Broadband impedance tube measuring device and applications thereof

Abstract

The application discloses a broadband impedance tube measuring device and application thereof, adopts a straight pipe with a square cross section as a measuring pipe, and arranges an acoustic source and a test sample at two ends of the measuring pipe; eight microphones are arranged on the pipe wall; the eight microphones are divided into two groups, namely a first microphone group and a second microphone group; four microphones in the first microphone group are arranged at the midpoints of four edges of the pipe cross section at a set position; four microphones in the second microphone group are arranged at the midpoints of four edges of the pipe cross section at another set position; and the transfer function method is adopted to calculate and obtain the normal incidence reflection coefficient and the sound absorption coefficient according to the superimposed sound pressure of the first microphone group and the superimposed sound pressure of the second microphone group. Compared with the impedance tube measuring device based on double microphones, the application significantly expands the applicable frequency range without reducing the cross section size of the impedance tube; and can be used for the measurement of the normal incidence reflection coefficient and the sound absorption coefficient within the (2,2) order acoustic mode frequency of the straight pipe with a square cross section.

Description

Technical Field

[0001] This invention relates to the field of acoustic measurement technology, and more specifically to a broadband impedance tube measuring device. Background Technology

[0002] Currently, the measurement of material reflection and absorption coefficients mainly employs the dual-microphone transfer function method. This method uses a pipe with a rigid wall, placing microphones at two locations near the test sample. The perpendicular incident reflection and absorption coefficients are calculated based on the transfer function of the sound pressure signals measured by the two microphones. However, this method can only measure the reflection and absorption coefficients at the (0,0) order sound mode frequency within the pipe, requiring the pipe to contain only plane waves. If the sound source frequency increases, higher-order sound modes will be present within the pipe, rendering the dual-microphone transfer function method ineffective. Expanding the test frequency requires reducing the cross-sectional size of the impedance tube, increasing measurement complexity and reducing the size of the measured material. The inhomogeneity of the sample material and the constraint effect of the sample on the pipe wall can negatively impact the measurement results. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, this invention provides a broadband impedance tube measuring device and its application. Without reducing the cross-sectional size of the impedance tube, the upper limit of the test frequency inside the pipe is extended from the (0,1) order acoustic mode frequency to the (2,2) order acoustic mode frequency, thereby improving the simplicity of measurement and the accuracy of testing.

[0004] The present invention adopts the following technical solution to solve the technical problem:

[0005] The broadband impedance tube measuring device of this invention is characterized by using a straight tube with a square cross-section as the measuring tube. The sound source is placed at the center of one end of the measuring tube, and the test sample is placed at the other end of the measuring tube. Eight microphones are set on the tube wall. The eight microphones are divided into two groups: a first microphone group and a second microphone group. In the first microphone group, four microphones are arranged at the midpoints of the four sides of the cross-section of the tube at a set position. In the second microphone group, four microphones are arranged at the midpoints of the four sides of the cross-section of the tube at another set position. The vertical incident reflection coefficient and the sound absorption coefficient are calculated using the transfer function method based on the superimposed sound pressure signal of the first microphone group and the superimposed sound pressure signal of the second microphone group.

[0006] The broadband impedance tube measuring device of the present invention is also characterized in that: the pipe wall of the measuring tube is a rigid wall, the sound source is used to generate sound wave signals inside the measuring tube, and the test sample is a sound-absorbing material.

[0007] The broadband impedance tube measuring device of the present invention is also characterized in that: the superimposed sound pressure signal of the first microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the first microphone group, p(z1); the superimposed sound pressure signal of the second microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the second microphone group, p(z2); z1 represents the distance between the cross section of the pipe where the first microphone group is located and the sound source; z2 represents the distance between the cross section of the pipe where the second microphone group is located and the sound source, and z1 and z2 are not equal.

[0008] The wideband impedance tube measuring device of this invention is also characterized by:

[0009] The calculation of the perpendicular incident reflection coefficient and sound absorption coefficient using the transfer function method refers to:

[0010] With H 12 Characterizing the transfer function, in:

[0011] p(z1) is the superimposed sound pressure of the first microphone group; p(z2) is the superimposed sound pressure of the second microphone group;

[0012] Based on the transfer function H 12 The calculated vertical incident reflection coefficient R is:

[0013]

[0014] Based on the vertical incident reflection coefficient R, the vertical incident sound absorption coefficient α is calculated as: α = 1 - R 2

[0015] in f0 is the frequency of the sound source, c0 is the speed of sound in air, and j is the imaginary unit.

[0016] The feature of the application of the broadband impedance tube measuring device of the present invention is that the measuring device is used to measure the vertical incident reflection coefficient and sound absorption coefficient within the (2,2)th order acoustic mode frequency of a straight tube with a square cross-section.

[0017] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0018] 1. Without changing the cross-sectional dimensions of the impedance tube, the present invention extends the upper limit of the test frequency from the (0,1) order acoustic mode frequency of the pipe to the (2,2) order acoustic mode frequency. The superimposed sound pressure is obtained by summing, and the summing process eliminates the influence of higher order (2,2) order and lower order acoustic wave in the pipe.

[0019] 2. Compared with the dual-microphone method, when measuring at the same upper frequency limit, the measuring tube size used in this invention is approximately three times that of the traditional dual-microphone method. This avoids the non-uniformity and tube wall constraint effect caused by the reduction in sample size to a certain extent, and effectively improves the test accuracy.

[0020] 3. This invention eliminates the need for multiple impedance transistors during use, enabling it to cover a wider frequency range and effectively simplifying the testing process. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the measuring device of the present invention.

[0022] Figure 2 This is a comparison between the sound absorption coefficient obtained by simulation testing using the device of the present invention and the theoretical value.

[0023] The numbers in the diagram are: 1 First microphone, 2 Second microphone, 3 Third microphone, 4 Fourth microphone, 5 Fifth microphone, 6 Sixth microphone, 7 Seventh microphone, 8 Eighth microphone, 9 Measuring tube, 10 Sound source, 11 Test sample. Detailed Implementation

[0024] See Figure 1 In this embodiment, the broadband impedance tube measuring device uses a straight tube with a square cross-section as the measuring tube 9. The sound source 10 is placed at the center of one end of the measuring tube to ensure a symmetrical sound field is generated inside the tube. The test sample 11 is placed at the other end of the measuring tube. Eight microphones are set on the tube wall of the measuring tube 9. The eight microphones are divided into two groups, namely the first microphone group and the second microphone group. In the first microphone group, four microphones are arranged at the midpoints of the four sides of the cross-section of the pipe at a set position. In the second microphone group, four microphones are arranged at the midpoints of the four sides of the cross-section of the pipe at another set position. The vertical incident reflection coefficient and the sound absorption coefficient are calculated using the transfer function method based on the superimposed sound pressure signal of the first microphone group and the superimposed sound pressure signal of the second microphone group.

[0025] Microphones are placed at the midpoints of the four sides of the pipe cross-section to extract sound pressure signals. After superposition, these signals can cancel out all higher-order modes within the (2,2) order sound mode frequency, leaving only the (0,0) order mode wave. The result can be obtained using the transfer function method. The pipe wall of the measuring tube 9 is a rigid wall. The sound source 10 is used to generate sound wave signals inside the measuring tube. The test sample 11 is a sound-absorbing material, placed at the other end of the measuring tube 9. In actual measurement, the sound-absorbing material is placed inside the other end of the measuring tube, and a rigid backing is placed at the port of the corresponding end of the measuring tube to seal the port and reduce transmitted waves. In the simulation model, the surface of the sound-absorbing material facing outwards from the tube is set to rigid.

[0026] In specific implementation, the superimposed sound pressure signal of the first microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the first microphone group, p(z1); the superimposed sound pressure signal of the second microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the second microphone group, p(z2).

[0027] z1 represents the distance between the cross-section of the pipe where the first microphone group is located and the sound source;

[0028] z2 represents the distance between the cross-section of the pipe where the second microphone group is located and the sound source, and z1 and z2 are not equal.

[0029] The calculation of the perpendicular incident reflection coefficient and sound absorption coefficient using the transfer function method refers to:

[0030] With H 12 The transfer function characterizing the superimposed sound pressures p(z1) and p(z2) is...

[0031] Based on the transfer function H 12 The calculated vertical incident reflection coefficient R is:

[0032]

[0033] The vertical incident sound absorption coefficient α is calculated based on the vertical incident reflection coefficient R as follows: α = 1 - R 2

[0034] in f0 is the frequency of the sound source, c0 is the speed of sound in air, and j is the imaginary unit.

[0035] In this embodiment, the measuring device can be used to measure the vertical incident reflection coefficient and sound absorption coefficient within the (2,2)th order acoustic modal frequency of a straight pipe with a square cross-section.

[0036] Simulation test

[0037] To verify the effectiveness of the device of the present invention, the testing process was simulated using COMSOL software, and the test results were compared with the theoretical values. In the simulation, the side length of the square cross-section of the measuring tube was set to 75 mm, the length of the measuring tube was 400 mm, and the sound source was a rigid piston with a moving speed of 1 m / s. Figure 1 The first microphone 1, the second microphone 2, the third microphone 3, and the fourth microphone 4 together constitute the first microphone group; the fifth microphone 5, the sixth microphone 6, the seventh microphone 7, and the eighth microphone 8 together constitute the second microphone group.

[0038] Taking the center point of the pipe end face where the sound source is located as the origin of the coordinate system, the axis along the measuring pipe as the z-axis, one side parallel to the cross-section of the measuring pipe as the x-axis, and the other side parallel to the cross-section of the measuring pipe as the y-axis, and let 'a' represent the side length of the square on the cross-section of the measuring pipe, then:

[0039] The coordinates of each microphone in the first microphone group are as follows:

[0040] The coordinates of each microphone in the second microphone group are as follows: In this simulation experiment, z1 was set to 300 mm and z2 to 325 mm; the flow resistance of the test sample was 10000 Pa·s / m. 2 The thickness is 25mm. The theoretical value of the sound absorption coefficient of this test sample was obtained by calculation using the Miki model. Figure 2 The image shows a comparison between the theoretical and simulated values ​​of the sound absorption coefficient. Figure 2 It can be seen that the theoretical and simulated values ​​of the sound absorption coefficient are in perfect agreement within the (2,2) modal frequency range, realizing accurate measurement of the high-frequency vertical incident sound absorption coefficient without reducing the cross-sectional size of the pipe.

Claims

1. A broadband impedance tube measuring device, characterized in that: A straight tube with a square cross-section is used as the measuring tube (9). The sound source (10) is placed at the center of one end of the measuring tube, and the test sample (11) is placed at the other end of the measuring tube. Eight microphones are set on the tube wall of the measuring tube (9). The eight microphones are divided into two groups, namely the first microphone group and the second microphone group. Four microphones in the first microphone group are arranged at the midpoints of the four sides of the cross-section of the pipe at a set position. Four microphones in the second microphone group are arranged at the midpoints of the four sides of the cross-section of the pipe at another set position. The vertical incident reflection coefficient and the sound absorption coefficient are calculated by the transfer function method based on the superimposed sound pressure signal of the first microphone group and the superimposed sound pressure signal of the second microphone group. The pipe wall of the measuring tube (9) is a rigid wall. The sound source (10) is used to generate sound wave signals in the measuring tube. The test sample (11) is a sound-absorbing material. The superimposed sound pressure signal of the first microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the first microphone group. The superimposed sound pressure signal of the second microphone group is obtained by summing the sound pressure measurement signals of the four microphones in the second microphone group. ;by The distance between the cross-section of the pipe containing the first microphone group and the sound source is represented by; The distance between the cross-section of the pipe containing the second microphone group and the sound source is characterized, and and They are not equal.

2. The broadband impedance tube measuring device according to claim 1, characterized in that: The calculation of the perpendicular incident reflection coefficient and sound absorption coefficient using the transfer function method refers to: Characterizing the transfer function, ;in: The superimposed sound pressure of the first microphone group; The superimposed sound pressure of the second microphone group; Based on the transfer function Calculate the vertical incident reflection coefficient for: Based on the stated vertical incident reflection coefficient Calculate the vertical incident sound absorption coefficient for: ; in , For the frequency of the sound source, The speed of sound in air. It is the imaginary unit.

3. An application of the broadband impedance tube measuring device according to claim 1, characterized in that: The measuring device is used to measure the vertical incident reflection coefficient and sound absorption coefficient of a straight tube with a square cross-section within the (2,2)th order acoustic modal frequency.

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