A slit-type extremely short-necked helmholtz resonant sound absorbing unit

Abstract

This invention discloses a slit-type ultra-short-neck Helmholtz resonant sound-absorbing unit, comprising a resonant back cavity and a slit-type ultra-short neck communicating with the resonant back cavity; the resonant back cavity is a hollow cavity with one end open and the remaining end faces closed, used to provide the acoustic spring required for resonance. This invention can precisely control the resonant frequency, peak absorption, and effective bandwidth of the structure through three dimensionless parameters: the neck depth-to-slit width ratio, the slit width-to-boundary layer ratio, and the slit length-to-width ratio, achieving co-source enhancement of inertia and damping, breaking through the dependence of traditional Helmholtz resonators on the geometric neck length; within an extremely compact structural scale, it simultaneously achieves high-efficiency low-frequency sound absorption and broadband performance improvement, providing a novel structural solution for low-frequency noise control in extremely confined spaces such as aerospace, deep-sea equipment, rail transportation, and engineering machinery, and also opening up new ideas and broader prospects for the application of Helmholtz resonant sound-absorbing structures.

Description

Technical Field

[0001] This invention mainly relates to the field of acoustic noise reduction technology, specifically a slit-type ultra-short neck Helmholtz resonant sound-absorbing unit. Background Technology

[0002] Low-frequency noise control is a core challenge that restricts the performance improvement of major equipment such as aerospace, deep-sea equipment, and rail transportation. In precision equipment such as rocket fairings, satellite instrument compartments, and submarine pressure compartments, low-frequency noise has extremely long wavelengths and strong transmission capabilities, which can easily lead to instrument inaccuracies and equipment performance degradation. Such scenarios have strict limitations on the thickness and volume of sound-absorbing structures, usually requiring the thickness to be controlled within 30-50mm.

[0003] Helmholtz resonators, due to their simple structure, mature theory, and strong engineering adaptability, are the core unit for deep subwavelength low-frequency sound absorption and are widely used in various silencers, sound-absorbing metamaterials, and sound insulation structures. The traditional sound absorption mechanism of a Helmholtz resonator is mass-spring resonance, with the air column in the neck acting as the acoustic mass and the air in the back cavity acting as the acoustic spring. Sound energy absorption is achieved through viscous dissipation of the air in the neck during resonance. In existing technologies, to achieve low-frequency resonance, structures such as extended necks, spiral necks, and curved necks are typically used to extend the acoustic path within a limited space, or multiple units are connected in parallel, or porous materials are used to expand the sound absorption bandwidth.

[0004] However, existing technologies have significant drawbacks: on the one hand, their low-frequency resonance relies on the geometric neck length to accumulate acoustic mass. The longer the neck, the narrower the absorption peak, making it impossible to balance low-frequency resonance and broadband sound absorption. Moreover, the structure is large and cannot be adapted to extremely confined spaces. On the other hand, existing theories are based on the inviscid fluid assumption, treating viscous heat dissipation as a perturbation term. The physical sources of acoustic mass and acoustic impedance are completely decoupled, naturally resulting in the inherent contradiction that "low-frequency resonance requires a long neck and broadband sound absorption requires large damping." This makes it impossible to cover the extreme working conditions where the neck depth-to-slit width ratio is less than 1 and the slit width is close to the boundary layer thickness. There is also a lack of corresponding acoustic impedance analytical models and design methods. Summary of the Invention

[0005] The present invention addresses the problem that existing technical solutions are too simplistic and provides a solution that is significantly different from existing technologies. It mainly provides a slit-type ultra-short neck Helmholtz resonant sound-absorbing unit to solve the technical problems mentioned in the background art, such as the need for long necks for low-frequency resonance and large damping for broadband sound absorption, which cannot cover the extreme working conditions where the neck depth-to-slit width ratio is less than 1 and the slit width is close to the boundary layer thickness.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit includes a resonant back cavity and a slit-type ultra-short neck communicating with the resonant back cavity; The resonant back cavity is a hollow cavity with one end open and the other ends closed, used to provide the acoustic spring required for resonance; The slit-type extremely short neck is a slit structure opened at the opening end of the resonant back cavity. The internal space of the slit is completely connected with the hollow cavity of the resonant back cavity, which is used to provide the acoustic quality and acoustic impedance required for resonance.

[0007] Further preferably, the slit-type extremely short neck satisfies the following dimensionless parameter constraints: neck depth to slit width ratio Λ=L / w<1, where L is the slit neck length, i.e., the wall thickness at the opening end of the resonant back cavity, and w is the slit width; slit width to boundary layer ratio Γ=w / δv≤1, where δv is the characteristic thickness of the air viscous boundary layer; slit length to width ratio , where b is the slit length.

[0008] More preferably, the neck depth-to-slit width ratio Λ is 0.1-0.8; the slit width-to-boundary layer ratio Γ is 0.2-1.0; The aspect ratio of the slit is ≥20.

[0009] Further preferred, when the slit aspect ratio Π≥20, the flow inside the slit exhibits pure two-dimensional boundary layer flow characteristics, and the end effect is negligible; When the slit width-to-boundary layer ratio Γ is in the range of 0.2-1.0, the viscous boundary layer completely occupies the slit flow cross section, and the boundary layer effect dominates.

[0010] More preferably, the number of the slit-shaped extremely short necks is one or more;

[0011] When multiple slits are set, each slit is arranged in parallel at the opening end of the resonant back cavity, and the internal space of all slits is connected to the resonant back cavity.

[0012] Further preferred, multiple slits use the same or different parameters to extend the sound absorption bandwidth through multi-resonant peak coupling; The center-to-center distance d between adjacent slits satisfies d≥2·w and d≤0.5·D, where w is the width of a single slit and D is the total width of the resonant back cavity opening surface in the slit arrangement direction.

[0013] More preferably, the opening edge of the slit-type very short neck is treated with at least one of chamfering, rounding, or stepped flaring structure.

[0014] More preferably, the chamfer depth is 0.1-0.5mm, the fillet radius R=0.1-0.3mm, and the stepped flare width is 1.2-2.0 times the original seam width.

[0015] More preferably, the cross-section of the resonant back cavity is any one of a rectangle, a circle, a regular polygon, or an irregular shape.

[0016] More preferably, the resonant back cavity and the slit-shaped ultra-short neck are integrally molded structures, prepared by any one of 3D printing, laser processing, mechanical milling, or mold injection molding, and the material is any one of metal, resin, plastic, or acrylic.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. This invention breaks through the dependence of traditional Helmholtz resonators on geometric neck length. Through the virtual neck length effect dominated by the boundary layer, low-frequency resonance is achieved in the case of extremely short neck with a neck depth-to-width ratio of less than 1. At the same resonance frequency, the structural volume is reduced by more than 30% compared with traditional Helmholtz resonators, which is perfectly adapted to the extremely confined space scenario with a thickness of 30-50mm.

[0019] 2. This invention achieves co-source enhancement of inertia and damping, simultaneously improving equivalent acoustic quality and viscous heat dissipation within the same slit structure. It can transform the high-Q narrowband response of traditional Helmholtz resonators into a low-Q broadband response without the need for additional damping structures, breaking the inherent contradiction of "high peak value and narrow bandwidth" in traditional structures, while ensuring both low-frequency absorption peak value and effective bandwidth.

[0020] 3. The present invention has a simple structure, without complex folding or bending structures, and without porous materials or other easily failed components. It is easy to process, cost-controllable, and can be mass-produced. At the same time, it can be flexibly adapted to various engineering integration methods such as series, parallel, and array, and has strong engineering adaptability. It can be widely used in various low-frequency noise control scenarios such as aerospace, deep-sea equipment, rail transportation, and engineering machinery.

[0021] 4. This invention establishes a design system based on three dimensionless parameters: neck depth-to-width ratio, slit width-to-boundary layer ratio, and slit length-to-width ratio. The resonant frequency, sound absorption peak value, and bandwidth can be precisely controlled by adjusting these parameters, achieving globally optimal design of sound absorption performance under limited space constraints. This provides a standardized design method for engineering applications.

[0022] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0023] Figure 1 This is a three-dimensional structural diagram of the present invention;

[0024] Figure 2 This is a schematic diagram of the transverse full-section structure of the present invention;

[0025] Figure 3 This is a schematic diagram of the longitudinal full-section structure of the present invention;

[0026] Figure 4 For the present invention Figure 2 Enlarged structural diagram at point A in the diagram;

[0027] Figure 5 For the present invention Figure 3 A magnified structural diagram at point B in the diagram.

[0028] The diagram is labeled as follows: 1. Resonance back cavity; 2. Slit-shaped very short neck; 3. Opening end. Detailed Implementation

[0029] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below with reference to the accompanying drawings, which illustrate several embodiments of the present invention. However, the present invention can be implemented in different forms and is not limited to the embodiments described in the text. Rather, these embodiments are provided to make the disclosure of the present invention more thorough and complete.

[0030] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used in this document are for illustrative purposes only.

[0031] Please refer to the appendix carefully. Figures 1-5 A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit includes a resonant back cavity and a slit-type ultra-short neck communicating with the resonant back cavity. The resonant back cavity is a hollow cavity with one end open and the other ends closed, used to provide the acoustic spring required for resonance; The internal dimensions are 120mm long * 120mm wide * 106mm high, and the wall thickness at the opening end, i.e. the slit neck length L = 0.08mm; The slit-type extremely short neck is a slit structure opened at the opening end of the resonant back cavity. The internal space of the slit is completely connected to the hollow cavity of the resonant back cavity, which is used to provide the acoustic quality and acoustic resistance required for resonance.

[0032] In this embodiment, as Figure 2 , Figure 3 , Figure 4 and Figure 5 As shown, the slit-type extremely short neck satisfies the following dimensionless parameter constraints: neck depth to slit width ratio Λ = L / w < 1, where L is the neck length of the slit, i.e., the wall thickness at the opening end of the resonant back cavity, and w is the slit width; slit width to boundary layer ratio Γ = w / δ_v ≤ 1, where δ_v is the characteristic thickness of the air viscous boundary layer; slit length to width ratio , where b is the length of the slit.

[0033] In this embodiment, as Figure 2 , Figure 3 , Figure 4 and Figure 5 As shown, the neck depth-to-width ratio Λ is 0.1-0.8, and within this range, virtual sound quality enhancement can be stably achieved under extremely short neck conditions; The slit width-to-boundary layer ratio Γ is set to 0.2-1.0, with a lower limit of 0.2 to ensure that the boundary layer completely occupies the slit flow cross section, with no central inviscid flow region, thus ensuring sufficient dissipation of viscous heat. When the slit aspect ratio Π≥20, the flow inside the slit exhibits fully developed pure two-dimensional boundary layer flow characteristics, and the three-dimensional vortex effect at the end can be completely ignored. The calculated acoustic impedance is in high agreement with the actual measurement. When the slit width-to-boundary layer ratio Γ≤1 and Γ≥0.2, the viscous boundary layer completely occupies the slit flow cross section within this range, and the boundary layer effect dominates.

[0034] In this embodiment, as Figure 2 and Figure 3 As shown, the number of slit-type very short necks is one or more; When multiple slits are set, each slit is arranged in parallel at the opening end of the resonant back cavity. The internal space of all slits is connected to the resonant back cavity. Multiple slits use the same or different parameters, and the sound absorption bandwidth is extended through multi-resonance peak coupling.

[0035] When the slit-type extremely short neck is a single rectangular slit, it is located at the center of the opening end of the resonant back cavity. The slit width w = 0.12 mm, the slit length b = 30 mm, and the internal space of the slit is completely connected to the hollow cavity of the resonant back cavity. After verification, the neck depth to slit width ratio Λ = L / w = 0.08 / 0.12 ≈ 0.67 < 1, the air viscous boundary layer thickness δv ≈ 0.15 mm at 200 Hz, the slit width to boundary layer ratio Γ = w / δv ≈ 0.12 / 0.15 = 0.8 ≤ 1, and the slit length to width ratio Π = b / w ≈ 30 / 0.12 = 250 >> 20, which fully meets the core parameter constraints of this invention.

[0036] In this embodiment, as Figure 2 and Figure 3 As shown, the center-to-center distance d between adjacent slits satisfies d≥2·w and d≤0.5·D, where w is the width of a single slit and D is the total width of the resonant back cavity opening surface in the slit arrangement direction.

[0037] In this embodiment, as Figure 2 and Figure 3 As shown, the opening edge of the slit-type extremely short neck is treated with at least one of chamfering, rounding, or stepped flaring structure; The chamfer depth is 0.1-0.5mm, the fillet radius R=0.1-0.3mm, and the stepped flare width is 1.2-2.0 times the original seam width.

[0038] In this embodiment, as Figure 1, Figure 2 and Figure 3 As shown, the cross-section of the resonant back cavity can be any of the following: rectangular, circular, regular polygonal, or irregular. The resonant back cavity and the slit-type extremely short neck are integrally molded structures. They are prepared by any of the following methods: 3D printing, laser processing, mechanical milling, or mold injection molding. The material can be any of the following: metal, resin, plastic, or acrylic.

[0039] The specific operation process of this invention is as follows: When the incident sound wave acts on the sound-absorbing unit, the sound wave enters the resonant back cavity through the slit-shaped extremely short neck, exciting the structural resonance; since the slit width is close to or less than the thickness of the viscous boundary layer, the flow in the slit is completely controlled by the boundary layer, and the air particles form extremely high vibration velocities in the slit, significantly increasing the kinetic energy density and generating an equivalent acoustic mass far exceeding the contribution of the geometric neck length, realizing low-frequency resonance under the extremely short neck; at the same time, the strong shear effect in the boundary layer simultaneously and significantly enhances the viscous heat dissipation, increases the equivalent acoustic impedance, and can broaden the sound absorption bandwidth without the need for additional damping structures, realizing the co-source enhancement of inertia and damping.

[0040] Tested using the B&K impedance tube dual-microphone method, the resonant frequency of the sound-absorbing unit in this embodiment is 320Hz, and the peak sound absorption coefficient reaches 0.96, achieving perfect sound absorption. The effective sound absorption bandwidth, i.e. the frequency band with a sound absorption coefficient ≥0.5, is 260Hz-410Hz, with a relative bandwidth of 45%. Compared with the traditional extended neck Helmholtz resonator with the same resonant frequency and the same back cavity volume, the effective sound absorption bandwidth of this embodiment is increased by 120%, and the overall structural thickness is only 30mm, which is perfectly suited to the low-frequency sound absorption requirements of confined spaces.

[0041] In another embodiment, the resonant back cavity is a rectangular cavity with internal dimensions of 50mm (length) * 50mm (width) * 48mm (thickness). The wall thickness at the opening end, i.e., the slit neck length L = 0.8mm, is also present. The extremely short slit neck consists of three parallel rectangular slits, all connected to the back cavity. The parameters of the three slits are as follows: slit width w1 = 0.8mm, length b1 = 45mm; slit width w2 = 1.0mm, length b2 = 45mm; slit width w3 = 1.2mm, length b3 = 45mm. The Λ value of all three slits is less than 1, the Γ value is less than or equal to 1, and the Π value is much greater than 1, satisfying the core parameter constraints of this invention. Impedance tube testing shows that this embodiment achieves an average sound absorption coefficient ≥ 0.6 and a peak sound absorption coefficient of up to 0.98 in the 120Hz-550Hz frequency band, realizing low-frequency broadband high-efficiency sound absorption. The overall thickness is only 50mm, making it suitable for direct integration into the inner walls of satellite instrument compartments and submarine pressure chambers.

[0042] The present invention has been described by way of example in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvement made by adopting the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, shall be within the protection scope of the present invention.

Claims

1. A slit-type very short neck Helmholtz resonator sound absorbing unit, characterized by: It includes a resonant back cavity (1) and a slit-shaped very short neck (2) connected to the resonant back cavity (1). The resonant back cavity (1) is a hollow cavity with one end open and the other end faces closed, used to provide the acoustic spring required for resonance; The slit-type extremely short neck (2) is a slit structure opened at the opening end (3) of the resonant back cavity (1). The internal space of the slit is completely connected with the hollow cavity of the resonant back cavity (1) to provide the acoustic quality and acoustic resistance required for resonance.

2. The slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 1, characterized in that: The slit-type extremely short neck (2) satisfies the following dimensionless parameter constraints: neck depth to slit width ratio Λ=L / w<1, where L is the slit neck length, i.e., the wall thickness of the opening end (3) of the resonant back cavity (1), and w is the slit width; slit width to boundary layer ratio Γ=w / δv≤1, where δv is the characteristic thickness of the air viscous boundary layer; slit length to width ratio , where b is the slit length.

3. The slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 2, characterized in that: The neck depth-to-slit width ratio Λ is 0.1-0.8; the slit width-to-boundary layer ratio Γ is 0.2-1.0; The aspect ratio of the slit is ≥20.

4. The slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 3, characterized in that: When the slit aspect ratio Π≥20, the flow inside the slit exhibits the characteristics of a pure two-dimensional boundary layer flow, and the end effect is negligible. When the slit width-to-boundary layer ratio Γ is in the range of 0.2-1.0, the viscous boundary layer completely occupies the slit flow cross section, and the boundary layer effect dominates.

5. A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 4, characterized in that: The number of the slit-type very short neck (2) is one or more; When multiple slits are set, each slit is arranged in parallel at the opening end (3) of the resonant back cavity (1), and the internal space of all slits is connected to the resonant back cavity (1).

6. The slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 5, characterized in that: Multiple slits using the same or different parameters extend the sound absorption bandwidth through multi-resonance peak coupling; The center-to-center distance d between adjacent slits satisfies d≥2·w and d≤0.5·D, where w is the width of a single slit and D is the total width of the opening face of the resonant back cavity (1) in the direction of slit arrangement.

7. The slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 1, characterized in that: The opening edge of the slit-type very short neck (2) is treated with at least one of chamfering, rounding, or stepped flaring structure.

8. A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 7, characterized in that: The chamfer depth is 0.1-0.5mm, the fillet radius R=0.1-0.3mm, and the stepped flare width is 1.2-2.0 times the original seam width.

9. A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 1, characterized in that: The cross-section of the resonant back cavity (1) can be any one of a rectangle, a circle, a regular polygon, or an irregular shape.

10. A slit-type ultra-short neck Helmholtz resonant sound-absorbing unit according to claim 1, characterized in that: The resonant back cavity (1) and the slit-type extremely short neck (2) are integrally molded structures, and are prepared by any one of the following methods: 3D printing, laser processing, mechanical milling, and mold injection molding. The material is any one of metal, resin, plastic, or acrylic.

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