Apparatus and method for measuring acoustic properties of a variable length back cavity

Abstract

The application provides a variable-length back cavity acoustic characteristic measuring device and method, relates to the technical field of acoustic characteristic measurement, and the device comprises a back cavity adjusting component, a measuring component and a data processing module; the back cavity adjusting component is used for providing a variable-length back cavity for a measured object and providing an unbiased flow measuring environment or a biased flow measuring environment for the measured object; the measuring component is used for generating an acoustic wave of a preset frequency and collecting sound pressure signals at different positions; and the data processing module processes the sound pressure signals at the different positions to obtain acoustic characteristic parameters of the measured object under the preset-length back cavity and the preset-frequency acoustic wave in the unbiased flow measuring environment or the biased flow measuring environment. The device has the advantages of high measurement precision and high automation level.

Description

Technical Field

[0001] This application relates to the field of acoustic property measurement technology, and in particular to an acoustic property measurement device and method for a variable-length back cavity. Background Technology

[0002] Acoustic structures such as perforated plates exhibit varying sound absorption or emission effects depending on the length of the back cavity and the presence or absence of flow deviation. Physically, this manifests as differences in acoustic characteristic parameters such as resonant frequency, acoustic impedance, and absorption coefficient. The resonant frequency can be measured using a microphone, while the acoustic impedance and absorption coefficient are typically measured using an impedance tube. An impedance tube is a long, tubular structure, typically square or circular in cross-section. A loudspeaker is placed on one side of the tube, and the object under test is placed on the other. The loudspeaker generates standing wave sound fields of different frequencies within the tube. By placing two or more microphones along the tube's axis near the object under test, changes in sound pressure within the tube can be collected. The acoustic impedance and absorption coefficient of the object under test can then be calculated.

[0003] Traditional impedance tube back cavity length adjustment typically uses manual adjustment, which cannot simultaneously provide convenient and precise adjustment. Each adjustment requires manual determination of the back cavity length. During research, it is often necessary to obtain acoustic parameters of acoustic structures such as perforated plates under different back cavity lengths. Due to the inconvenience of this adjustment method, measurements are usually only performed on a limited number of operating conditions, and measurements of continuous back cavity changes are often neglected. Furthermore, when the back cavity length changes, if deflected flow enters from the piston and is incident perpendicularly into the back cavity, it can cause insufficient flow development within the back cavity, leading to uneven deflected flow in the acoustic structure and introducing experimental errors. Simultaneously, during measurements, microphones are usually arranged in a straight line along the tube direction; however, when the deflected flow effect is significant, the resulting disturbances can affect the sound field within the tube, thus impacting the microphone measurement results. Summary of the Invention

[0004] In view of this, this application provides an acoustic characteristic measurement device and method for a variable-length back cavity to solve the above-mentioned technical problems.

[0005] In a first aspect, embodiments of this application provide an acoustic characteristic measurement device for a variable-length back cavity, comprising: a back cavity adjustment component, a measurement component, and a data processing module;

[0006] The back cavity adjustment component is used to provide a variable length back cavity for the object under test, and to provide a non-biased flow measurement environment or a biased flow measurement environment for the object under test.

[0007] The measuring component is used to generate sound waves of a preset frequency and collect sound pressure signals at different locations;

[0008] The data processing module is used to process the sound pressure signals at different locations to obtain the acoustic characteristic parameters of the object under test in a back cavity of a preset length and a sound wave of a preset frequency under a measurement environment with or without biased flow, or under a measurement environment with biased flow.

[0009] Furthermore, the back cavity adjustment component includes: a back cavity sleeve, a piston, a piston head, a push rod, an air source, an air inlet pipe, a stepper motor, and a stepper motor drive controller;

[0010] The back cavity sleeve is a cylindrical cavity, and the piston is set inside the cavity. The piston is cylindrical and its diameter is smaller than the inner diameter of the back cavity sleeve. The upper end face of the piston is connected to the air inlet pipe and the push rod that extend into the cavity. The lower end face of the piston is connected to the piston head. The air source is used to inject gas at a preset flow rate into the air inlet pipe.

[0011] The stepper motor is installed at the tail of the push rod. The stepper motor drive controller controls the stepper motor to rotate, thereby driving the push rod to extend and retract, causing the piston to move up and down along the axis of the back cavity sleeve.

[0012] Furthermore, a cover plate is fixed to the upper end face of the back cavity sleeve through a flange structure. The cover plate is provided with two holes, which allow the push rod and the air inlet pipe to pass through and enter the inner cavity of the back cavity sleeve, respectively.

[0013] Furthermore, the piston has a threaded through hole at its center, and the head of the air intake pipe is fitted with a pneumatic connector, which is connected to the central threaded through hole on the upper end face of the piston via threads; the head of the push rod is provided with multiple rotatable external threaded screws, which are connected to multiple threaded holes around the central threaded through hole on the upper end face of the piston.

[0014] Furthermore, the piston head is disc-shaped, with an external thread structure on its upper surface, which mates with the central threaded through hole on the lower end face of the piston.

[0015] Furthermore, when the object being measured is in a measurement environment without bias flow, the piston head is a non-air-intake piston head with a through hole in the center; when the object being measured is in a measurement environment with bias flow, the piston head is an air-intake piston head with multiple air inlets on the side wall of the piston head, which are connected to the central threaded through hole of the piston.

[0016] Furthermore, the measuring component includes: a measuring sleeve, four microphones, a speaker, and an exhaust sleeve;

[0017] Both the measuring sleeve and the exhaust sleeve are cylindrical cavities; the inner diameters of the measuring sleeve, the back cavity sleeve, and the exhaust sleeve are all the same.

[0018] The upper end face of the measuring sleeve and the lower end face of the back cavity sleeve are connected by their respective flange structures; the object to be measured is fixed at the connection between the measuring sleeve and the back cavity sleeve.

[0019] When the object being measured is in a measurement environment with biased flow, the lower end face of the measuring sleeve and the upper end face of the exhaust sleeve are connected by their respective flange structures; the lower end face of the exhaust sleeve and the loudspeaker are connected by their respective flange structures, and a sealing gasket is provided between them; two exhaust pipes extending outward are provided symmetrically on the side wall of the exhaust sleeve; when the object being measured is in a measurement environment without biased flow, the lower end face of the measuring sleeve and the loudspeaker are connected by their respective flange structures.

[0020] The loudspeaker is used to generate sound waves of different frequencies; three measuring holes are sequentially opened along the axial straight line on the outer wall of the measuring sleeve for installing a first microphone, a second microphone and a third microphone, wherein the first microphone is close to the upper end face of the measuring sleeve; a measuring hole is provided on the outer wall of the second microphone on the opposite side in the transverse direction for installing a fourth microphone.

[0021] Furthermore, the data processing module is used for:

[0022] Acquire the sound pressure signals P1, P2, P3, and P4 collected by four microphones.

[0023] Calculate the transfer function H of the first and second microphones. 12 :

[0024]

[0025] in, The initial transfer function between the first and second microphones:

[0026]

[0027] Among them, S 12 Let S be the cross-power spectral density of sound pressure signals P1 and P2 at the sound wave angular frequency ω. 11 Let P1 be the self-power spectral density of the sound pressure signal P1 at the sound wave angular frequency ω; where ω = 2πf, and f is the frequency of the sound wave generated by the loudspeaker.

[0028] C 1,2,4 The correlation coefficients for the first, second, and fourth microphones:

[0029]

[0030] Among them, D 24 The coherence coefficients of the second and fourth microphones:

[0031]

[0032] Among them, S 24S is the cross-power spectral density of sound pressure signals P2 and P4 at the sound wave angular frequency ω. 22 Let S be the self-power spectral density of the sound pressure signal P2 at the sound wave angular frequency ω. 44 Let P4 be the autopower spectral density of the sound pressure signal P4 at the sound wave angular frequency ω.

[0033] D 12 The coherence coefficients of the first and second microphones:

[0034]

[0035] D 42 The coherence coefficients of the fourth and second microphones:

[0036]

[0037] Among them, S 42 Let P4 be the cross power spectral density of sound pressure signal P4 and sound pressure signal P2 at the sound wave angular frequency ω.

[0038] Calculate the transfer function H of the third microphone and the second microphone. 32 ;

[0039] Establish the following overdetermined equations:

[0040]

[0041] Where L1, L2, and L3 are the distances between the first microphone and the object being measured, the second microphone and the object being measured, and the third microphone and the object being measured, respectively; k + Forward propagation wavenumber: k + =ω / (c+u), where c is the speed of sound at the current temperature, and u is the gas velocity inside the measuring sleeve. In a measurement environment without bias, u = 0; in a measurement environment with bias, u is a preset velocity. k - For negative propagation wavenumber: k - =ω / (cu); A *+ and A *- All are equivalent amplitudes;

[0042] By optimizing the above overdetermined equations, we can obtain A. *+ and A *- ;

[0043] The acoustic impedance value Z of the measured object is:

[0044]

[0045] The sound absorption coefficient α of the object being measured is:

[0046]

[0047] Secondly, embodiments of this application provide a method for measuring the acoustic characteristics of a variable-length back cavity, applied to the apparatus of this application, for measuring the acoustic characteristic parameters of a test object in a non-biased flow measurement environment, including:

[0048] Install the non-air-entry piston head on the piston; place the object to be measured at the connection between the measuring sleeve and the back cavity sleeve; connect the lower end face of the measuring sleeve to the speaker through their respective flange structures.

[0049] Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the back cavity of the preset length;

[0050] Control the speaker to generate sound waves at a preset frequency;

[0051] Acquire sound pressure signals from four microphones and send them to the data processing module;

[0052] The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test under a preset length of back cavity and preset frequency sound waves in a non-biased measurement environment.

[0053] Thirdly, embodiments of this application provide a method for measuring the acoustic characteristics of a variable-length back cavity, applied to the apparatus described in this application, for measuring the acoustic characteristic parameters of the object under test in a biased flow measurement environment, including:

[0054] Install the air intake piston head on the piston; place the object to be measured at the connection between the measuring sleeve and the back cavity sleeve; fix the air intake sleeve to the measuring sleeve and the speaker respectively;

[0055] Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the back cavity of the preset length;

[0056] Turn on the air source and inject gas at the preset flow rate into the air intake pipe;

[0057] Control the speaker to generate sound waves at a preset frequency;

[0058] Acquire sound pressure signals from four microphones and send them to the data processing module;

[0059] The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test in a biased measurement environment, under a back cavity of a preset length and a sound wave of a preset frequency.

[0060] The device described in this application has the advantages of high measurement accuracy and high level of automation. Attached Figure Description

[0061] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0062] Figure 1 A structural diagram of the acoustic characteristic measurement device for a variable-length back cavity provided in an embodiment of this application;

[0063] Figure 2 This is a structural diagram of the piston position provided in an embodiment of this application;

[0064] Figure 3 A flowchart of a method for measuring the acoustic characteristics of a variable-length back cavity provided in an embodiment of this application;

[0065] Figure 4 A flowchart illustrating the method for measuring the acoustic characteristics of a variable-length back cavity provided in this application embodiment.

[0066] icon:

[0067] 101-Back cavity sleeve; 102-Piston; 103-Piston head; 104-Push rod;

[0068] 105 - Intake pipe; 106 - Stepper motor; 107 - Stepper motor drive controller;

[0069] 108 - Measuring sleeve; 109 - First microphone; 110 - Second microphone;

[0070] 111 - Third microphone; 112 - Fourth microphone; 113 - Speaker;

[0071] 114 - Exhaust sleeve; 1031 - Intake piston head; 1032 - Non-intake piston head. Detailed Implementation

[0072] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0073] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0074] First, a brief introduction to the design concept of the embodiments of this application will be given.

[0075] Traditional impedance tube back cavity length adjustment is usually done manually, which cannot simultaneously provide convenient and precise adjustment. Therefore, this application provides an acoustic characteristic measurement device for a variable length back cavity, which can effectively solve the shortcomings of existing devices.

[0076] The device provided in this application controls the movement of a piston within the back cavity using a programmable stepper motor and push rod. This allows for the measurement of the piston's continuously changing position and precise, efficient adjustment of the back cavity length to measure the acoustic characteristics of cavities with different lengths. Installing the air inlet on the piston for lateral air intake avoids uneven flow at different locations within the acoustic structure. Deploying four microphones for sound pressure level monitoring eliminates the influence of internal flow on the measurement results, improving measurement accuracy.

[0077] The device described in this application has a simple structure, low cost, and high practicality, and can replace manually adjusted devices currently on the market. Furthermore, the device described in this application can significantly improve the measurement results and efficiency of acoustic characteristics of acoustic structures such as perforated plates; it has the advantages of high measurement accuracy and a high level of automation.

[0078] After introducing the application scenarios and design concepts of the embodiments of this application, the technical solutions provided by the embodiments of this application will be described below.

[0079] like Figure 1 As shown, this application provides an acoustic characteristic measuring device for a variable-length back cavity, including: a back cavity sleeve 101, a piston 102, a piston head 103, a push rod 104, an air source (not shown), an air inlet pipe 105, a stepper motor 106, a stepper motor drive controller 107, a measuring sleeve 108, a first microphone 109, a second microphone 110, a third microphone 111, a fourth microphone 112, a speaker 113, an exhaust sleeve 114, and an electronic device (not shown), on which a data processing module is provided.

[0080] The back cavity sleeve 101 is a cylindrical cavity, and the piston 102 is disposed in the cavity. The piston 102 is cylindrical, and its diameter is smaller than the inner diameter of the back cavity sleeve 101. The upper end face of the piston 102 is connected to the air inlet pipe 105 and the push rod 104 that extend into the back cavity sleeve 101, respectively. The lower end face of the piston 102 is threadedly assembled with the piston head 103. The air source is used to inject gas at a preset flow rate into the air inlet pipe 105.

[0081] The back cavity sleeve 101 can be made of materials such as stainless steel or acrylic. A fixing device is provided on the outer side wall of the back cavity sleeve 101 to prevent the entire device from moving due to piston movement. A cover plate is fixed to the upper end face of the back cavity sleeve 101 through a flange structure. The cover plate is provided with two holes for the push rod 104 and the air inlet pipe 105 to pass through and enter the inner cavity of the back cavity sleeve 101, respectively.

[0082] The piston 102 is cylindrical and has a certain gap with the inner wall of the back cavity sleeve 101. After installation, the end face of the piston 102 is parallel to the cross-section of the back cavity sleeve 101. A sealing ring is placed in the sealing groove of the side wall of the piston 102 to keep the back cavity sealed and prevent air leakage during piston movement. Nitrile O-rings are generally used as sealing rings.

[0083] The piston 102 has a threaded through hole in the center, and the piston head 103 is disc-shaped. The upper surface of the piston head 103 is provided with an external thread structure, which is connected to the central threaded through hole on the lower end face of the piston. During assembly, in order to prevent air leakage, raw material can be wrapped around the threaded mating area.

[0084] like Figure 2 As shown, the piston head 103 has two structures: an air-intake piston head 1031 and a non-air-intake piston head 1032, suitable for measurement environments with and without bias flow. The air-intake piston head 1031 has multiple air inlets on its sidewall, communicating with the central threaded hole of the piston. These openings in the sidewall of the air-intake piston head 1031 ensure more uniform flow when bias flow passes through the acoustic structure. The non-air-intake piston head 1032 has a central through-hole, ensuring communication with the outside environment during piston 102 movement and allowing for smooth piston 102 operation.

[0085] A pneumatic connector is installed at the head of the intake pipe 105, which is connected to the central threaded through hole on the upper end face of the piston 102 via threads; the head of the push rod 104 is provided with multiple rotatable external threaded screws, which are connected to multiple threaded holes around the central threaded hole on the upper end face of the piston 102.

[0086] A stepper motor 106 is mounted at the tail of the push rod 104. A stepper motor drive controller 107 controls the rotation of the stepper motor 106, thereby causing the push rod 104 to extend and retract, and causing the piston 102 to move up and down along the axis of the back cavity sleeve 101. The stepper motor 106 can rotate clockwise and counterclockwise, thereby moving the push rod 104 up and down, changing the position of the piston 102, and achieving the purpose of adjusting the length of the back cavity. The stepper motor drive controller 107 can adjust the speed and angle of the stepper motor 106 to control the movement trajectory of the piston 102.

[0087] Both the measuring sleeve 108 and the exhaust sleeve 114 are cylindrical cavities; the inner diameters of the measuring sleeve 108, the back cavity sleeve 101, and the exhaust sleeve 114 are the same; the measuring sleeve 108 and the exhaust sleeve 114 are made of stainless steel or acrylic, etc.

[0088] The upper end face of the measuring sleeve 108 and the lower end face of the back cavity sleeve 101 are connected by their respective flange structures. The object to be measured is fixed at the connection between the measuring sleeve and the back cavity sleeve. When the object to be measured is a perforated plate, it can be cut into a circular plate with the same diameter as the inner diameter of the measuring sleeve. Then, the object to be measured is clamped between the flange structure of the upper end face of the measuring sleeve 108 and the flange structure of the lower end face of the back cavity sleeve 101.

[0089] Three measuring holes are sequentially formed along the axial direction on the outer wall of the measuring sleeve 108 for mounting the first microphone 109, the second microphone 110, and the third microphone 111. A measuring hole is also provided on the outer wall of the measuring sleeve 108 on the radially opposite side of the second microphone 110 for mounting the fourth microphone 112. All four microphones continuously sample and record signals via a data acquisition card. The loudspeaker is used to generate sound waves of different frequencies.

[0090] When the object being measured is in a measurement environment with bias flow, the lower end face of the measuring sleeve 108 and the upper end face of the exhaust sleeve 114 are connected by their respective flange structures; the lower end face of the exhaust sleeve 114 and the speaker 113 are connected by their respective flange structures, and a sealing gasket is provided between them; two exhaust pipes are provided symmetrically on the side wall of the exhaust sleeve 114, which can exhaust air when bias flow is introduced; when the object being measured is in a measurement environment without bias flow, the lower end face of the measuring sleeve 108 and the speaker 113 are connected by their respective flange structures.

[0091] By adjusting the stepper motor drive controller 107, the stepper motor 106 can move at different speeds and in different ways, enabling the device to measure the acoustic characteristics of the acoustic structure under the dynamic effect of the back cavity. Furthermore, by setting the stepper motor drive controller 107, the piston 102 can be moved to a specified position to form a back cavity of a specified length, with a control accuracy of up to 0.1 mm for the back cavity length.

[0092] The data processing module is used to process sound pressure signals at different locations to obtain acoustic characteristic parameters of the object under test in a pre-defined cavity and under pre-defined frequency sound waves, whether under a biased or biased flow measurement environment. The specific steps are as follows:

[0093] Acquire the sound pressure signals P1, P2, P3, and P4 collected by four microphones.

[0094] Calculate the transfer function H of the first and second microphones. 12 :

[0095]

[0096] in, The initial transfer function between the first and second microphones:

[0097]

[0098] Among them, S 12 Let S be the cross-power spectral density of sound pressure signals P1 and P2 at the sound wave angular frequency ω. 11 Let P1 be the self-power spectral density of the sound pressure signal P1 at the sound wave angular frequency ω; where ω = 2πf, and f is the frequency of the sound wave generated by the loudspeaker.

[0099] C 1,2,4 The correlation coefficients for the first, second, and fourth microphones:

[0100]

[0101] Among them, D 24 The coherence coefficients of the second and fourth microphones:

[0102]

[0103] Among them, S 24 S is the cross-power spectral density of sound pressure signals P2 and P4 at the sound wave angular frequency ω. 22 Let S be the self-power spectral density of the sound pressure signal P2 at the sound wave angular frequency ω. 44 Let P4 be the autopower spectral density of the sound pressure signal P4 at the sound wave angular frequency ω.

[0104] D12 The coherence coefficients of the first and second microphones:

[0105]

[0106] D 42 The coherence coefficients of the fourth and second microphones:

[0107]

[0108] Among them, S 42 Let P4 be the cross power spectral density of sound pressure signal P4 and sound pressure signal P2 at the sound wave angular frequency ω.

[0109] The transfer functions H of the third and second microphones can be calculated using the same method described above. 32 ;

[0110] Establish the following overdetermined equations:

[0111]

[0112] Where L1, L2, and L3 are the distances between the first microphone and the object being measured, the second microphone and the object being measured, and the third microphone and the object being measured, respectively; k + Forward propagation wavenumber: k + =ω / (c+u), where c is the speed of sound at the current temperature, and u is the gas velocity inside the measuring sleeve. In a measurement environment without bias, u = 0; in a measurement environment with bias, u is a preset velocity. k - For negative propagation wavenumber: k - =ω / (cu); A *+ and A *- All are equivalent amplitudes;

[0113] By optimizing the above overdetermined equations, we can obtain A. *+ and A *- ;

[0114] The acoustic impedance value Z of the measured object is:

[0115]

[0116] The sound absorption coefficient α of the object being measured is:

[0117]

[0118] Based on the above-mentioned device, such as Figure 3 As shown, this application provides a method for measuring the acoustic characteristics of a variable-length back cavity, used to measure the acoustic characteristic parameters of a test object in a non-biased flow measurement environment, including:

[0119] Step 201: Install the non-air-entry piston head on the piston; fix the object to be measured at the connection between the measuring sleeve and the back cavity sleeve; connect the lower end face of the measuring sleeve to the speaker through their respective flange structures;

[0120] Step 202: Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the preset length of the back cavity;

[0121] Step 203: Control the speaker to generate sound waves of a preset frequency;

[0122] Step 204: Acquire the sound pressure signals collected by the four microphones and send them to the data processing module;

[0123] Step 205: The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test in a non-biased measurement environment, under the conditions of a back cavity of a preset length and a sound wave of a preset frequency.

[0124] Based on the above-mentioned device, such as Figure 4 As shown, this application provides a method for measuring the acoustic characteristics of a variable-length back cavity, used to measure the acoustic characteristic parameters of an object under test in a biased flow measurement environment, including:

[0125] Step 301: Install the air intake piston head onto the piston; fix the object to be measured at the connection between the measuring sleeve and the back cavity sleeve; fix the air intake sleeve to the measuring sleeve and the speaker respectively;

[0126] Step 302: Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the preset length of the back cavity;

[0127] Step 303: Turn on the air source and inject gas at a preset flow rate into the air inlet pipe;

[0128] Step 304: Control the speaker to generate sound waves of a preset frequency;

[0129] Step 305: Acquire the sound pressure signals collected by the four microphones and send them to the data processing module;

[0130] Step 306: The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test in a biased measurement environment, under the sound waves of a preset length and a preset frequency.

[0131] Furthermore, although the operations of the method of this application are described in a specific order in the accompanying drawings, this does not require or imply that these operations must be performed in that specific order, or that all the operations shown must be performed to achieve the desired result. Additionally or alternatively, certain steps may be omitted, multiple steps may be combined into one step, and / or one step may be broken down into multiple steps.

[0132] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0133] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A device for measuring the acoustic characteristics of a variable-length back cavity, characterized in that, include: Back cavity adjustment components, measuring components, and data processing module; The back cavity adjustment component is used to provide a variable length back cavity for the object under test, and to provide a non-biased flow measurement environment or a biased flow measurement environment for the object under test. The measuring component is used to generate sound waves of a preset frequency and collect sound pressure signals at different locations; The data processing module is used to process the sound pressure signals at different locations to obtain the acoustic characteristic parameters of the object under test under a preset length of back cavity and a preset frequency of sound waves, whether under a bias-free or bias-flow measurement environment. The measuring component includes a measuring sleeve and four microphones. Three measuring holes are sequentially formed along the axial direction on the outer wall of the measuring sleeve for mounting the first microphone, the second microphone, and the third microphone, wherein the first microphone is located near the upper end face of the measuring sleeve. A measuring hole is provided on the outer wall of the second microphone on the opposite side along the lateral direction for mounting the fourth microphone. The data processing module is used for: Acquire sound pressure signals from four microphones and , Calculate the transfer functions of the first and second microphones. : in, The initial transfer function between the first and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at; where, , The frequency at which the loudspeaker produces sound waves; The correlation coefficients for the first, second, and fourth microphones: in, The coherence coefficients of the second and fourth microphones: ， in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at that location, Sound pressure signal At the angular frequency of sound waves The self-power spectral density at the location; The coherence coefficients of the first and second microphones: The coherence coefficients of the fourth and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at; Calculate the transfer functions of the third and second microphones. ; Establish the following overdetermined equations: in, , , These are the distances between the first microphone and the object being measured, the distances between the second microphone and the object being measured, and the distances between the third microphone and the object being measured, respectively. Forward propagation wavenumber: , The speed of sound at the current temperature, To measure the gas velocity inside the sleeve, under a non-biased flow measurement environment, In a biased flow measurement environment, The preset flow rate; For negative propagation wavenumber: ; and All are equivalent amplitudes; The optimal solution to the above overdetermined equations can be obtained as follows: and ; Then the acoustic impedance value of the object being measured for: Sound absorption coefficient of the object being tested for: 。 2. The acoustic characteristic measuring device for a variable-length back cavity according to claim 1, characterized in that, The back cavity adjustment component includes: a back cavity sleeve, a piston, a piston head, a push rod, an air source, an air inlet pipe, a stepper motor, and a stepper motor drive controller; The back cavity sleeve is a cylindrical cavity, and the piston is set inside the cavity. The piston is cylindrical and its diameter is smaller than the inner diameter of the back cavity sleeve. The upper end face of the piston is connected to the air inlet pipe and the push rod that extend into the cavity. The lower end face of the piston is connected to the piston head. The air source is used to inject gas at a preset flow rate into the air inlet pipe. The stepper motor is installed at the tail of the push rod. The stepper motor drive controller controls the stepper motor to rotate, thereby driving the push rod to extend and retract, causing the piston to move up and down along the axis of the back cavity sleeve.

3. The acoustic characteristic measuring device for a variable-length back cavity according to claim 2, characterized in that, The upper end face of the back cavity sleeve is fixed with a cover plate by a flange structure. The cover plate is provided with two holes for the push rod and the air inlet pipe to pass through and enter the inner cavity of the back cavity sleeve, respectively.

4. The acoustic characteristic measuring device for a variable-length back cavity according to claim 3, characterized in that, The piston has a threaded through hole in the center, and the head of the air intake pipe is equipped with a pneumatic connector, which is connected to the central threaded through hole on the upper end face of the piston through a thread; the head of the push rod is provided with multiple rotatable external threaded screws, which are connected to multiple threaded holes around the central threaded through hole on the upper end face of the piston.

5. The acoustic characteristic measuring device for a variable-length back cavity according to claim 4, characterized in that, The piston head is disc-shaped, with an external thread structure on its upper surface, which mates with the central threaded through hole on the lower end face of the piston.

6. The acoustic characteristic measuring device for a variable-length back cavity according to claim 5, characterized in that, When the object being measured is in a measurement environment without bias flow, the piston head is a non-air-intake piston head with a through hole in the center; when the object being measured is in a measurement environment with bias flow, the piston head is an air-intake piston head with multiple air inlets on the side wall of the piston head, which are connected to the central threaded through hole of the piston.

7. The acoustic characteristic measuring device for a variable-length back cavity according to claim 6, characterized in that, The measuring components also include: a speaker and an exhaust sleeve; Both the measuring sleeve and the exhaust sleeve are cylindrical cavities; the inner diameters of the measuring sleeve, the back cavity sleeve, and the exhaust sleeve are all the same. The upper end face of the measuring sleeve and the lower end face of the back cavity sleeve are connected by their respective flange structures; the object to be measured is fixed at the connection between the measuring sleeve and the back cavity sleeve. When the object being measured is in a measurement environment with biased flow, the lower end face of the measuring sleeve and the upper end face of the exhaust sleeve are connected by their respective flange structures; the lower end face of the exhaust sleeve and the loudspeaker are connected by their respective flange structures, and a sealing gasket is provided between them; two exhaust pipes extending outward are provided symmetrically on the side wall of the exhaust sleeve; when the object being measured is in a measurement environment without biased flow, the lower end face of the measuring sleeve and the loudspeaker are connected by their respective flange structures. The loudspeaker is used to generate sound waves of different frequencies.

8. A method for measuring the acoustic characteristics of a variable-length back cavity, applied to the device described in claim 7, for measuring the acoustic characteristic parameters of a test object under a bias-free flow measurement environment, characterized in that, include: Install a non-air-intake piston head on the piston; The object to be measured is fixed at the connection between the measuring sleeve and the back cavity sleeve; the lower end face of the measuring sleeve is connected to the speaker through their respective flange structures. Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the back cavity of the preset length; Control the speaker to generate sound waves at a preset frequency; Acquire sound pressure signals from four microphones and send them to the data processing module; The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test in a back cavity of a preset length and under sound waves of a preset frequency under a bias-free measurement environment. Acquire sound pressure signals from four microphones and , Calculate the transfer functions of the first and second microphones. : in, The initial transfer function between the first and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at; where, , The frequency at which the loudspeaker produces sound waves; The correlation coefficients for the first, second, and fourth microphones: in, The coherence coefficients of the second and fourth microphones: ， in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at that location, Sound pressure signal At the angular frequency of sound waves The self-power spectral density at the location; The coherence coefficients of the first and second microphones: The coherence coefficients of the fourth and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at; Calculate the transfer functions of the third and second microphones. ; Establish the following overdetermined equations: in, , , These are the distances between the first microphone and the object being measured, the distances between the second microphone and the object being measured, and the distances between the third microphone and the object being measured, respectively. Forward propagation wavenumber: , The speed of sound at the current temperature, To measure the gas velocity inside the sleeve, under a non-biased flow measurement environment, In a biased flow measurement environment, The preset flow rate; For negative propagation wavenumber: ; and All are equivalent amplitudes; The optimal solution to the above overdetermined equations can be obtained as follows: and ; Then the acoustic impedance value of the object being measured for: Sound absorption coefficient of the object being tested for: 。 9. A method for measuring the acoustic characteristics of a variable-length back cavity, applied to the device described in claim 7, for measuring the acoustic characteristic parameters of a test object under a biased flow measurement environment, characterized in that, include: Install the intake piston head on the piston; Fix the object to be measured at the connection between the measuring sleeve and the back cavity sleeve; fix the exhaust sleeve to the measuring sleeve and the speaker respectively; Adjust the stepper motor drive controller to control the stepper motor to rotate, and push the piston to reach the preset position, thereby achieving the back cavity of the preset length; Turn on the air source and inject gas at the preset flow rate into the air intake pipe; Control the speaker to generate sound waves at a preset frequency; Acquire sound pressure signals from four microphones and send them to the data processing module; The data processing module processes the sound pressure signals collected by the four microphones to obtain the acoustic characteristic parameters of the object under test in a biased current measurement environment, under a back cavity of a preset length and a sound wave of a preset frequency. Acquire sound pressure signals from four microphones and , Calculate the transfer functions of the first and second microphones. : in, The initial transfer function between the first and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at; where, , The frequency at which the loudspeaker produces sound waves; The correlation coefficients for the first, second, and fourth microphones: in, The coherence coefficients of the second and fourth microphones: ， in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at , Sound pressure signal At the angular frequency of sound waves The self-power spectral density at that location, Sound pressure signal At the angular frequency of sound waves The self-power spectral density at the location; The coherence coefficients of the first and second microphones: The coherence coefficients of the fourth and second microphones: in, Sound pressure signal With sound pressure signal At the angular frequency of sound waves Cross-power spectral density at; Calculate the transfer functions of the third and second microphones. ; Establish the following overdetermined equations: in, , , These are the distances between the first microphone and the object being measured, the distances between the second microphone and the object being measured, and the distances between the third microphone and the object being measured, respectively. Forward propagation wavenumber: , The speed of sound at the current temperature, To measure the gas velocity inside the sleeve, under a non-biased flow measurement environment, In a biased flow measurement environment, The preset flow rate; For negative propagation wavenumber: ; and All are equivalent amplitudes; The optimal solution to the above overdetermined equations can be obtained as follows: and ; Then the acoustic impedance value of the object being measured for: Sound absorption coefficient of the object being tested for: 。

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