Near-field acoustical holography measurement system and method for silencing capacity of silencer

Abstract

The invention particularly relates to a near-field acoustical holography measurement system and method for the noise reduction amount of a silencer. The near-field acoustical holography measurement system comprises a sound production chamber, a receiving chamber, a sound source device, a sound pressure acquisition device, a universal computer and a test pipeline located between the sound production chamber and the receiving chamber. The sound source device comprises a signal generator and a power amplifier which are installed in the general-purpose computer, and a non-directional sound source located in the sound production room; the sound pressure acquisition device comprises first microphones located at one or more measuring point positions in the sound production chamber, a plane microphone array, a multi-channel data acquisition instrument and a data recording and processing module, wherein the plane microphone array and the multi-channel data acquisition instrument are located in the receiving chamber and are connected in sequence, and the data recording and processing module is installed on a general-purpose computer; and a silencer test piece or an alternative pipe is arranged in the test pipeline. The device has the advantages of being high in precision, good in measurement environment adaptability, capable of obtaining sound energy distribution of the end face of the tail end of the silencer and the like, and provides a new reference for laboratory measurement, application effect evaluation and structure optimization of the silencer.

Description

Technical Field

[0001] The invention belongs to the field of noise and vibration control, and in particular relates to a near-field acoustic holographic measurement system and method for the muffler muffler volume. Background Art

[0002] Silencer is widely used in the field of noise control, and the sound attenuation is an important indicator to measure its acoustic performance. At present, the methods for measuring the sound attenuation of silencers in the laboratory are mainly divided into sound pressure method and sound intensity method. The sound pressure method estimates the sound power level by the spatial energy average sound pressure level at the inlet and outlet of the silencer, and then obtains the transmission loss and insertion loss of the silencer. However, the sound pressure method has a poor suppression effect on lateral sound transmission, so it is impossible to obtain the sound energy information radiated by the silencer itself to the receiving room. The sound intensity method calculates the sound power level at the outlet of the silencer by measuring the sound intensity on the envelope measurement surface at the outlet of the silencer, and then obtains the transmission loss and insertion loss of the silencer. Although the sound intensity method can effectively suppress lateral sound transmission and obtain the energy information radiated by the silencer itself to the receiving room, since the distance between the measurement surface and the outlet of the silencer is at least greater than 0.25m, this method cannot obtain the sound energy distribution on the end surface of the outlet of the silencer. At the same time, the directivity of the sound source is also an important factor affecting the measurement accuracy of the sound intensity method. Therefore, when detecting the muffler's noise reduction, it is generally necessary to use the sound field non-uniformity indicator value calculated based on the sound intensity measurement results to make a judgment. When the indicator value does not meet the requirements, it is necessary to increase the sampling or scanning density of the muffler end envelope measurement surface, which will lead to a decrease in the measurement efficiency of the sound intensity method.

[0003] At present, as a powerful tool for noise source positioning, identification, free-field reconstruction and sound field visualization, near-field acoustic holography based on the equivalent source method has the advantages of not being restricted by the shape of the noise source and high computational efficiency, and has been widely used in industrial production and low-noise product design. The basic principle of the equivalent source near-field acoustic holography technology is to use a microphone array to collect the sound pressure signal at the end of the muffler, and then use the equivalent source near-field acoustic holography algorithm to reconstruct the normal sound intensity on the radiation surface of the muffler end, and then obtain the radiated sound power of its transmitted sound, so as to achieve the measurement of the muffler's sound reduction. At the same time, an acoustic image of the radiated sound energy at the end of the muffler with high spatial resolution can also be obtained, providing a new technical path for comprehensively evaluating the noise reduction effect of the muffler and guiding the optimization of the muffler structure. Summary of the invention

[0004] The purpose of the present invention is to address the shortcomings of the existing silencer sound attenuation measurement technology and provide a near-field acoustic holographic measurement system and method for silencer sound attenuation to achieve silencer sound attenuation measurement, especially the measurement of silencer transmission loss and insertion loss, while realizing the visualization of the radiated sound field of the silencer outlet end face.

[0005] In a first aspect, the present invention provides a near-field acoustic holographic measurement system for the muffler muffler, comprising a sound-generating chamber, a receiving chamber, a sound source device, a sound pressure collection device, a general-purpose computer, and a test pipe located between the sound-generating chamber and the receiving chamber;

[0006] The sound source device includes a signal generator and a power amplifier installed in a general-purpose computer and a non-directional sound source located in the sound generation room;

[0007] The sound pressure collection device includes a first microphone located at one or more measuring points in the sound emission chamber, a planar microphone array located in the receiving chamber and connected in sequence, a multi-channel data acquisition instrument, and a data recording and processing module installed on a general-purpose computer;

[0008] Wherein, a muffler test piece or a replacement pipe is installed in the test pipeline.

[0009] Furthermore, the muffler specimen has an effective length of 0.5 m, a width of 0.42 m, and a height of 0.6 m. The thickness of the muffler blade of the muffler specimen is 2.5 cm. The muffler shell is made of an aluminum alloy plate with a thickness of 1.5 mm, and the inside of the metal shell is provided with a sound-absorbing lining. The muffler specimen is at least one of a resistive muffler, a reactive muffler, an impedance composite muffler, and an active noise reduction muffler.

[0010] Furthermore, the planar microphone array includes a supporting structure and second microphones fixed on the supporting structure at equal intervals, wherein the second microphone is at least one of a free-field microphone, a pressure-field microphone, and a diffuse-field microphone.

[0011] Furthermore, the diameter of the second microphone is 3 mm to 26 mm; and the fixed distance between the second microphones on the supporting structure is 2 cm to 10 cm.

[0012] Furthermore, the data recording and processing module includes a data recorder and an equivalent source near-field sound holographic anechoic output module.

[0013] Furthermore, the one or more measuring points meet the following conditions when deployed: the first microphone is not facing the reflecting wall, the first microphone is at least 1.2 m above the ground, the first microphone is at least 1.1 m away from the nearest wall, the first microphone is at least 1.5 m away from the non-directional sound source, and if there are two or more measuring points, the distance between the first microphones at different measuring points is at least 1 m.

[0014] In a second aspect, the present invention further provides a method for measuring a near-field acoustic holographic measurement system for a muffler muffler, comprising the following steps:

[0015] Step 1: starting the near-field acoustic holographic measurement system of the muffler attenuation based on the muffler test piece installed;

[0016] Step 2: the signal generator generates a white noise signal or a pink noise signal, which is amplified by the power amplifier and fed back to the omnidirectional sound source. After the omnidirectional sound source stably emits sound, the multi-channel data acquisition instrument synchronously measures the first sound pressure data in the sound emission room and the second sound pressure data at the position of the planar microphone array in the receiving room;

[0017] Step 3: Through the data recording and processing module, the first sound pressure data is processed by the frequency band filter to calculate the average sound pressure level of the sound energy in the sound chamber, and then the incident sound power level at the inlet of the muffler specimen is obtained; the second sound pressure data is used as the holographic surface input value, and the sound pressure distribution data of the end face of the muffler specimen outlet is obtained through inverse reconstruction;

[0018] Step 4: using the Euler formula and two-dimensional Fourier transform to solve the sound pressure distribution data of the end surface normal sound intensity distribution data of the muffler test piece outlet, and outputting the sound image after processing by the frequency band filter;

[0019] Step 5: Generate the radiated sound power level of the muffler specimen outlet under the corresponding frequency band after performing frequency band filtering based on the sound pressure distribution data and the sound intensity distribution data, and generate the transmission loss of the muffler specimen according to the incident power level and the radiated sound power level;

[0020] Step 6: turn off the muffler near-field acoustic holographic measurement system, remove the muffler specimen, install a replacement tube in the test tube, start the muffler near-field acoustic holographic measurement system, and then repeat steps 2 to 5 to obtain the radiation power level at the outlet of the replacement tube, and generate the insertion loss of the muffler specimen based on the obtained radiation power at the outlet of the muffler specimen and the radiation sound power level at the outlet of the replacement tube;

[0021] The calculation formula of the incident sound power level at the inlet of the muffler specimen is expressed as:

[0022]

[0023] Where, L Wi is the incident sound power level, is the spatial energy average sound pressure level at the measuring point in the sound chamber, P0 is the reference sound pressure, W0 is the reference sound power, S is the cross-sectional area of ​​the muffler specimen inlet, and ρ0c is the characteristic impedance of air.

[0024] Furthermore, the step of obtaining the sound pressure distribution data of the end face of the muffler specimen outlet through inverse reconstruction is specifically as follows:

[0025] The equivalent source surface is arranged near the outlet of the muffler specimen, with the geometric center of the outlet end surface of the muffler specimen as the origin. The arrangement distance of the equivalent source surface is -1cm to -10cm. The number of equivalent source points on the equivalent source surface is at least greater than the number of measurement points on the plane array.

[0026] The calculation formula of the source strength of the equivalent source point on the equivalent source surface is expressed as:

[0027]

[0028] Where q is the source strength of the equivalent source point, G(r|r m ) is the Green function of the free sound field, the subscript m represents the mth equivalent source point, p h is the holographic surface sound pressure data, which is measured by the set planar microphone array;

[0029] The reconstructed surface is a conformal geometric plane at the outlet of the muffler specimen and coincides with the end surface of the outlet of the muffler specimen;

[0030] The number of reconstruction points on the reconstruction surface is at least greater than the number of measurement points on the plane array;

[0031] The calculation formula of the complex sound pressure on the reconstruction surface is expressed as:

[0032]

[0033] In the formula, p(r s ) is the reconstructed surface r s The complex sound pressure at , i is the imaginary unit, k is the wave number.

[0034] Furthermore, the normal sound intensity distribution data in step 4 is calculated based on the normal particle velocity, where:

[0035] The normal particle velocity calculation formula is expressed as:

[0036]

[0037] In the formula, u n (r s ) is the surface r of the muffler specimen at the outlet s The normal component of the particle velocity at , is the normal component of the wave number on the end surface of the muffler specimen outlet, FFT is the two-dimensional Fourier transform, FFT -1 is the inverse Fourier transform in two-dimensional space.

[0038] The calculation formula of the normal sound intensity distribution data is expressed as:

[0039]

[0040] In the formula, In is the normal sound intensity on the outlet end face of the muffler specimen, Re represents the real part, and “*” represents the complex conjugate.

[0041] The acoustic image is at least one of the sound pressure distribution, normal sound intensity distribution, and radiated sound power on the end surface of the muffler outlet.

[0042] Furthermore, the transmission loss of the muffler is calculated by the following formula:

[0043]

[0044] Where, L Wi is the incident sound power level at the inlet of the muffler specimen, L Wt is the radiated sound power level at the muffler outlet, is the average sound pressure level of the sound indoor space, P0 is the basic sound pressure, N1 and N2 are the number of radial and axial reconstruction points on the outlet plane of the muffler specimen, respectively, and the subscripts i and j are the corresponding reconstruction point numbers. n,i,j is the sound intensity distribution on the outlet end surface of the muffler specimen, ΔS i,j is the value on the outlet end of the muffler specimen corresponding to I n,i,j The area of ​​the surface element;

[0045] Among them, the radiated sound power level L at the outlet of the muffler specimen is Wt It is expressed as:

[0046]

[0047] Where W0 is the reference sound power;

[0048] The insertion loss calculation formula is expressed as:

[0049]

[0050] Where IL is the insertion loss of the muffler specimen, I″ n,i,j I′ is the normal sound intensity at the outlet of the test pipe after the silencer specimen is installed. n,i,j Normal sound intensity at the outlet of the test pipe after installing a replacement tube.

[0051] Beneficial effects of the present invention:

[0052] The advantages and beneficial effects of the muffler noise reduction measurement system and method provided by the present invention are as follows:

[0053] (1) The present invention uses a multi-channel data acquisition instrument, a microphone and a planar microphone array to realize the synchronous acquisition of sound pressure information in the sound emission room and the receiving room, and the data acquisition efficiency is higher.

[0054] The calculation result error is smaller;

[0055] (2) The present invention uses a planar microphone array at the outlet of the muffler specimen and based on the equivalent source method near-field acoustic holography technology, the radiated sound field at the outlet of the muffler specimen can be acquired through one acquisition, including the acquisition of sound pressure distribution, vector sound intensity distribution, sound power distribution, etc.;

[0056] (3) The present invention uses near-field acoustic holography technology to visualize the normal sound intensity on the end surface of the muffler outlet and obtain an acoustic image of the normal sound intensity, thereby intuitively and clearly obtaining the sound radiation characteristics of the muffler at different frequencies, providing a reference for the optimization of the muffler structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a schematic diagram of a near-field acoustic holographic measurement system for the muffler noise reduction of the present invention;

[0058] Figure 2 It is a schematic diagram of the structure of a muffler test piece in an implementation case of the present invention;

[0059] Figure 3 It is a schematic diagram of the structure of the replacement tube in the implementation case of the present invention;

[0060] Figure 4 It is a schematic diagram of the arrangement of measuring points of a planar microphone array in an implementation case of the present invention;

[0061] Figure 5 It is a method flow chart of the near-field acoustic holographic measurement system of the muffler noise reduction of the present invention;

[0062] Figure 6 It is a schematic diagram of the relative position relationship between the holographic surface, the equivalent source surface, and the reconstruction surface of the equivalent source near-field acoustic holography method in the implementation case of the present invention;

[0063] Figure 7 The sonograms of the normal sound intensity levels at 160 Hz, 315 Hz, 800 Hz, 1250 Hz, 1600 Hz and 3150 Hz in the implementation cases of the present invention;

[0064] Figure 8 It is a schematic diagram of the transmission loss results of the muffler specimen in the frequency range of 63Hz to 3150Hz (1 / 3 octave band) in the implementation case of the present invention;

[0065] Fig. 9 It is a schematic diagram of the insertion loss results of the muffler specimen in the frequency range of 63Hz to 3150Hz (1 / 3 octave band) in the implementation case of the present invention;

[0066] The following are marked in the figure: 1. Sound-generating chamber (reverberation chamber); 2. Receiving chamber (full anechoic chamber); 3. Multi-channel data acquisition instrument; 4. Planar microphone array; 5. First microphone; 6. Non-directional sound source; 7. General-purpose computer; 8. Signal generator; 9. Power amplifier; 10. Test pipe; 11. Anechoic blades; 12. Anechoic specimen; 13. Uploader on the planar microphone array; 14. Holographic surface; 15. Equivalent source surface; 16. Reconstruction surface; T1. Replacement tube. DETAILED DESCRIPTION

[0067] The technical solution of the present invention is described in detail below with reference to the accompanying drawings, but the protection scope of the present invention is not limited to the embodiments.

[0068] Implementation Cases:

[0069] like Figure 1-Figure 4 A near-field acoustic holographic measurement system for the muffler noise reduction is shown.

[0070] A near-field acoustic holographic measurement system for the muffler muffler comprises a sound-generating chamber 1, a receiving chamber 2, a sound source device, a sound pressure collection device, a general-purpose computer 7, and a test pipe 10 located between the sound-generating chamber and the receiving chamber;

[0071] The sound source device includes a signal generator (VA-Lab) 8 installed in a general-purpose computer, a power amplifier (P2500S) 9, and a non-directional sound source (OS003A) 6 located in the sound room;

[0072] The sound pressure collection device includes a first microphone (MPA231) 5 located at one or more measuring points in the sound emission room 1, a planar microphone array 4 connected in sequence in the receiving room, a multi-channel data acquisition instrument (DH5960) 3, and a data recording and processing module installed on a general-purpose computer;

[0073] The test pipe 10 is installed with a muffler test piece 12 or a replacement pipe T1.

[0074] In this embodiment, a test pipe is set between the sound chamber 1 and the receiving chamber 2. The cross-section of the test pipe is rectangular or circular. The length of the test pipe is 0.5m, the cross-section height is 0.61m, the width is 0.43m, and the area is 0.2m. 2 ~2.25m 2 ;

[0075] In this implementation case, the silencer specimen is a sheet-type resistive silencer 12 (hereinafter referred to as: silencer). The effective length of the silencer specimen 12 is 0.5m, the width is 0.42m, the height is 0.6m, the thickness of the silencer blade 11 is 2.5cm, and the silencer shell 12 is made of an aluminum alloy plate with a thickness of 1.5mm. The metal shell has a sound-absorbing lining. In this embodiment, the silencer specimen is at least one of a resistive silencer, a reactive silencer, an impedance composite silencer, and an active noise reduction silencer.

[0076] In this embodiment, the muffler specimen 12 is installed in the test pipe, and the outlet end of the muffler specimen 12 is aligned with the end of the test pipe; the size of the replacement pipe is consistent with the size of the muffler specimen, and the material of the replacement pipe is at least one of steel plate, aluminum plate, and wooden plate; the general-purpose computer serves as an installation platform for the signal generator, data recording and processing module.

[0077] In this embodiment, the sound signal emitted by the signal generator 8 in the sound source device is broadband white noise. The sound signal is amplified by the digital power amplifier (P2500S) 7 and then fed back to the non-directional sound source (OS003A) 6. After stable sound generation, a diffuse sound field is formed in the sound generation room 1, and the sound pressure level in the sound field is 95dB to 100dB. It should be noted that the signal generator emits at least one of white noise and pink noise; the power amplifier is at least one of an analog power amplifier and a digital power amplifier; and the sound source is a non-directional sound source installed in the sound generation room.

[0078] In this embodiment, the non-directional sound source (OS003A) 6 is installed at the center of the generating room 1, and the distance from the microphone (MPA231) 5 at the nearest measuring point is 1.6m, and the distance from the inlet end of the muffler specimen 12 is 2.0m;

[0079] In this embodiment, the size of the planar microphone array 4 of the sound pressure collection device in the receiving room 2 is 0.42m×0.6m, 15 free-field microphones (MPA416) 13 with a spacing of 3cm are arranged on the shorter side, and the microphone diameter is about 6.35mm, and 21 free-field microphones (MPA416) 13 with a spacing of 3cm are arranged on the longer side, and the microphone diameter is about 6.35mm;

[0080] In this embodiment, the planar microphone array 4 is arranged in parallel at a position 5 cm away from the outlet end of the muffler specimen 12 and is aligned with the center of the end face of the outlet end of the muffler specimen 12 .

[0081] In this embodiment, four measuring points are arranged in the sound chamber 1, and the interval between the measuring points is greater than 1.2m. The first microphone (MPA231) 5 is arranged at the measuring point. The first microphone (MPA231) 5 is 1.2m away from the nearest reflective wall, 1.5m above the ground, and not facing the reflective wall. The first microphone is at least one of a free field microphone, a pressure field microphone, and a diffuse field microphone, and the diameter of the first microphone is 3mm to 26mm.

[0082] like Figure 5 , Figure 6 The method of the near-field acoustic holographic measurement system of the muffler noise reduction shown in the figure includes using a microphone (MPA231) 5 to collect the sound pressure at four measuring points in the sound chamber 1, and calculating the average sound pressure level of the spatial sound energy in the sound chamber 1, and then calculating the incident sound power level L at the inlet end of the muffler specimen 12. Wi ; The sound pressure data at 5 cm from the outlet of the muffler specimen 12 is collected by the planar microphone array 4, and the sound pressure data collected by the planar microphone array 4 is used as the input value of the holographic surface 14 of the equivalent source near-field acoustic holography technology to reconstruct the equivalent source surface 15 near the outlet end face of the muffler specimen 12, and then the sound pressure distribution and normal sound intensity distribution on the reconstructed surface 16 are obtained, and the radiated sound power level L at the outlet of the muffler specimen 12 is calculated. Wt2 , and then the transmission loss TL of the muffler specimen 12 is obtained; similarly, the sound power level L radiated from the outlet of the test channel 10 when the replacement tube T1 is installed in the test channel 10 can be obtained. Wt1 , and then obtain the insertion loss IL of the muffler; at the same time, use the near-field acoustic holographic visualization technology to obtain the acoustic image of the radiated sound field on the reconstruction surface 16 (the end surface of the outlet end of the muffler specimen 12);

[0083] In this embodiment, the air characteristic impedance ρ0c is 415kg / (m 2 ·s), the cross-sectional area S of the inlet end of the muffler 10 is 0.252m 2 , the reference sound pressure P0 is 2.0×10 -5 Pa, the reference sound power W0 is 10 -12 W, the sound power level at the inlet of the muffler specimen 12 in the sound chamber 1 can be calculated by the following formula:

[0084]

[0085] In the formula, It is the average sound pressure level value in the frequency range of 63Hz to 4000Hz after the sound pressure level results measured by the microphone 5 at the four measuring points in the receiving room 1 are processed by a 1 / 3 octave band filter.

[0086] In this embodiment, the geometric center of the end face of the outlet end of the muffler specimen 12 or the end face of the outlet end of the substitute pipe T1 is taken as the origin, and the equivalent source surface 15 is set inside the muffler specimen 12 or at -5 cm of the end face of the outlet end of the substitute pipe T1. The size of the equivalent source surface 15 is 0.42m×0.6m, and 15×42 equivalent source intensity points are set, with a total of 630. The source intensity q of the equivalent source point can be calculated by the following formula:

[0087]

[0088] Where q is the source strength of the equivalent source point, G(r|r m ) is the free sound field Green’s function, the subscript m represents the mth (m=1,2,…,630) equivalent source point, p h is the sound pressure data of the holographic surface 14, which is measured by the set planar microphone array 4.

[0089] In this embodiment, the outlet end face of the muffler specimen 12 or the outlet end face of the substitute pipe T1 is used as the reconstruction surface 16. The size of the reconstruction surface 16 is 0.42m×0.6m. The spacing between the reconstruction points is 3cm. The number of reconstruction points is 15×21, a total of 315. The complex sound pressure p(r s ) can be calculated by the following formula:

[0090]

[0091] In the formula, p(r s ) is the reconstruction surface 17 on r s The complex sound pressure at the reconstruction point is, i is the imaginary unit, and k is the wave number.

[0092] In this embodiment, the normal particle velocity on the outlet end face of the muffler specimen 12 or the outlet end face of the substitute pipe T1 is calculated using the Euler formula and two-dimensional Fourier transform, which can be calculated by the following formula:

[0093]

[0094] In the formula, u n (r s ) is the surface r of the muffler outlet end s The normal component of the particle velocity at , is the normal component of the wave number on the end surface of the muffler outlet, FFT is the two-dimensional Fourier transform, FFT -1 is the inverse Fourier transform in two-dimensional space.

[0095] In this embodiment, the normal sound intensity is calculated by the normal particle velocity and the reconstructed complex sound pressure, which can be calculated by the following formula:

[0096]

[0097] In the formula, In is the normal sound intensity on the end face of the muffler outlet, Re represents the real part, and “*” represents the complex conjugate.

[0098] The radiated sound power level L at the outlet of the muffler specimen 12 in this embodiment is Wt2 And the radiated sound power level L at the outlet of the replacement tube T1 Wt1 It can be calculated from the normal sound intensity and the corresponding surface area, L Wt1 and L Wt2 It can be calculated by the following formula:

[0099]

[0100] In the formula, I′ n,i,j It is the normal sound intensity at the outlet of the test pipeline after the replacement pipe is installed, I″ n,i,j The normal sound intensity at the outlet of the muffler installed in the test pipe, the reference sound power W0 = 10 -12 W, surface area ΔS = 0.0009 m 2 , the number of reconstruction points N1×N2=15×21, where W represents watts.

[0101] In this embodiment, the insertion loss IL of the anechoic test piece 12 is determined by the radiated sound power level L at the outlet when the replacement tube T1 is installed. Wt1 and the radiated sound power level L at the outlet of the muffler specimen 12 when it is installed Wt2 The difference between , can be calculated by the following formula:

[0102]

[0103] Where IL is the insertion loss of the muffler, I′ n,i,j The normal sound intensity at the outlet of the test pipeline after the replacement pipe is installed, I″ n,i,j The normal sound intensity at the outlet of the test pipe after the silencer is installed is the number of reconstruction points N1×N2=15×21.

[0104] The transmission loss TL of the muffler in this embodiment is determined by the incident sound power level L at the inlet of the muffler specimen 12 in the sound chamber 1 after the muffler specimen 12 is installed. wi and the sound power level L at the outlet of the muffler specimen 12 in the receiving chamber 2 wt2 The difference is expressed as follows:

[0105]

[0106] Where, L Wi is the incident sound power level at the muffler inlet, L Wt is the radiated sound power level at the muffler outlet, is the average sound pressure level of the sound indoor space energy, the reference sound pressure P0 = 2.0 × 10 -5 Pa, the number of reconstruction points N1×N2=15×21, the subscripts i,j are the corresponding reconstruction point numbers, I n,i,j is the sound intensity distribution on the end surface of the muffler outlet, the surface area ΔS = 0.0009m 2 .

[0107] like Figure 7 Shown are the acoustic images of the normal sound intensity level distribution on the outlet end surface of the muffler specimen 12 calculated and obtained in this embodiment when the frequencies are 160 Hz, 315 Hz, 800 Hz, 1250 Hz, 1600 Hz, and 3150 Hz respectively;

[0108] In this implementation case, the size and distribution of the radiated sound energy at the outlet of the muffler 10 can be clearly understood through the acoustic image at the outlet of the muffler test piece 12;

[0109] like Figure 8 The figure shows the frequency curve of the sound attenuation of the muffler specimen 12 in this embodiment, specifically the frequency curve of the insertion loss IL;

[0110] like Fig. 9 The figure shows the frequency curve of the sound attenuation of the muffler specimen 12 in this embodiment, specifically the frequency curve of the transmission loss TL;

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A near-field acoustic holographic measurement system for muffler noise reduction, characterized in that: It includes a sound-generating chamber, a receiving chamber, a sound source device, a sound pressure collection device, a general-purpose computer, and a test pipe located between the sound-generating chamber and the receiving chamber; The sound source device includes a signal generator and a power amplifier installed in a general-purpose computer and a non-directional sound source located in the sound generation room; The sound pressure collection device includes a first microphone located at one or more measuring points in the sound emission chamber, a planar microphone array located in the receiving chamber and connected in sequence, a multi-channel data acquisition instrument, and a data recording and processing module installed on a general-purpose computer; Wherein, a muffler test piece or a replacement pipe is installed in the test pipeline.

2. The near-field acoustic holographic measurement system for muffler sound attenuation according to claim 1 is characterized in that: The muffler specimen has an effective length of 0.5 m, a width of 0.42 m, and a height of 0.6 m. The thickness of the muffler blade of the muffler specimen is 2.5 cm. The muffler shell is made of an aluminum alloy plate with a thickness of 1.5 mm, and the inside of the metal shell is provided with a sound-absorbing lining. The muffler specimen is at least one of a resistive muffler, a reactive muffler, an impedance composite muffler, and an active noise reduction muffler.

3. The near-field acoustic holographic measurement system for muffler sound attenuation according to claim 1 is characterized in that: The planar microphone array comprises a supporting structure and second microphones fixed on the supporting structure at equal intervals, wherein the second microphone is at least one of a free-field microphone, a pressure-field microphone, and a diffuse-field microphone.

4. The near-field acoustic holographic measurement system for muffler noise reduction according to claim 3 is characterized in that: The diameter of the second microphone is 3 mm to 26 mm; the fixed distance between the second microphones on the supporting structure is 2 cm to 10 cm.

5. The near-field acoustic holographic measurement system for muffler sound attenuation according to claim 1, characterized in that: The data recording and processing module includes a data recorder and an equivalent source near-field sound holographic anechoic output module.

6. The near-field acoustic holographic measurement system for muffler noise reduction according to claim 1, characterized in that: The one or more measuring points meet the following requirements when deployed: the first microphone is not facing the reflecting wall, the first microphone is at least 1.2 m above the ground, the first microphone is at least 1.1 m away from the nearest wall, the first microphone is at least 1.5 m away from the non-directional sound source, and if there are two or more measuring points, the distance between the first microphones at different measuring points is at least 1 m.

7. A method for measuring the near-field acoustic holographic measurement system of a muffler based on any one of claims 1 to 6, comprising the following steps: Step 1: starting the near-field acoustic holographic measurement system of the muffler attenuation based on the muffler test piece installed; Step 2: the signal generator generates a white noise signal or a pink noise signal, which is amplified by the power amplifier and fed back to the omnidirectional sound source. After the omnidirectional sound source stably emits sound, the multi-channel data acquisition instrument synchronously measures the first sound pressure data in the sound emission room and the second sound pressure data at the position of the planar microphone array in the receiving room; Step 3: Through the data recording and processing module, the first sound pressure data is processed by the frequency band filter to calculate the average sound pressure level of the sound energy in the sound chamber, and then the incident sound power level at the inlet of the muffler specimen is obtained; the second sound pressure data is used as the holographic surface input value, and the sound pressure distribution data of the end face of the muffler specimen outlet is obtained through inverse reconstruction; Step 4: using the Euler formula and two-dimensional Fourier transform to solve the sound pressure distribution data of the end surface normal sound intensity distribution data of the muffler test piece outlet, and outputting the sound image after processing by the frequency band filter; Step 5: Generate the radiated sound power level of the muffler specimen outlet under the corresponding frequency band after performing frequency band filtering based on the sound pressure distribution data and the sound intensity distribution data, and generate the transmission loss of the muffler specimen according to the incident power level and the radiated sound power level; Step 6: turn off the muffler near-field acoustic holographic measurement system, remove the muffler specimen, install a replacement tube in the test tube, start the muffler near-field acoustic holographic measurement system, and then repeat steps 2 to 5 to obtain the radiation power level at the outlet of the replacement tube, and generate the insertion loss of the muffler specimen based on the obtained radiation power at the outlet of the muffler specimen and the radiation sound power level at the outlet of the replacement tube; The calculation formula of the incident sound power level at the inlet of the muffler specimen is expressed as: Where, L Wi is the incident sound power level, is the spatial energy average sound pressure level at the measuring point in the sound chamber, P0 is the reference sound pressure, W0 is the reference sound power, S is the cross-sectional area of ​​the muffler specimen inlet, and ρ0c is the characteristic impedance of air.

8. The measuring method according to claim 7, characterized in that: The specific steps of obtaining the sound pressure distribution data of the end face of the muffler outlet through inverse reconstruction are: The equivalent source surface is arranged near the outlet of the muffler specimen, with the geometric center of the outlet end surface of the muffler specimen as the origin. The arrangement distance of the equivalent source surface is -1cm to -10cm. The number of equivalent source points on the equivalent source surface is at least greater than the number of measurement points on the plane array. The calculation formula of the source strength of the equivalent source point on the equivalent source surface is expressed as: Where q is the source strength of the equivalent source point, G(r|r m ) is the Green function of the free sound field, the subscript m represents the mth equivalent source point, p h is the holographic surface sound pressure data, which is measured by the set planar microphone array; The reconstructed surface is a conformal geometric plane at the outlet of the muffler specimen and coincides with the end surface of the outlet of the muffler specimen; The number of reconstruction points on the reconstruction surface is at least greater than the number of measurement points on the plane array; The calculation formula of the complex sound pressure on the reconstruction surface is expressed as: In the formula, p(r s ) is the reconstructed surface r s The complex sound pressure at , i is the imaginary unit, k is the wave number.

9. The measuring method according to claim 7, characterized in that: The normal sound intensity distribution data in step 4 is calculated based on the normal particle velocity, where: The normal particle velocity calculation formula is expressed as: In the formula, u n (r s ) is the surface r of the muffler specimen at the outlet s The normal component of the particle velocity at , is the normal component of the wave number on the end surface of the muffler specimen outlet, FFT is the two-dimensional Fourier transform, FFT -1 is the inverse Fourier transform in two-dimensional space; The calculation formula of the normal sound intensity distribution data is expressed as: In the formula, I n is the normal sound intensity on the outlet end surface of the muffler specimen, Re represents the real part, and "*" represents the complex conjugate; The acoustic image is at least one of the sound pressure distribution, normal sound intensity distribution, and radiated sound power on the end surface of the outlet end of the muffler specimen.

10. The measuring method according to claim 7, characterized in that: The transmission loss of the muffler specimen is calculated by the following formula: Where, L Wi is the incident sound power level at the inlet of the muffler specimen, L Wt is the radiated sound power level at the outlet of the muffler specimen, is the average sound pressure level of the sound chamber space energy, P0 is the basic sound pressure, N1 and N2 are the number of radial and axial reconstruction points on the plane of the muffler specimen outlet, respectively, and the subscripts i and j are the corresponding reconstruction point numbers. n,i,j is the sound intensity distribution on the outlet end surface of the muffler specimen, ΔS i,j is the value on the outlet end of the muffler specimen corresponding to I n,i,j The area of ​​the surface element; Among them, the radiated sound power level L at the outlet of the muffler specimen is Wt It is expressed as: Where W0 is the reference sound power; The calculation formula of the insertion loss of the muffler specimen is expressed as: Where IL is the insertion loss of the muffler specimen, I″ n,i,j I′ is the normal sound intensity at the outlet of the test pipe after the silencer specimen is installed. n,i,j Normal sound intensity at the outlet of the test pipe after installing a replacement tube.

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