A low-frequency sound absorption structure and sound absorption method

Abstract

The present invention discloses a low-frequency sound absorption structure and a sound absorption method, comprising a sound absorption unit, wherein the sound absorption unit comprises a base, a cover plate, and an annular nested structure arranged between the base and the cover plate, wherein the annular nested structure comprises a plurality of layers of split resonant cavities from the outside to the inside, and the split resonant cavities are provided with longitudinal openings. The present invention has the advantage of being able to adjust internal components according to the noise spectrum of the actual environment to absorb noises of different frequencies, overcoming the technical bottleneck that the existing acoustic metamaterial cannot change the sound absorption frequency band once printed; the overall structure is compact, the low-frequency sound absorption performance is good, and the sound waves incident at different angles can be absorbed, and the sound absorption and noise reduction of small integrated equipment are suitable.

Description

Technical Field

[0001] The invention relates to the field of acoustic technology, in particular to a low-frequency sound absorption structure and a sound absorption method. Background Art

[0002] Noise cancellation plays an important role in our daily life, especially for low-frequency noise. Due to its high penetration, it is still a very difficult task to achieve effective absorption of low-frequency noise.

[0003] Traditional sound absorbers mostly absorb noise by adding fiber materials to the structure, but the sound absorption effect is poor in the low-frequency range, and fiber materials are prone to accumulate dust and bacteria, which will have an impact on the environment and human health. In recent years, with the rapid development of acoustic metamaterials, their acoustic properties that are different from natural materials have received widespread attention. Many sound absorbers with subwavelength sizes have been proposed and used for low-frequency noise control. In addition, due to the complexity and changeability of noise sources, the frequency of noise often changes. However, once the current sound-absorbing metamaterials are manufactured, their structure and sound absorption frequency band cannot be changed. This feature hinders the application of sound-absorbing metamaterials in actual noise reduction places. Summary of the invention

[0004] Purpose of the invention: In order to overcome the deficiencies in the prior art, one of the purposes of the present invention is to provide a low-frequency sound absorbing structure that can efficiently absorb complex low-frequency noise in the environment in a targeted manner, and has a compact structure that meets the needs of existing low-frequency noise control; the second purpose of the present invention is to provide a sound absorption method for a low-frequency sound absorbing structure.

[0005] Technical solution: A low-frequency sound absorbing structure described in the present invention includes a sound absorbing unit. The low-frequency sound absorbing structure can be composed of several sound absorbing units; the sound absorbing unit includes a base, a cover plate and an annular nested structure arranged between the base and the cover plate, the annular nested structure includes several layers of split resonance cavities from the outside to the inside, and the split resonance cavity is provided with a longitudinal opening, and the opening is a gap along the axial direction of the resonance cavity.

[0006] Preferably, the openings of the inner and outer adjacent split resonant cavities are arranged in opposite directions; in the initial state of the sound absorbing structure, the openings arranged in opposite directions from the outside to the inside are located on the same horizontal line.

[0007] That is: the outer resonant cavity is sequentially sleeved on the outside of the inner resonant cavity, and is sequentially sleeved to form a ring-shaped nested structure; and the gap openings of the adjacent inner and outer resonant cavities are sequentially set in reverse order; the base and cover are used to ensure the sealing of the structure, and the split resonant cavity has the same shape, which is annular, and can be square ring, circular ring, cross ring, polygonal ring and other different forms; the inner and outer multi-layer resonant cavities can be coaxially arranged or non-coaxially arranged. The opening angle α of the annular resonant cavity is the angle between the two ends of the opening and the center (axis) of the sound-absorbing structure; the rotation angle β of the inner resonant cavity is the angle between the rotated resonant cavity and the initial position. The height of the resonant cavity opening is h1, and the height of the internal sound channel formed by the adjacent resonant cavities is h2, h1≤h2.

[0008] The annular nested structure includes at least one layer of rotatable resonant cavity, which is located inside the annular nested structure. The annular resonant cavity is in the shape of a circular ring. By rotating any one or more layers of the split resonant cavity inside, the rotation angle ranges from 0° to 180°, the acoustic impedance of the sound absorbing structure can be continuously changed, thereby continuously adjusting the sound absorption frequency band within a wide frequency band.

[0009] Preferably, the rotatable resonant cavity is connected to the cover plate, and the other resonant cavities are fixedly connected to the base; and a split printing scheme is adopted, and the two are nested to form an integral sound-absorbing structure.

[0010] The number of layers of the above split resonant cavity is recorded as N layers, and the number of layers of the resonant cavity is arranged from the outside to the inside. The specific number of layers can be changed and adjusted according to actual needs. The N layers of annular split resonant cavities placed in reverse order form N-1 internal sound channels. The sound wave enters from the opening of the first layer of the split resonant cavity, passes through the internal sound channel and the opening of the inner layer resonant cavity, and finally enters the internal cavity; the rotating split resonant cavity is connected to the upper cover plate, and the other split resonant cavities are connected to the lower cover plate (base), and the two are nested to form an overall sound absorption structure. And by rotating the split resonant cavity connected to the upper cover plate, the continuous change of the acoustic impedance of the sound absorption structure can be achieved, thereby achieving continuous adjustment of the sound absorption frequency band.

[0011] The spacing d between the split resonant cavities, the cavity thickness t, the cavity height h, and the cavity opening size α are all variable, and the spacing d between adjacent split resonant cavities satisfies The opening angle α of the split resonant cavity satisfies Among them, η is the shear viscosity coefficient of the background medium, ρ0 is the mass density of the background medium, ω is the angular frequency, r1 is the radius of the outermost resonance cavity, and the preferred range is to ensure that the opening of the resonance cavity and the internal sound channel meet the requirements of a narrow slit.

[0012] Furthermore, the radius r1 of the outermost split resonant cavity is 1-5 cm, the thickness t of the resonant cavity is 1-3 mm, the spacing d between the resonant cavities is 1-3 mm, and the opening angle α is 1-6°. The sound absorption band is mainly a low-frequency band, that is, the sound wave frequency f < 1000 Hz.

[0013] Preferably, the split resonant cavities have the same shape and thickness, the spacing between adjacent split resonant cavities is the same, and the opening angle of each split resonant cavity is the same.

[0014] The length of the sound absorbing structure along the direction of sound wave propagation is 2r1, and the total height of the sound absorbing structure (the sum of the height of the resonance cavity and the height of the cover plate and the base) is H, which is much smaller than the wavelength of the low-frequency sound wave, belongs to the sub-wavelength scale, and has a compact structure.

[0015] The material of the sound absorbing structure is an acoustic rigid material; preferably, the manufacturing material of the sound absorbing structure is metal or organic plastic. Preferably, the acoustic impedance of the wall surface (inner and outer surfaces) of the sound absorbing structure is much greater than the background medium, at least 100 times the acoustic impedance of the background medium.

[0016] The present invention also provides a sound absorption method for a low-frequency sound absorption structure, which adjusts the acoustic impedance of the sound absorption structure by rotating at least one layer of split resonance cavity inside the annular nested structure.

[0017] Optionally, the rotation angle of the split resonant cavity is adjusted so that the openings of the inner and outer adjacent split resonant cavities are arranged in opposite directions; in the initial state of the sound absorbing structure, the openings arranged in opposite directions from the outside to the inside are located on the same horizontal line.

[0018] The acoustic impedance of the sound absorbing structure is adjusted by rotating at least one layer of split resonant cavity inside the above-mentioned annular nested structure. The openings of each resonant cavity can be set to be on the same straight line in the initial state, the acoustic impedance of the initial internal sound channel is the largest, and the acoustic impedance of each opening is constant; the impedance of the internal sound channel is adjusted by rotating the resonant cavity, so that the acoustic impedance of the internal sound channel changes continuously, thereby effectively absorbing sound waves of different frequencies.

[0019] The sound-absorbing structure of the present invention nests N layers of annular split resonant cavities, changes the direction of sound wave propagation inside the structure, increases the effective length of sound wave propagation, and thus increases the sound quality of the entire structure, allowing the structure to resonate at low frequencies. This solution reduces the volume of the structure and makes the structure more compact. In addition, the opening of the split resonant cavity and the internal sound channel are both micro-slit sound absorbers, which increase the thermal viscous friction between the background medium and the sound-absorbing structure, thereby achieving efficient sound absorption at the resonant frequency. The split resonant cavity is in the form of a circular ring, and the rotating split resonant cavity is connected to the upper cover plate, and the other split resonant cavity is connected to the lower cover plate. The two are nested to form an integral sound-absorbing structure. By rotating the inner split resonant cavity, the acoustic impedance of the internal sound channel changes continuously, thereby effectively absorbing sound waves of different frequencies.

[0020] Beneficial effects: The present invention is a continuously adjustable low-frequency sound absorption structure, which solves the technical problems that the existing sound absorption structure has poor low-frequency sound absorption performance and cannot adjust the sound absorption frequency according to the complex noise spectrum. The present invention has the advantage of being able to adjust the internal components according to the noise spectrum of the actual environment to absorb noise of different frequencies, overcoming the technical bottleneck that the existing acoustic metamaterial cannot change the sound absorption frequency band once printed; the overall structure is compact, the low-frequency sound absorption performance is good, and it can absorb sound waves incident at different angles, which is suitable for sound absorption and noise reduction of small integrated equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural schematic diagram of the sound absorbing unit.

[0022] Figure 2 It is a schematic diagram of the sound absorbing unit structure with the upper cover removed.

[0023] Figure 3 It is a schematic cross-sectional view of the sound absorbing unit.

[0024] Figure 4 It is a schematic diagram of the structure of the second-layer resonant cavity and the upper cover.

[0025] Figure 5 It is a schematic diagram of the structure of the first-layer resonant cavity, the third-layer resonant cavity and the lower cover.

[0026] Figure 6 It is the acoustic circuit diagram of the present invention.

[0027] Figure 7 1 is a comparison diagram of the theoretical results and simulation results of the present invention; (a) is the theoretical result, and (b) is the simulation result.

[0028] Figure 8 1 is a comparison diagram of the experimental results and the simulation results of the present invention; (a) is the simulation result, and (b) is the experimental result.

[0029] Fig. 9 It is the simulated sound absorption spectrum of the sound absorption structure of the present invention under oblique incidence. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the embodiments.

[0031] A low-frequency sound absorption structure includes a sound absorption unit, the sound absorption unit includes a base 2, a cover plate 1, and an annular nested structure 6 arranged between the base and the cover plate, the annular nested structure 6 includes a plurality of layers of split resonant cavities from outside to inside, and the split resonant cavities are provided with longitudinal openings 7. The thickness of the split resonant cavities is the same, the spacing between adjacent split resonant cavities is the same, and the opening angle of each split resonant cavity is the same. This embodiment takes a coaxially arranged three-layer split resonant cavity as an example, and all use an annular resonant cavity.

[0032] like Figure 1 The figure shows a schematic diagram of the structure of the sound absorbing unit. The sound absorbing unit is cylindrical in appearance, with a total height of H, upper and lower bottom surfaces being circular with a radius of r1, and a longitudinal opening 7 with a height of h1 being provided on the side wall.

[0033] like Figures 2 to 5 As shown, a continuously adjustable low-frequency sound absorption structure is composed of three layers of annular split resonant cavities, namely, a first layer of split resonant cavity 3, a second layer of split resonant cavity 4, and a third layer of split resonant cavity 5, and each split resonant cavity is provided with a longitudinal gap with a height of h1 and an opening angle of α to form an opening of the resonant cavity; the height of the internal sound channel formed by adjacent resonant cavities is h2 (not shown in the figure), and h1≤h2. In this embodiment, the opening height h1 and the internal sound channel h2 are equal.

[0034] The radii of the three layers of resonance cavities from outside to inside are r1, r2, and r3, respectively, and the thickness is t. The three layers of annular split resonance cavities are placed in reverse order with a spacing of d to form two layers of internal sound channels. The second layer of split resonance cavity 4 can rotate to change the acoustic impedance of the internal sound channel. The rotation angle is β. The sound wave enters from the opening of the first layer of split resonance cavity 2, passes through the internal sound channel and the openings of the second and third layers of resonance cavities, and finally enters the internal cavity. Figure 3 As shown, the resonant cavity opening angle α is the angle between the two ends of the opening and the line connecting the center (axis) of the sound absorbing structure; the rotation angle β of the inner resonant cavity is the angle between the rotated resonant cavity and the initial position.

[0035] like Figure 4 , 5As shown, the first-layer split resonant cavity 3 and the third-layer split resonant cavity 5 are connected to the base 2, and the second-layer split resonant cavity 4 is connected to the upper cover 1, and a split printing scheme is adopted, and the two are nested to form an overall sound absorption structure; by rotating the second-layer split resonant cavity 4, the combination of internal sound channels will change, so that the acoustic impedance of the sound absorption structure changes continuously, thereby effectively absorbing sound waves of different frequencies.

[0036] The following is a theoretical explanation of the continuously adjustable low-frequency sound absorption structure:

[0037] like Figure 6 As shown, the continuously adjustable low-frequency sound absorption structure can be equivalent to an acoustic circuit diagram, and the openings of the N split resonant cavities are equivalent to N series-connected acoustic impedances Z ai (i=1, 2, ..., N), the acoustic impedance Z of the internal acoustic channel of each layer axy (x=1,2,...,N-1;y=1,2) are determined by the acoustic impedance Z of the acoustic channels on both sides. ax1 and Z ax2 The acoustic impedance of the two layers of internal sound channels and the acoustic impedance of the micro-slit tube opening are connected in series in sequence. When the inner split resonant cavity rotates, the acoustic impedance Z of each parallel sound channel ax1 and Z ax2 changes, thus affecting the acoustic impedance Z of the entire internal acoustic channel axy , C a It represents the acoustic capacity of the internal cavity;

[0038] The acoustic impedance of the three split resonant cavity openings is Z ai =R ai +jωM ai (i=1,2,...,N), the acoustic impedance is The sound quality is M ai =ρ0t km , Where η is the shear viscosity coefficient of the background medium, ρ0 is the mass density of the background medium, and d i is the equivalent length of the resonant cavity opening, F(e) is the full elliptic function, t and h1 are the thickness of the resonant cavity and the height of the resonant cavity opening respectively;

[0039] The acoustic impedance of the two internal acoustic channels is Z axy =R axy +jωM axy (x=1,2,...,N-1;y=1,2), acoustic impedance Sound quality M axy =ρ0L xy k m , in is the equivalent length of the internal sound channel, β is the rotation angle of the inner split resonant cavity, d is the spacing of the resonant cavity, r x is the radius of the x-th split resonant cavity, a0 is half the spacing of the resonant cavity, and h2 is the height of the sound channel inside the resonant cavity;

[0040] The acoustic capacity of the internal cavity can be expressed as where r N is the radius of the internal cavity, c0 is the speed of sound in air;

[0041] It can be deduced that when the inner split resonant cavity rotates, the overall acoustic impedance of the sound absorbing structure is

[0042] From this, we get the reflection coefficient r of the sound intensity I The expression of absorption coefficient α is

[0043] Z0=ρ0c0 / S,S=(2r1+0.01)H,α=1-r I , where H is the height of the overall structure.

[0044] The entire sound-absorbing structure nests three layers of annular split resonant cavities together to change the direction of sound wave propagation inside the structure, increase the effective length of sound wave propagation, and thus increase the sound quality of the entire structure, allowing the structure to resonate at low frequencies. This solution reduces the volume of the structure and makes the structure more compact. Its size is much smaller than the wavelength of low-frequency sound waves and is on a sub-wavelength scale. The opening of the split resonant cavity and the internal sound channel are both micro-slit sound absorbers, which increase the thermal viscous friction between the background medium and the sound-absorbing structure, thereby achieving efficient sound absorption at the resonant frequency. By changing the rotation angle of the second layer of the split resonant cavity, the sound quality of the internal sound channel changes, so that the resonant frequency can be changed continuously, and because its acoustic resistance changes very little, the sound absorption coefficient remains very high.

[0045] When designing the above structural parameters, the parameters can be adjusted according to the actual environmental requirements. In this embodiment, mainly for low-frequency 280-580Hz frequency band sound absorption, the height, thickness, opening angle, and spacing of the three-layer split resonant cavity are kept consistent. The radii r1, r2, and r3 of the three-layer split resonant cavity are 25mm, 21mm, and 17mm respectively, and the height h, thickness t, spacing d, and opening angle α are 60mm, 2mm, 2mm, and 3° respectively.

[0046] Figure 7This is a comparison chart of the theoretical results and simulation results of the sound absorption structure of this embodiment under vertical incidence of sound waves. The horizontal axis in the figure is the rotation angle of the second-layer split resonant cavity, and the vertical axis is the frequency of the incident sound wave. The color depth in the figure represents the sound absorption coefficient of the structure. The darker the color, the larger the sound absorption coefficient, the closer it is to 1; the lighter the color, the smaller the sound absorption coefficient, the closer it is to 0. There is a good consistency between the theory and the simulation. The sound absorption coefficient exceeds 0.9 in the frequency range of 280-580Hz, and perfect sound absorption can be achieved at 280-380Hz.

[0047] Figure 8 The following are comparison diagrams of simulation results and experimental results of samples made of ABS plastic. They are all simulation and experimental comparison diagrams of 7 angles with vertical incident sound waves, rotation angle β from 0° to 180°, and step length of 30°. The horizontal axis in the figure is the frequency of the incident sound wave, and the vertical axis is the sound absorption coefficient of the structure. Figure (a) is the simulation result, and Figure (b) is the experimental result. The angle of the curve in the figure increases from left to right. Figure 8 It can be seen that the experiment is in good agreement with the simulation. By changing the rotation angle of the second layer of split resonant cavity, the sound absorption frequency can be continuously changed without changing the original structure.

[0048] Fig. 9 It is the simulated sound absorption spectrum of the sound-absorbing structure under oblique incidence. The sound wave incident angle ranges from 0° to 60°, with a total of three incident directions. Fig. 9 It can be seen that the sound absorption spectrum at small oblique incidence is almost the same as that at normal incidence, while the absorption performance at large oblique incidence is slightly weakened. Therefore, it can be shown that the sound absorption structure of the present invention can efficiently absorb sound waves from different directions.

Claims

1. A low-frequency sound absorbing structure, characterized in that: The sound absorbing unit includes a base, a cover plate, and an annular nested structure arranged between the base and the cover plate. The annular nested structure includes a plurality of layers of split resonant cavities from outside to inside. The split resonant cavities are provided with longitudinal openings. The spacing d between adjacent split resonant cavities satisfies The opening angle α of the split resonant cavity satisfies η is the shear viscosity coefficient of the background medium, ρ0 is the mass density of the background medium, ω is the angular frequency, and r1 is the radius of the outermost resonant cavity.

2. The low-frequency sound absorbing structure according to claim 1, characterized in that: At least one layer of rotatable split resonance cavity is arranged inside the annular nested structure.

3. The low-frequency sound absorbing structure according to claim 1, characterized in that: The split resonant cavity is in the shape of a circular ring.

4. The low-frequency sound absorbing structure according to claim 1, characterized in that: The radius r1 of the outermost split resonant cavity is 1-5 cm, the thickness t of the resonant cavity is 1-3 mm, the spacing d between the resonant cavities is 1-3 mm, and the opening angle α is 1-6°.

5. The low-frequency sound absorbing structure according to claim 1, characterized in that: The split resonant cavities have the same shape and thickness, the spacing between adjacent split resonant cavities is the same, and the opening angle of each split resonant cavity is the same.

6. The low-frequency sound absorbing structure according to claim 1, characterized in that: The sound absorbing structure is made of metal or organic plastic.

7. The low-frequency sound absorbing structure according to claim 1, characterized in that: The acoustic impedance of the wall of the sound-absorbing structure is at least 100 times the acoustic impedance of the background medium.

8. A sound absorption method of a low-frequency sound absorption structure, applied to the low-frequency sound absorption structure according to any one of claims 1 to 7, characterized in that: The acoustic impedance of the sound absorbing structure is adjusted by rotating at least one layer of the split resonance cavity inside the annular nested structure.

9. The sound absorbing method of the low-frequency sound absorbing structure according to claim 8, characterized in that: The rotation angle of the split resonant cavity is adjusted so that the openings of the inner and outer adjacent split resonant cavities are arranged in opposite directions.

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