency sound signal , set it to time zero, and for frequency The direct and reflected sounds are re-expressed;

[0013] Step 5: Use time domain resampling and scaling to obtain is the reference, the frequency is The waveform is resampled to obtain the resampled direct sound and reflected sound, and realize waveform scaling;

[0014] Step 6: Setup Transmitting frequency , after the above-mentioned time domain alignment, waveform resampling and time domain scaling processing, the waveform is averaged and the energy ratio DRR of the direct sound and the reflected sound is calculated;

[0015] Step 7: Perform low-frequency data processing based on the time domain scaling averaging method. Apply the time domain scaling averaging method to the laser demodulation velocity waveform and the hydrophone output voltage waveform to suppress the reflected sound, where uniform weighting is used between each frequency point.

[0016] Preferably, the calculation formula of the sound pressure amplitude p is:

[0017] ;

[0018] in is the medium density, is the speed of sound, is the vibration amplitude, is the equivalent refractive index of water medium, which is 1.01.

[0019] Preferably, the received waveform at the sound pressure measurement point is:

[0020] ;

[0021] in For direct sound and For reflected sound.

[0022] Preferably, the direct sound and reflected sound It is expressed as:

[0023] ;

[0024] ;

[0025] in is the baseband signal corresponding to the frequency of the transmitted signal, is the direct sound envelope, is the modulation of the waveform envelope by the acoustic reflection, , Respectively represent the arrival time of the direct sound and the first reflected sound, is the reverberation time.

[0026] Preferably, the direct sound and reflected sound after re-expression in step 4 are:

[0027]

[0028]

[0029] in , Indicates the time delay difference between the first reflected sound and the direct sound.

[0030] Preferably, the time domain waveforms of the direct sound and reflected sound resampled in step 5 are expressed as:

[0031]

[0032]

[0033] in is the sound reflection coefficient, It is the direct sound envelope.

[0034] Preferably, the energy ratio DRR of the direct sound to the reflected sound is calculated as follows:

[0035] .

[0036] Preferably, if the frequency band The frequency of the transmitted signal is selected uniformly and densely, and the energy ratio DRR of the direct sound to the reflected sound can be further simplified as:

[0037] .

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. Compared with the complex moving weighted average processing scheme, the proposed scheme of the present invention carries out measurement by emitting steady-state sinusoidal pulses and directly performs frequency averaging in the time domain, thereby avoiding problems such as spectrum leakage and improving the accuracy of direct sound amplitude estimation; compared with the existing spatial averaging scheme, this scheme performs variable frequency measurement at the same measurement point, which is convenient for realizing automatic data collection and analysis processing, without changing the measurement position, avoiding processes such as laser refocusing, and significantly improving measurement efficiency;

[0040] 2. The present invention does not rely on the prior electroacoustic parameters of the transducer and the hydrophone, and can achieve good suppression effects on various complex and multi-path reflected sounds, expanding the calibration low-frequency limit. The scheme has strong applicability and feasibility. Based on laser interferometry calibration, the reflected sound is suppressed and the direct sound is restored for the signals received by the hydrophone and the reflective sound-transmitting film to be tested. The given measurement scheme and signal processing method can also be combined with calibration methods such as the reciprocity method and the comparison method to expand the calibration frequency lower limit of such methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the laser interferometer sound pressure reproduction system of the present invention;

[0042] Figure 2 It is a laser demodulation vibration velocity waveform diagram of the acoustic emission aliasing phenomenon of the low-frequency receiving waveform of the present invention;

[0043] Figure 3 A hydrophone output waveform diagram showing the acoustic emission aliasing phenomenon of the low-frequency receiving waveform of the present invention;

[0044] Figure 4 This is a waveform comparison diagram of the time domain scaling and averaging processing of the present invention;

[0045] Figure 5 This is a laser demodulation vibration velocity waveform diagram under the condition of a signal frequency of 10 kHz in the present invention;

[0046] Figure 6 This is a waveform diagram of the hydrophone output voltage under the condition of a signal frequency of 10 kHz;

[0047] Figure 7 This is a laser demodulation vibration velocity waveform diagram under the condition of a signal frequency of 3 kHz in the present invention;

[0048] Figure 8 This is a waveform diagram of the hydrophone output voltage under the signal frequency of 3 kHz of the present invention;

[0049] Fig. 9 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0051] See also Figures 1 to 9 An embodiment of the present invention provides a method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing, comprising the following steps:

[0052] Step 1: Perform sound pressure reproduction based on laser interferometry: Place the reflective sound-transmitting film in the water body, and the transmitting transducer emits a high-frequency pulse signal to excite the sound field and cause the water body to vibrate. The film follows the movement of the water particles, and the laser heterodyne interferometer system measures the film vibration and outputs the vibration velocity waveform through the demodulation system, and reads the vibration velocity amplitude from the vibration velocity waveform. , the sound pressure amplitude at the measurement point , sound pressure amplitude The calculation formula is:

[0053] ;

[0054] in is the medium density, is the speed of sound, is the vibration amplitude, is the equivalent refractive index of water medium.

[0055] Then take out the film frame, place the hydrophone to be tested at the same point in the water body, read the voltage amplitude, and divide the two to get the sound pressure sensitivity of the hydrophone;

[0056] Step 2: For the low frequency band, collect the particle velocity waveform demodulated by laser and the voltage waveform output by the hydrophone;

[0057] Here we take 3 kHz as an example: when the transmit signal frequency is set to 3 kHz and the signal cycle number is set to 5, the received waveform is as follows: Figure 2 and Figure 3 As shown in the figure, it can be seen that the received waveform shows obvious acoustic reflection aliasing phenomenon. Due to the introduction of film and frame acoustic reflection, the laser data is more seriously affected by reflection superposition. The steady-state direct sound waveform is less than one cycle, making it difficult to accurately estimate the signal amplitude.

[0058] Step 3: Use the time domain scaling average method to calculate the frequency , the number of cycles is The low-frequency transmission signal is measured, and the received waveform at the sound pressure measurement point is obtained as follows:

[0059] ;

[0060] in For direct sound and For reflected sound;

[0061] Direct sound and reflected sound It is expressed as:

[0062] ;

[0063] ;

[0064] in is the baseband signal corresponding to the frequency of the transmitted signal; is the direct sound envelope, which is approximately constant except for the peaks at the beginning and end of the waveform caused by the transient state of the transducer; Represents the modulation of the waveform envelope by acoustic reflections; , Respectively represent the arrival time of the direct sound and the first reflected sound, It is the reverberation time, that is, the time required for the sound pressure level to decrease by 60 dB after the sound source stops making sound. It can be considered that the reflected sound ends at this time.

[0065] An important observation is that the propagation time of the direct sound and the reflected sound of each path is determined only by the geometric position of the transmitting transducer, the sound pressure measurement point and the reflecting surface, and has nothing to do with the frequency of the sound signal. High-frequency sound signals usually have higher time resolution than low-frequency sound signals, which means that the arrival time of the direct sound is It can be accurately measured by emitting high-frequency sound pulses;

[0066] Step 4: Use high-frequency sound pulses to measure the arrival time of the direct sound , set it to time zero, and for frequency The direct sound and reflected sound are re-expressed as follows:

[0067]

[0068]

[0069] in , Indicates the time delay difference between the first reflected sound and the direct sound.

[0070] If the transmission frequencies are and For the two groups of sound pulses, due to the different frequencies, the coherence of the direct sound components of the received waveform is usually low.

[0071] Step 5: Use time domain resampling and scaling to obtain is the reference, the frequency is The waveform is resampled to obtain the resampled direct sound and reflected sound. The time domain waveforms of the resampled direct sound and reflected sound are expressed as follows:

[0072]

[0073]

[0074] in is the sound reflection coefficient, It is the direct sound envelope.

[0075] Acoustic reflection coefficient Direct sound envelope The value is related to the frequency, but for the frequency band with limited bandwidth and far away from the resonance point between the transducer and the hydrophone, and Can be considered constant.

[0076] Step 6: Setup Transmitting frequency , after the above-mentioned time domain alignment, waveform resampling and scaling processing, the waveform is averaged, and the energy ratio DRR of the direct sound and the reflected sound is calculated;

[0077] The energy ratio DRR of direct sound to reflected sound is calculated as follows:

[0078] ;

[0079] If in the frequency band The frequency of the transmitted signal is selected uniformly and densely, and the energy ratio DRR of the direct sound to the reflected sound can be further simplified as:

[0080] ;

[0081] When the bandwidth selected for time domain scaling averaging is When the reflected sound is almost eliminated, the DRR of the received waveform is greatly improved, such as Figure 4 As shown, a processing diagram of the time domain scaling averaging method is given, from which it can be intuitively seen that this method realizes the coherent superposition of direct sound and the random superposition of reflected sound.

[0082] Step 7: Perform low-frequency data processing based on the time-domain scaling and averaging method. Apply the time-domain scaling and averaging method to the laser demodulation velocity waveform and the hydrophone output voltage waveform to suppress the reflected sound. Perform laser interferometry sound pressure reproduction and hydrophone signal collection in turn, and perform time-domain scaling and averaging on the laser data and the hydrophone data, with uniform weighting between the frequency points.

[0083] Here we take 10 kHz and 3 kHz as examples: first set the signal frequency to 10 kHz, select 11 frequency points at 500 Hz intervals in the frequency range of 7.5 kHz~12.5 kHz, and the transducer emits signal pulses of corresponding frequencies in turn. Each pulse contains 12 signal cycles. Laser interferometry sound pressure reproduction and hydrophone signal acquisition are carried out, and time domain scaling and averaging processing is performed on the laser method data and hydrophone data. Uniform weighting is used between each frequency point. The waveform of the laser demodulation signal, the waveform of the hydrophone output signal, and the time domain scaling and averaging processing results are shown in Figure 2. Figure 5 and Figure 6As shown in the figure, in the 1 ms~2 ms range, the reflected sound and the direct sound are superimposed, the signal fluctuates greatly, there are many transient disturbances, and it is difficult to estimate the amplitude. After time domain scaling and averaging, the transient and steady-state components of the reflected sound are suppressed, and the signal presents a stable sinusoidal state, that is, the direct sound signal is well restored at this time.

[0084] Continue to investigate the direct sound recovery effect at 3 kHz: In the frequency range of 2 kHz~4 kHz, 9 frequency points are selected at an interval of 250 Hz. The transducer emits signal pulses of corresponding frequencies in turn. Each pulse contains 5 signal cycles. Laser interferometry sound pressure reproduction and hydrophone signal collection are carried out in turn. The laser method data and hydrophone data are processed by time domain scaling and averaging, and uniform weighting is used between each frequency point. The laser demodulation signal waveform, the hydrophone output signal waveform and their respective time domain scaling and averaging processing results are shown in Figure 2. Figure 7 and Figure 8 As shown in the figure, it can be seen from the processing results that the time domain scaling averaging method effectively suppresses the reflected sound waveform and restores the sinusoidal direct sound waveform.

[0085] The above processing method can be used for low-frequency data of other frequencies to effectively suppress the reflected sound waveform and restore the direct sound waveform.

[0086] In summary, the present application accurately determines the arrival time of direct sound by emitting high-frequency sound pulses, thereby aligning the starting points of waveform data of different frequencies; when conducting low-frequency measurements, not only the data of the frequency under investigation is collected, but a narrow frequency band near the frequency under investigation is selected, and multi-frequency point data is collected in turn and subsequently processed by algorithm; coherent superposition of direct sound waveforms of different frequencies is achieved through waveform resampling and scaling, and the reflected sound waveform is misaligned due to the time domain scaling processing, thereby achieving random superposition or cancellation.

[0087] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A method for calibrating low-frequency sensitivity of a hydrophone based on time domain scaling and averaging processing, characterized in that: The steps include: Step 1: Perform sound pressure reproduction based on laser interferometry: Place the reflective sound-transmitting film in the water, place the transducer and film, and perform laser focusing. The transmitting transducer emits a high-frequency pulse signal to excite the sound field and cause the water to vibrate. The film follows the movement of the water particles. The laser heterodyne interferometer system measures the film vibration and outputs the vibration velocity waveform through the demodulation system. The vibration velocity amplitude is read from the vibration velocity waveform. , calculate the sound pressure amplitude p at the measuring point, then take out the film frame, place the hydrophone to be tested at the same point in the water body, read the voltage amplitude, and divide the two to get the sound pressure sensitivity of the hydrophone; Step 2: For the low frequency band, collect the particle velocity waveform demodulated by laser and the voltage waveform output by the hydrophone; Step 3: Use the time domain scaling average method and set the frequency to , the number of cycles is Measure and collect waveforms of low-frequency transmission signals; Step 4: Measure the arrival time of the direct sound using a high-frequency sound signal , set it to time zero, and for frequency The direct and reflected sounds are re-expressed; Step 5: Use time domain resampling and scaling to obtain is the reference, the frequency is The waveform is resampled to obtain the resampled direct sound and reflected sound, and realize waveform scaling; Step 6: Setup Transmitting frequency , the above-mentioned time domain scaling and averaging method, waveform resampling and scaling are sequentially performed, and then waveform averaging is performed to calculate the energy ratio DRR of the direct sound and the reflected sound; Step 7: Apply the time domain scaling averaging method to the laser demodulated velocity waveform and the hydrophone output voltage waveform to achieve the suppression of reflected sound, wherein uniform weighting is used between each frequency point.

2. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 1, characterized in that: The calculation formula of the sound pressure amplitude p is: ； in is the medium density, is the speed of sound, is the vibration amplitude, is the equivalent refractive index of water medium, which is 1.

01.

3. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 1, characterized in that: The received waveform at the sound pressure measurement point is: ； in For direct sound, For reflected sound.

4. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 3, characterized in that: The direct sound and reflected sound It is expressed as: ； ； in is the baseband signal corresponding to the frequency of the transmitted signal, is the direct sound envelope, is the modulation of the waveform envelope by the acoustic reflection, , Respectively represent the arrival time of the direct sound and the first reflected sound, is the reverberation time.

5. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 4, characterized in that: The direct sound and reflected sound after re-expression in step 4 are: ； ； in , Indicates the time delay difference between the first reflected sound and the direct sound.

6. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 5, characterized in that: The time domain waveforms of the direct sound and reflected sound resampled in step 5 are expressed as follows: ； ； in is the sound reflection coefficient, It is the direct sound envelope.

7. The method for calibrating low-frequency sensitivity of a hydrophone based on time-domain scaling and averaging processing according to claim 6, characterized in that: The energy ratio DRR of the direct sound to the reflected sound is calculated as follows: 。

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