A Low-frequency Tube Measurement Method for the Sound Absorption Coefficient of an Underwater Acoustic Material Sample

Abstract

The present invention relates to the field of metrology and testing, and discloses a low-frequency tube measurement method and measurement device for the sound absorption coefficient of an underwater acoustic material sample. A low-frequency transmitting transducer is installed on the bottom flange of the low-frequency tube, and the sample to be measured can be horizontally suspended at any position in the tube. The space between the sample to be measured and the transmitting transducer is filled with deaerated water; a rigid backing is pasted on the upper surface of the sample to be measured, and the pressure, vibration velocity on the surface of the low-frequency transmitting transducer and the vibration velocity on the rigid backing are recorded; the sound pressure reflection coefficient and sound absorption coefficient of the sample to be measured are calculated. Through the unique design of the low-frequency tube, the integration of the low-frequency piston-type excitation sound source and the pressure and acceleration sensors, the present invention establishes a low-frequency tube measurement system for the sound absorption coefficient of an underwater acoustic material sample, avoiding the detection of the sound field in the tube by the two hydrophones and the calculation of the transfer function in the measurement by the two-hydrophone transfer function method, and well solving the problem of large measurement errors caused by factors such as the sensitivity consistency of the two hydrophones and the extreme points of the standing wave sound field.

Description

Technical Field

[0001] The present invention relates to the field of metrology and testing, and more particularly to a method for measuring the sound absorption coefficient of an underwater acoustic material sample in a low-frequency tube. Background Art

[0002] According to different main functions, submarine acoustic coatings can be divided into anechoic tiles, decoupling tiles, and multi-functional tiles, which are used to absorb the detection sound waves of the enemy's active sonar and shield the radiation of the submarine's own noise into the sea water respectively. The multi-functional tile combines the functions of sound absorption and decoupling. In theoretical research, it is assumed that the submarine hull model with the coating is a planar multi-layer structure, simulating the structure of a double-hull submarine (with ribs between two steel plates and a water layer in the middle) and a single-hull submarine (water layer - steel plate - air). Since the acoustic and mechanical characteristic parameters of underwater acoustic materials are very sensitive to the working frequency and hydrostatic pressure, with the decrease of the sonar working frequency and the increase of the working depth, the importance of measuring the sound absorption characteristics at low frequencies and high hydrostatic pressures becomes more prominent. At present, it is still relatively difficult to measure the acoustic performance of underwater acoustic material samples in the frequency band below 1 kHz in a free field. Because the wavelength in water is much larger than that of the material sample, the edge diffraction and diffraction interference of the sample are serious. In order to quickly and correctly measure the sound absorption performance of the anechoic tile, various research on underwater acoustic tube measurement methods has been carried out. An ideal plane wave sound field can be generated in the underwater acoustic tube. The required sample size is small, the boundary conditions are simple, and it is easy to compare with the theoretical calculation results. For the measurement of underwater acoustic materials in the low-frequency band, standing wave tubes and traveling wave tubes are generally used. In theory, their lowest working frequency is not limited by the length of the acoustic tube. As long as there is enough low-frequency signal-to-noise ratio in the tube, the measurement requirements can be met, and the sample diameter is close to the inner diameter of the acoustic tube. When the traveling wave tube measurement device measures the reflection and transmission coefficients of the sample, the front and rear boundaries of the sample are both water media, which is suitable for evaluating the acoustic performance of the sample itself and can simulate the working conditions of the anechoic tile of a double-hull submarine. However, it cannot simulate the stratification situation of a single-hull submarine such as "sea water - anechoic tile - hull - air". The low-frequency tube equipment with nitrogen pressurization using the two-hydrophone transfer function method can simulate the stratification situation of "sea water - anechoic tile - hull - air" and measure the sound absorption coefficient of the anechoic tile sample pasted on the steel plate under an air backing. However, when separating the two incident and reflected waves by using the two-hydrophone transfer function method, there are very high requirements for the consistency of the receiving sensitivities of the two hydrophones, especially the consistency of the sensitivity phases. Even through the control of components, materials, processes, etc. during the production process of the hydrophone and the calibration and screening of the finished products, it is very difficult to ensure good consistency, which directly affects the measurement error of the sound pressure reflection coefficient of the measured sample. At some frequency points, there are also axial sound field peaks and valleys formed by the superposition of the incident wave and the reflected wave in the standing wave tube, and the measurement error is large. Summary of the Invention

[0003] The present invention aims at the drawbacks in the prior art and provides a low-frequency tube measurement method and measurement device for the sound absorption coefficient of an underwater acoustic material sample.

[0004] To solve the above technical problems, the present invention is solved by the following technical solutions:

[0005] A low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample, comprising the following steps:

[0006] 1) Fabricate the sample to be measured into a cylinder with a clearance fit to the inner wall of the low-frequency tube;

[0007] 2) Open the low-frequency tube, place the sample to be measured into the tube and fix it, ensure it is flat, and then close the low-frequency tube;

[0008] 3) Start the pressure control system connected to the low-frequency tube (1), turn on the vacuum pumping device to create a negative pressure in the low-frequency tube, and use the external atmospheric pressure to inject pure water into the low-frequency tube to a certain height to form a measurement section, so that the sample to be measured is just immersed in the water column of the measurement section;

[0009] 4) Turn on the measurement system, connect the first acceleration sensor and the first pressure sensor installed on the piston vibration surface of the low-frequency transmitting transducer (2), connect the second acceleration sensor installed on the rigid back lining of the sample to be measured, and run the measurement software on the computer. The software control process is as follows: ① Input measurement information, including the thickness, density and sound velocity of the rigid back lining, the height, density and sound velocity of the water column, water temperature, water pressure and other information; ② Select the required measurement frequency points, including the starting frequency, ending frequency and frequency interval. The measurement frequency range of the device is: 100 Hz to 4 kHz, and equal intervals or one-third octave can be used; ③ Set the parameters of the programmable instrument, including the amplitude of the sine signal output by the signal source, which is generally not greater than 1 V according to the technical requirements of the power amplifier; for the received measurement signal, adjust the output impedance of the power amplifier and the gain of the preamplifier to make the received measurement signal have a signal-to-noise ratio greater than 20 dB under the condition that the instrument does not overload; make a stable plane wave sound field have been formed between the piston of the low-frequency transmitting transducer and the sample to be measured;

[0010] 5) Measure the sound pressure p0 and vibration velocity v0 of the piston vibration surface through the first acceleration sensor and the first pressure sensor installed on the low-frequency transmitting transducer, and the vibration velocity v2 of the rigid back lining on the sample to be measured, and finally obtain the sound absorption coefficient a of the sample to be measured;

[0011] 6) Adjust different hydrostatic pressures and repeat step 5);

[0012] 7) After the measurement is completed, start the pressure control system, open the pressure relief valve, and release the pressure in the low-frequency tube;

[0013] 8) Open the low-frequency tube (1) and take out the sample to be measured.

[0014] Preferably, under a certain hydrostatic pressure condition in step 5), the sound pressure p0, vibration velocity v0 of the piston vibration surface of the low-frequency transmitting transducer (2), and the vibration velocity v2 of the rigid backing on the sample to be measured are successively measured at set frequency points. By performing a fast Fourier transform on the signals, the amplitude and phase are obtained. Given the water column height, sound velocity, and density between the radiation surface of the low-frequency transmitting transducer and the sample to be measured, based on the measured sound pressure p0 and vibration velocity v0 of the piston vibration surface of the low-frequency transmitting transducer, the input impedances Z0 and Z1 at the interface of the water column and the sample to be measured are calculated, thereby measuring the low-frequency sound pressure reflection coefficient r of the underwater acoustic material sample to be measured within a certain hydrostatic pressure range P and sound absorption coefficient a.

[0015] A low-frequency tube measuring device for the sound absorption coefficient of an underwater acoustic material sample, comprising a vertically placed low-frequency tube. A low-frequency transmitting transducer is installed at the lower part of the low-frequency tube, and a rigid backing is installed in the middle of the low-frequency tube. The tube section between the low-frequency transmitting transducer and the rigid backing is a measurement section filled with water. The sample to be measured is fixedly connected to the rigid backing and is located in the measurement section. A first acceleration sensor and a first pressure sensor are integrated on the piston vibration surface of the low-frequency transmitting transducer, and a second acceleration sensor is installed on the rigid backing.

[0016] Preferably, the sample to be measured is a cylinder, and the gap between the sample to be measured and the inner wall of the low-frequency tube is not greater than 1 mm.

[0017] Preferably, the low-frequency tube includes an upper tube body and a lower tube body arranged coaxially. The upper tube body and the lower tube body are connected by an open cover flange. A bracket is also included, and the bracket is installed on the open cover flange, and the rigid backing is installed on the bracket.

[0018] Preferably, the low-frequency transmitting transducer successively includes a piston, a rare earth rod, and a shock-absorbing spring from top to bottom. A head mass and a tail mass are installed on the rare earth rod.

[0019] Preferably, a water inlet hole communicating with the measurement section is provided on the tube wall of the low-frequency tube. The water inlet hole is connected to a water inlet pipe and is connected to a water source through the water inlet pipe.

[0020] Preferably, a top cover flange is installed at the upper opening of the low-frequency tube, and a bottom cover flange is installed at the lower opening of the low-frequency tube. The tube section between the top cover flange and the rigid backing is an inflated upper air cavity, and the tube section between the piston on the low-frequency transmitting transducer and the bottom cover flange is an inflated lower air cavity. Both the upper air cavity and the lower air cavity are high-pressure cavities.

[0021] Preferably, air holes communicating with the upper cavity and the lower cavity are respectively provided on the top cover flange and the bottom cover flange, and an air inlet pipe is connected to the air holes; cable holes for cables to pass through are provided on both the top cover flange and the bottom cover flange, and the cables connected to the first acceleration sensor and the first pressure sensor extend out through the cable holes on the bottom cover flange, and the cable connected to the second acceleration sensor extends out through the cable holes on the top cover flange.

[0022] Since the present invention adopts the above technical solutions, it has remarkable technical effects: the present invention realizes a measurement method for the sound absorption coefficient of an underwater acoustic material sample in a low-frequency tube. Through the unique design of the low-frequency tube, the integration of a low-frequency piston-type excitation sound source, sound pressure, and acceleration sensors, the configuration of electronic instruments, and the preparation of measurement-specific software, a measurement system for the sound absorption coefficient of an underwater acoustic material sample is established, avoiding the detection of the sound field in the tube by two hydrophones and the calculation of the transfer function in the method of measuring the transfer function of two hydrophones, and well solving the problem of large measurement errors caused by factors such as the sensitivity consistency of the two hydrophones and the extreme points of the standing wave sound field. Description of the Drawings

[0023] Figure 1 is a schematic structural diagram of the present invention.

[0024] Figure 2 is a plane wave model for measuring the sound absorption coefficient.

[0025] The names of the parts referred to by each numerical label in the drawings are as follows: 1 - low-frequency tube, 2 - low-frequency transmitting transducer, 3 - rigid backing, 4 - sample to be measured, 5 - water inlet pipe, 6 - air inlet pipe, 7 - cable, 8 - bracket, 11 - upper tube body, 12 - lower tube body, 101 - top cover flange, 102 - bottom cover flange, 103 - opening cover flange, 1001 - upper cavity, 1002 - lower cavity, 1003 - measurement section, 201 - first acceleration sensor, 202 - first pressure sensor, 203 - piston, 204 - rare earth rod, 205 - shock-absorbing spring, 206 - tail mass, 207 - head mass, 301 - second acceleration sensor. Detailed Embodiments

[0026] The present invention will be further described in detail below with reference to the drawings and embodiments. Embodiment 1

[0027] A low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample includes the following steps:

[0028] 1) The sample to be measured 4 is made into a cylinder with a clearance fit with the inner wall of the low-frequency tube 1, a cylinder with a diameter meeting the requirements, and the clearance with the tube wall is not greater than 1 mm;

[0029] 2) Open the low-frequency tube 1, place the sample to be measured 4 on the bracket 8 inside the tube, ensure it is flat, and close the low-frequency tube 1;

[0030] 3) Start the pressure control system connected to the low-frequency tube 1, turn on the vacuum pumping equipment to form a negative pressure in the low-frequency tube 1, and use the external atmospheric pressure to inject pure water into the low-frequency tube 1 to a certain height to form a measurement section 1003, so that the measured sample 4 is just immersed in the water column of the measurement section 1003;

[0031] 4) Turn on the computer in the measurement system, turn on the measurement system, turn on the first acceleration sensor 201 and the first pressure sensor 202 installed on the piston vibration surface of the low-frequency transmitting transducer 2, turn on the second acceleration sensor 301 installed on the rigid backing 3 of the measured sample 4, run the measurement software on the computer, select the required measurement frequency points, set the signal source output amplitude on the measurement software interface, adjust the power amplifier gain and impedance, so that a standing wave field has been formed between the piston 203 of the low-frequency transmitting transducer 2 and the measured sample 4;

[0032] 5) Measure the sound pressure p0 and vibration velocity v0 of the piston vibration surface through the first acceleration sensor 201 and the first pressure sensor 202 installed on the low-frequency transmitting transducer 2, and the vibration velocity v2 of the rigid backing 3 on the measured sample 4, and finally obtain the sound absorption coefficient a of the measured sample;

[0033] 6) Adjust different hydrostatic pressures and repeat step 5);

[0034] 7) After the measurement is completed, start the pressure control system, open the pressure relief valve, and release the pressure in the low-frequency tube 1;

[0035] 8) Open the low-frequency tube 1 and take out the measured sample.

[0036] The measurement signal in step 4) is a sine continuous signal, and the software control flow of the measurement system in step 4) is as follows: ① Input measurement information, including the thickness, density and sound velocity of the rigid backing, the height, density and sound velocity of the water column, water temperature and water pressure; ② Select the required measurement frequency points, including the starting frequency, ending frequency and frequency interval, and the measurement frequency range of the measuring device is: 100 Hz to 4 kHz, using equal intervals or one-third octave; ③ Set the parameters of the programmable instrument, including the amplitude of the sine signal output by the signal source, not greater than 1 V according to the technical requirements of the power amplifier; for the received measurement signal, adjust the output impedance of the power amplifier and the gain of the preamplifier, and make the received measurement signal have a signal-to-noise ratio greater than 20 dB under the condition that the instrument does not overload.

[0037] In step 5), under a certain hydrostatic pressure condition, the sound pressure, vibration velocity of the piston vibration surface of the low-frequency transmitting transducer 2, and the vibration velocity of the rigid backing 3 on the measured sample 4 are successively measured at the set frequency points. By performing a fast Fourier transform on the signals, the amplitude and phase are obtained. Given the water column height, sound velocity, and density between the radiation surface of the low-frequency transmitting transducer 2 and the measured sample 4, the input impedance of the interface between the water column and the measured sample 4 is calculated based on the measured bottom sound pressure and vibration velocity, thereby measuring the low-frequency sound pressure reflection coefficient and sound absorption coefficient of the underwater acoustic material measured sample 4 within a certain hydrostatic pressure range.

[0038] The medium in the low-frequency tube 1 has a multi-layer acoustic structure form, which consists of the piston vibration radiation surface on the low-frequency transmitting transducer 2, the water layer, the measured sample 4, and the rigid backing 3. The plane wave model is as Figure 2 shown. The height of the water layer in the measurement section 1003 is H w , the sound velocity and density in water are ρ w , c w respectively, the sound pressure and vibration velocity on the water layer of the piston vibration surface on the low-frequency transmitting transducer 2 are p0 and v0 respectively; the sound pressure and vibration velocity of the measured sample 4 on the water surface are p1 and v1 respectively. The measured sound pressure and accelerometer signals are input into the dynamic signal analysis system collected under the control of a computer after conditioning and amplification. After processing, the sound pressure p0 and vibration velocity v0 of the piston vibration radiation surface on the low-frequency transmitting transducer 2 can be obtained, the input impedance of the sample layer is calculated, and finally the sound pressure reflection coefficient and sound absorption coefficient of the measured sample are obtained.

[0039] The input impedance of the water layer can be directly calculated from the measured sound pressure p0 and vibration velocity v0:

[0040]

[0041] Given the acoustic characteristic parameters of the water layer, the sound velocity c w , density ρ w and thickness H w , the output impedance can be calculated by the following formula:

[0042]

[0043] where k w ——wave number in water, k w = 2πf / c w , Z1 is also the input impedance of the measured sample layer. From this, the sound pressure reflection coefficient r p can be calculated by the following formula:

[0044]

[0045]

[0046] where R is the sound pressure reflection coefficient rp The amplitude value, and the sound absorption coefficient a is calculated by the following formula:

[0047] a = (1 - R) 2 (5). Example 2

[0048] A low-frequency tube measuring device for the sound absorption coefficient of an underwater acoustic material sample, including a vertically placed low-frequency tube 1. A low-frequency transmitting transducer 2 with a piston vibration mode excited by a magnetostrictive rare-earth element is installed at the lower part of the low-frequency tube 1. A rigid backing 3 that is hermetically fitted with the inner wall of the low-frequency tube 1 is installed in the middle of the low-frequency tube 1. The tube section between the piston on the low-frequency transmitting transducer 2 and the rigid backing 3 is a measurement section 1003 filled with water. The sample under test 4 is fixedly connected to the rigid backing 3 and is located in the measurement section 1003. A first acceleration sensor 201 and a first pressure sensor 202 are integrated on the piston vibration surface of the low-frequency transmitting transducer 2, and a second acceleration sensor 301 is installed on the rigid backing 3.

[0049] An inlet hole communicating with the measurement section 1003 is provided on the tube wall of the low-frequency tube 1. The inlet hole is connected to an inlet pipe 5 and is connected to a water source through the inlet pipe 5. Example 3

[0050] Same as Example 1, except that the sample under test 4 is a cylinder, and the gap between the sample under test 4 and the inner wall of the low-frequency tube 1 is not greater than 1 mm.

[0051] The low-frequency tube 1 includes an upper tube body 11 and a lower tube body 12 arranged coaxially. The upper tube body 11 and the lower tube body 12 are connected by an open cover flange 103. It also includes a bracket 8. The bracket 8 is installed on the open cover flange 103, and the rigid backing 3 is installed on the bracket.

[0052] The low-frequency transmitting transducer 2 includes a piston 203, a rare-earth rod 204, and a shock-absorbing spring 205 from top to bottom in sequence. A tail mass 206 and a head mass 207 are installed on the rare-earth rod 204. Example 4

[0053] Same as Example 1, except that a top cover flange 101 is installed at the upper end opening of the low-frequency tube 1, and a bottom cover flange 102 is installed at the lower end opening of the low-frequency tube 1. The tube section between the top cover flange 101 and the rigid backing 3 is an inflated upper cavity 1001, and the tube section between the piston 203 on the low-frequency transmitting transducer 2 and the bottom cover flange 102 is an inflated lower cavity 1002. Both the upper cavity 1001 and the lower cavity 1002 are high-pressure air cavities with adjustable air pressure.

[0054] The measurement section 1003 is a sealed section formed by the cooperation of the rigid backing 3 and the piston 203 on the low-frequency transmitting transducer 2 with the tube wall of the low-frequency tube 1.

[0055] The top cover flange 101 and the bottom cover flange 102 are respectively provided with air holes communicating with the upper cavity 1001 and the lower cavity 1002. The air holes are connected with an air inlet pipe 6, and the air inlet pipe 6 is connected with a pressure control system to control the air pressure in the upper cavity 1001 and the lower cavity 1002 through the pressure system. Both the top cover flange 101 and the bottom cover flange 102 are provided with cable holes for the cables to pass through. The cables 7 connected to the first acceleration sensor 201 and the first pressure sensor 202 extend out through the cable holes in the bottom cover flange 102, and the cable 7 connected to the second acceleration sensor 301 extends out through the cable holes in the top cover flange 101.

[0056] In summary, the above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the patent of the present invention.

Claims

1. A method for measuring the sound absorption coefficient of an underwater acoustic material sample using a low-frequency tube, characterized in that: It includes the following steps: Make the sample under test (4) into a cylinder with a clearance fit with the inner wall of the low-frequency tube (1); 2) Open the low-frequency tube (1), put the sample under test (4) into the tube and fix it inside, ensure it is flat, and close the low-frequency tube (1); 3) Start the pressure control system connected to the low-frequency tube (1), turn on the vacuum pumping device to create a negative pressure in the low-frequency tube (1), and use the external atmospheric pressure to inject pure water into the low-frequency tube (1) to a certain height to form a measurement section (1003), so that the sample under test (4) is just immersed in the water column of the measurement section (1003); 4) Turn on the measurement system, connect the first acceleration sensor (201) and the first pressure sensor (202) installed on the piston vibration surface of the low-frequency transmitting transducer (2), connect the second acceleration sensor (301) installed on the rigid backing (3) of the sample under test (4), run the measurement software on the computer, so that a stable plane wave sound field has been formed between the piston (203) of the low-frequency transmitting transducer (2) and the sample under test (4); 5) Measure the sound pressure p0 and vibration velocity v0 of the piston vibration surface through the first acceleration sensor (201) and the first pressure sensor (202) installed on the low-frequency transmitting transducer (2), and the vibration velocity v2 of the rigid backing (3) on the sample under test (4), and finally obtain the sound absorption coefficient a of the sample under test; 6) Adjust different hydrostatic pressures and repeat step 5); 7) After the measurement is completed, start the pressure control system, open the pressure relief valve to release the pressure in the low-frequency tube (1); 8) Open the low-frequency tube (1) and take out the sample under test (4); In step 5), under a certain hydrostatic pressure condition, the sound pressure p0, vibration velocity v0 of the piston vibration surface of the low-frequency transmitting transducer (2), and the vibration velocity v2 of the rigid backing (3) on the sample under test (4) are measured successively at the set frequency points. By performing a fast Fourier transform on the signals, the amplitude and phase are obtained. Given the water column height, sound velocity, and density between the radiation surface of the low-frequency transmitting transducer (2) and the sample under test (4), based on the measured sound pressure p0 and vibration velocity v0 of the piston vibration surface of the low-frequency transmitting transducer (2), the input impedances Z0 and Z1 at the interface of the water column and the sample under test (4) are calculated, thereby measuring the sound pressure reflection coefficient r P and the sound absorption coefficient a of the underwater acoustic material sample under test (4) within a certain hydrostatic pressure range; The input impedance of the water layer can be directly calculated from the measured sound pressure p0 and vibration velocity v0: ， The acoustic characteristic parameters of the known water layer, the sound speed c in water w , density ρ w and thickness H w , the output impedance can be calculated by the following formula: ， where k w —— wave number in water, k w = 2πf / c w , and Z1 is also the input impedance of the sample layer to be measured. Thus, the sound pressure reflection coefficient r p can be calculated by the following formula: ， ， where R is the magnitude of the sound pressure reflection coefficient r p and the sound absorption coefficient a is calculated by the following formula: a = (1 - R) 2 (5).

2. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 1, characterized in that: The software control process of the measurement system in step 4) is as follows: ① Input measurement information, including the thickness, density and sound velocity of the rigid backing (3), the height, density and sound velocity of the water column, water temperature and water pressure; ② Select the required measurement frequency points, including the starting frequency, ending frequency and frequency interval, and the measurement frequency range of the measuring device is: 100 Hz~4 kHz, using equal intervals or one-third octave; ③ Set the parameters of the programmable instrument, including the amplitude of the sine signal output by the signal source, which is not greater than 1 V according to the technical requirements of the power amplifier; for the received measurement signal, adjust the output impedance of the power amplifier and the gain of the preamplifier to make the received measurement signal have a signal-to-noise ratio greater than 20 dB under the condition that the instrument does not overload.

3. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 1, characterized in that: Adopt a low-frequency tube measuring device for the sound absorption coefficient of an underwater acoustic material sample. The low-frequency tube measuring device includes a vertically placed low-frequency tube (1), a low-frequency transmitting transducer (2) is installed at the lower part of the low-frequency tube (1), a rigid backing (3) is installed in the middle of the low-frequency tube (1), the tube section between the low-frequency transmitting transducer (2) and the rigid backing (3) is a measurement section (1003) filled with water, the sample under test (4) is fixedly connected to the rigid backing (3) and is located in the measurement section (1003), a first acceleration sensor (201) and a first pressure sensor (202) are integrated on the vibration surface of the piston (203) of the low-frequency transmitting transducer (2), and a second acceleration sensor (301) is installed on the rigid backing (3).

4. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 3, characterized in that: The measured sample (4) used in the low-frequency tube measurement method is a cylinder, and the gap between the measured sample (4) and the inner wall of the low-frequency tube (1) is not greater than 1 mm.

5. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 3, characterized in that: The low-frequency tube (1) of the low-frequency tube measurement device used includes an upper tube body (11) and a lower tube body (12) arranged coaxially. The upper tube body (11) and the lower tube body (12) are connected by an opening cover flange (103). A bracket (8) is installed on the opening cover flange (103), and a rigid backing (3) is installed on the bracket (8).

6. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 3, characterized in that: The low-frequency transmitting transducer (2) of the low-frequency tube measurement device used includes, from top to bottom, a piston (203), a rare earth rod (204), and a shock-absorbing spring (205). A tail mass (206) and a head mass (207) are installed on the rare earth rod (204).

7. A method for measuring the sound absorption coefficient of an underwater acoustic material sample at low frequencies according to claim 3, characterized in that: The tube wall of the low-frequency tube (1) of the low-frequency tube measurement device used is provided with a water inlet hole communicating with the measurement section (1003). The water inlet hole is connected to a water inlet pipe (5) and is connected to a water source through the water inlet pipe (5).

8. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 3, wherein: A top cover flange (101) is installed at the upper opening of the low-frequency tube (1) of the low-frequency tube measurement device used, and a bottom cover flange (102) is installed at the lower opening of the low-frequency tube (1). The tube section between the top cover flange (101) and the rigid backing (3) is an inflated upper cavity (1001), and the tube section between the piston (203) on the low-frequency transmitting transducer (2) and the bottom cover flange (102) is an inflated lower cavity (1002). Both the upper cavity (1001) and the lower cavity (1002) are high-pressure cavities.

9. The low-frequency tube measurement method for the sound absorption coefficient of an underwater acoustic material sample according to claim 8, characterized in that: The top cover flange (101) and the bottom cover flange (102) of the low-frequency tube measurement device used are respectively provided with air holes communicating with the upper cavity (1001) and the lower cavity (1002). The air holes are connected to an air inlet pipe (6); both the top cover flange (101) and the bottom cover flange (102) are provided with cable holes for cables to pass through. The cables (7) connected to the first acceleration sensor (201) and the first pressure sensor (202) extend out through the cable hole on the bottom cover flange (102), and the cable (7) connected to the second acceleration sensor (301) extends out through the cable hole on the top cover flange (101).

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