Sound tube measurement method for low-frequency sound performance of isotropic underwater acoustic material

Abstract

The invention relates to the technical field of metering and testing, in particular to a sound tube measuring method for low-frequency sound performance of an isotropic underwater acoustic material, in a sound tube, a double-section planar standing wave sound field is constructed by utilizing the characteristics of an isotropic underwater acoustic material sample, and the low-frequency sound performance of the isotropic underwater acoustic material is measured by quickly collecting the open-circuit voltage amplitude and phase of a four-channel hydrophone. The method realizes rapid and accurate measurement of acoustic performances such as a sound pressure reflection coefficient, a sound pressure transmission coefficient and a sound absorption coefficient of a low-frequency isotropic underwater acoustic material sample, can be directly applied to an existing thick-wall sound tube device in China, and greatly improves the measurement efficiency of the low-frequency acoustic performances of the underwater acoustic material.

Description

Technical Field

[0001] The invention relates to the field of measurement and testing technology, and specifically relates to a method for measuring the low-frequency acoustic performance of an isotropic hydroacoustic material using a sound tube. Background Art

[0002] The test of the acoustic performance of small samples of water acoustic materials is mostly carried out in thick-walled sound tubes that can establish a plane wave sound field environment. Sound tube test methods can be divided into pulse sound tube tests, standing wave sound tube tests, and traveling wave tube sound tube tests according to the test frequency band and test conditions. The pulse sound tube test method is limited by the time domain separation of reflection, transmission and other pulse signals, and the lower limit of the test frequency depends on the length of the sound tube. Lower frequency tests are carried out in standing wave tubes and traveling wave tubes: the standing wave tube method forms a stable standing wave sound field in the sound tube to evaluate the low-frequency sound reflection, sound absorption and other properties of the material under the working condition of "water-material-air"; the traveling wave tube method constructs a stable traveling wave sound field with unidirectional propagation in the sound tube through active and passive silencing technology, and the water acoustic material is placed in the middle of the sound tube to form a sound field mode of "standing wave sound field + material + traveling wave sound field". The traveling wave tube method can be used to evaluate the low-frequency sound reflection, sound insulation and sound absorption performance of the material under the working condition of "water-material-water". When the operating frequency of underwater acoustic materials is as low as below 500Hz, people often use active anechoic traveling wave tubes to test the acoustic properties of the materials. The method of continuous iterative approximation is used to make the reflection coefficient of the transducer surface at the upper end of the sound tube approach 0. This is not only time-consuming, but also there is a situation where the iteration does not converge, that is, a stable traveling wave sound field cannot be formed in the sound tube at certain frequency points. Summary of the invention

[0003] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for measuring the low-frequency acoustic performance of isotropic hydroacoustic materials using a sound tube, and to use a double-segment standing wave acoustic field in various existing sound tubes to measure the acoustic performance of various isotropic hydroacoustic materials, thereby greatly improving work efficiency while ensuring measurement accuracy. The method proposed by the present invention does not require the construction of a traveling wave acoustic field, and the upper tube end does not require a transducer to send a signal. The backing material of the upper tube end can be an existing transducer, a sound-absorbing wedge, air, a rigid backing, or any other solid material with stable physical properties.

[0004] To achieve the above object, the present invention provides the following technical solution: a method for measuring the low-frequency acoustic performance of an isotropic hydroacoustic material using a sound tube, the steps of which are as follows:

[0005] 1) The isotropic hydroacoustic material sample to be tested is placed in the middle of the sound tube, and the upper and lower sections of the sound tube separated by the material sample each have at least two hydrophones;

[0006] 2) Fill the sound tube with water, evacuate it, and eliminate bubbles in the water;

[0007] 3) As needed, start the temperature and pressure control system of the sound tube, control the water temperature and hydrostatic pressure in the tube to a certain value, and after stabilization, measure the acoustic properties of the material sample;

[0008] 4) Control the transmitting transducer at the bottom of the sound tube to transmit a single-frequency long pulse signal, forming a plane standing wave sound field in the upper and lower sections of the sound tube respectively;

[0009] 5) Read the open-circuit voltage amplitude and phase of each hydrophone of the sound tube, combine the sensitivity amplitude and phase value of the hydrophone under the current environmental conditions and the current frequency, and calculate the sound pressure amplitude and phase at each hydrophone position according to the formula;

[0010] 6) Select a suitable pair of hydrophones and calculate the acoustic performance parameters of the sample material under the current environmental conditions and at the current measurement frequency point according to the formula, including the sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient;

[0011] 7) Transmit the next frequency signal and repeat steps 4) to 6) to complete the acoustic performance measurement of the material sample under the current environmental conditions at all frequency points;

[0012] 8) Adjust the water temperature and hydrostatic pressure of the acoustic tube as needed, and repeat steps 3) to 7) until the acoustic performance of the material sample under all environmental conditions and at all frequency points is measured.

[0013] 9) Release the hydrostatic pressure to normal pressure.

[0014] In some of the embodiments, when measuring the acoustic properties of an isotropic hydroacoustic material sample in an acoustic tube, two sections of standing wave acoustic fields are quickly constructed in the acoustic tube;

[0015] By measuring the phase of the open-circuit voltage amplitude of four hydrophones distributed in the acoustic tube, the sound wave components in the standing wave sound field are separated, thereby obtaining the acoustic properties of the isotropic hydroacoustic material samples, such as the sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient.

[0016] In some embodiments, the acoustic tube is a water-filled thick-walled stainless steel acoustic tube, which is placed vertically, with a transmitting transducer installed at the bottom, and the isotropic hydroacoustic material sample to be tested is placed in the middle of the acoustic tube.

[0017] The acoustic tube is divided into an upper tube section and a lower tube section. The hydrophones are respectively installed on the walls of the acoustic tube by an embedded method. There are at least two hydrophones in each of the upper tube section and the lower tube section. The size of the hydrophone is less than 1 / 10 of the minimum measurement wavelength in the tube.

[0018] In some of the embodiments, a backing is provided on the top of the sound tube, and the backing is a transducer, a sound absorbing wedge, air, a rigid backing or any other solid material that is insoluble in water and has stable physical properties.

[0019] In some of the embodiments, the tested hydroacoustic material sample is a cylindrical isotropic sample, that is, the sample is symmetrical along the axial direction of the cylinder, and the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample when the sound wave is incident from the upper circular surface are equal to the sound pressure reflection coefficient and sound pressure transmission coefficient when the sound wave is incident from the lower circular surface.

[0020] In some embodiments, under certain water temperature and hydrostatic pressure conditions, only one single-frequency long pulse signal is required for each measurement frequency point, and two standing wave sound fields can be quickly formed in the sound tube. The sound signal in the lower sound tube can be considered to be composed of three parts: the signal p incident on the lower surface of the material sample i 、p i p reflected by the lower surface of the material sample r , the transmission signal p' of the upper tube acoustic signal through the material sample t The acoustic signal in the upper sound tube can also be considered to be composed of three parts: the signal p' incident on the upper surface of the material sample i 、p' i p' reflected from the upper surface of the material sample r , the transmission signal p of the acoustic signal from the lower tube through the material sample t .

[0021] In some of the embodiments, a one-dimensional coordinate system is constructed with the bottom surface of the material to be measured as the origin of the coordinates, and the positions of the 1# to 4# hydrophones are respectively x1, x2, x3 and x4;

[0022] The transmitting transducer at the bottom of the tube emits a single-frequency long pulse p in , the acoustic signal propagates in the lower acoustic tube, and after reaching the lower surface of the material sample, part of it is absorbed by the material sample, part of it is reflected, and part of it is transmitted;

[0023] The reflected sound propagates in the reverse direction in the lower tube, and is reflected or absorbed by the transmitting transducer. The reflected wave is incident on the lower surface of the material sample in the forward direction, and this process is repeated until a steady state is reached.

[0024] The sound signal passing through the sample enters the upper tube, is transmitted to the backing on the top of the sound tube, is partially absorbed and partially reflected back to the upper surface of the material sample, and is reflected, transmitted and absorbed again until the sound field in the upper sound tube reaches a steady state. r =R·p i 、p t =T·p i 、p r '=R·p i '、p t '=T·p i ',

[0025] R and T are the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample under test respectively;

[0026] The hydrophones in the sound tube are used to pick up the sound field characteristics in the sound tube. The sound pressure at the 1# to 4# hydrophones can be expressed by formulas (1) to (4) respectively:

[0027]

[0028]

[0029]

[0030]

[0031] The sound pressure reflection coefficient of the measured hydroacoustic material is calculated as:

[0032]

[0033] The sound pressure transmission coefficient is:

[0034]

[0035] Here, l 12 is the distance between hydrophone #1 and hydrophone #2, l 34 is the distance between 3# hydrophone and 4# hydrophone, the sound pressure p1, p2, p3 and p4 at the positions of the four hydrophones, and the hydrophone open circuit voltage signal and hydrophone sensitivity are calculated as follows:

[0036]

[0037]

[0038]

[0039]

[0040] Among them, A1, A2, A3, and A4 are the open-circuit voltage signal amplitudes of each hydrophone, is the phase value of the open-circuit voltage signal of each hydrophone, M1, M2, M3, and M4 are the sensitivity amplitudes of each hydrophone under the current water temperature and water pressure conditions and the current measurement frequency, and θ1, θ2, θ1, and θ2 are the sensitivity phase values ​​of the four hydrophones under the current environment and frequency conditions;

[0041] The sound pressure reflection coefficient and sound pressure transmission coefficient of each isotropic hydroacoustic material are obtained, and the echo reduction ER, insertion loss IL and sound absorption coefficient α can be calculated according to formulas (11) to (13).

[0042] ER=20lg(1 / R) (11)

[0043] IL=20lg(1 / T) (12)

[0044] α=1-R 2 -T 2 (13).

[0045] In some embodiments, more hydrophones are arranged in the acoustic tube, which results in more measurement selectivity. The spacing l between the hydrophone groups is selected to be mn The relationship with wavelength λ should satisfy:

[0046]

[0047] Among the hydrophones that satisfy formula (14), two hydrophones with the largest open-circuit voltage amplitude deviation are selected in each standing wave acoustic field to form a hydrophone group;

[0048] The spacing between the hydrophone groups selected in each standing wave acoustic field should be between 0.05 and 0.45 of the measurement wavelength, and the two hydrophones with the largest open-circuit voltage amplitude deviation should be selected to form a hydrophone group to reduce the uncertainty of the acoustic performance measurement results.

[0049] Compared with the prior art, the beneficial effects of the present invention are as follows: in view of the low efficiency of traveling wave generation and the inability to form a traveling wave field at individual frequency points when testing the acoustic properties of hydroacoustic materials using the above-mentioned traveling wave tube method, a method for measuring the acoustic properties of various isotropic hydroacoustic materials using a double-segment standing wave acoustic field in various existing acoustic tubes is proposed, which greatly improves work efficiency while ensuring measurement accuracy. The method proposed by the present invention does not require the construction of a traveling wave acoustic field, and the upper tube end does not require a transducer to send a signal. The backing material of the upper tube end can be an existing transducer, a sound-absorbing wedge, air, a rigid backing, or any other solid material with stable physical properties.

[0050] Details of one or more embodiments of the present application are presented in the following drawings and descriptions to make other features, purposes and advantages of the present application more concise and easy to understand. The present application is fully described and understood through the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a double-section standing wave field acoustic tube structure of the present invention. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] See also Figure 1The present invention provides a technical solution: a sound tube measurement method for the low-frequency acoustic performance of isotropic hydroacoustic materials. The double-section standing wave field sound tube structure in the figure includes a thick-walled water-filled stainless steel tube body, a transmitting transducer and a group of hydrophones, and a backing material at the upper end of the tube. The sound tube is placed vertically, the transmitting transducer is placed at the bottom, and the isotropic hydroacoustic material sample to be measured is placed in the middle of the sound tube to separate the sound tube into an upper tube section and a lower tube section. The hydrophones are respectively installed on the walls of the sound tube by an embedded method, and there are at least two hydrophones in each of the upper tube section and the lower tube section. A one-dimensional coordinate system is given in the figure, and the bottom surface of the material sample is x=0. A single-frequency long pulse signal is emitted by the transmitting transducer, and an upper and lower double-section planar standing wave sound field is formed in the sound tube: the standing wave sound field of the lower tube section can be regarded as consisting of three parts: the signal p incident on the lower surface of the material sample i 、p i p reflected by the lower surface of the material sample r , the transmission signal p' of the upper tube acoustic signal through the material sample t The standing wave acoustic field of the upper tube section can also be considered to be composed of three parts: the signal p' incident on the upper surface of the material sample i 、p' i p' reflected from the upper surface of the material sample r , the transmission signal p of the acoustic signal from the lower tube through the material sample t According to the measurement frequency, a suitable hydrophone group is selected to collect the acoustic signal. After signal separation, the acoustic performance parameters such as the sound pressure reflection coefficient (echo reduction), sound pressure transmission coefficient (insertion loss) and sound absorption coefficient of the isotropic hydroacoustic material components are calculated.

[0054] Through the technical solution of this application, Figure 1 In the device shown, two upper and lower plane standing wave sound fields are constructed with the measured hydroacoustic material as the boundary, so as to realize the rapid and accurate measurement of the acoustic properties such as the sound reflection, sound transmission (sound insulation) and sound absorption of various isotropic hydroacoustic materials.

[0055] The acoustic tube used to construct a double-stage standing wave field of the present invention is a water-filled thick-walled stainless steel acoustic tube. The bottom of the tube body is a low-frequency transmitting transducer, and the sample of the hydroacoustic material to be measured is placed in the middle of the acoustic tube. The upper and lower sections of the acoustic tube each have at least two hydrophones, and the size of the hydrophone is less than 1 / 10 of the minimum measurement wavelength in the tube, and the influence on the sound field can be ignored. The backing on the top of the acoustic tube can be a transducer, a sound-absorbing wedge, air, a rigid backing or any other solid material that is insoluble in water and has stable physical properties.

[0056] The tested hydroacoustic material sample described in the present invention is a cylindrical isotropic sample, that is, the sample is symmetrical along the axial direction of the cylinder, and the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample when the sound wave is incident from the upper circular surface are equal to the sound pressure reflection coefficient and sound pressure transmission coefficient when the sound wave is incident from the lower circular surface.

[0057] Take the bottom surface of the material to be measured as the coordinate origin and construct a one-dimensional coordinate system, such as Figure 1 As shown in the figure, the positions of hydrophones 1# to 4# are x1, x2, x3 and x4 respectively. The transmitting transducer at the bottom of the tube emits a single-frequency long pulse p in (The upper frequency limit is determined by the inner diameter of the sound tube), the sound signal propagates in the lower sound tube, and after reaching the lower surface of the material sample, part of it is absorbed by the material sample, part of it is reflected, and part of it is transmitted. The reflected sound propagates in the reverse direction in the lower tube, and is reflected or absorbed again when it encounters the transmitting transducer. The reflected wave is then incident on the lower surface of the material sample in the forward direction, and this is repeated until a steady state is reached. The sound signal that passes through the sample enters the upper tube, is transmitted to the top backing of the sound tube, is partially absorbed and partially reflected back to the upper surface of the material sample, and is reflected, transmitted and absorbed again until the sound field of the upper sound tube reaches a steady state. Although the sound waves propagate back and forth in the sound tube, and the characteristic impedances of the two interfaces are different, the sound wave components in the sound tube are extremely complex, but as long as the physical properties of the radiating surface material of the transmitting transducer, the measured hydroacoustic material, and the backing material on the top of the sound tube are stable, two stable plane standing wave sound fields can be quickly formed in the upper and lower sound tubes. The sound signal in the lower sound tube can be regarded as consisting of three parts: the signal p incident on the lower surface of the material sample i 、p i p reflected by the lower surface of the material sample r , the transmission signal p' of the upper tube acoustic signal through the material sample t The acoustic signal in the upper sound tube can also be considered to be composed of three parts: the signal p' incident on the upper surface of the material sample i 、p' i p' reflected from the upper surface of the material sample r , the transmission signal p of the acoustic signal from the lower tube through the material sample t Here, p r =R·p i 、p t =T·p i 、p r '=R·p i '、p t '=T·p i ', R and T are the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample under test respectively.

[0058] The hydrophones in the sound tube are used to pick up the sound field characteristics in the sound tube. The sound pressure at the 1# to 4# hydrophones can be expressed by formulas (1) to (4) respectively:

[0059]

[0060]

[0061]

[0062]

[0063] The sound pressure reflection coefficient of the measured hydroacoustic material is calculated as:

[0064]

[0065] The sound pressure transmission coefficient is:

[0066]

[0067] Here, l 12 is the distance between hydrophone #1 and hydrophone #2, l 34 is the distance between hydrophone #3 and hydrophone #4. The sound pressures p1, p2, p3 and p4 at the positions of the four hydrophones are calculated as follows:

[0068]

[0069]

[0070]

[0071]

[0072] Among them, A1, A2, A3, and A4 are the open-circuit voltage signal amplitudes of each hydrophone, is the phase value of the open-circuit voltage signal of each hydrophone, M1, M2, M3, and M4 are the sensitivity amplitudes of each hydrophone under the current water temperature and water pressure conditions and the current measurement frequency, and θ1, θ2, θ1, and θ2 are the sensitivity phase values ​​of the four hydrophones under the current environment and frequency conditions.

[0073] The sound pressure reflection coefficient and sound pressure transmission coefficient of each isotropic hydroacoustic material are obtained, and the echo reduction ER, insertion loss IL and sound absorption coefficient α can be calculated according to formulas (11) to (13).

[0074] ER=20lg(1 / R) (11)

[0075] IL=20lg(1 / T) (12)

[0076] α=1-R 2 -T 2 (13)

[0077] If more hydrophones are arranged in the above-mentioned sound tube, there will be more measurement selectivity. The spacing l between the hydrophone groups is selected as mn The relationship with wavelength λ should satisfy:

[0078]

[0079] Among the hydrophones satisfying formula (14), two hydrophones with the largest open-circuit voltage amplitude deviation are selected in each standing wave acoustic field to form a hydrophone group.

[0080] The spacing between the hydrophone groups selected in each standing wave acoustic field should be between 0.05 and 0.45 of the measurement wavelength. The two hydrophones with the largest open-circuit voltage amplitude deviation can be selected to form a hydrophone group to reduce the uncertainty of the acoustic performance measurement results.

[0081] Acoustic tube measurement technology, in the attached Figure 1 The method is carried out in a water-filled thick-walled stainless steel acoustic tube as shown, and the specific implementation method is as follows:

[0082] 1) The isotropic hydroacoustic material sample to be tested is placed in the middle of the sound tube, and the upper and lower sections of the sound tube separated by the material sample each have at least two hydrophones;

[0083] 2) Fill the sound tube with water, evacuate it, and eliminate bubbles in the water;

[0084] 3) As needed, start the temperature and pressure control system of the sound tube, control the water temperature and hydrostatic pressure in the tube to a certain value, and after stabilization, measure the acoustic properties of the material sample;

[0085] 4) Control the transmitting transducer at the bottom of the sound tube to transmit a single-frequency long pulse signal, forming a plane standing wave sound field in the upper and lower sections of the sound tube respectively;

[0086] 5) Read the open-circuit voltage amplitude and phase of each hydrophone of the sound tube, and calculate the sound pressure amplitude and phase at each hydrophone position according to formulas (7) to (10) based on the sensitivity amplitude and phase value of the hydrophone under the current environmental conditions and the current frequency;

[0087] 6) Select a suitable hydrophone pair and calculate the acoustic performance parameters such as the sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient of the sample material under the current environmental conditions and at the current measurement frequency point according to formulas (8), (9) and (11) to (13);

[0088] 7) Transmit the next frequency signal and repeat steps 4) to 6) to complete the acoustic performance measurement of the material sample under the current environmental conditions at all frequency points;

[0089] 8) Adjust the water temperature and hydrostatic pressure of the acoustic tube as needed, and repeat steps 3) to 7) until the acoustic performance of the material sample under all environmental conditions and at all frequency points is measured.

[0090] 9) Release the hydrostatic pressure to normal pressure.

[0091] The above embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

[0092] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring the low-frequency acoustic performance of an isotropic hydroacoustic material using a sound tube, characterized in that: The steps are: 1) The isotropic hydroacoustic material sample to be tested is placed in the middle of the sound tube, and the upper and lower sections of the sound tube separated by the material sample each have at least two hydrophones; 2) Fill the sound tube with water, evacuate it, and eliminate bubbles in the water; 3) As needed, start the temperature and pressure control system of the sound tube, control the water temperature and hydrostatic pressure in the tube to a certain value, and after stabilization, measure the acoustic properties of the material sample; 4) Control the transmitting transducer at the bottom of the sound tube to transmit a single-frequency long pulse signal, forming a plane standing wave sound field in the upper and lower sections of the sound tube respectively; 5) Read the open-circuit voltage amplitude and phase of each hydrophone of the sound tube, combine the sensitivity amplitude and phase value of the hydrophone under the current environmental conditions and the current frequency, and calculate the sound pressure amplitude and phase at each hydrophone position according to the formula; 6) Select a suitable pair of hydrophones and calculate the acoustic performance parameters of the sample material under the current environmental conditions and at the current measurement frequency point according to the formula, including the sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient; 7) Transmit the next frequency signal and repeat steps 4) to 6) to complete the acoustic performance measurement of the material sample under the current environmental conditions at all frequency points; 8) Adjust the water temperature and hydrostatic pressure of the acoustic tube as needed, and repeat steps 3) to 7) until the acoustic performance of the material sample under all environmental conditions and at all frequency points is measured. 9) Release the hydrostatic pressure to normal pressure.

2. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using an acoustic tube according to claim 1, characterized in that: When measuring the acoustic properties of isotropic hydroacoustic material samples in an acoustic tube, two sections of standing wave acoustic fields are quickly constructed in the acoustic tube; By measuring the phase of the open-circuit voltage amplitude of four hydrophones distributed in the acoustic tube, the sound wave components in the standing wave sound field are separated, thereby obtaining the acoustic properties of the isotropic hydroacoustic material samples, such as the sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient.

3. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using an acoustic tube according to claim 2, characterized in that: The acoustic tube is a water-filled thick-walled stainless steel acoustic tube, which is placed vertically and has a transmitting transducer installed at the bottom. The isotropic hydroacoustic material sample to be tested is placed in the middle of the acoustic tube. The acoustic tube is divided into an upper tube section and a lower tube section. The hydrophones are respectively installed on the walls of the acoustic tube by an embedded method. There are at least two hydrophones in each of the upper tube section and the lower tube section. The size of the hydrophone is less than 1 / 10 of the minimum measurement wavelength in the tube.

4. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using a sound tube according to claim 3, characterized in that: A backing is arranged on the top of the sound tube, and the backing is a transducer, a sound absorbing wedge, air, a rigid backing or any other solid material that is insoluble in water and has stable physical properties.

5. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using a sound tube according to claim 4, characterized in that: The tested hydroacoustic material sample is a cylindrical isotropic sample, that is, the sample is symmetrical along the axial direction of the cylinder, and the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample when the sound wave is incident from the upper circular surface are equal to the sound pressure reflection coefficient and sound pressure transmission coefficient when the sound wave is incident from the lower circular surface.

6. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using a sound tube according to claim 5, characterized in that: Under certain water temperature and hydrostatic pressure conditions, only one single-frequency long pulse signal is needed for each measurement frequency point, and two standing wave sound fields can be quickly formed in the sound tube. The sound signal in the lower sound tube can be regarded as composed of three parts: the signal p incident on the lower surface of the material sample i 、p i p reflected by the lower surface of the material sample r , the transmission signal p' of the upper tube acoustic signal through the material sample t The acoustic signal in the upper sound tube can also be considered to be composed of three parts: the signal p' incident on the upper surface of the material sample i 、p' i p' reflected by the upper surface of the material sample r , the transmission signal p of the acoustic signal from the lower tube through the material sample t .

7. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using a sound tube according to claim 6, characterized in that: With the bottom surface of the material under test as the origin, a one-dimensional coordinate system is constructed, and the positions of hydrophones 1# to 4# are x1, x2, x3 and x4 respectively; The transmitting transducer at the bottom of the tube emits a single-frequency long pulse p in , the acoustic signal propagates in the lower acoustic tube, and after reaching the lower surface of the material sample, part of it is absorbed by the material sample, part of it is reflected, and part of it is transmitted; The reflected sound propagates in the reverse direction in the lower tube, and is reflected or absorbed by the transmitting transducer. The reflected wave is incident on the lower surface of the material sample in the forward direction, and this process is repeated until a steady state is reached. The sound signal passing through the sample enters the upper tube, is transmitted to the backing on the top of the sound tube, is partially absorbed and partially reflected back to the upper surface of the material sample, and is reflected, transmitted and absorbed again until the sound field in the upper sound tube reaches a steady state. r =R·p i 、p t =T·p i 、p r '=R·p i '、p t '=T·p i ', R and T are the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample under test respectively; The hydrophones in the sound tube are used to pick up the sound field characteristics in the sound tube. The sound pressure at the 1# to 4# hydrophones can be expressed by formulas (1) to (4) respectively: The sound pressure reflection coefficient of the measured hydroacoustic material is calculated as: The sound pressure transmission coefficient is: Here, l 12 is the distance between hydrophone #1 and hydrophone #2, l 34 is the distance between hydrophone #3 and hydrophone #4. The sound pressures p1, p2, p3 and p4 at the positions of the four hydrophones are calculated as follows: Among them, A1, A2, A3, and A4 are the open-circuit voltage signal amplitudes of each hydrophone, is the phase value of the open-circuit voltage signal of each hydrophone, M1, M2, M3, and M4 are the sensitivity amplitudes of each hydrophone under the current water temperature and water pressure conditions and the current measurement frequency, and θ1, θ2, θ1, and θ2 are the sensitivity phase values ​​of the four hydrophones under the current environment and frequency conditions; The sound pressure reflection coefficient and sound pressure transmission coefficient of each isotropic hydroacoustic material are obtained, and the echo reduction ER, insertion loss IL and sound absorption coefficient α can be calculated according to formulas (11) to (13). ER=20lg(1 / R) (11) IL=20lg(1 / T) (12) α=1-R 2 -T 2 (13)。 8. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material by using a sound tube according to claim 7, characterized in that: The more hydrophones are arranged in the above-mentioned sound tube, the more measurement selectivity there is. The spacing l between the hydrophone groups is selected. mn The relationship with wavelength λ should satisfy: Among the hydrophones that satisfy formula (14), two hydrophones with the largest open-circuit voltage amplitude deviation are selected in each standing wave acoustic field to form a hydrophone group; The spacing between the hydrophone groups selected in each standing wave acoustic field should be between 0.05 and 0.45 of the measurement wavelength, and the two hydrophones with the largest open-circuit voltage amplitude deviation should be selected to form a hydrophone group to reduce the uncertainty of the acoustic performance measurement results.

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