A subwavelength ventilated asymmetric sound absorber

Abstract

The present invention discloses a subwavelength ventilated asymmetric sound absorber, which is composed of one or more absorption cells. The absorption cell includes a metamaterial resonator and a backing impedance boundary. The backing impedance boundary includes a rear panel, a top panel, a bottom panel, a long plate and a short plate, which together form an L-shaped structure; a flow port is provided on the upper portion of the rear panel to facilitate airflow; the bottom of the metamaterial resonator is placed on the short plate of the backing impedance boundary, and a channel is left between it and the bottom plate of the backing impedance boundary; the back of the metamaterial resonator is fixedly connected to the side of the long plate of the backing impedance boundary, and a channel is left between it and the rear panel of the backing impedance boundary. The deep subwavelength absorber of the present invention can almost perfectly absorb the sound energy incident from the loss end and almost perfectly reflect the sound energy incident from the opposite end, has a good suppressive effect on broadband noise, and can achieve ventilation and heat dissipation functions while controlling noise.

Description

Technical Field

[0001] The present invention belongs to the technical field of acoustic noise reduction and relates to a ventilated asymmetric sound absorber for low-frequency sound waves, in particular to a subwavelength broadband ventilated asymmetric sound absorber based on coupling of a Fabry-Perot resonator (FFP) with a back-propagating impedance boundary. Background Art

[0002] Suppressing low-frequency sound waves has always been an important and challenging topic in the acoustics community. Traditional acoustic absorbers, such as porous sound-absorbing materials, are limited by linear response theory, making their ability to absorb low-frequency sound waves heavily dependent on increasing material thickness. To construct subwavelength-scale, low-frequency sound absorbers, acoustic metamaterials based on localized resonance mechanisms have emerged. These absorbers highly localize sound energy within the resonator and dissipate it as heat through frictional losses and molecular relaxation.

[0003] Compared to traditional porous sound-absorbing materials, absorbers constructed based on metamaterials have advantages such as small size and high absorption properties. However, the dense rigid backing in such absorbers hinders airflow and heat exchange between the absorber and the surrounding space, making them unable to meet many requirements of practical applications. To overcome this drawback, a ventilated absorber was constructed based on multiple cascaded resonant metaatoms in a dual-port open system. In this type of absorption system, an acoustically equivalent soft boundary can be constructed through a resonance mechanism, and asymmetric absorption can be achieved by coupling with lossy resonant supercells [Appl. Phys. Lett., 111, 143502 (2017), Phys. Rev. Appl. 11, 024022 (2019)]. Due to the dual-end openness of the absorption system, asymmetric absorption is achieved while allowing free airflow, resulting in excellent ventilation and heat dissipation performance. However, the equivalent soft boundary based on the resonance mechanism is limited to a narrow bandwidth, making it difficult to expand the operating frequency band to a broadband one. Currently, existing broadband ventilated asymmetric sound absorbers are mostly focused on duct systems [Appl. Phys. Lett., 111, 143502 (2017), J. Sound Vib. 479, 115371 (2020)]. However, there is a lack of ventilated asymmetric absorbers for free space, which greatly limits their application. Furthermore, the equivalent soft boundary constructed based on the resonance mechanism complicates the structure of ventilated asymmetric absorption systems. Summary of the Invention

[0004] In view of the shortcomings of the existing equivalent soft boundary based on the resonance mechanism, such as narrow band limitation and complex absorber structure, the present invention proposes a ventilated asymmetric absorber with deep subwavelength thickness.

[0005] The technical solution adopted by the present invention to solve the problem is:

[0006] A subwavelength ventilated asymmetric sound absorber is composed of one or more absorption cells, each of which includes a metamaterial resonator and a backing impedance boundary. The backing impedance boundary includes a rear panel, a top panel, a bottom panel, a long plate, and a short plate, which together form an L-shaped structure. The upper portion of the rear panel is provided with a flow opening to facilitate airflow. The bottom of the metamaterial resonator is placed on the short plate of the backing impedance boundary, with a channel left between it and the bottom plate of the backing impedance boundary. The back of the metamaterial resonator is fixedly connected to the side of the long plate of the backing impedance boundary, and a channel left between it and the rear panel of the backing impedance boundary.

[0007] Furthermore, the bottom of the metamaterial resonator is fixedly connected to the short plate backing the impedance boundary.

[0008] Furthermore, the backing impedance boundary also includes a front panel, which is fixedly connected to the long plate side of the backing impedance boundary and has a channel between the front panel and the back panel; the back of the metamaterial resonator is fixedly connected to the front panel; and a flow port is provided at the bottom of the front panel.

[0009] Furthermore, a plurality of absorption cells are arranged in an array to form the sound absorber, and each absorption cell has a metamaterial resonator with the same or different resonance frequency.

[0010] Furthermore, the metamaterial resonator adopts a folded Fabry-Perot resonator, a Helmholtz resonator or a curled space resonator.

[0011] Furthermore, the absorption cell is an integrated structure formed by 3D printing or injection molding, or is obtained by mechanical processing.

[0012] Furthermore, the widths of the bottom opening of the metamaterial resonator in the propagation direction and the perpendicular propagation direction are w and w respectively. x =20mm~32mm and w y = 24mm ~ 29.4mm; the channel length of the backing impedance boundary is (H-2h) = 140mm ~ 180mm, and the width of the backing impedance boundary in the propagation direction is w sx =5~20mm, the short plate height of the backing impedance boundary is h=10mm~30mm.

[0013] The absorber of this invention couples lossy metaatoms with a backing impedance boundary, enabling the realization of a deep subwavelength ventilated absorber. Furthermore, by coupling multiple absorber cells with different operating frequencies, a broadband subwavelength ventilated absorber can be constructed. This invention offers the advantages of deep subwavelength, broadband, efficient absorption, ventilation and heat dissipation, and a simple structure, providing a viable solution for low-frequency noise control. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A three-dimensional view of the deep subwavelength asymmetric absorber designed for this invention. The left inset shows a three-dimensional view of the FFP resonator, where 1 is the opening of the FFP resonator, 2 is the internal channel of the FFP resonator, and 3 is the FFP resonator cover (transparent here to clearly show its internal channel). The right inset shows a three-dimensional view of the backing impedance boundary, where 4 is the channel of the impedance boundary, 5 is the bottom plate, 6 is the short plate, 7 is the long plate, 8 is the top plate, 9 is the back plate, and 10 is the front plate (transparent here).

[0015] Figure 2 A three-dimensional view of the broadband vented asymmetric absorber designed for this invention. The Roman numerals in the figure represent FFP resonators with different resonant frequencies, with I to VIII representing the resonant frequencies from low to high. In this invention, the resonant frequency is determined by the length of the internal folded channel.

[0016] Figure 3 is the scattering coefficient of the single-band ventilated asymmetric absorber, (a) is the scattering curve when the sound wave is incident from the left, and (b) is the sound energy scattering curve when the sound wave is incident from the right.

[0017] Figure 4 is the scattering coefficient of the broadband ventilated asymmetric absorber, (a) is the sound energy scattering curve when the sound wave is incident from the left, and (b) is the sound energy scattering curve when the sound wave is incident from the right. DETAILED DESCRIPTION

[0018] The present invention couples the acoustic resonance meta-atom with large loss to the backing impedance surface to obtain a highly symmetric asymmetric absorber (such as Figure 1 In this embodiment, the lossy meta-atom is an FFP resonator (as shown in FIG. Figure 1 The width of the bottom opening of the resonator is denoted as w x and w y , the width of the internal channel is denoted as w x and w z The thickness of the structure wall is t; the lengths of the external sides are W and (Hh). The structure of the backing impedance boundary is as follows Figure 1As shown in the illustration on the right side, it includes a front panel 10, a rear panel 9, a top panel 8, a bottom panel 5, a long panel 7 and a short panel 6, which together form an L-shaped structure. Among them, the front panel 10 can be omitted. In this embodiment, the front panel 10 is fixed to the side of the long panel 7, and a gap channel is left between the front panel 10 and the rear panel 9. A flow opening is provided on the upper part of the rear panel 9 and the lower part of the front panel 10. In this embodiment, gaps of the same height are set between the top of the rear panel 9 and the top panel 8, and between the bottom of the front panel 10 and the bottom panel 5 as flow openings. Of course, through holes and other structures can also be set on the two panels to facilitate airflow. The back of the metamaterial resonator is fixed to the front panel 10 of the backing impedance boundary, and the bottom of the metamaterial resonator is placed on the short panel 6 of the backing impedance boundary, and a gap channel is retained between it and the bottom panel 5 of the backing impedance boundary, so that an L-shaped channel is formed with the flow openings of the two panels, allowing airflow to circulate on the left and right sides of the absorber. The widths of the channels are w and w, respectively. sx and w sy , length is H-2h, and the total thickness of the absorber is T.

[0019] By coupling FFP resonators with the same resonant frequency with the backing impedance boundary, a deep subwavelength ventilated asymmetric absorber can be obtained. This working mechanism can be extended to broadband, that is, by connecting multiple FFP resonators with different resonant frequencies in parallel and coupling them with the backing impedance boundary, a broadband ventilated asymmetric absorber (such as Figure 2 (Figure 2 shows a single cell of a broadband absorber). The absorber of the present invention can nearly perfectly absorb acoustic energy incident from the left and nearly perfectly reflect acoustic energy incident from the right. The deep subwavelength ventilated asymmetric absorber of the present invention can be manufactured using materials such as epoxy resin, photosensitive resin, and nylon through 3D printing, injection molding, and other methods, or using materials such as wood and metal through conventional machining methods. The present invention is described in detail below using specific examples.

[0020] Example 1

[0021] The asymmetric absorber was fabricated using 3D printing technology, and the geometric parameters of the structure are shown in Table 1. The external dimensions of the FFP resonator are W = 160 mm, (Hh) = 190 mm; the width of the bottom opening is w x =22mm, w y =29.4mm; the width of the internal channel is w x =22mm, w z =29.4mm; the width of the backing channel is w sx =10mm,w sy = 158mm; the wall thickness of the structure is t = 1mm, and the total thickness of the absorber is T = 34mm. The resonant frequency of the FFP resonator is 100Hz. The absorption curve obtained by the absorber is as follows Figure 3As shown, Figure 3 (a) and (b) show the absorber's scattering curves for sound energy incident from the left and right, respectively. When the sound wave is incident from the left, the system absorbs the 100 Hz sound wave nearly perfectly (the wavelength is 100.8 times the absorber thickness); however, the sound wave incident from the right is almost perfectly reflected. Furthermore, because the absorber's backing boundary, front panel 10 and rear panel 9, are both equipped with airflow openings, ventilation is achieved.

[0022] Example 2

[0023] Use 3D printing technology to process Figure 2 The geometric parameters of the absorber cell are shown in Table 1. In the absorber, the external dimensions of each FFP resonator are W = 40 mm, (Hh) = 95 mm; the width of the bottom opening is w x =24mm, w y =11.3mm, the width of the internal channel is w x =24mm, w z =11.3 mm; the equivalent lengths of the folded channels inside the FFP resonator (marked with Roman numerals I to VIII) are [256, 253, 250, 244, 237, 233.5, 231, 226.5] mm respectively; the width of the backing channel is w sx =5mm,w sy =38mm; the wall thickness of the structure is t=1mm, and the total thickness of the absorber is T=32mm. The absorption curve obtained by the absorber is as follows Figure 4 As shown in the figure, for sound waves incident from the left, the absorber exhibits an absorption coefficient greater than 85% within the frequency range of 308Hz to 352Hz (wavelengths corresponding to 33.2 to 29.1 times the absorber's thickness). For sound waves incident from the right, the theoretical absorption coefficient within this frequency range is less than 1.5%. Furthermore, because the absorber's backing edges, front panel 10 and rear panel 9, are provided with ventilation openings, ventilation is achieved.

[0024] Table 1 Geometric parameters of ventilated asymmetric absorber (unit: mm)

[0025] W H h <![CDATA[w sx ]]> <![CDATA[w sy ]]> <![CDATA[w x ]]> <![CDATA[w y ]]> <![CDATA[w z ]]> t T Example 1 160 200 10 10 158 22 29.4 29.4 1 34 Example 2 40 100 5 5 38 24 11.3 11.3 1 32

Claims

1. A subwavelength ventilated asymmetric sound absorber, comprising one or more absorption cells, each of which includes a metamaterial resonator and a backing impedance boundary, characterized in that: The backing impedance boundary includes a rear panel, a top panel, a bottom panel, a long panel, and a short panel, which together form an L-shaped structure. A flow opening is provided on the top of the rear panel to facilitate airflow. The bottom of the metamaterial resonator is placed on the short panel of the backing impedance boundary, and a channel is left between the bottom panel and the backing impedance boundary. The back of the metamaterial resonator is fixedly connected to the side of the long panel of the backing impedance boundary, and a channel is left between the back panel and the backing impedance boundary.

2. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: The bottom of the metamaterial resonator is fixedly connected to the short plate backing the impedance boundary.

3. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: The backing impedance boundary also includes a front panel, which is fixedly connected to the long plate side of the backing impedance boundary and has a channel between the front panel and the back panel; the back of the metamaterial resonator is fixedly connected to the front panel; and a flow port is provided at the bottom of the front panel.

4. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: A plurality of absorption cells are arranged in an array to form the sound absorber, and each absorption cell has a metamaterial resonator with the same or different resonance frequency.

5. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: The metamaterial resonator is a folded Fabry-Perot resonator, a Helmholtz resonator or a curled space resonator.

6. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: The absorption cell is an integrated structure formed by 3D printing or injection molding, or is obtained by mechanical processing.

7. The subwavelength ventilated asymmetric sound absorber according to claim 1, characterized in that: The metamaterial resonator is composed of an absorption cell, and the width of the bottom opening in the propagation direction and the vertical propagation direction are w x =20mm~32mm and w y = 24mm ~ 29.4mm; the channel length of the backing impedance boundary is (H-2h) = 140mm ~ 180mm, and the width of the backing impedance boundary in the propagation direction is w sx =5~20mm, the short plate height of the backing impedance boundary is h=10mm~30mm.

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