Acoustic flow tube test platform and method of testing thereof

Abstract

The present application belongs to the technical field of acoustic measurement, and particularly relates to an acoustic flow tube test platform and a test method thereof, wherein the transfer loss measurement of an acoustic metasurface under a flow field working condition is performed through the acoustic flow tube test platform, the test piece installation tube of the acoustic flow tube test platform can install most of the test pieces of the acoustic metasurface, and is beneficial to the acoustic metasurface related technology research; meanwhile, the test platform creates a flow acoustic measurement environment, based on the scattering matrix theory, the reference signal, the self-spectrum and the cross-spectrum signal processing method are used to weaken the influence of the turbulent flow pressure disturbance, the double load method is used to solve the parameters of the scattering matrix and calculate the acoustic characteristics of the acoustic metasurface to be measured, and the transfer loss measurement problem of the acoustic metasurface under the low Mach number (<0.3 Mach) tangential flow and in the low-middle frequency range (300-2000 Hz) is solved, and the technology will be widely used in the fields requiring noise reduction such as unmanned aerial vehicles, eVTOL, household appliances, automobiles and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of acoustic measurement, and particularly relates to an acoustic flow tube test platform and a test method thereof. BACKGROUND

[0002] Acoustic metasurface has attracted extensive attention in recent years due to its ability to manipulate the phase, size and direction of sound waves in space and frequency, and its strong designability. With the advancement of practical application research of acoustic metasurface, more complex working conditions need to be considered. Among them, the flow field condition is inevitable, and the introduction of flow field influence will greatly increase the complexity of predicting and evaluating the acoustic characteristics of acoustic metasurface. It is particularly important to provide basic acoustic information of the flow acoustic metasurface / material through experimental measurement, in order to verify the feasibility of non-accurate analysis / numerical prediction, and evaluate the overall performance of the system configuration.

[0003] Common acoustic measurement methods include reverberation chamber method, far-field method and acoustic impedance tube method. Since a specific flow field condition requires a reasonable aerodynamic layout, the acoustic impedance tube, which is essentially a fluid pipe, can meet the ventilation demand through ingenious design, and this method can measure the acoustic information at the front and back of the acoustic metasurface to be measured.

[0004] For acoustic research under flow conditions, there are some mature flow tube acoustic experimental platforms designed abroad. The flow tube acoustic experimental platform built by NASALaSR in 2009 uses a rectangular pipe with a cross section of 50.8x63.5mm, the flow field in the pipe is provided by a negative fan (0-0.6 Mach), the sound field is generated by 18 array loudspeakers (frequency range 0.4-3kHz, single frequency up to 150 decibels), and a variable cavity depth local resonance silencer is arranged at the end of the pipe to simulate the non-reflective outlet boundary condition. This device is mainly based on the target function method, and the acoustic impedance characteristics of the test piece are accurately measured by means of 53 microphones installed on the opposite inner wall of the test piece; the cross section of the flow tube acoustic experimental platform of the German Aerospace Center (DLR) is 80x80mm, compared with NASALaSR, the maximum Mach number in the pipe is 0.3, and a single frequency sound of 120dB can be generated in the frequency range of 0.21-2.11kHz, 5 microphones arranged in an exponential pattern are used to measure the scattering matrix parameters, and 10 microphones with the same interval are arranged on the opposite inner wall of the test piece to measure the acoustic impedance characteristics, and similar devices are also available at the Royal Institute of Technology (KTH) in Sweden; the flow tube acoustic experimental platform of the French Aerospace Institute (ONERA) considers the influence of air temperature, and the highest controllable temperature is 570K; there are also similar researches in China, and Beijing University of Aeronautics and Astronautics designs a flow tube acoustic experimental platform, which is based on Prony method, uses 16 evenly distributed microphones to measure the acoustic impedance characteristics in the range of 0-0.2 Mach and 0.4-3kHz, and uses 4 microphones arranged in a group to measure the sound pressure information upstream and downstream of the test piece and calculate the transmission loss.

[0005] However, the above flow tube acoustic experimental platform focuses on the impedance characteristics of the test piece under compressible flow field and high sound intensity, and is mainly aimed at the nonlinear acoustic extreme application places such as aerospace. However, most of the current flow-acoustic super surface research working conditions are linear acoustics for actual applications, considering incompressible flow and low sound intensity. At the same time, most of the above devices do not consider the measurement of important acoustic parameters (transmission loss). In addition, due to the need to measure the impedance characteristics under high sound intensity, a large number of microphones, high-power loudspeakers and wind tunnels are used, and the experimental cost is high. SUMMARY

[0006] In order to solve the above technical problems, the present application provides an acoustic flow tube test platform and a test method thereof to complete the transmission loss measurement of the acoustic super surface under the flow field condition.

[0007] The acoustic flow tube test platform of the present application comprises a support frame and an acoustic flow tube mounted on the support frame, wherein the acoustic flow tube comprises a test piece mounting tube in the middle, the left end of the test piece mounting tube is connected in sequence with a sound field measurement tube, a sound source tube, a flow field measurement tube and a muffler tube, and the end of the muffler tube is connected with an upstream fan; the right end of the test piece mounting tube is also connected in sequence with a sound field measurement tube, a flow field measurement tube and a muffler tube, and the end of the muffler tube is connected with a downstream fan.

[0008] The acoustic flow tube is a square cross-section waveguide tube, and the pipes are connected through flanges.

[0009] The square cross-section waveguide tube of the muffler tube is provided with a partition plate on the inner and outer sides, and a gradient back plate is mounted outside the partition plate.

[0010] The partition plate is a micro-perforated plate or a solid plate.

[0011] The flow field measurement tube is provided with a Pitot tube insertion hole on the surface of the central axis for collecting the flow field information in the pipe.

[0012] A through hole is formed in the center of the upper surface of the sound source tube, a perforated plate and a metal mesh are mounted in the through hole from top to bottom in sequence, and a loudspeaker is connected to the top of the through hole.

[0013] Three microphone insertion holes are arranged on the central axis of the upper surface of the sound field measurement tube in the axial direction for mounting microphones, and the receiving sound signal end of the microphone is flush with the inner wall of the pipe.

[0014] The upper surface of the test piece mounting tube is a pipe wall-free slot for mounting the acoustic super surface.

[0015] The present application also provides a test method for the above acoustic flow tube test platform, which comprises the following steps:

[0016] S1: Perform microphone amplitude and phase calibration, install microphones on the surface of the sound field measurement tube, install the test piece on the test piece installation tube, and then connect the microphone measurement system and the computer;

[0017] S2: Turn on the fan and adjust the flow rate, insert the pitot tube into the pitot tube hole on the flow field measurement tube, and calculate the average flow rate and velocity profile of the two end flow field measurement tube sections;

[0018] S3: Control the speaker on the sound source tube to emit sound, insert the micro-perforated plate at the muffler tube, and start the experimental measurement;

[0019] S4: Remove the micro-perforated plate and replace it with a solid plate, and start the experimental measurement again;

[0020] S5: Obtain the test piece transmission loss curve and save the relevant data.

[0021] The beneficial effects of the present application are:

[0022] Compared with the prior art, the present application measures the transmission loss of the acoustic super surface under the flow field working condition through the constructed acoustic flow tube test platform, and the test piece installation tube of the acoustic flow tube test platform can install most of the test pieces of the acoustic super surface, which is conducive to the research of the acoustic super surface related technology. At the same time, the test platform creates a flow acoustic measurement environment, based on the scattering matrix theory, with the help of reference signal, autospectrum and cross-spectrum signal processing method to weaken the influence of turbulent pressure disturbance, and adopts double load method to solve the parameters of scattering matrix and calculate the acoustic characteristics of the measured acoustic super surface, transmission loss (proportional relationship between input energy and output energy, which can represent energy loss), solve the transmission loss measurement problem of acoustic super surface under low Mach number (<0.3 Mach) tangential flow in low frequency range (300-2000Hz), and the technology will be widely used in the fields of unmanned aerial vehicles, eVTOL, household appliances, automobiles and other noise reduction needs. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed for the embodiment description will be briefly introduced as follows.

[0024] Figure 1 It is the overall schematic diagram of the acoustic flow tube test platform in the embodiment;

[0025] Figure 2 It is the explosion schematic diagram of the muffler tube in the embodiment;

[0026] Figure 3 It is the noise reduction result schematic diagram of the muffler tube in the embodiment;

[0027] Figure 4 It is the flow field measurement tube schematic diagram in the embodiment;

[0028] Figure 5 Figure 1 is a schematic diagram of the sound source tube in the embodiment;

[0029] Figure 6 Figure 2 is a sectional view of the sound source tube in the embodiment;

[0030] Figure 7 Figure 3 is a schematic diagram of the sound field measurement tube in the embodiment;

[0031] Figure 8 Figure 4 is a schematic diagram of the test piece installation tube in the embodiment;

[0032] Figure 9 Figure 5 is a schematic diagram of the measurement principle in the embodiment;

[0033] Figure 10 Figure 6 is a flow chart of the measurement in the embodiment.

[0034] 1 - support frame, 2 - upstream fan, 3 - sound attenuation tube, 301 - square cross-section waveguide tube, 302 - baffle, 303 - gradient backplate, 4 - flow field measurement tube, 401 - Pitot tube socket, 5 - sound source tube, 501 - through hole, 502 - perforated plate, 503 - wire mesh, 6 - sound field measurement tube, 601 - microphone socket, 7 - test piece installation tube, 8 - downstream fan, 9 - loudspeaker, 10 - flange. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application.

[0036] Embodiment 1

[0037] Referring to Figure 1 As shown in the figure, the acoustic flow tube test platform comprises a support frame 1 and an acoustic flow tube installed on the support frame 1. The acoustic flow tube comprises a middle test piece installation tube 7. The left end of the test piece installation tube 7 is sequentially connected with an upstream sound field measurement tube 6, an upstream sound source tube 5, an upstream flow field measurement tube 4 and an upstream sound attenuation tube 3. The end of the upstream sound attenuation tube 3 is connected with an upstream fan 2. The right end of the test piece installation tube 7 is also sequentially connected with a downstream sound field measurement tube 6, a downstream flow field measurement tube 4 and a downstream sound attenuation tube 3. The end of the downstream sound attenuation tube 3 is connected with a downstream fan 8.

[0038] The acoustic flow tube is a square cross-section waveguide tube 301 with an inner wall of 80x80mm and a wall thickness of 20mm. There is a uniform flow of u=0~90m / s in the tube. The sound source emits white noise. In order to reduce the measurement method error, the white noise emitted by the sound source is required to propagate in the tube in the form of a plane wave only. Therefore, the side length a is determined by the waveguide cutoff frequency f c Determination:

[0039]

[0040] Wherein, c0=343m / s is the sound speed of air at 20℃, M0=u / c0 represents the Mach number. When the maximum measurement frequency (i.e. the cut-off frequency) is designed to be 2000Hz, the maximum side length of the pipe satisfying the design requirement can be obtained from formula (1) as 81.8mm, and the present application selects a=80mm.

[0041] The flanges 10 and concave-convex grooves are arranged at both ends of each section of the acoustic flow pipe, and are used for connection and positioning with other pipes.

[0042] In order to ensure the measurement accuracy, the inner wall of the pipe satisfies the rigid condition, and at the same time, the sound wave leakage amount can be reduced, and generally, the pipe wall needs to have a large area density. In the present application, the thickness of the acrylic material used in each section of the acoustic flow pipe is t=20mm, and the commonly used acrylic density is 1190kg / m 3 , and according to the selected cross-sectional side length, the area density is m=5.95kg / m 2 . According to the mass law [7] , the minimum sound insulation amount of the pipe in the range of 300-2000Hz can be estimated as:

[0043] TL=20lgmf-42.5≈22.5-39dB (2)

[0044] The cross-sectional side length a of each subsequent section is designed to be 80mm, and the thickness t is designed to be 20mm.

[0045] ①Silencer 3

[0046] During the measurement process, the environmental noise and the wind source noise have a certain influence on the measurement results, and the silencer 3 can be installed at the inlet and outlet of the acoustic flow pipe. At the same time, the measurement principle scattering matrix is solved by the double load method, and two kinds of loads with large impedance difference are required to be installed at the end of the acoustic flow pipe as different working conditions. The present application creates two load conditions with / without silencers.

[0047] Based on this, as shown in Figure 1 , two silencers 3 are prepared and arranged at the inlet and outlet of the acoustic flow pipe. As shown in Figure 2 , the square cross-section waveguide pipe of the silencer 3 is provided with a baffle plate 302 on the inner and outer sides, and a gradient back plate 303 is installed outside the baffle plate 302; the baffle plate 302 is a micro-perforated plate or a solid plate, and the micro-perforated plate and the solid plate can be directly inserted into or extracted from the silencing section, and have the ability to switch with / without silencing. When the micro-perforated plate is used, due to the large acoustic resistance and the connection between the pipe and the gradient back cavity, an acoustic black hole is coupled, and a large range of noise reduction can be achieved. This is Figure 3The results are verified; when the solid plate is used, the rigid wall is formed, the sound wave continues to propagate along the pipeline axis, and almost no sound energy is attenuated. The application adopts a switchable design, which not only meets certain noise reduction requirements, but also provides conditions for two different working conditions of the double load method.

[0048] ②Flow field measurement pipe 4

[0049] The acoustic flow pipe contains two flow field measurement sections with a length of 500 mm, which are located at the rear end of the upstream muffler 3 and the front end of the downstream muffler 3, respectively. As shown in Figure 4 , the surface of the axis of the flow field measurement pipe 4 is provided with a pitot tube insertion hole 401, which is used to collect the flow field information in the pipeline, such as the center flow velocity and the velocity type.

[0050] ③Sound source pipe 5

[0051] As shown in Figure 5 , a 300 mm long sound source section is designed to install a BMS-4592nd loudspeaker 9, and a through hole 501 with a diameter comparable to the diaphragm of the loudspeaker 9 is arranged at the center of the upper surface to allow sound waves to enter the pipeline. Generally, when the sound power generated by the loudspeaker 9 is much larger than the background noise, the background noise can be ignored. The application works in a soundproof room, and the background noise is lower than 30 dB, while the incident sound wave of the loudspeaker 9 is set to 94 dB, which can meet the measurement requirements.

[0052] Due to the thickness of the pipe wall, the installation of the loudspeaker 9 will form a cylindrical cavity. The uniform flow in the pipeline through the cavity may produce flow-induced noise, which will have a certain impact on the sound field measurement. As shown in Figure 6 , the application designs a perforated plate 502 and a wire mesh 503 coupling structure with small acoustic resistance, which can make the uniform flow in the pipeline flow smoothly without disturbance, and ensure that most of the sound energy can pass through the structure with good sound transmission. Among them, the perforated plate 502 is used to support the wire mesh 503 to reduce the vibration of the wire mesh 503 in the flow field.

[0053] ④Sound field measurement pipe 6

[0054] The scattering matrix principle adopted by the application requires four microphones, two of which are installed upstream and downstream of the test piece installation pipe 7 to collect the sound field information before and after the test piece. Based on this, two identical sound field measurement pipes 6 are designed. As shown in Figure 7 , three front, middle and rear microphone insertion holes 601 are arranged on the axis of the sound field measurement pipe 6, which are 30 mm apart along the axis, which is convenient for installing microphones and making the receiving sound signal end of the microphone flush with the inner wall of the pipeline. Generally, when measuring, two microphones are installed in the front and rear microphone insertion holes 601 to measure the sound field information.

[0055] The distance between microphones affects the measurement accuracy. To avoid large sensitivity errors, the sensor spacing l needs to satisfy the following mathematical relationship:

[0056]

[0057] Right now

[0058] Formula (4) shows that the larger the microphone spacing, the lower the upper and lower frequency limits, and vice versa. Considering that the designed measurement frequency upper limit is 2000Hz, and the uniform flow velocity is u=0~90m / s, it can be obtained from formula (4) that l<62.43mm. In order to ensure a wider measurement range, the microphone spacing l cannot be too small. The present invention selects l=60mm, that is, the front and rear microphone jacks 601 are used. The corresponding more accurate measurement frequency range is: 285.83~2286.67Hz when there is no flow; 260.11~2080.67Hz when u=90m / s, and 300~2000Hz is used as the measurement range of the present invention.

[0059] Furthermore, in actual pipe installations, gaps may exist at joints, causing sound scattering and generating multiple acoustic modes, which can affect the plane wave signals collected by the microphone. The pipe cross-section design takes into account that at frequencies not exceeding 2000 Hz, sound waves propagate as plane waves within the pipe. Therefore, a sufficiently long 80×80 mm pipe is sufficient to effectively attenuate multiple acoustic modes. Based on the design standard for impedance tubes, which requires a distance between the microphone and the sound source greater than three times the pipe side length and a distance from the test piece greater than two times the side length, the present invention selects a 700 mm length for the sound field measurement tube 6 to meet these requirements.

[0060] ⑤Test piece installation tube 7

[0061] like Figure 8 As shown, a 500 mm long wall-free surface is reserved on the upper surface of the test piece mounting tube 7, which can meet the installation requirements of most acoustic metasurfaces. At the same time, the longer distance can also weaken the multi-order modes caused by the scattering effect of the acoustic metasurface, ensuring measurement accuracy.

[0062] Working principle: Figure 9 As shown, when there is tangential flow and white noise in the acoustic flow tube, the present invention collects the sound pressure information of two points upstream and two points downstream of the test piece under two different end load conditions through a microphone measurement system (double load method), solves the element values ​​of the scattering matrix, and calculates the transmission loss of the test piece from the scattering matrix elements.

[0063] Assume that there are uniform flow and plane wave along the axial direction in the pipe, and the subscripts 'i' and 'o' represent the sound wave incident end and the outgoing end respectively, '+' and '-' represent the same and opposite direction of the uniform flow respectively, the superscripts '' and '' represent the end impedance Z' and Z" respectively, '*' represents complex conjugate, and '^' represents data in the frequency domain. The transmission loss formula is as follows:

[0064]

[0065] Where S represents the cross-sectional area of the pipe, p represents the density of the flow medium in the pipe, c represents the sound velocity, M represents the Mach number, S 21 is an element of the scattering matrix, which can be represented as:

[0066]

[0067] In the formula, H represents the transfer function with the positive sound pressure at the i port as the reference sound pressure, and the autospectrum G x·x and the cross spectrum G x·y are used to weaken the influence of turbulent pressure disturbance, and the data is also averaged to reduce data fluctuations when solving

[10] . Wherein, each p value is solved by the row wave formula:

[0068]

[0069] Where p1-p4 correspond to the sound pressure data measured by microphones 1-4 respectively. k + and k - represent the wave vectors in the same and opposite directions of the uniform flow, which can be solved by the following formula:

[0070]

[0071] Where k0=ω / c0 is the free field wave number, D is the ratio of the cross-sectional area to the perimeter of the pipe, μ is the dynamic viscosity of the flow medium (Pa·s), γ is the adiabatic constant, Pr is the Prandtl number, Re is the Reynolds number, and ψ is the turbulent friction coefficient.

[0072] Therefore, the sound pressure values measured by the four microphones under different end load conditions are substituted into formulas (5)-(8) to obtain the transmission loss of the test piece.

[0073] Referring to Figure 10 , a test method of an acoustic flow pipe test platform, which comprises the following steps:

[0074] S1: Perform microphone amplitude and phase calibration, install microphones on the surface of the sound field measurement pipe 6, install a test piece on the test piece installation pipe 7, and then connect the microphone measurement system and the computer;

[0075] S2: Turn on the fan and adjust the flow rate, insert the pitot tube into the pitot tube hole 401 on the flow field measuring tube 4 to measure and calculate the average flow rate and velocity profile of the two end flow field measuring tube 4 cross section;

[0076] S3: Control the loudspeaker 9 on the sound source tube 5 to emit sound, insert the micro-perforated plate at the sound absorbing tube 3, and start the experimental measurement;

[0077] S4: Remove the micro-perforated plate and replace it with a solid plate, and start the experimental measurement again;

[0078] S5: Obtain the transmission loss curve of the test piece and save the relevant data.

[0079] The preferred embodiments of the application disclosed above are only used to help illustrate the application. The preferred embodiments do not describe all the details and do not limit the application to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of the specification.

Claims

1. An acoustic flow tube test platform, characterized by, The application relates to an acoustic flow pipe comprising a support frame (1) and an acoustic flow pipe installed on the support frame (1), wherein the acoustic flow pipe comprises a middle test piece installation pipe (7), the left end of the test piece installation pipe (7) is sequentially connected with a sound field measurement pipe (6), a sound source pipe (5), a flow field measurement pipe (4) and a muffling pipe (3), the end of the muffling pipe (3) is connected with an upstream fan (2), the right end of the test piece installation pipe (7) is sequentially connected with the sound field measurement pipe (6), the flow field measurement pipe (4) and the muffling pipe (3), the end of the muffling pipe (3) is connected with a downstream fan (8), and the inside and outside of the square cross-section waveguide pipe of the muffling pipe (3) is provided with a baffle (302), the baffle (302) is a perforated plate (502) or a solid plate, and a gradient back plate (303) is arranged outside the baffle (302). A through hole (501) is formed in the center of the upper surface of the sound source pipe (5), a perforated plate (502) and a metal wire mesh (503) are sequentially arranged on the bottom of the through hole (501) from top to bottom, and a loudspeaker (9) is connected to the top of the through hole (501).

2. The acoustic flow tube test platform of claim 1, wherein, The acoustic flow pipe is a square cross-section waveguide pipe, and the pipes are connected through flanges (10).

3. The acoustic flow tube test platform of claim 1, wherein, A pitot tube insertion hole (401) is arranged on the upper surface of the flow field measurement pipe (4) on the central axis, and is used for collecting flow field information in the pipe.

4. The acoustic flow tube test platform of claim 1, wherein, Three microphone insertion holes (601) are arranged on the upper surface of the sound field measurement pipe (6) on the central axis in the axial direction, and are used for mounting microphones and making the sound signal receiving ends of the microphones flush with the inner wall of the pipe.

5. The acoustic flow tube test platform of claim 1, wherein, The upper surface of the test piece installation pipe (7) is a pipe wall-free groove, and is used for mounting an acoustic super surface.

6. The method of testing an acoustic flow tube test platform of any of claims 1-5, wherein, The application further discloses a test method comprising the following steps: S1: performing microphone amplitude and phase calibration, mounting microphones on the upper surface of the sound field measurement pipe (6), mounting test pieces on the test piece installation pipe (7), and connecting a microphone measurement system and a computer; S2: starting the fan and adjusting the flow rate, inserting a pitot tube into the pitot tube insertion hole (401) on the flow field measurement pipe (4) to measure and calculate the cross-section average flow rate and velocity type of the flow field measurement pipe (4) at both ends; S3: controlling the loudspeaker (9) on the sound source pipe (5) to emit sound, inserting a micro-perforated plate (502) into the muffling pipe (3), and starting experimental measurement; S4: dismounting the micro-perforated plate (502) and replacing it with a solid plate, and starting experimental measurement again; S5: obtaining a test piece transmission loss curve and saving relevant data.

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