Acoustic flow tube testing platform and testing method thereof

Abstract

The invention belongs to the technical field of acoustic measurement, and particularly relates to an acoustic flow tube test platform and a test method thereof, transmission loss measurement of an acoustic metasurface under a flow field working condition is carried out through the constructed acoustic flow tube test platform, and a test piece mounting tube of the acoustic flow tube test platform can be used for mounting most acoustic metasurface test pieces. The related technical research of the acoustic metasurface is facilitated; meanwhile, the test platform creates a flow acoustic measurement environment, weakens the turbulence pressure disturbance influence by means of a reference signal, self-spectrum and cross-spectrum signal processing method based on a scattering matrix theory, and adopts a double-load method to perform parameter solution on a scattering matrix and calculate a class of acoustic characteristics of the acoustic metasurface to be tested, thereby solving the problem of low Mach number (llt; the method can be used for measuring the transmission loss of the acoustic metasurface in the low and medium frequency range (300-2000Hz) under the tangential flow (0.3 Mach), and the technology can be widely applied to the fields of unmanned aerial vehicles, eVTOLs, household appliances, automobiles and the like with noise reduction requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of acoustic measurement, and in particular relates to an acoustic flow tube test platform and a test method thereof. Background Art

[0002] Acoustic metasurfaces have received extensive attention in recent years because of their ability to manipulate the phase, size, and direction of sound waves in space and frequency, and their strong designability. As the research on the practical application of acoustic metasurfaces advances, it is necessary to consider the influence of more complex working conditions. Among them, flow field conditions are inevitable, and the introduction of flow field effects will greatly increase the complexity of predicting and evaluating the acoustic properties of acoustic metasurfaces. It is particularly important to provide basic acoustic information of flow acoustic metasurfaces / materials through experimental measurements to verify the feasibility of non-precise analysis / numerical predictions and to evaluate the overall performance of the system configuration.

[0003] Commonly used acoustic measurement experimental methods include reverberation chamber method, far field method and acoustic impedance tube method. Since specific flow field conditions require reasonable aerodynamic layout, the acoustic impedance tube, which is essentially a fluid pipeline, can meet the ventilation needs through sophisticated design, and this method can measure the acoustic information of the front and rear ends of the acoustic metasurface to be measured.

[0004] For acoustic research under flow conditions, there are some mature flow tube acoustic experimental platform designs abroad. The flow tube acoustic experimental platform built by NASALaSR in 2009 uses a rectangular pipe with a cross-section of 50.8×63.5mm. The flow field in the pipe is provided by a negative fan (0-0.6 Mach), and the sound field is generated by 18 array speakers (frequency range 0.4-3kHz, single frequency up to 150 decibels), and a variable cavity depth local resonance muffler is set at the end of the pipe to approximately simulate the non-reflection outlet boundary condition. The device is mainly based on the objective function method, and uses 53 microphones installed on the inner wall of the opposite side of the test piece to accurately measure the acoustic impedance characteristics of the test piece; the cross-section of the German Space Agency (DLR) flow tube acoustic experimental platform is 80×80mm. Compared with NASALaSR, its maximum Mach number in the tube is 0.3, and it can generate 120dB single-frequency sound in the frequency range of 0.21~2.11kHz. Microphones with exponential spacing in a group of 5 are arranged upstream and downstream of the test piece to measure the scattering matrix parameters, and 10 microphones with the same spacing are used on the opposite side of the test piece. The inner wall of the side is used to measure the acoustic impedance characteristics. Similar equipment includes the Royal Institute of Technology (KTH) in Sweden; and the flow tube acoustic experimental platform of the French Space Agency (ONERA) takes into account the influence of airflow temperature and can be controlled at up to 570K. There are similar studies in China. The Beijing University of Aeronautics and Astronautics has designed a flow tube acoustic experimental platform, which is based on the Prony method and uses 16 evenly distributed microphones to measure the acoustic impedance characteristics in the range of 0 to 0.2 Mach and 0.4 to 3kHz. It also uses microphones in groups of 4 to measure the upstream and downstream sound pressure information of the test piece and calculate the transmission loss.

[0005] However, the above-mentioned flow tube acoustic experimental platform focuses on the study of impedance characteristics of test pieces under compressible flow fields and high sound intensity, and is mainly aimed at extreme applications of nonlinear acoustics such as aerospace. However, at present, most of the research conditions of flow acoustic metasurfaces are more linear acoustics with practical applications, considering incompressible flows and low sound intensity. At the same time, most of the above-mentioned equipment does 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 speakers and wind tunnels are used, and the experimental cost is relatively high. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes an acoustic flow tube test platform and a test method thereof to complete the transmission loss measurement of the acoustic metasurface under flow field conditions.

[0007] The acoustic flow tube test platform of the present invention comprises a supporting frame and an acoustic flow tube installed on the supporting 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 sequentially connected to a sound field measurement tube, a sound source tube, a flow field measurement tube, and a silencer tube, and the end of the silencer tube is connected to an upstream fan; the right end of the test piece mounting tube is also sequentially connected to a sound field measurement tube, a flow field measurement tube, and a silencer tube, and the end of the silencer tube is connected to a downstream fan.

[0008] The acoustic flow tube is a square cross-section waveguide tube, and each tube is connected by a flange.

[0009] Partition plates are installed on both sides of the square-section waveguide tube of the muffler, and a gradient back plate is installed outside the partition plate.

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

[0011] The flow field measuring tube is provided with a pitot tube jack on the upper surface of the central axis for collecting flow field information in the pipeline.

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

[0013] Three microphone jacks are axially arranged on the central axis of the upper surface of the sound field measurement tube for installing the microphone, and the sound signal receiving 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 through groove without a tube wall, which is used for mounting an acoustic metasurface.

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

[0016] S1: Perform microphone amplitude and phase calibration, install the microphone on the upper 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 jack on the flow field measuring tube to measure and calculate the average flow velocity and velocity type of the flow field measuring tube cross section at both ends.

[0018] S3: Control the speaker on the sound source tube to make sound, insert the micro-perforated plate into 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 transmission loss curve of the test piece and save the relevant data.

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

[0022] Compared with the prior art, the present invention measures the transmission loss of acoustic metasurfaces under flow field conditions by constructing an acoustic flow tube test platform, and the test piece mounting tube of the acoustic flow tube test platform can mount most test pieces of acoustic metasurfaces, which is beneficial to the research on acoustic metasurface related technologies; 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 methods to weaken the influence of turbulent pressure disturbance, and uses the double load method to solve the parameters of the scattering matrix and calculate a type of acoustic characteristics of the acoustic metasurface to be tested, the transmission loss (the proportional relationship between input energy and output energy, which can characterize energy loss), solves the transmission loss measurement problem of acoustic metasurfaces under low Mach number (<0.3 Mach) tangential flow and in the medium and low frequency range (300-2000Hz). This technology will be widely used in fields with noise reduction needs such as drones, eVTOL, home appliances, and automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments are briefly introduced below.

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

[0025] Figure 2 An explosion diagram of the muffler pipe in the embodiment;

[0026] Figure 3 Schematic diagram of noise reduction results of the muffler pipe in the embodiment;

[0027] Figure 4 Schematic diagram of a flow field measuring tube in an embodiment;

[0028] Figure 5 Schematic diagram of the sound source tube in the embodiment;

[0029] Figure 6 is a cross-sectional view of a sound source tube in an embodiment;

[0030] Figure 7 Schematic diagram of the sound field measurement tube in the embodiment;

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

[0032] Fig. 9 It is a schematic diagram of the measurement principle in the embodiment;

[0033] Fig.10 It is a measurement flow chart in the embodiment.

[0034] 1-support frame, 2-upstream fan, 3-muffler, 301-square section waveguide, 302-partition, 303-gradient back plate, 4-flow field measurement tube, 401-pitot tube jack, 5-sound source tube, 501-through hole, 502-perforated plate, 503-metal wire mesh, 6-sound field measurement tube, 601-microphone jack, 7-test piece mounting tube, 8-downstream fan, 9-speaker, 10-flange. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0036] Example 1

[0037] See also Figure 1 As shown, the acoustic flow tube test platform includes a support frame 1 and an acoustic flow tube installed on the support frame 1, the acoustic flow tube includes a test piece mounting tube 7 in the middle, the left end of the test piece mounting tube 7 is connected to the upstream sound field measurement tube 6, the upstream sound source tube 5, the upstream flow field measurement tube 4, and the upstream silencer tube 3 in sequence, and the end of the upstream silencer tube 3 is connected to the upstream fan 2; the right end of the test piece mounting tube 7 is also connected to the downstream sound field measurement tube 6, the downstream flow field measurement tube 4, and the downstream silencer tube 3 in sequence, and the end of the downstream silencer tube 3 is connected to the downstream fan 8.

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

[0039]

[0040] Wherein, c0=343m / s is the air sound speed at 20°C, and M0=u / c0 represents the Mach number. When the design maximum measurement frequency (i.e., the cutoff frequency) is 2000Hz, the maximum side length of the pipeline that meets the design requirements can be obtained from formula (1) to be 81.8mm. The present invention selects a=80mm.

[0041] Both ends of each section of the acoustic flow tube are provided with flanges 10 and concave-convex grooves for connection and positioning with other pipelines.

[0042] In order to ensure the measurement accuracy, the inner wall of the pipe meets the rigidity condition and can also reduce the amount of sound wave leakage. Usually, the pipe wall needs to have a larger surface density. Each section of the acoustic flow pipe in the present invention uses acrylic material with a minimum thickness of t=20mm. The commonly used acrylic density is known to be 1190kg / m 3 According to the selected cross-section length, the surface density is m = 5.95 kg / m 2 , according to the law of mass [7] The minimum sound insulation of pipes between 300 and 2000 Hz can be estimated as:

[0043] TL=20lgmf-42.5≈22.5~39dB (2)

[0044] The subsequent sections are designed with a cross-section side length a = 80 mm and a thickness t = 20 mm.

[0045] ①Muffler 3

[0046] During the measurement process, environmental noise and wind source noise have a certain impact on the measurement results. A muffler 3 can be installed at both ends of the acoustic flow tube inlet and outlet. At the same time, the measurement principle scattering matrix is ​​solved by the double load method, requiring two loads with large impedance differences to be installed at the ends of the acoustic flow tube as different working conditions. The present invention uses the presence / absence of mufflers to create two load conditions.

[0047] Based on this, Figure 1 As shown, two muffler pipes 3 are prepared and arranged at the inlet and outlet of the acoustic flow pipe respectively. Figure 2 As shown, the square-section waveguide of the muffler tube 3 is provided with partitions 302 on both sides inside and outside, and a gradient back plate 303 is provided outside the partition 302; the partition 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 pulled out of the muffler section, and have the ability to switch between muffler and non-muffler. When the micro-perforated plate is used, due to the large acoustic resistance and the connection between the pipe and the gradient back cavity, coupling into an acoustic black hole, a large range of noise reduction can be achieved, which is Figure 3The results show that when a solid plate is used, a rigid wall is formed, and the sound wave continues to propagate along the axial direction of the pipe with almost no sound energy attenuation. The present invention adopts a switchable design, which not only meets certain noise reduction requirements, but also provides conditions for two different working conditions of the dual load method.

[0048] ②Flow field measurement tube 4

[0049] The acoustic flow tube includes two flow field measurement sections with a length of 500 mm, which are located at the rear end of the upstream muffler tube 3 and the front end of the downstream muffler tube 3 respectively. Figure 4 As shown, a Pitot tube socket 401 is provided on the upper surface of the central axis of the flow field measuring tube 4 for collecting flow field information in the pipeline, such as central flow velocity and velocity type.

[0050] ③Sound source tube 5

[0051] like Figure 5 As shown, a 300 mm long sound source section is designed to install the BMS-4592nd speaker 9, and a through hole 501 equivalent to the diameter of the diaphragm of the speaker 9 is set in the center of the upper surface so that the sound wave can enter the pipe. Generally, when the sound wave power generated by the speaker 9 is much greater than the background noise, the influence of the background noise can be ignored. The present invention works in an anechoic chamber, the background noise is less than 30 dB, and the incident sound wave of the speaker 9 is set to 94 dB, which can meet the measurement requirements.

[0052] Due to the thickness of the pipe wall, installing the speaker 9 will form a cylindrical cavity. The uniform flow in the pipe passing through the cavity may generate flow-induced noise, which will have a certain impact on the sound field measurement. Figure 6 As shown, the present invention designs a coupling structure of a perforated plate 502 and a metal mesh 503 with relatively small acoustic resistance, which can make the uniform flow in the tube 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 metal mesh 503 to reduce its vibration under the flow field.

[0053] ④ Sound field measurement tube 6

[0054] The scattering matrix principle adopted in the present invention requires four microphones, two in a group, installed in the upstream and downstream of the test piece installation tube 7 to collect the sound field information before and after the test piece. Based on this, two identical sound field measurement tubes 6 are designed. Figure 7 As shown, three microphone jacks 601 are arranged 30 mm apart along the axial direction at the center of the upper surface of the middle axis, so as to facilitate the installation of the microphone and make its receiving end of the sound signal flush with the inner wall of the pipe. In general, the front and rear microphone jacks 601 are used to install the microphone and 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 limits of the frequency, and vice versa. Considering that the upper limit of the designed measurement frequency 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, and 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] In addition, in the actual installation of the pipeline, there may be gaps at the joints, which may cause sound scattering, thereby generating multi-order sound modes, affecting the plane wave signal collected by the microphone. The design of the pipeline cross section takes into account that under the condition of a frequency not exceeding 2000Hz, the sound waves in the pipeline propagate in the form of plane waves. Therefore, only a sufficiently long 80×80mm pipeline is needed to well attenuate the multi-order sound modes. According to the design standard of the impedance tube, that is, the distance between the microphone and the sound source is greater than 3 times the side length of the pipeline, and the distance from the test piece is greater than 2 times the side length, the length of the sound field measurement tube 6 of the present invention is selected to be 700mm to meet the 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, thereby ensuring the measurement accuracy.

[0062] Working principle: Fig. 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] Assuming that there is uniform flow and plane wave along the axial direction in the pipeline, the subscripts 'i' and 'o' are defined to represent the incident and exit ends of the sound wave, '+' and '-' represent the same and opposite flow directions as the uniform flow, respectively, the superscripts '''' and '"' represent the end impedances as Z' and Z", '*' 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, ρ represents the density of the flowing medium in the pipe, c represents the speed of sound, M represents the Mach number, S 21 is an element of the scattering matrix and can be expressed as:

[0066]

[0067] In the formula, H represents the transfer function with the forward sound pressure of port i as the reference sound pressure, and with the help of the autospectrum G in signal processing x·x and cross spectrum G x·y Weaken the influence of turbulent pressure disturbances and average the data to reduce data fluctuations when solving.

[10] . Among them, each p value is solved by the traveling wave formula:

[0068]

[0069] Among them, p1~p4 correspond to the sound pressure data measured by microphones 1~4 respectively. + and k - They represent the wave vectors with the same and opposite directions as the uniform flow, respectively, and can be solved by the following formula:

[0070]

[0071] Among them, k0=ω / c0 is the free field wave number, D is the ratio of the pipe cross-sectional area to the circumference, μ is the dynamic viscosity of the flowing 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 transmission loss of the test piece can be obtained by substituting the sound pressure values ​​measured by the four microphones under different end load conditions into formulas (5) to (8).

[0073] See also Fig.10 As shown, the testing method of the acoustic flow tube test platform comprises the following steps:

[0074] S1: perform microphone amplitude and phase calibration, install the microphone on the upper surface of the sound field measurement tube 6, install the test piece on the test piece installation tube 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 jack 401 on the flow field measuring tube 4 to measure and calculate the average flow velocity and velocity type of the cross section of the flow field measuring tube 4 at both ends;

[0076] S3: Control the loudspeaker 9 on the sound source tube 5 to make sound, insert the micro-perforated plate into the muffler 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 present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all details in detail, nor do they limit the invention to only the specific embodiments described. Obviously, many modifications and changes can be made according to the content of this specification.

Claims

1. Acoustic flow tube test platform, characterized in that: The invention comprises a support frame (1) and an acoustic flow tube installed on the support frame (1), wherein the acoustic flow tube comprises a test piece installation tube (7) in the middle, the left end of the test piece installation tube (7) is connected in sequence to a sound field measurement tube (6), a sound source tube (5), a flow field measurement tube (4), and a silencer tube (3), and the end of the silencer tube (3) is connected to an upstream fan (2); the right end of the test piece installation tube (7) is also connected in sequence to a sound field measurement tube (6), a flow field measurement tube (4), and a silencer tube (3), and the end of the silencer tube (3) is connected to a downstream fan (8).

2. The acoustic flow tube test platform according to claim 1, characterized in that: The acoustic flow tube is a square cross-section waveguide tube, and each tube is connected via a flange (10).

3. The acoustic flow tube test platform according to claim 1, characterized in that: Partition plates (302) are installed on both the inner and outer sides of the square-section waveguide tube of the muffler tube (3), and a gradient back plate (303) is installed outside the partition plate (302).

4. The acoustic flow tube test platform according to claim 3, characterized in that: The partition (302) is a micro-perforated plate (502) or a solid plate.

5. The acoustic flow tube test platform according to claim 1, characterized in that: The flow field measuring tube (4) is provided with a Pitot tube socket (401) on the upper surface of the central axis, which is used to collect flow field information in the pipeline.

6. The acoustic flow tube test platform according to claim 1, characterized in that: A through hole (501) is provided at the center of the upper surface of the sound source tube (5), a perforated plate (502) and a metal wire mesh (503) are installed in sequence from top to bottom at the bottom of the through hole (501), and a loudspeaker (9) is connected to the top of the through hole (501).

7. The acoustic flow tube test platform according to claim 1, characterized in that: Three microphone jacks (601) are axially arranged on the central axis of the upper surface of the sound field measurement tube (6) for installing microphones, and the sound signal receiving end of the microphone is flush with the inner wall of the pipe.

8. The acoustic flow tube test platform according to claim 1, characterized in that: The upper surface of the test piece installation tube (7) is a through groove without a tube wall and is used for installing an acoustic metasurface.

9. The testing method of the acoustic flow tube testing platform according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1: Perform microphone amplitude and phase calibration, install the microphone on the upper surface of the sound field measurement tube (6), install the test piece on the test piece installation tube (7), and then connect the microphone measurement system and the computer; S2: Turn on the fan and adjust the flow rate, insert the Pitot tube into the Pitot tube socket (401) on the flow field measuring tube (4) to measure and calculate the average flow rate and velocity type of the cross section of the flow field measuring tube (4) at both ends; S3: Control the loudspeaker (9) on the sound source tube (5) to make sound, insert the micro-perforated plate (502) into the muffler tube (3), and start the experimental measurement; S4: dismantle the micro-perforated plate (502) and replace it with a solid plate, and start the experimental measurement again; S5: Obtain the transmission loss curve of the test piece and save the relevant data.

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