An indirect test method for the sound absorption coefficient of acoustic covering layer under double-layer shell conditions

Abstract

The present invention discloses an indirect testing method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition, comprising the following steps: preparing an outer shell sample and an inner shell sample respectively based on a double-layer shell structure, and laying an acoustic covering layer on the front and / or back of the outer shell sample, and laying an acoustic covering layer on the front of the inner shell sample; conducting an in-tube acoustic test on the outer shell sample to obtain a complex reflection coefficient and a complex transmission coefficient of the outer shell sample under the condition that sound waves of different frequencies are incident from the front and back of the outer shell sample; placing the inner shell sample at the end of an underwater acoustic tube, and conducting a tube-end acoustic test on the inner shell sample to obtain a complex reflection coefficient of the inner shell sample under the condition that sound waves of different frequencies are incident from the front of the inner shell; calculating the complex reflection coefficient of the double-layer shell structure laid with an acoustic covering layer; and calculating the sound absorption coefficient of the acoustic covering layer under the double-layer shell condition.

Description

Technical Field

[0001] The invention relates to the technical field of underwater acoustic testing, and in particular to an indirect testing method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition. Background Art

[0002] Acoustic coating refers to a special underwater acoustic material or component laid or installed on the surface of a submarine hull to reduce the target intensity and radiation noise of the submarine. It is usually a type of polymer viscoelastic material with an internal acoustic structure. It is mainly used to achieve sound absorption, while taking into account vibration suppression, sound insulation, decoupling and other functions. It is a key component on a submarine that can effectively counter the enemy's active and passive sonar at the same time. At present, the main structural form of my country's submarines is a double-shell structure, which consists of an inner pressure hull and an outer non-pressure hull (usually also called an outer hull). The space between the inner and outer hulls is filled with seawater. The hull medium is generally steel, and the inside of the boat is air. The design idea of ​​laying acoustic coatings with specific functions on the surfaces of double-shell submarines to reduce the target reflection intensity and suppress the noise radiation intensity, thereby improving the comprehensive acoustic stealth capability of the submarine has gradually become a consensus in the industry.

[0003] Conducting experimental tests on the sound absorption coefficient of acoustic covering layers is the most direct means to verify the accuracy of relevant design theories and methods and the effectiveness of design schemes. At present, the test of the sound absorption coefficient of acoustic covering layers mainly refers to the ship industry standard CB / T 3674-2019 "Standing wave tube measurement method for underwater acoustic materials", which uses the dual hydrophone transfer function method to measure its complex reflection coefficient under the total reflection backing, and obtains the sound absorption coefficient of the acoustic covering layer according to the energy conservation relationship. In addition, the national standards GB / T32523-2016 "Traveling wave tube method for measuring sound pressure reflection coefficient, sound pressure transmission coefficient and sound absorption coefficient of acoustic underwater acoustic material samples" and GB / T 14369-2011 "Measurement method for insertion loss, echo reduction and sound absorption coefficient of acoustic underwater acoustic material samples" respectively give the traveling wave tube and pulse tube test methods for the sound absorption coefficient of acoustic covering layers.

[0004] The above method is relatively mature in carrying out the sound absorption test of the acoustic covering layer under simple backing conditions, but there are still certain difficulties in testing the sound absorption coefficient of the acoustic covering layer under the condition of a double-layer shell. The reason is: due to the large spacing between the double-layer shells, when directly using the existing underwater acoustic sound tube test device to carry out the sound absorption coefficient test of the acoustic covering layer under the condition of a double-layer shell, it is necessary to place the test sample simulating the double-layer shell structure in the underwater acoustic sound tube. This process will bring about the installation difficulties of the test sample, and it is usually necessary to add an extension tube to the existing test device. However, since it is often necessary to inflate and pressurize the sound tube during the test to simulate the hydrostatic pressure environment at different depths, the extension tube needs to be sealed with high pressure resistance, which makes the design and preparation of the extension tube difficult; in addition, once the extension tube is set, the distance between the double-layer shells will not be adjustable. If it is necessary to experimentally study the influence of the spacing between the double-layer shells on the sound absorption coefficient of the acoustic covering layer, it is necessary to design extension tubes with different spacings respectively, and the testing cost is high. Summary of the invention

[0005] In view of this, the present invention provides an indirect testing method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition, which is used to at least solve the problems of sample installation and difficulty in adjusting the distance between double-layer shells when directly conducting the sound absorption coefficient test of the acoustic covering layer under the double-layer shell condition in the existing testing method.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] An indirect test method for the sound absorption coefficient of an acoustic cover layer under a double-shell condition is based on a double-shell structure. The double-shell structure includes an outer shell and an inner shell. The outer side of the outer shell contacts the external seawater. The outer shell and the inner shell are between the side seawater and the inner shell. The inner side of the inner shell contacts the internal air. The two sides of the outer shell in contact with the external seawater and the side seawater are called the front side of the outer shell and the back side of the outer shell, respectively. The two sides of the inner shell in contact with the side seawater and the internal air are called the front side of the inner shell and the back side of the inner shell. The method includes the following steps:

[0008] S1. Prepare the sample to be tested; prepare the outer shell sample and the inner shell sample respectively based on the double-layer shell structure, and lay the acoustic covering layer on the front and / or back of the outer shell sample, and lay the acoustic covering layer on the front of the inner shell sample;

[0009] S2. Place the outer shell sample in an underwater acoustic tube, conduct acoustic tests on the outer shell sample under different frequency sound wave incidence, and obtain the complex reflection coefficient and complex transmission coefficient of the outer shell sample under the conditions of sound wave incidence from the front and back of the outer shell sample respectively; the complex reflection coefficient and complex transmission coefficient under the condition of front incidence are recorded as and The complex reflection coefficient and complex transmission coefficient under reverse incidence conditions are denoted as and ;

[0010] S3. Place the inner shell sample at the end of the hydroacoustic tube and conduct acoustic tests on the inner shell sample under different frequency sound wave incidence to obtain the complex reflection coefficient of the inner shell sample under the condition that the sound wave is incident from the front of the inner shell. ;

[0011] S4. Preset spacing between ships , the complex reflection coefficient of the double-layer shell structure with acoustic covering layer is calculated :

[0012] ,

[0013] ,

[0014] ,

[0015] In the formula, is the wave number in water, is the circular frequency of the incident sound wave, is the incident sound wave frequency, is the circumference of a circle, is the speed of sound in water, is an imaginary unit;

[0016] S5. Based on the complex reflection coefficient Calculation of the sound absorption coefficient of the acoustic cover under double shell conditions .

[0017] Preferably, S2 obtains and The specific contents of the in-tube acoustic test include:

[0018] The outer shell sample is placed in the hydroacoustic tube, with the front side of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back sides of the outer shell sample are in contact with water through the acoustic covering layer. The hydrophone test is used to obtain the acoustic waves of different frequencies. and .

[0019] Preferably, S2 obtains and The specific contents of the in-tube acoustic test include:

[0020] The outer shell sample is placed in the hydroacoustic tube, with the back of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back of the outer shell sample are in contact with water through the acoustic covering layer. The test results are as follows: and .

[0021] Preferably, the specific contents of the pipe end acoustic test under different frequency sound wave incidence on the inner shell sample in S3 include:

[0022] The inner shell sample is placed at the end of the hydroacoustic sound tube, with the front of the inner shell sample facing the position of the transducer in the hydroacoustic sound tube, and the front of the inner shell sample is in contact with water through the acoustic covering layer, and the back of the inner shell sample is in contact with the air at the end of the hydroacoustic sound tube, and the complex reflection coefficient of the inner shell sample is obtained under the condition that the sound wave is incident from the front of the inner shell. .

[0023] Preferably, the sound absorption coefficient The calculation method is:

[0024] .

[0025] Preferably, a transducer is arranged at the opening of the hydroacoustic tube, and hydrophones are respectively arranged on the walls of the hydroacoustic tube, wherein two hydrophones are evenly arranged on both sides of the outer shell sample when conducting an in-tube acoustic test, and two hydrophones are arranged on the front side of the inner shell sample when conducting a pipe end acoustic test.

[0026] Preferably, the outer shell sample and the inner shell sample are both steel discs.

[0027] Preferably, an internal acoustic structure is provided inside the acoustic cover layer.

[0028] It can be seen from the above technical solution that, compared with the prior art, the present invention discloses an indirect test method for the sound absorption coefficient of the acoustic covering layer under the condition of a double-layer shell, which has the following beneficial effects:

[0029] 1. When the present invention is used to test the sound absorption coefficient of the acoustic covering layer under the condition of a double-layer shell, it can be carried out based on the test device currently widely used in the test of the sound absorption coefficient of the acoustic covering layer under the condition of a single-layer shell, and no special modification such as adding an extension pipe is required, which is simple and easy;

[0030] 2. The test method proposed in this invention can obtain the different side spacings of the double-layer hull through one test. The sound absorption coefficient under certain conditions can be given, and the sound absorption coefficient test results of different acoustic covering layer combinations can be given through a small number of test results, which saves multiple repeated test operations and significantly improves the test efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 A flow chart of an indirect test method for the sound absorption coefficient of an acoustic covering layer under double-layer shell conditions provided by the present invention;

[0033] Figure 2 This is a schematic diagram of a double-layer shell structure with an acoustic covering layer according to the present invention; wherein 1 is an outer shell, 2 is an acoustic covering layer, 3 is an inner shell, and 4 is an internal acoustic structure;

[0034] Figure 3 A schematic diagram of a simplified calculation method for a sound reflection (sound absorption) characteristic analysis model provided in an embodiment of the present invention;

[0035] Figure 4 Schematic diagram of the outer shell sample provided in the embodiment of the present invention for in-tube acoustic testing; wherein, 5-transducer; 6-sound tube wall; 7-first hydrophone; 8-second hydrophone; 9-outer shell sample; 10-third hydrophone; 11-fourth hydrophone; 12-sound absorbing end;

[0036] Figure 5 Schematic diagram of the pipe end acoustic test of the inner shell sample provided in the embodiment of the present invention; wherein 5-transducer, 13-fifth hydrophone, 14-sixth hydrophone, 15-inner shell sample;

[0037] Figure 6 The test results of the sound absorption coefficient of the three-layer acoustic covering layer under the double-layer shell condition obtained by the method of the present invention and the direct testing method are compared. DETAILED DESCRIPTION

[0038] 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.

[0039] The present invention provides an indirect test method for the sound absorption coefficient of an acoustic cover layer under a double-shell condition, based on a double-shell structure, the double-shell structure comprises an outer shell and an inner shell, the outer side of the outer shell contacts with external seawater, the space between the outer shell and the inner shell is between the sideboard seawater, the inner side of the inner shell contacts with internal air, the two sides of the outer shell contacting with the external seawater and the sideboard seawater are respectively referred to as the outer shell front side and the outer shell back side, and the two sides of the inner shell contacting with the sideboard seawater and the internal air are respectively referred to as the inner shell front side and the inner shell back side. Figure 2 As shown, the following steps are included:

[0040] S1. Prepare the sample to be tested; prepare the outer shell sample and the inner shell sample respectively based on the double-layer shell structure, and lay the acoustic covering layer on the front and / or back of the outer shell sample, and lay the acoustic covering layer on the front of the inner shell sample;

[0041] S2. Place the outer shell sample in an underwater acoustic tube, conduct acoustic tests on the outer shell sample under different frequency sound wave incidence, and obtain the complex reflection coefficient and complex transmission coefficient of the outer shell sample under the conditions of sound wave incidence from the front and back of the outer shell sample respectively; the complex reflection coefficient and complex transmission coefficient under the condition of front incidence are recorded as and The complex reflection coefficient and complex transmission coefficient under reverse incidence conditions are denoted as and ;

[0042] S3. Place the inner shell sample at the end of the hydroacoustic tube and conduct acoustic tests on the inner shell sample under different frequency sound wave incidence to obtain the complex reflection coefficient of the inner shell sample under the condition that the sound wave is incident from the front of the inner shell. ;

[0043] S4. Preset spacing between ships , the complex reflection coefficient of the double-layer shell structure with acoustic covering layer is calculated :

[0044] ,

[0045] ,

[0046] ,

[0047] In the formula, is the wave number in water, is the circular frequency of the incident sound wave, is the incident sound wave frequency, is the circumference of a circle, is the speed of sound in water, is an imaginary unit;

[0048] S5. Based on the complex reflection coefficient Calculation of the sound absorption coefficient of the acoustic cover under double shell conditions .

[0049] It should be noted that:

[0050] The acoustic covering layer applied on the front and / or back side of the outer shell sample can be selected as needed, and each acoustic covering layer can be different.

[0051] like Figure 2 As shown in the figure, consider a beam of external plane detection sound wave incident vertically from the semi-infinite water area on the left to the surface of the acoustic cover layer. The sound pressure of the incident wave can be expressed as:

[0052] (1)

[0053] in, is the incident wave sound pressure radiation, is the incident sound wave circular frequency, is the wave number in water; for simplicity, the time factor will be omitted below .

[0054] After the sound wave is incident on the double-layer shell surface, part of the sound wave is reflected on the surface of the acoustic cover layer to form a reflected sound wave. ; Part of the sound wave passes through the composite double-layer shell to form a transmitted sound wave that propagates into the air space inside the boat .

[0055] When an external simple harmonic sound wave is incident vertically, a reflected sound wave will be formed in the incident medium, and a forward propagating wave will exist in the water layer between the sides. (along the +z direction) and the counter-propagating waves (along the -z direction), the expressions are:

[0056] (2)

[0057] (3)

[0058] in, and Respectively The forward and reverse propagating sound pressures at an interface, usually a complex quantity.

[0059] First, consider the composite double-layer shell analysis model, such as Figure 3As shown in the figure, the specific structure of the composite double-layer shell analysis model includes two parts: external water area-acoustic cover layer I-outer shell-acoustic cover layer II-interboard water layer, interboard water layer-acoustic cover layer III-inner shell-air domain inside the boat. The sound pressure reflection and transmission coefficients of the total system are given below ( ) and the acoustic coefficients of the two subsystems ( and ) coupling relationship.

[0060] From the stress continuity relationship on each fluid-structure coupling boundary, we can obtain:

[0061] exist The interface has,

[0062] (4)

[0063] exist have,

[0064] (5)

[0065] exist have,

[0066] (6)

[0067] exist have,

[0068] (7)

[0069] in, and represent the complex sound pressure reflection coefficient and the complex sound pressure transmission coefficient of the total system respectively; , , and are the forward sound pressure reflection coefficient, forward sound pressure transmission coefficient, reverse sound pressure reflection coefficient and reverse sound pressure transmission coefficient of the outer shell, respectively. The forward and reverse here refer to the cases where the sound wave is incident from the external water area of ​​the outer shell (along the +z direction) or the between-board water layer (along the -z direction), respectively. and are the forward reflection coefficient and forward transmission coefficient of the inner shell respectively. The forward direction here refers to the situation where the sound wave is incident from the between-board water layer (along the +z direction).

[0070] From equations (5) and (6), we can get

[0071] (8)

[0072] (9)

[0073] Furthermore, by combining equations (4), (7), (8) and (9), we can obtain

[0074] (10)

[0075] In formula (10), the forward reflection coefficient of the inner shell is The acoustic parameters of the outer shell can be obtained by preparing an acoustic tube test sample for the inner shell and using the pipe end test conditions (simulating the "incident water area-test sample-air domain" conditions) through the underwater acoustic tube. and ) can be prepared by preparing the acoustic tube test sample on the outer shell, and in the underwater acoustic tube adopting the test condition in the tube (simulating the "incident water area-test sample-transmission water area" condition), the test sample is placed in the sound tube in the forward and reverse directions, and the forward complex reflection and complex transmission coefficients ( and ) and reverse complex reflection and complex transmission coefficients ( and ); Based on the above tests, the total complex reflection coefficient of the double-layer shell system is obtained based on expression (10).

[0076] In order to further implement the above technical solution, S2 obtains and The specific contents of the in-tube acoustic test include:

[0077] The outer shell sample is placed in the hydroacoustic tube, with the front side of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back sides of the outer shell sample are in contact with water through the acoustic covering layer. The hydrophone test is used to obtain the acoustic waves of different frequencies. and .

[0078] In order to further implement the above technical solution, S2 obtains and The specific contents of the in-tube acoustic test include:

[0079] The outer shell sample is placed in the hydroacoustic tube, with the back of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back of the outer shell sample are in contact with water through the acoustic covering layer. The test results are as follows: and .

[0080] In order to further implement the above technical solution, the specific contents of the pipe end acoustic test under different frequency sound wave incidence on the inner shell sample in S3 include:

[0081] The inner shell sample is placed at the end of the hydroacoustic sound tube, with the front of the inner shell facing the position of the transducer in the hydroacoustic sound tube, and the front of the inner shell sample is in contact with water through the acoustic covering layer, and the back of the inner shell sample is in contact with air through the acoustic covering layer at the end of the hydroacoustic sound tube, and the complex reflection coefficient of the inner shell sample is obtained under the condition that the sound wave is incident from the front of the inner shell. .

[0082] In order to further implement the above technical solution, the sound absorption coefficient The calculation method is:

[0083] .

[0084] It should be noted that:

[0085] Due to the serious impedance mismatch between the pressure hull and the air inside the boat, the energy transmission coefficient of the composite double-layer shell under external sound wave incidence is very small and can be approximately taken as According to the law of energy conservation, the sound absorption coefficient of the composite double-layer shell is It can be expressed as:

[0086] (11)

[0087] In order to further implement the above technical scheme, a transducer is arranged at the opening of the hydroacoustic tube, and hydrophones are respectively arranged on the walls of the hydroacoustic tube, wherein two hydrophones are evenly arranged on both sides of the outer shell sample when conducting an in-tube acoustic test, and two hydrophones are arranged on the front side of the inner shell sample when conducting a pipe end acoustic test.

[0088] In order to further implement the above technical solution, the outer shell sample and the inner shell sample are both steel discs.

[0089] It should be noted that:

[0090] In this embodiment, the outer shell sample and the inner shell sample can be selected from materials according to actual needs, such as steel, titanium alloy, aluminum alloy and hard composite material plate, etc. The shapes of the outer shell sample and the inner shell sample are designed according to the shape of the hydroacoustic tube. For example, if the current hydroacoustic tube is cylindrical, the test sample is set to a disc shape in this embodiment.

[0091] In order to further implement the above technical solution, an internal acoustic structure is provided inside the acoustic covering layer.

[0092] It should be noted that:

[0093] The internal acoustic structure can be a simple air cavity, a heavy material or other specially designed structure that performs a specific acoustic function. That is, the interior of the acoustic covering layer can be a cavity that can be provided with an acoustic structure, or can be other solid or porous structures. When the test method disclosed in the present invention is specifically implemented, specific settings can be made according to actual test requirements.

[0094] In this embodiment, refer to Figure 2 The double-layer shell structure schematic diagram shown in the figure is used to prepare the outer shell and inner shell cylindrical sound tube test samples for underwater sound tube testing. and are the distances between the centers of the two internal acoustic structures in each acoustic covering layer in the figure.

[0095] In the actual testing process, the shapes of the inner and outer shell samples are determined according to the shape of the hydroacoustic sound tube. In this embodiment, a cylindrical sound tube is selected for testing. The outer shell sample is made into a steel disc with a thickness of 6-10 mm, which can be determined according to the actual test requirements. The inner shell sample is a steel disc with a thickness of 30 mm, which can also be determined according to the actual test requirements.

[0096] The number of layers of acoustic covering layers applied to the inner and outer shell samples is determined according to the actual test requirements, and the diameter of the cylindrical sample to be tested is determined by the test requirements of the selected hydroacoustic tube.

[0097] The inner diameter of the cylindrical sound tube is 120mm, according to the test tube requirements, the outer diameter of the selected cylindrical sample for: 118mm. This embodiment considers applying an acoustic covering layer on both the front and back sides of the outer shell and the front side of the inner shell. The selected outer shell thickness is 10mm, the pressure shell thickness is 30mm, the acoustic covering layer thicknesses on the outer and inner surfaces of the outer shell are 14mm and 29.5mm respectively, and the acoustic covering layer thickness on the outer surface of the pressure shell is 36.5mm. Ignoring the thickness of the bonding layer between the acoustic covering layer and the shell, the total thickness of the outer shell and the inner shell to be tested is 53.5mm and 66.5mm, and the shipboard spacing between the outer shell and the inner shell is 400mm.

[0098] The acoustic covering layers of this embodiment are all made of butyl rubber, and the acoustic covering layers are embedded with periodically arranged cylindrical acoustic cavities, and the axis of the cylindrical cavity is perpendicular to the surface of the acoustic covering layer. Among them, the cavity radius of the acoustic covering layer outside the outer shell is 7.25 mm, the cavity height is 5 mm, the upper surface of the cylindrical cavity is 5 mm away from the incident end face of the acoustic covering layer, and the lateral arrangement period of the cavity in the acoustic covering layer is 32.5 mm; the cavity radius of the acoustic covering layer inside the outer shell is 7.65 mm, the cavity height is 13 mm, the upper surface of the cylindrical cavity is 12.5 mm away from the incident end face of the acoustic covering layer, and the lateral arrangement period of the cavity in the acoustic covering layer is 44.3 mm; the cavity radius of the acoustic covering layer outside the inner shell is 8 mm, the cavity height is 12.5 mm, the upper surface of the cylindrical cavity is 20 mm away from the incident end face of the acoustic covering layer, and the lateral arrangement period of the cavity in the acoustic covering layer is 29 mm.

[0099] Reference Figure 4 The outer shell sample was tested in the tube, and the four-hydrophone transfer function method was used to perform the in-tube acoustic test. The four hydrophones were the first hydrophone, the second hydrophone, the third hydrophone, and the fourth hydrophone. is the incident sound pressure, is the reflected sound pressure, is the transmitted sound pressure, is the total reflected sound pressure; is the distance between the first hydrophone and the second hydrophone, is the distance between the second hydrophone and the outer shell sample, is the thickness of the outer shell sample, is the distance between the outer shell sample and the third hydrophone, is the distance between the third hydrophone and the fourth hydrophone, refer to Figure 5 The acoustic tube end test of the inner shell sample is carried out, and the dual hydrophone transfer function method is used to carry out the tube end acoustic test. The dual hydrophones are the fifth hydrophone and the sixth hydrophone. This is the inner shell sample.

[0100] In this embodiment, the speed of sound in water is .

[0101] Figure 6 The results of measuring the sound absorption coefficient of the acoustic cover layer combination determined in this embodiment using the indirect test method proposed by the present invention and the traditional direct test method are compared. As can be seen from the figure, the two groups of sound absorption coefficient curves can be well matched in the frequency range of 500Hz-6000Hz, proving the effectiveness of the indirect test method proposed by the present invention.

[0102] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. An indirect test method for the sound absorption coefficient of an acoustic covering layer under double-layer shell conditions, characterized in that: Based on a double-shell structure, the double-shell structure includes an outer shell and an inner shell, the outer side of the outer shell is in contact with external seawater, the outer shell and the inner shell are between the seawater, and the inner side of the inner shell is in contact with internal air, and the two sides of the outer shell in contact with the external seawater and the seawater between the ships are respectively called the front side of the outer shell and the back side of the outer shell, and the two sides of the inner shell in contact with the seawater between the ships and the internal air are respectively called the front side of the inner shell and the back side of the inner shell, including the following steps: S1. Prepare the sample to be tested; prepare the outer shell sample and the inner shell sample respectively based on the double-layer shell structure, and lay the acoustic covering layer on the front and / or back of the outer shell sample, and lay the acoustic covering layer on the front of the inner shell sample; S2. Place the outer shell sample in an underwater acoustic tube, conduct acoustic tests on the outer shell sample under different frequency sound wave incidence, and obtain the complex reflection coefficient and complex transmission coefficient of the outer shell sample under the conditions of sound wave incidence from the front and back of the outer shell sample respectively; the complex reflection coefficient and complex transmission coefficient under the condition of front incidence are recorded as and The complex reflection coefficient and complex transmission coefficient under reverse incidence conditions are denoted as and ; S3. Place the inner shell sample at the end of the hydroacoustic tube and conduct acoustic tests on the inner shell sample under different frequency sound wave incidence to obtain the complex reflection coefficient of the inner shell sample under the condition that the sound wave is incident from the front of the inner shell. ; S4. Preset spacing between ships , the complex reflection coefficient of the double-layer shell structure with acoustic covering layer is calculated : ， ， ， In the formula, is the wave number in water, is the circular frequency of the incident sound wave, is the incident sound wave frequency, is the circumference of a circle, is the speed of sound in water, is an imaginary unit; S5. Based on the complex reflection coefficient Calculation of the sound absorption coefficient of the acoustic cover under double shell conditions .

2. The indirect test method for the sound absorption coefficient of an acoustic covering layer under double-layer shell conditions according to claim 1 is characterized in that: Get from S2 and The specific contents of the in-tube acoustic test include: The outer shell sample is placed in the hydroacoustic tube, with the front side of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back sides of the outer shell sample are in contact with water through the acoustic covering layer. The hydrophone test is used to obtain the acoustic waves of different frequencies. and .

3. The indirect test method for the sound absorption coefficient of an acoustic covering layer under double-layer shell conditions according to claim 1 is characterized in that: Get from S2 and The specific contents of the in-tube acoustic test include: The outer shell sample is placed in the hydroacoustic tube, with the back of the outer shell sample facing the position of the transducer in the hydroacoustic tube, and the front and back of the outer shell sample are in contact with water through the acoustic covering layer. The test results are as follows: and .

4. The indirect test method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition according to claim 1 is characterized in that: The specific contents of the pipe end acoustic test of the inner shell sample under different frequency sound wave incidence in S3 include: The inner shell sample is placed at the end of the hydroacoustic sound tube, with the front of the inner shell sample facing the position of the transducer in the hydroacoustic sound tube, and the front of the inner shell sample is in contact with water through the acoustic covering layer, and the back of the inner shell sample is in contact with the air at the end of the hydroacoustic sound tube, and the complex reflection coefficient of the inner shell sample is obtained under the condition that the sound wave is incident from the front of the inner shell. .

5. The indirect test method for the sound absorption coefficient of an acoustic covering layer under double-layer shell conditions according to claim 1 is characterized in that: Sound absorption coefficient The calculation method is: 。 6. The indirect test method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition according to claim 1 is characterized in that: A transducer is arranged at the opening of the hydroacoustic tube, and hydrophones are respectively arranged on the walls of the hydroacoustic tube, wherein two hydrophones are evenly arranged on both sides of the outer shell sample when conducting an in-tube acoustic test, and two hydrophones are arranged on the front side of the inner shell sample when conducting a pipe end acoustic test.

7. The indirect test method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition according to claim 1 is characterized in that: The outer shell sample and the inner shell sample are both steel discs.

8. The indirect test method for the sound absorption coefficient of an acoustic covering layer under a double-layer shell condition according to claim 1 is characterized in that: An internal acoustic structure is provided inside the acoustic cover.

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