A method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material using an acoustic tube

Abstract

The present application relates to the technical field of metrological test, especially to a sound tube measurement method for low-frequency acoustic performance of isotropic underwater acoustic material, in the sound tube, a double-section plane standing wave sound field is constructed by using the characteristics of the isotropic underwater acoustic material sample, the open-circuit voltage amplitude and phase of four-channel hydrophone are quickly collected, and the sound pressure reflection coefficient, sound pressure transmission coefficient and sound absorption coefficient of the low-frequency isotropic underwater acoustic material sample are quickly and accurately measured, the present application can be directly applied to the existing thick-wall sound tube device in China, and the measurement efficiency of the low-frequency acoustic performance of the underwater acoustic material is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of metrology and testing technology, specifically to a method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials using a acoustic tube. Background Technology

[0002] The acoustic performance testing of small-scale underwater acoustic materials is mostly conducted in thick-walled acoustic tubes that can establish a plane wave acoustic field environment. Acoustic tube testing methods can be categorized into pulsed acoustic tube testing, standing wave acoustic tube testing, and traveling wave tube testing based on the test frequency band and operating conditions. Pulsed acoustic tube testing is limited by time-domain separation of reflection and transmission pulse signals, and the lower limit of the test frequency depends on the tube length. Lower frequency tests are conducted in standing wave tubes and traveling wave tubes: the standing wave tube method evaluates the low-frequency sound reflection and absorption performance of materials in a "water-material-air" operating condition by creating a stable standing wave acoustic field within the tube; the traveling wave tube method constructs a stable unidirectional traveling wave acoustic field within the tube using active and passive anechoic techniques, with the underwater acoustic material placed in the middle of the tube, forming a "standing wave acoustic field + material + traveling wave acoustic field" sound field mode. The traveling wave tube method can be used to evaluate the low-frequency sound reflection, sound insulation, and sound absorption performance of materials in a "water-material-water" operating condition. When the operating frequency of underwater acoustic materials is below 500Hz, people often use active silencing traveling wave tubes to test the acoustic performance of the materials. The method of repeatedly iterating to approximate the material to make the reflection coefficient of the transducer surface at the top of the tube approach 0 is not only time-consuming, but also has the problem of non-convergence of iteration, that is, a stable traveling wave sound field cannot be formed in the tube at certain frequency points. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials in acoustic tubes. This method utilizes a two-segment standing wave sound field to measure the acoustic performance of isotropic underwater acoustic materials in various existing acoustic tubes, significantly improving work efficiency while ensuring measurement accuracy. The method proposed in this invention eliminates the need to construct a traveling wave sound field, and the upper tube end does not require a transducer to transmit signals. The backing material at the upper tube end can be any existing transducer, sound-absorbing wedge, air, rigid backing, or other solid materials with stable physical properties.

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

[0005] 1) Place the isotropic underwater acoustic material sample to be tested in the middle of the acoustic tube, and each of the upper and lower sections of the acoustic tube separated by the material sample has at least 2 hydrophones.

[0006] 2) Fill the sound tube with water, create a vacuum, and eliminate air bubbles in the water;

[0007] 3) As needed, activate the temperature and pressure control system of the acoustic tube to control the water temperature and static pressure in the tube to a certain value. After stabilization, the acoustic performance of the material sample can be measured.

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

[0009] 5) Read the open-circuit voltage amplitude and phase of each hydrophone in the sound tube, and combine the sensitivity amplitude and phase of the hydrophone under the current environmental conditions and frequency to 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, such as sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss, and sound absorption coefficient, according to the formula.

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

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

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

[0014] In some embodiments, when measuring the acoustic performance of isotropic underwater acoustic material samples in a acoustic tube, two standing wave sound fields are rapidly 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 acoustic wave components in the standing wave sound field are separated, thereby obtaining the acoustic properties of isotropic underwater acoustic material samples, such as 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, placed vertically with a transmitting transducer installed at the bottom, and the isotropic underwater acoustic material sample to be tested is placed in the middle of the acoustic tube.

[0017] The acoustic tube is divided into an upper section and a lower section. Hydrophones are installed on the tube wall using an embedded method. There are at least two hydrophones in each of the upper and lower sections. The size of the hydrophones is less than 1 / 10 of the smallest measurement wavelength in the tube.

[0018] In some embodiments, the top of the sound tube is provided with a backing, which may 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.

[0019] In some embodiments, the underwater acoustic material sample being tested is a cylindrical isotropic sample, meaning that the sample is symmetrical along the cylindrical axis, 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 those 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 needs to be emitted at each measurement frequency point. Two standing wave sound fields can be quickly formed in the acoustic tube. The acoustic signal in the lower acoustic tube can be considered to consist of three parts: the signal p incident on the lower surface of the material sample. i p i p reflected from the lower surface of the material sample r The transmitted signal p' of the upper tube acoustic signal through the material sample t The acoustic signal inside the upper acoustic tube can also be considered to consist 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 transmitted signal p of the acoustic signal through the material sample. t .

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

[0022] A single-frequency long pulse p is emitted by the transmitting transducer at the bottom of the tube. in The acoustic signal propagates in the lower section of the acoustic tube. After reaching the lower surface of the material sample, part of it is absorbed by the material sample, part is reflected, and part is transmitted.

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

[0024] The acoustic signal passing through the sample enters the upper tube, is transmitted to the backing at the top of the tube, undergoes partial absorption and reflection, and is transmitted back to the upper surface of the sample material. It then undergoes further reflection, transmission, and absorption until the acoustic field in the upper 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 tested material sample, respectively;

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

[0027]

[0028]

[0029]

[0030]

[0031] The calculated sound pressure reflection coefficient of the tested underwater acoustic material is:

[0032]

[0033] The sound pressure transmission coefficient is:

[0034]

[0035] Here, l 12 The distance between hydrophone #1 and hydrophone #2 is l 34 The distance between hydrophone #3 and hydrophone #4 is given. The sound pressures p1, p2, p3, and p4 at the locations of the four hydrophones are calculated using the open-circuit voltage signal and hydrophone sensitivity of the hydrophones.

[0036]

[0037]

[0038]

[0039]

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

[0041] The sound pressure reflection coefficient and sound pressure transmission coefficient of isotropic underwater acoustic materials were obtained. Their echo reduction ER, insertion loss IL, and absorption coefficient α can all 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 inside the acoustic tube, thus providing greater measurement selectivity, and the spacing l of the hydrophone groups can be selected. mn The relationship with wavelength λ should satisfy:

[0046]

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

[0048] The spacing between the hydrophone groups selected in each standing wave sound field should be between 0.05 and 0.45 of the measurement wavelength. 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 existing technologies, the beneficial effects of this invention are as follows: Addressing the problems of low traveling wave generation efficiency and the inability to form traveling wave fields at certain frequency points when testing the acoustic performance of underwater acoustic materials using the traveling wave tube method, this invention proposes a method for measuring the acoustic performance of isotropic underwater acoustic materials using a dual-segment standing wave acoustic field in various existing acoustic tubes. This significantly improves work efficiency while ensuring measurement accuracy. The method proposed in this invention does not require the construction of a traveling wave acoustic field, and the upper tube end does not require a transducer to transmit signals. The backing material at 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 this application are set forth in the following drawings and description to make other features, objects and advantages of this application more readily apparent. The embodiments of this application will provide a detailed description and understanding of the application. Attached Figure Description

[0051] Figure 1 This invention relates to a dual-segment standing wave field acoustic tube structure. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] Please see Figure 1This invention provides a technical solution: a method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials using a acoustic tube. The diagram shows a dual-segment standing wave field acoustic tube structure, comprising a thick-walled, water-filled stainless steel tube, a transmitting transducer, and a set of hydrophones, with a backing material at the upper end. The acoustic tube is placed vertically, with the transmitting transducer at the bottom. The isotropic underwater acoustic material sample to be tested is placed in the middle of the acoustic tube, dividing it into an upper and lower segment. The hydrophones are embedded in the tube wall, with at least two hydrophones in each segment. A one-dimensional coordinate system is shown in the diagram, with the bottom surface of the material sample at x=0. A single-frequency long pulse signal is emitted through the transmitting transducer, forming a dual-segment planar standing wave sound field within the acoustic tube. The standing wave sound field of the lower segment can be considered to consist of three parts: the signal p incident on the lower surface of the material sample... i p i p reflected from the lower surface of the material sample r The transmitted signal p' of the upper tube acoustic signal through the material sample t The standing wave acoustic field of the upper pipe section can also be considered as consisting 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 transmitted signal p of the acoustic signal through the material sample. t Based on the measurement frequency, a suitable hydrophone array is selected for acoustic signal acquisition. After signal separation, acoustic performance parameters such as sound pressure reflection coefficient (echo reduction), sound pressure transmission coefficient (insertion loss), and sound absorption coefficient of isotropic underwater acoustic materials are calculated.

[0054] The technical solution of this application addresses the following: Figure 1 The device shown constructs two planar standing wave sound fields, one above the other, bounded by the underwater acoustic material being measured, to achieve rapid and accurate measurement of the acoustic properties of isotropic underwater acoustic materials, such as sound reflection, sound transmission (sound insulation), and sound absorption.

[0055] The acoustic tube used to construct a dual-segment standing wave field according to the present invention is a water-filled, thick-walled stainless steel acoustic tube. A low-frequency transmitting transducer is located at the bottom of the tube. The underwater acoustic material sample to be tested is placed in the middle of the acoustic tube. At least two hydrophones are present in both the upper and lower sections of the acoustic tube. The size of the hydrophones is less than 1 / 10 of the smallest measurement wavelength in the tube, and their influence on the sound field is negligible. The backing at 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 underwater acoustic material sample to be tested in this invention is a cylindrical isotropic sample, that is, the sample is symmetrical along the cylindrical axis. When the sound wave is incident from the upper circular surface, the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample are equal to those of the sound pressure reflection coefficient and sound pressure transmission coefficient when the sound wave is incident from the lower circular surface.

[0057] A one-dimensional coordinate system is constructed with the bottom surface of the material being measured as the origin, such as... Figure 1 As shown, the positions of hydrophones #1 to #4 are x1, x2, x3, and x4, respectively. A single-frequency long pulse p is emitted from the transmitting transducer at the bottom of the tube. in (The upper frequency limit is determined by the inner diameter of the acoustic tube). The acoustic signal propagates in the lower section of the acoustic tube. After reaching the lower surface of the material sample, part of it is absorbed by the material sample, part is reflected, and part is transmitted. The reflected sound propagates in the opposite direction in the lower tube, and is reflected or absorbed again when it encounters the transmitting transducer. The reflected wave is then incident forward to the lower surface of the material sample, and this process repeats until a steady state is reached. The acoustic signal that passes through the sample enters the upper tube, is transmitted to the backing at the top of the acoustic tube, and is partially absorbed and partially reflected back to the upper surface of the material sample. It is then reflected, transmitted, and absorbed again until the sound field in the upper acoustic tube reaches a steady state. Although the sound wave propagates back and forth in the acoustic tube, and the characteristic impedances of the two interfaces are different, the composition of the sound wave in the acoustic tube is extremely complex. However, as long as the physical properties of the radiating surface material of the transmitting transducer, the underwater acoustic material being measured, and the backing material at the top of the acoustic tube are stable, two stable planar standing wave sound fields can be quickly formed in the upper and lower acoustic tubes. The acoustic signal in the lower acoustic tube can be considered to consist of three parts: the signal p incident on the lower surface of the material sample. i p i p reflected from the lower surface of the material sample r The transmitted signal p' of the upper tube acoustic signal through the material sample t The acoustic signal inside the upper acoustic tube can also be considered to consist 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 transmitted signal p of the acoustic signal 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 tested material sample, respectively.

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

[0059]

[0060]

[0061]

[0062]

[0063] The calculated sound pressure reflection coefficient of the tested underwater acoustic material is:

[0064]

[0065] The sound pressure transmission coefficient is:

[0066]

[0067] Here, l 12 The distance between hydrophone #1 and hydrophone #2 is l 34 The distance between hydrophone #3 and hydrophone #4 is given. The sound pressures p1, p2, p3, and p4 at the locations of the four hydrophones are obtained from the open-circuit voltage signal and hydrophone sensitivity calculations.

[0068]

[0069]

[0070]

[0071]

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

[0073] The sound pressure reflection coefficient and sound pressure transmission coefficient of isotropic underwater acoustic materials were obtained. 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 inside the aforementioned acoustic tube, there are more measurement options available, allowing for the selection of the hydrophone group spacing l. mn The relationship with wavelength λ should satisfy:

[0078]

[0079] In the hydrophones that satisfy formula (14), the two hydrophones with the largest open-circuit voltage amplitude deviation in each segment of the standing wave sound field are selected to form a hydrophone group.

[0080] The spacing between hydrophone groups selected in each standing wave sound field should be between 0.05 and 0.45 of the measurement wavelength. 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] Sound tube measurement technology, in the appendix Figure 1 The procedure is carried out in the water-filled thick-walled stainless steel acoustic tube shown, and the specific implementation method is as follows:

[0082] 1) Place the isotropic underwater acoustic material sample to be tested in the middle of the acoustic tube, and there should be at least 2 hydrophones in each of the upper and lower sections of the acoustic tube separated by the material sample.

[0083] 2) Fill the sound tube with water, create a vacuum, and eliminate air bubbles in the water;

[0084] 3) As needed, activate the temperature and pressure control system of the acoustic tube to control the water temperature and static pressure in the tube to a certain value. After stabilization, the acoustic performance of the material sample can be measured.

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

[0086] 5) Read the open-circuit voltage amplitude and phase of each hydrophone in the sound tube, and combine the sensitivity amplitude and phase of the hydrophone under the current environmental conditions and current frequency. Calculate the sound pressure amplitude and phase of each hydrophone position according to formulas (7) to (10).

[0087] 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, such as sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss and sound absorption coefficient, 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 test material sample at all frequency points under the current environmental conditions.

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

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

[0091] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

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

Claims

1. A method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials using a acoustic tube, characterized in that: The steps are as follows: 1) Place the isotropic underwater acoustic material sample to be tested in the middle of the acoustic tube, and each of the upper and lower sections of the acoustic tube separated by the material sample has at least 2 hydrophones. 2) Fill the sound tube with water, create a vacuum, and eliminate air bubbles in the water; 3) As needed, activate the temperature and pressure control system of the acoustic tube to control the water temperature and static pressure in the tube to a certain value. After stabilization, the acoustic performance of the material sample can be measured. 4) Control the transmitting transducer at the bottom of the sound tube to emit a single-frequency long pulse signal, forming a planar standing wave sound field in each of the upper and lower sections of the sound tube; 5) Read the open-circuit voltage amplitude and phase of each hydrophone in the sound tube, and combine the sensitivity amplitude and phase of the hydrophone under the current environmental conditions and frequency to 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, including sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss, and sound absorption coefficient, according to the formula. 7) Transmit the next frequency signal and repeat steps 4) to 6) to complete the acoustic performance measurement of the material sample at all frequency points under the current environmental conditions; 8) Adjust the water temperature and static water pressure in the acoustic tube as needed, and repeat steps 3) to 7) until the acoustic performance of the material sample at all frequencies under all environmental conditions is measured. 9) Release the hydrostatic pressure to normal atmospheric pressure; The calculation formula in step 5) is: , in, φ is the sound pressure at the location of the hydrophone, A is the amplitude of the open-circuit voltage signal of the corresponding hydrophone, φ is the phase value of the open-circuit voltage signal of the corresponding hydrophone, M is the sensitivity amplitude of the corresponding hydrophone under the current water temperature and pressure conditions and the current measurement frequency, and θ is the sensitivity phase value of the corresponding hydrophone under the current environmental and frequency conditions. The acoustic performance parameters for sound pressure reflection coefficient, sound pressure transmission coefficient, echo reduction, insertion loss, and absorption coefficient in step 6) are calculated using the following formulas: ， Where R is the sound pressure reflection coefficient of the tested material sample, T is the sound pressure transmission coefficient of the tested material sample, ER is the echo reduction parameter of the tested material sample, IL is the insertion loss parameter of the tested material sample, α is the sound absorption coefficient of the tested material sample, and x1, x2, x3, and x4 are the locations of hydrophones #1 to #4, respectively. , , and These represent the sound pressure levels at the locations of hydrophones #1 through #4. The distance between hydrophone #1 and hydrophone #2 The distance between hydrophone #3 and hydrophone #4.

2. The method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials according to claim 1, characterized in that: When measuring the acoustic performance of isotropic underwater acoustic material samples in a acoustic tube, two standing wave sound fields are rapidly constructed in the acoustic tube. By measuring the phase of the open-circuit voltage amplitude of four hydrophones distributed in the acoustic tube, the acoustic wave components in the standing wave sound field are separated, thereby obtaining the acoustic properties of isotropic underwater acoustic material samples, such as 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 according to claim 2, characterized in that: The acoustic tube is a water-filled, thick-walled stainless steel acoustic tube, placed vertically with a transmitting transducer installed at the bottom. The isotropic underwater acoustic material sample to be tested is placed in the middle of the acoustic tube. The acoustic tube is divided into an upper section and a lower section. Hydrophones are installed on the tube wall using an embedded method. There are at least two hydrophones in each of the upper and lower sections. The size of the hydrophones is less than 1 / 10 of the smallest measurement wavelength in the tube.

4. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material according to claim 3, characterized in that: The top of the sound tube is provided with a backing, which 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.

5. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material according to claim 4, characterized in that: The underwater acoustic material sample under test is a cylindrical isotropic sample, meaning that the sample is symmetrical along the cylindrical axis. When the sound wave is incident from the upper circular surface, the sound pressure reflection coefficient and sound pressure transmission coefficient of the material sample are equal to those 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 according to claim 5, characterized in that: Under certain water temperature and hydrostatic pressure conditions, only one single-frequency long pulse signal needs to be emitted at each measurement frequency point. 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 consisting of three parts: the signal p incident on the lower surface of the material sample. i p i p reflected from the lower surface of the material sample r The transmitted signal p' of the upper tube acoustic signal through the material sample t The acoustic signal inside the upper acoustic tube can also be considered to consist 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 transmitted signal p of the acoustic signal through the material sample. t .

7. The method for measuring the low-frequency acoustic performance of an isotropic underwater acoustic material according to claim 6, characterized in that: A one-dimensional coordinate system is constructed with the bottom surface of the material being tested as the origin. The positions of hydrophones #1 to #4 are x1, x2, x3, and x4, respectively. A single-frequency long pulse p is emitted by the transmitting transducer at the bottom of the tube. in The acoustic signal propagates in the lower section of the acoustic tube. After reaching the lower surface of the material sample, part of it is absorbed by the material sample, part is reflected, and part is transmitted. The reflected sound propagates in the reverse direction inside the lower tube, and is reflected or absorbed by the transmitting transducer. The reflected wave is then incident in the forward direction onto the lower surface of the material sample, and this process repeats until a steady state is reached. The acoustic signal passing through the sample enters the upper tube, is transmitted to the backing at the top of the tube, undergoes partial absorption and reflection, and is transmitted back to the upper surface of the sample material. It then undergoes further reflection, transmission, and absorption until the acoustic field in the upper tube reaches a steady state. , , , , R and T are the sound pressure reflection coefficient and sound pressure transmission coefficient of the tested material sample, respectively; The hydrophones in the sound tube are used to pick up the sound field characteristics inside the sound tube. The sound pressure at hydrophones #1 to #4 is expressed by formulas (1) to (4): （1） （2） （3） （4） The calculated sound pressure reflection coefficient of the tested underwater acoustic material is: (5) The sound pressure transmission coefficient is: （6） here, The distance between hydrophone #1 and hydrophone #2 The distance between hydrophone #3 and hydrophone #4, and the sound pressure at the locations of the four hydrophones. , , and The open-circuit voltage signal and hydrophone sensitivity of the hydrophone are calculated as follows: （7） （8） （9） （10） Where A1, A2, A3, and A4 are the open-circuit voltage signal amplitudes of each hydrophone, φ1, φ2, φ3, and φ4 are the open-circuit voltage signal phases of each hydrophone, M1, M2, M3, and M4 are the sensitivity amplitudes of each hydrophone under the current water temperature and pressure conditions and at the current measurement frequency, and θ1, θ2, θ1, and θ2 are the sensitivity phases of the four hydrophones under the current environmental and frequency conditions. The sound pressure reflection coefficient and sound pressure transmission coefficient of isotropic underwater acoustic materials were obtained. Their echo reduction ER, insertion loss IL, and absorption coefficient α can all be calculated according to formulas (11) to (13). （11） （12） （13）。 8. The method for measuring the low-frequency acoustic performance of isotropic underwater acoustic materials according to claim 7, characterized in that: If more hydrophones are arranged inside the aforementioned acoustic tube, there is a greater range of measurement options, allowing for the selection of the spacing between the hydrophone groups. With wavelength The relationship should satisfy: （14） In the hydrophones that satisfy formula (14), the two hydrophones with the largest open-circuit voltage amplitude deviation are selected in each segment of the standing wave sound field to form a hydrophone group. The spacing between hydrophone groups selected in each standing wave sound field should be between 0.05 and 0.45 of the measurement wavelength. 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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