A micro-perforated panel sound absorption structure with a variable cross-section back cavity structure and its design method

Abstract

The present invention relates to a micro-perforated panel sound absorption structure with a variable cross-section back cavity structure, which is characterized in that it includes a micro-perforated panel, a bottom rigid wall, an outer rigid wall, and an inner rigid wall. The outer rigid wall is a hollow cylinder, and a back cavity is formed between the micro-perforated panel, the bottom rigid wall, and the outer rigid wall. The inner rigid wall is a hollow frustum rigid wall with a gradually changing cross-section. The inner rigid wall divides the back cavity into a first back cavity and a second back cavity. The first back cavity is located inside the inner rigid wall, and the second back cavity is between the inner rigid wall and the outer rigid wall. The volumes of the first back cavity and the second back cavity are different. By using the inner rigid wall in the shape of a frustum with a variable cross-section to divide the conventional back cavity into two air back cavities with different volumes, the present invention realizes the absorption of noises with different frequencies by the micro-perforated panel sound absorption structure, achieves the purpose of broadening the sound absorption frequency band, improves the sound absorption performance, and also enhances the structural strength of the sound absorber.

Description

Technical Field

[0001] The present invention relates to the design of a broadband noise reduction structure. By improving the traditional micro-perforated panel absorber through a variable cross-section back cavity structure, a micro-perforated panel sound absorption structure with a variable cross-section back cavity structure is designed and its design method is given. Through simulation and experiments, it is proved that by reasonably designing the back cavity structure of the micro-perforated panel absorber, the sound absorption frequency band of the absorber can be effectively broadened. Background Art

[0002] The micro-perforated panel was proposed by Professor Ma Dayou in the last century and has a development process of more than 40 years. The micro-perforated panel is composed of a thin plate with a perforated grid of sub-millimeter size distributed on its surface. By reducing the aperture size to the sub-millimeter level, the perforations themselves provide the acoustic resistance and low acoustic reactance required by the sound absorber without using any porous materials. As a substitute for traditional porous sound absorption materials in recent years, the micro-perforated panel is widely used in the field of noise control. The reasons are as follows: First, there is a complete theoretical system and design data. In 1975, Professor Ma Dayou first proposed the theory and design of the micro-perforated sound absorption structure. After more than 40 years of research and development, its theoretical system has been improved and the design data has become more complete. Second, compared with traditional porous sound absorption materials, the micro-perforated panel has better structural strength, can adapt to more environmental places, and improves its usage level. And it will not have the disadvantages of secondary pollution and poor durability like traditional porous materials. Third, the micro-perforated panel sound absorption structure has good sound absorption performance (sound absorption coefficient and frequency range), and at the same time has the advantages of convenient design, simple structure, low cost, and easy processing.

[0003] However, in actual applications, considering the complex noise environment, the traditional single-layer micro-perforated panel sound absorption structure is difficult to meet the actual application. At the same time, due to certain difficulties in manufacturing micro-perforated panels with smaller apertures and in actual applications, the sound absorption performance of the traditional single-layer micro-perforated panel absorber will be limited by the environmental place, its own parameters, etc. Although the micro-perforated sound absorption structure is called a broadband sound absorption structure, the sound absorption frequency bandwidth of the conventional single-layer micro-perforated panel sound absorption structure is limited, making it difficult to become a general sound absorption and noise reduction structure. How to achieve broadband sound absorption of the single-layer micro-perforated panel sound absorption structure is an urgent problem to be solved at present. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention aims to provide a micro-perforated panel sound absorption structure with a variable cross-section back cavity structure. By using an inner rigid wall with a variable cross-section frustum shape to divide the conventional back cavity into two air back cavities with different volumes, the micro-perforated panel sound absorption structure can absorb noises of different frequencies, achieving the purpose of broadening the sound absorption frequency band, improving the sound absorption performance, and at the same time enhancing the structural strength of the sound absorber.

[0005] The present invention is realized through the following technical solutions.

[0006] A micro-perforated panel sound absorption structure with a variable cross-section back cavity structure, characterized in that it includes a micro-perforated panel, a bottom rigid wall, an outer rigid wall, and an inner rigid wall. The outer rigid wall is a hollow cylinder. The upper open end of the outer rigid wall is connected to the micro-perforated panel, and the lower open end of the outer rigid wall is connected to the bottom rigid wall. A back cavity is formed between the micro-perforated panel, the bottom rigid wall, and the outer rigid wall. The inner rigid wall is a hollow frustum rigid wall with a gradually changing cross-section. The upper open end of the inner rigid wall is connected to the micro-perforated panel, and the lower open end of the inner rigid wall is connected to the bottom rigid wall. The inner rigid wall divides the back cavity into a first back cavity and a second back cavity. The first back cavity is located inside the inner rigid wall, and the second back cavity is between the inner rigid wall and the outer rigid wall. The volumes of the first back cavity and the second back cavity are different, which changes their equivalent back cavity depth and thus changes the structural resonance frequency, absorbs noises of different frequencies, and achieves the purpose of broadening the sound absorption frequency band.

[0007] Preferably, the radii of the lower open ends of the outer rigid wall and the inner rigid wall are equal.

[0008] Preferably, the inner diameter of the lower open end of the outer rigid wall is equal to the outer diameter of the lower open end of the inner rigid wall.

[0009] Preferably, the micro-perforated panel is a panel with a single pore diameter, perforation rate, and uniform perforation.

[0010] Preferably, the micropores on the micro-perforated panel are uniformly arranged in an equilateral triangle pattern, and the micro-perforated panel is a rigid panel.

[0011] Preferably, the bottom rigid wall and the outer rigid wall are of an integral structure.

[0012] A design method for a micro-perforated panel sound absorption structure with a variable cross-section back cavity structure, characterized by including the following steps:

[0013] (1). According to the micro-perforated panel theory of Professor Ma Dayou and the computer calculation of the sound absorption performance of a conventional micro-perforated panel sound absorption structure, verify the accuracy of the model;

[0014] The relative acoustic impedance of a single-layer micro-perforated panel consists of a real part acoustic resistivity and an imaginary part acoustic reactance, which respectively represent the viscous effect of air and the inner wall surface of the hole and the inertial motion of air in the hole; the mathematical expression of the relative acoustic impedance of the micro-perforated panel can be written as:

[0015] z mmp =z 声阻 +z 声抗 =r + jωm (1)

[0016] Where:

[0017]

[0018]

[0019] In the above formula, r is the relative acoustic resistance of the micro-perforated plate, m is the relative acoustic mass of the micro-perforated plate, is the perforated plate constant, p is the perforation rate, d is the diameter of the micro-holes, t is the thickness of the perforated plate, c = 343 m / s is the sound speed, ω is the angular frequency, μ = 1.84×10 -5 Pa·s is the dynamic viscosity coefficient of air;

[0020] The relative acoustic impedance generated by the air back cavity with a depth of D can be written as:

[0021] z D = -jcot(ωD / c) (4)

[0022] Therefore, the total relative acoustic impedance Z of the sound absorber can be written as:

[0023] Z = z mmp + z D = r + j(ωm - cot(ωD / c)) (5)

[0024] When the sound wave is vertically incident, the sound absorption coefficient α of the sound absorption structure can be written as:

[0025]

[0026] where Re(Z) represents taking the real part of the impedance Z, and Im(Z) represents taking the imaginary part of the impedance Z;

[0027] The sound absorption coefficient of the micro-perforated sound absorber reaches the maximum value at resonance, and the maximum sound absorption coefficient value is:

[0028]

[0029] The resonance frequency f0 satisfies:

[0030] 2πf0m - cot(2πf0D / c) = 0 (8)

[0031] For the micro-perforated plate sound absorption structure with a variable cross-section back cavity structure designed in the present invention, the inner rigid wall divides the back cavity into two parts; according to the micro-perforated plate theory, its relative acoustic impedance can be written as:

[0032]

[0033] Among them:

[0034] Z1 = r1 + j(ωm1 - cot(ωD1 / c)) (10)

[0035] Z2 = r2 + j(ωm2 - cot(ωD2 / c)) (11)

[0036] Where Z1 and Z2 are the impedances of the first back cavity (5) and the corresponding region of the micro-perforated panel and the second back cavity (6) and the corresponding region of the micro-perforated panel respectively, r1 and r2 are the relative acoustic resistances of the two regions of the micro-perforated panel (the two regions represented by a1 and a2), m1 and m2 are the relative acoustic masses of the two regions of the perforated panel respectively, D1 = V1 / a1 and D2 = V2 / a2 are the equivalent cavity depths of the first back cavity (5) and the second back cavity (6) respectively, where V1 and a1 are the volume of the first back cavity and the area of the corresponding top region respectively, V2 and a2 are the volume of the second back cavity and the area of the corresponding top region respectively, and Z is the relative total impedance of the absorber; after obtaining the total acoustic impedance of the absorber, the absorption coefficient of the absorber can be obtained according to Equation (6);

[0037] As can be seen from the above theoretical formula, the impedance of the micro-perforated panel absorption structure is affected by the parameters of the micro-perforated panel itself and the height D of the air back cavity. Existing data show that the height D of the air back cavity can significantly affect the absorption peak frequency. By using the inner rigid wall of a frustum of a cone with a variable cross-section to divide the conventional circular air back cavity with a constant cross-section into two air back cavities with different volumes, and ensuring that the area ratios of the micro-perforated panels in the inner and outer regions are equivalent, different equivalent cavity depths can be obtained, thereby achieving multiple absorption peaks to broaden the absorption frequency band;

[0038] (2) By reasonably designing the top and bottom radii of the inner variable cross-section back cavity wall according to the micro-perforated panel theory verified in the previous step, different equivalent cavity depths D can be obtained, thereby achieving the absorption of noises with different frequencies; as can be seen from Equation (7), for the areas a1 and a2 of the two regions into which the micro-perforated panel is divided by the inner rigid wall of a frustum of a cone with a variable cross-section, their relative sizes affect the magnitude of the total impedance, and further affect the absorption performance of the absorption structure.

[0039] Preferably, through theoretical research and calculation, when the aperture, perforation rate and hole arrangement of the top micro-perforated panel are uniform, when a1 / a2≈1, there is a wider absorption frequency band and a higher absorption coefficient, and the relative magnitudes of the absorption peaks are close to 1.

[0040] Preferably, it further includes step (3) of verifying the designed micro-perforated panel absorption structure with a variable cross-section back cavity structure through simulation.

[0041] Compared with the prior art, the advantages of the present invention are:

[0042] 1. The micro-perforated panel absorption structure with a variable cross-section frustum wall of the present invention obtains air back cavities with different volumes by dividing the back cavity, and changes the resonance frequency of the structure by changing the equivalent back cavity depth to achieve the absorption of noises with different frequencies.

[0043] 2. The micro-perforated panel sound absorption structure with a frustum wall of variable cross-section according to the invention has better structural strength and can meet the strength requirements in certain specific occasions.

[0044] 3. The micro-perforated panel sound absorption structure with a frustum wall of variable cross-section according to the invention has the advantages of convenient design, simple structure, low cost, easy processing, etc., and has a wide application prospect in the field of noise reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is the longitudinal sectional view of the back cavity of the present invention;

[0046] Figure 2 is the top view of the back cavity of the present invention;

[0047] Figure 3 is the schematic diagram of the sound absorption structure of the present invention;

[0048] Figure 4 is the schematic diagram of the frustum wall of variable cross-section of the inner back cavity of the present invention;

[0049] Figure 5 is the schematic diagram of the side wall of the outer back cavity of the present invention;

[0050] Figure 6 is the schematic diagram of the micro-perforated panel of the present invention;

[0051] Figure 7 is the schematic sectional view of the overall structure of the present invention;

[0052] Figure 8 is the comparison diagram of the sound absorption effect diagrams of the present invention and the traditional structure;

[0053] In the figure: 1. Micro-perforated panel, 2. Bottom rigid wall, 3. Outer rigid wall, 4. Inner rigid wall, 5. Back cavity one, 6. Back cavity two. DETAILED DESCRIPTION OF THE INVENTION

[0054] The present invention will be further described below in conjunction with the drawings and specific embodiments, but it is not intended to limit the present invention.

[0055] As Figure 1 、 Figure 2 shown, it is the variable cross-section back cavity structure of the micro-perforated panel sound absorber, which is composed of an outer rigid wall 3 of equal cross-section cylindrical shape and an inner rigid wall 4 of frustum shape with variable cross-section. The air back cavity is divided into two air back cavities with different volumes by the inner rigid wall 4, namely back cavity one 5 and back cavity two 6. V1 in the figure is the volume of back cavity one 5, V2 is the volume of back cavity two 6, D is the height of the back cavity; r1 is the top radius of the inner rigid wall 4, and the area of the top region corresponding to back cavity one 5 is a1 = π(r1) 2 ; r2 is the radius of the outer rigid wall, and the area of the top region corresponding to back cavity two 6 is a2 = π(r2)2 -π(r1 + 1) 2 ;

[0056] As Figure 3 shown, it is a schematic diagram of the micro-perforated panel sound absorption structure, which consists of the micro-perforated panel at the top and the designed variable cross-section back cavity structure. In the figure, b is the hole pitch of the micro-perforated panel 1, and its relationship with the perforation rate p is t is the thickness of the micro-perforated panel 1, d is the hole diameter of the micro-perforated panel 1, and D is the back cavity height.

[0057] As Figure 4 shown, it is a schematic diagram of the inner layer variable cross-section frustum wall structure. Inside the inner rigid wall 4 of the variable cross-section frustum type is the back cavity one 5.

[0058] As Figure 5 shown, it is a schematic diagram of the outer layer equal cross-section cylindrical wall structure. The inner rigid wall 4 and the outer rigid wall 3 form an overall back cavity structure, and the outer rigid wall 3 and the bottom rigid wall 2 are an integral whole; the area between the inner rigid wall 4 and the outer rigid wall 3 is the back cavity two 6.

[0059] As Figure 6 shown, it is a schematic diagram of the micro-perforated panel. The circular micro-perforated panel and the overall back cavity form a sound absorption structure. The micro-perforated panel and the back cavity should be sealed to prevent sound leakage to ensure the effective sound absorption of the sound absorber.

[0060] As Figure 7 shown, it is a schematic longitudinal section diagram of the micro-perforated panel sound absorption structure designed by the present invention.

[0061] As Figure 8 shown, it is a comparison diagram of the sound absorption effects between the sound absorption structure designed by the present invention and the traditional sound absorption structure. It can be seen that the sound absorption structure designed by this patent has a wider sound absorption bandwidth than the conventional sound absorption structure.

[0062] As Figures 1 to 8As shown in the figure, a microperforated panel sound absorption structure with a variable cross-section back cavity structure, characterized in that it includes a microperforated panel 1, a bottom rigid wall 2, an outer rigid wall 3, and an inner rigid wall 4. The outer rigid wall 3 is a hollow cylinder. The upper open end of the outer rigid wall 3 is connected to the microperforated panel 1, and the lower open end of the outer rigid wall 3 is connected to the bottom rigid wall 2. A back cavity is formed between the microperforated panel 1, the bottom rigid wall 2, and the outer rigid wall 3. The inner rigid wall 4 is a hollow frustum rigid wall with a variable cross-section. The upper open end of the inner rigid wall 4 is connected to the microperforated panel 1, and the lower open end of the inner rigid wall 4 is connected to the bottom rigid wall 2. The inner rigid wall 4 divides the back cavity into a first back cavity 5 and a second back cavity 6. The first back cavity 5 is located inside the inner rigid wall 4, and the second back cavity 6 is between the inner rigid wall 4 and the outer rigid wall 3. The volumes of the first back cavity 5 and the second back cavity 6 are different, changing its equivalent back cavity depth and thus changing the structural resonance frequency to absorb noises of different frequencies and achieve the purpose of broadening the sound absorption frequency band.

[0063] Preferably, the radii of the lower open ends of the outer rigid wall 3 and the inner rigid wall 4 are equal.

[0064] Preferably, the inner diameter of the lower open end of the outer rigid wall 3 is equal to the outer diameter of the lower open end of the inner rigid wall 4.

[0065] Preferably, the microperforated panel 1 is a panel with a single pore diameter, perforation rate, and uniform perforations.

[0066] Preferably, the micropores on the microperforated panel 1 are uniformly arranged in an equilateral triangle pattern, and the microperforated panel 1 is a rigid panel.

[0067] Preferably, the bottom rigid wall 2 and the outer rigid wall 3 are of an integral structure.

[0068] A design method for a microperforated panel sound absorption structure with a variable cross-section back cavity structure, characterized in that it includes the following steps:

[0069] (1) According to the microperforated panel theory of Professor Ma Dayou and the computer calculation of the sound absorption performance of a conventional microperforated panel sound absorption structure, verify the accuracy of the model;

[0070] The relative acoustic impedance of a single-layer microperforated panel consists of a real part acoustic resistivity and an imaginary part acoustic reactance, respectively representing the viscous effect of air on the inner wall surface of the pores and the inertial motion of air in the pores; the mathematical expression of the relative acoustic impedance of the microperforated panel can be written as:

[0071] z mmp =z 声阻 +z 声抗 =r + jωm (1)

[0072] Where:

[0073]

[0074]

[0075] In the above formula, r is the relative acoustic resistance of the micro-perforated panel, m is the relative acoustic mass of the micro-perforated panel, is the perforated panel constant, p is the perforation rate, d is the diameter of the micro-holes, t is the thickness of the perforated panel, c = 343 m / s is the speed of sound, ω is the angular frequency, μ = 1.84×10 -5 Pa·s is the dynamic viscosity coefficient of air;

[0076] The relative acoustic impedance generated by the air back cavity with a depth of D can be written as:

[0077] z D = -jcot(ωD / c) (4)

[0078] Therefore, the total relative acoustic impedance Z of the sound absorber can be written as:

[0079] Z = z mmp + z D = r + j(ωm - cot(ωD / c)) (5)

[0080] When the sound wave is vertically incident, the sound absorption coefficient α of the sound absorption structure can be written as:

[0081]

[0082] where Re(Z) represents taking the real part of the impedance Z, and Im(Z) represents taking the imaginary part of the impedance Z;

[0083] The sound absorption coefficient of the micro-perforated sound absorber reaches the maximum value at resonance, and the maximum sound absorption coefficient value is:

[0084]

[0085] The resonance frequency f0 satisfies:

[0086] 2πf0m - cot(2πf0D / c) = 0 (8)

[0087] For the micro-perforated panel sound absorption structure with a variable cross-section back cavity structure designed in the present invention, the inner rigid wall divides the back cavity into two parts; according to the micro-perforated panel theory, its relative acoustic impedance can be written as:

[0088]

[0089] where:

[0090] Z1 = r1 + j(ωm1 - cot(ωD1 / c)) (10)

[0091] Z2 = r2 + j(ωm2 - cot(ωD2 / c)) (11)

[0092] Where Z1 and Z2 are the impedances of the first back cavity (5) and the corresponding micro-perforated panel region, and the second back cavity (6) and the corresponding micro-perforated panel region respectively. r1 and r2 are the relative acoustic resistances of the two regions of the micro-perforated panel (the two regions represented by a1 and a2). m1 and m2 are the relative acoustic masses of the two regions of the perforated panel. D1 = V1 / a1 and D2 = V2 / a2 are the equivalent cavity depths of the first back cavity (5) and the second back cavity (6) respectively, where V1 and a1 are the volume of the first back cavity 5 and the area of the corresponding top region, and V2 and a2 are the volume of the second back cavity 6 and the area of the corresponding top region. Z is the relative total impedance of the sound absorber. After obtaining the total acoustic impedance of the sound absorber, the absorption coefficient of the sound absorber can be obtained according to Equation (6).

[0093] It can be seen from the above theoretical formula that the impedance of the micro-perforated panel sound absorption structure is affected by the parameters of the micro-perforated panel itself and the height D of the air back cavity. Existing data show that the height D of the air back cavity can significantly affect the absorption peak frequency. By using an inner rigid wall with a variable cross-section frustum shape to divide the conventional equal cross-section circular air back cavity into two air back cavities with different volumes, and ensuring that the area ratio of the micro-perforated panels in the inner and outer regions is quite equal, different equivalent cavity depths can be obtained, so as to achieve multiple absorption peaks to broaden the sound absorption frequency band.

[0094] (2) By reasonably designing the top and bottom radii of the inner variable cross-section back cavity wall according to the micro-perforated panel theory verified in the previous step, different equivalent cavity depths D can be obtained, so as to achieve the absorption of noises with different frequencies. It can be seen from Equation (7) that the relative sizes of the areas a1 and a2 of the two regions where the micro-perforated panel is divided by the inner rigid wall with a variable cross-section frustum shape affect the size of the total impedance, and further affect the sound absorption performance of the sound absorption structure.

[0095] Preferably, through theoretical research and calculation, when the aperture, perforation rate and hole arrangement of the top micro-perforated panel are all uniform, when a1 / a2 ≈ 1, there is a relatively wide sound absorption frequency band and a relatively high absorption coefficient, and the relative sizes of the absorption peaks are close to 1.

[0096] Preferably, it further includes step (3) of verifying the designed micro-perforated panel sound absorption structure with a variable cross-section back cavity structure through simulation.

[0097] Example:

[0098] The following is a specific simulation model;

[0099] The sound absorption structure is set as a cylinder with a diameter of 29 mm for convenient measurement in the impedance tube. That is, the outer rigid wall 3 is a cylinder with a diameter of 29 mm.

[0100] Figure 8 It is a comparison diagram of the sound absorption effects between the implementation case and the conventional sound absorption structure. The parameters of the implementation case are as follows: the top radius r1 of the inner rigid wall 4 is 9 mm, the bottom radius r2 is 13.5 mm. At this time, a1 / a2≈1. The wall thicknesses of both the inner rigid wall 4 and the outer rigid wall 3 are 1 mm. The radius of the outer rigid wall 3 is 14.5 mm. The plate thickness t of the micro-perforated plate is 1 mm, the perforation rate p is 0.01, and the pore diameter d is 0.5 mm. Except for the absence of the inner variable cross-section frustum wall, all other parameters of the comparison case are the same as those of the implementation case.

[0101] From Figure 8 As can be seen from the comparison of the sound absorption effects shown, the micro-perforated plate sound absorption structure described in the present invention has a good absorption effect on noise. By using the variable cross-section frustum-shaped inner rigid wall 4 to unevenly divide the back cavity, the purpose of broadening the sound absorption frequency band of the sound absorption structure is achieved, and it has a wider sound absorption frequency band compared with the conventional micro-perforated plate sound absorption structure.

[0102] The above are only the preferred embodiments of the present invention, and do not limit the implementation manners and protection scope of the present invention accordingly. For those skilled in the art, it should be able to realize that all the equivalent replacements and obvious changes made by using the description and illustrations of the present invention should be included in the protection scope of the present invention.

Claims

1. A microperforated panel sound-absorbing structure with a variable cross-section back cavity structure, characterized in that, It includes a micro-perforated plate (1), a bottom rigid wall (2), an outer rigid wall (3), and an inner rigid wall (4). The outer rigid wall (3) is a hollow cylinder. The upper open end of the outer rigid wall (3) is connected to the micro-perforated plate (1), and the lower open end of the outer rigid wall (3) is connected to the bottom rigid wall (2). A back cavity is formed among the micro-perforated plate (1), the bottom rigid wall (2), and the outer rigid wall (3). The inner rigid wall (4) is a hollow frustum rigid wall with a gradually changing cross-section. The upper open end of the inner rigid wall (4) is connected to the micro-perforated plate (1), and the lower open end of the inner rigid wall (4) is connected to the bottom rigid wall (2). The inner rigid wall (4) divides the back cavity into a first back cavity (5) and a second back cavity (6). The first back cavity (5) is located inside the inner rigid wall (4), and the second back cavity (6) is between the inner rigid wall (4) and the outer rigid wall (3). The volumes of the first back cavity (5) and the second back cavity (6) are different.

2. The micro-perforated panel sound absorption structure with a variable cross-section back cavity structure according to claim 1, characterized in that, The radii of the lower open ends of the outer rigid wall (3) and the inner rigid wall (4) are equal.

3. The microperforated panel sound absorption structure with a variable cross-section back cavity structure according to claim 1, characterized in that, The inner diameter of the lower open end of the outer rigid wall (3) is equal to the outer diameter of the lower open end of the inner rigid wall (4).

4. The micro-perforated panel sound absorption structure with a variable cross-section back cavity structure according to claim 1, characterized in that The micro-perforated plate (1) is a panel with a single pore diameter, perforation rate, and uniform perforations.

5. The micro-perforated panel sound absorption structure with a variable cross-section back cavity structure according to any one of claims 1-4, characterized in that The micropores on the micro-perforated plate (1) are uniformly arranged in equilateral triangles, and the micro-perforated plate (1) is a rigid panel.

6. The micro-perforated panel sound absorption structure with a variable cross-section back cavity structure according to any one of claims 1-4, characterized in that The bottom rigid wall (2) and the outer rigid wall (3) are of an integral structure.

7. A design method for a micro-perforated panel sound absorption structure with a variable cross-section back cavity structure, characterized in that, It includes the following steps: (1). According to the micro-perforated plate theory and computer calculation of the sound absorption performance of a conventional micro-perforated plate sound absorption structure, verify the accuracy of the model; The relative acoustic impedance of a single-layer micro-perforated plate consists of a real part acoustic resistivity and an imaginary part acoustic reactance, which respectively represent the viscous effect of air and the inner wall surface of the hole and the inertial motion of air in the hole; the mathematical expression of the relative acoustic impedance of the micro-perforated plate is written as: z mmp = z 声阻 + z 声抗 = r + jωm (1) Where: In the above formula, r is the relative acoustic resistance of the micro-perforated plate, m is the relative acoustic mass of the micro-perforated plate, is the perforated plate constant, p is the perforation rate, d is the diameter of the micro-holes, t is the thickness of the perforated plate, c = 343 m / s is the speed of sound, ω is the angular frequency, μ = 1.84×10 -5 Pa·s is the dynamic viscosity coefficient of air; The relative acoustic impedance generated by an air back cavity with a depth of D is written as: z D = -j cot(ωD / c) (4) Therefore, the total relative acoustic impedance Z of the sound absorber is written as: Z = z mmp + z D = r + j(ωm - cot(ωD / c)) (5) When a sound wave is vertically incident, the sound absorption coefficient α of the sound absorption structure is written as: Where Re(Z) represents taking the real part of the impedance Z, and Im(Z) represents taking the imaginary part of the impedance Z; The sound absorption coefficient of the micro-perforated sound absorber reaches the maximum value at resonance, and the maximum sound absorption coefficient value is: The resonance frequency f0 satisfies: 2πf0m - cot(2πf0D / c) = 0 (8) For a micro-perforated plate sound absorption structure with a variable cross-section back cavity structure, the inner rigid wall divides the back cavity into two parts; according to the micro-perforated plate theory, its relative acoustic impedance is written as: Where: Z1 = r1 + j(ωm1 - cot(ωD1 / c)) (10) Z2 = r2 + j(ωm2 - cot(ωD2 / c)) (11) In the formula, Z1 and Z2 are respectively the impedance of the first back cavity (5) and the corresponding area of the micro-perforated plate and the second back cavity (6) The impedance of the corresponding area of the micro-perforated panel, r1 and r2 are the relative acoustic resistances of the two areas of the micro-perforated panel respectively, m1 and m2 are the relative acoustic masses of the two areas of the perforated panel respectively, D1 = V1 / a1, D2 = V2 / a2 are the equivalent cavity depths of the first back cavity (5) and the second back cavity (6) respectively, where V1 and a1 are the volume of the first back cavity (5) and the area of the corresponding top area respectively, V2 and a2 are the volume of the second back cavity (6) and the area of the corresponding top area respectively, and Z is the relative total impedance of the sound absorber; after obtaining the total acoustic impedance of the sound absorber, according to Equation (6) the absorption coefficient of the sound absorber can be obtained; It can be seen from the above theoretical formula that the impedance of the micro-perforated panel sound absorption structure is affected by the parameters of the micro-perforated panel itself and the height D of the air back cavity. Existing data show that the height D of the air back cavity can significantly affect the absorption peak frequency. By using the inner rigid wall of the variable cross-section frustum to divide the conventional equal cross-section circular air back cavity into two air back cavities with different volumes, different equivalent cavity depths can be obtained by ensuring that the area ratios of the micro-perforated panels in the inner and outer regions are equivalent, so as to achieve multiple absorption peaks to broaden the sound absorption frequency band; (2) By reasonably designing the top and bottom radii of the inner variable cross-section back cavity wall according to the micro-perforated panel theory verified in the previous step, different equivalent cavity depths D can be obtained, so as to achieve the absorption of noises with different frequencies; it can be seen from Equation (7) that the relative sizes of the areas a1 and a2 of the two regions where the micro-perforated panel is divided by the inner rigid wall of the variable cross-section frustum affect the magnitude of the total impedance, and thus affect the sound absorption performance of the sound absorption structure.

8. The design method of the micro-perforated panel sound absorption structure with a variable cross-section back cavity structure according to claim 7, characterized in that, Through theoretical research and calculation, when the aperture, perforation rate and hole arrangement of the top micro-perforated panel are uniform, a relatively wide sound absorption frequency band and a relatively high absorption coefficient can be obtained when a1 / a2≈1, and the relative magnitudes of the absorption peaks are close to 1.

9. The design method of the micro-perforated panel sound-absorbing structure with a variable cross-section back cavity structure according to claim 7 or 8, characterized in that, It further includes step (3) of verifying the designed micro-perforated panel sound absorption structure with a variable cross-section back cavity structure through simulation.

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