Sound absorption unit and sound absorption structure based on micro crack and spiral coupling and preparation method of sound absorption unit and sound absorption structure

Abstract

The invention provides a sound absorption unit and structure based on micro crack and spiral coupling and a preparation method of the sound absorption unit and structure, and belongs to the technical field of acoustic metamaterials. The sound absorption unit comprises a housing with a rectangular inner cavity, the incidence side of the housing is provided with a micro-slit incidence channel communicated with the rectangular inner cavity, the rectangular inner cavity is internally provided with a spiral curly channel with the outer side end communicated with the micro-slit incidence channel, and the spiral curly channel is formed by a partition plate and the two opposite side walls of the rectangular inner cavity in a sealed surrounding mode. The partition plate is in the shape of a spiral curve on a section plane perpendicular to the incidence direction of the spiral curled channel and extends in the incidence direction. Broadband efficient sound absorption can be achieved, and the technical problem that in the prior art, a traditional sound absorption metamaterial is narrow in sound absorption frequency band is effectively solved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of acoustic metamaterials, and particularly relates to a sound absorption unit and a sound absorption structure based on micro-slits and spiral coupling, and a preparation method thereof. BACKGROUND

[0002] In the acoustic design of modern underwater facilities (such as deep-sea observation stations, underwater robots, and submarine energy devices), acoustic stealth performance is a key indicator to ensure their concealment, safety, and long-period reliable operation. Traditional underwater sound absorption materials (such as rubber-based sound absorption tiles and porous polymers) mainly rely on acoustic impedance matching and viscoelastic dissipation mechanisms to absorb sound wave energy. These materials achieve sound absorption effects by matching acoustic impedance within a specific frequency range. However, with the rapid development of modern sonar detection technology, especially the widespread application of low-frequency active sonar (frequency range typically 100Hz-5kHz) and wide-frequency detection technology, the deficiencies of traditional sound absorption materials, such as insufficient sound absorption efficiency at low frequencies and narrow frequency bands, have become increasingly prominent, severely restricting the stealth performance of underwater facilities.

[0003] In related technologies, acoustic metamaterials have brought revolutionary breakthroughs to the stealth technology of underwater facilities due to their unique subwavelength structural design and controllable acoustic properties. Acoustic metamaterials, through artificially designed periodic structures (such as Helmholtz resonators, spiral winding channels, or thin film resonance structures), can achieve acoustic parameters that natural materials cannot, including negative equivalent mass density and negative elastic modulus. These characteristics enable them to maintain extraordinary sound absorption performance at low frequencies (<1000Hz), effectively compensating for the shortcomings of traditional materials in low-frequency sound wave absorption.

[0004] However, traditional sound absorption metamaterials, such as micro-perforated panel sound absorbers, are a new type of sound absorption material made of micro-perforated panel sound structures. They are composed of a thin plate with regularly arranged micro-holes and the cavity behind it. When applied to low-frequency sound wave absorption, multiple winding cavities connected by micro-holes are set to achieve multiple resonance absorption peaks in the low-frequency range to improve sound absorption. However, the absorption peaks of this structure are relatively discrete, and the frequency range of a single absorption peak is usually less than 100Hz, resulting in a narrow sound absorption band, which is difficult to meet the acoustic stealth performance requirements of underwater facilities. SUMMARY

[0005] The present application provides a sound absorption unit and a sound absorption structure based on micro-slits and spiral coupling, and a preparation method thereof, which can solve the technical problem of narrow sound absorption band of traditional sound absorption metamaterials in the prior art. The technical solution is as follows:

[0006] In a first aspect, an embodiment of the present application provides a sound absorption unit based on microslit and spiral coupling, comprising a shell with a rectangular inner cavity, a microslit incident channel communicated with the rectangular inner cavity is arranged on the incident side of the shell, a spiral curling channel communicated with the microslit incident channel at the outer side is arranged in the rectangular inner cavity, the spiral curling channel is sealed and surrounded by a partition plate and opposite two side walls of the rectangular inner cavity, and the partition plate is a spiral curve on a cross-sectional plane perpendicular to the incident direction of the spiral curling channel and extends along the incident direction.

[0007] Optionally, the spiral curve is a logarithmic spiral curve, and the following formula is satisfied:

[0008] x=a·e (bθ) cosθ

[0009] y=a·e (bθ) sinθ

[0010] wherein x and y are rectangular coordinates in the plane where the spiral curve is located, a is an initial radius of the spiral curve relative to a pole, b is a spiral growth rate, and θ is a polar angle.

[0011] Optionally, the initial radius a is 0.5 mm, the spiral growth rate b is 0.098, the thickness of the spiral curve is 0.25 mm, and the polar angle θ is in the range of 0 to 10π.

[0012] Optionally, the shell is a rectangular body, the total length a0 of the sound absorption unit is 25 mm, the width b0 is 20 mm, the height h0 is 20 mm, the length a1 of the microslit incident channel is 5 mm, the height h1 is 0.4 mm, and the length a2 of the rectangular inner cavity is 20 mm.

[0013] Optionally, the inner side end of the spiral curling channel is closed to the geometric center of the rectangular inner cavity.

[0014] Optionally, the sound absorption unit is a photosensitive resin structure.

[0015] Optionally, the sound absorption unit based on microslit and spiral coupling is prepared by 3D printing.

[0016] In a second aspect, an embodiment of the present application provides a sound absorption structure, comprising the sound absorption unit based on microslit and spiral coupling in the first aspect, the sound absorption structure comprises a plurality of the sound absorption units based on microslit and spiral coupling, the plurality of the sound absorption units based on microslit and spiral coupling are arranged in an array, and the entrances of the microslit incident channels of the plurality of the sound absorption units are located on the same side.

[0017] In a third aspect, the embodiments of the present application provide a preparation method for preparing the sound absorption unit based on micro-slit and spiral coupling according to the first aspect, comprising:

[0018] Step 1, determining the size parameters of the shell, the micro-slit incident channel and the spiral curling channel, establishing a three-axis coordinate system of x, y and z axes, constructing a two-dimensional shape in the zx direction and generating a three-dimensional structure by stretching along the y axis;

[0019] Step 2, establishing a finite element simulation model, constructing an air domain and applying an incident sound pressure on the incident side, calculating the transfer function, reflection coefficient and sound absorption coefficient and performing parameterized optimization to obtain the performance of the target frequency band;

[0020] Step 3, exporting the model and adopting 3D printing forming.

[0021] Optionally, the step 2 comprises:

[0022] The micro-slit incident channel and the spiral curling channel are provided with a thermal adhesion boundary layer impedance, and the remaining boundaries in the rectangular inner cavity are provided as rigid boundaries.

[0023] The technical scheme provided by the embodiments of the present application has at least the following beneficial effects:

[0024] The sound absorption unit provided by the embodiments of the present application is directed towards sound waves through the incident side of the shell, and the transmission path of the sound waves contacting the sound absorption unit is: after entering the sound absorption unit from the inlet of the micro-slit incident channel, the sound waves first enter the rectangular inner cavity, then propagate to the outside end of the spiral curling channel and enter the spiral curling channel, and finally reach the inside end of the spiral curling channel, forming a complete sound wave dissipation path. This design optimizes the sound wave incident efficiency through the micro-slit incident channel, combines the lengthening and variable pitch characteristics of the spiral path of the spiral curling channel, enhances the multiple reflection and energy dissipation of the sound waves, and thus realizes wideband and high-efficiency sound absorption. The technical problem of narrow sound absorption frequency band of the traditional sound absorption metamaterial in the prior art is effectively solved. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0026] Figure 1 is a three-dimensional structure schematic diagram of a sound absorption unit based on micro-slit and spiral coupling provided by the embodiments of the present application;

[0027] Figure 2This is a front view of another sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention;

[0028] Figure 3 This is a simulation model diagram of the sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention;

[0029] Figure 4 This is a curve showing the change of sound absorption coefficient with frequency for a sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention.

[0030] Figure 5 This is a schematic diagram of the sound-absorbing structure provided in an embodiment of the present invention;

[0031] Figure 6 This is a flowchart of a preparation method provided in an embodiment of the present invention.

[0032] In the picture:

[0033] 1-Shell; 11-Rectangular inner cavity; 12-Micro-slit injection channel; 13-Helical coiled channel; 131-Baffle. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0035] Figure 1 This is a three-dimensional structural schematic diagram of a sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention; Figure 2 This is a front view of another sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention; Figure 3 This is a simulation model diagram of the sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention; Figure 4 This is a graph showing the sound absorption coefficient of the sound-absorbing unit based on micro-slit and helical coupling provided in an embodiment of the present invention as a function of frequency. Figures 1 to 4 As shown, this embodiment of the invention provides a sound-absorbing unit based on microslit and helical coupling, including a housing 1 with a rectangular inner cavity 11. A microslit incident channel 12 communicating with the rectangular inner cavity 11 is provided on the incident side of the housing 1. A helical coiled channel 13, with its outer end communicating with the microslit incident channel 12, is provided in the rectangular inner cavity 11. The helical coiled channel 13 is sealed and enclosed by a partition 131 and the opposite side walls of the rectangular inner cavity 11. The partition 131 has a helical curve on a cross-sectional plane perpendicular to the incident direction of the helical coiled channel 13 and extends along the incident direction.

[0036] In the embodiment of the present application, the sound absorption unit is a cuboid structure formed by a cover 1 with a rectangular inner cavity 11 and a spiral coiled channel 13 extending in the rectangular inner cavity 11. A micro slit incident channel 12 in the form of a flat slit is formed on the incident side of the cuboid structure, that is, the side for contacting sound waves, and is in communication with the rectangular inner cavity 11. Referring to Figure 1 and Figure 2 A three-axis coordinate system with x, y, and z axes is established for the sound absorption unit, and the micro slit incident channel 12 can be regarded as a horizontal channel parallel to the x axis. The spiral coiled channel 13 is sealed by a partition plate 131 and the two opposite side walls of the rectangular inner cavity 11 in the y axis direction. The partition plate 131 is arranged in a spiral curve on the cross-sectional plane perpendicular to the incident direction of the spiral coiled channel 13, that is, the two-dimensional cross section formed by the xz axis, and the spiral radius thereof changes nonlinearly with the polar angle. The micro slit incident channel 12 serves as an incident channel for sound waves, and its main function is to regulate acoustic impedance matching to enable sound waves to efficiently enter the interior of the sound absorption structure. The narrow design of the micro slit incident channel 12 can enhance the local resonance effect of sound waves, and by adjusting the geometric parameters (such as width and length) thereof, the coupling efficiency of sound waves in a specific frequency band can be optimized. The spiral coiled channel 13 realizes gradient regulation of the sound wave path through the nonlinearly changing spiral radius, thereby widening the sound absorption frequency band. The coupling design of the micro slit incident channel 12 and the spiral coiled channel 13 can achieve a synergistic effect: the micro slit incident channel 12 optimizes the incident efficiency of sound waves, and the spiral coiled channel 13 realizes wideband sound absorption through multiple resonance modes. However, separate use will weaken this synergistic effect, resulting in a narrower sound absorption frequency band or lower efficiency.

[0037] The sound absorption unit provided by the embodiment of the present application is used with the incident side of the cover 1 facing the sound waves. The transmission path of the sound waves contacting the sound absorption unit is as follows: after entering the sound absorption unit from the inlet of the micro slit incident channel 12, the sound waves first enter the rectangular inner cavity 11, then propagate to the outer side end of the spiral coiled channel 13 to enter the spiral coiled channel 13, and finally reach the inner side end of the spiral coiled channel 13 to form a complete sound wave dissipation path. This design optimizes the incident efficiency of sound waves through the micro slit incident channel 12, and combines the lengthening and variable pitch characteristics of the spiral path of the spiral coiled channel 13 to enhance the multiple reflections and energy dissipation of sound waves, thereby realizing wideband and efficient sound absorption.

[0038] Optionally, the spiral curve is a logarithmic spiral curve, and satisfies the following formula:

[0039] x = a·e (bθ) cosθ (1)

[0040] y = a·e (bθ) sinθ (2)

[0041] In the formula: x, y are the rectangular coordinates in the plane of the spiral curve, a is the initial radius of the spiral curve relative to the pole, b is the spiral growth rate, and θ is the polar angle.

[0042] Exemplarily, in the preparation of the sound absorption unit based on microslit and spiral coupling provided in the embodiments of the present application, first, the size of the sound absorption unit structure is determined, including the size parameters of the shell 1, the microslit incident channel 12 and the spiral winding channel 13, a three-axis coordinate system of x, y and z axes is established, a two-dimensional shape is constructed in the zx direction and stretched along the y axis to generate a three-dimensional structure, and the construction of the sound absorption unit structure is completed. Specifically, the sound absorption unit provided in the embodiments of the present application is as shown in Figure 1 and Figure 2 , the shell 1 is in the shape of a rectangular body as a whole, the total length of the sound absorption unit a0=25mm, the width b0=20mm, the height h0=20mm, the length of the microslit incident channel 12 a1=5mm, the height h1=0.4mm, and the length of the rectangular inner cavity 11 a2=20mm. That is, in the z-axis direction, the microslit incident channel 12 is located in the middle of the entire sound absorption unit in the height direction, and above and below it are densely packed structures. The rectangular inner cavity 11 communicating with the microslit incident channel 12 is a cube cavity with a length, width and height of 20mm. In the rectangular inner cavity 11, the inner side of the spiral winding channel 13 is closed to the geometric center of the rectangular inner cavity 11, and the extension of the partition plate 131 on the two-dimensional cross section formed by the xz axis can be seen as extending from the center of the rectangular inner cavity 11. In the embodiments of the present application, the logarithmic spiral curve of the extension of the partition plate 131 has an initial radius a=0.5mm and a spiral growth rate b=0.098, the thickness of the partition plate 131, that is, the thickness of the spiral curve is 0.25mm, and the polar angle θ ranges from 0 to 10π.

[0043] Then, a unit structure simulation model is established by using COMSOL Multiphysics 6.2 finite element simulation software, and acoustic characteristic analysis is performed by applying a stress acoustic module structure, and the simulation model is as shown in Figure 3 . The incident surface A is provided with plane wave radiation, the incident sound pressure of the simulated incident sound wave is 1Pa, and the incident direction is the positive direction of the x-axis in Figure 1 and Figure 2 . In order to more accurately calculate the attenuation and energy loss of the sound wave, the microslit incident channel 12 and the spiral winding channel 13 are provided with thermal viscous boundary impedance by the simulation software, and the remaining boundaries in the rectangular inner cavity 11 are provided with rigid boundaries. The calculation formula of the sound pressure reflection coefficient r and the sound absorption coefficient α of the variable structure in the assembly is as follows:

[0044]

[0045] α=1-|r| 2 (6)

[0046] wherein: f is frequency; c is sound velocity; k is wave number; P1, P2 are sound pressure values at the midpoint B and point C respectively; H is the distance between point B and point C. Figure 3 12 is the transfer function between point B and point C; x1, x2 are the distances from point B and point C to the sound absorption unit respectively.

[0047] Further, referring to Figure 4 , in the graph of the sound absorption coefficient of the micro-slit and the spiral-coupled sound absorption unit varying with frequency, the black curve is the calculation result under finite element simulation, and the red curve is the theoretical model calculation result. It can be seen that the two curves basically coincide. There is an absorption peak (650 Hz) in the low frequency range of 0 to 1500 Hz, the sound absorption coefficient at this position reaches 0.99 perfect sound absorption frequency, and the working frequency band width with sound absorption coefficient greater than 0.5 reaches 350 Hz. Compared with the existing sound absorption metamaterial, the absorption peak is relatively discrete under the same conditions, and the sound absorption frequency range of a single absorption peak is generally less than 100 Hz. It can realize wideband sound absorption, effectively solve the technical problem of narrow sound absorption frequency band of the traditional sound absorption metamaterial in the prior art, and meet the sound stealth performance requirements of underwater facilities.

[0048] Exemplarily, among other possible implementations, other forms of variable-pitch spiral curves can also increase the width of the absorption peak to a certain extent, but through the applicant's multiple simulation and test experiments, the effect of the spiral curl channel 13 designed in the form of the logarithmic spiral optimized in the foregoing embodiment is not as good as that.

[0049] Optionally, the sound absorption unit is a photosensitive resin structure. Exemplarily, in the embodiment of the present application, the sound absorption unit uses photosensitive resin as raw material. The photosensitive resin is usually a dense and hard plastic after curing, and does not have such a porous sound absorption property itself. Its role is to take advantage of its high-precision 3D printing characteristics to manufacture complex microstructures that cannot be processed by traditional methods. These structures can effectively capture, guide and dissipate sound energy. Using light-cured 3D printing technology for molding and preparation, while ensuring sound absorption performance, short-cycle mass production can be achieved.

[0050] Figure 5 is a structural schematic diagram of a sound absorption structure provided by the embodiment of the present application. As Figure 5 indicated, the embodiment of the present application further provides a sound absorption structure, which includes a sound absorption unit 10 and a micro-slit 20. Figures 1 to 2 ​The microslit and spiral coupling based sound absorption unit shown, the sound absorption structure includes a plurality of sound absorption units, the plurality of sound absorption units are arranged in an array splicing manner, and the entrances of the microslit incident channels 12 are located on the same side. Exemplarily, in the embodiment of the present application, by combining a plurality of sound absorption units to form an acoustic metamaterial sound absorption structure, a cladding layer with a certain thickness is formed to achieve wideband absorption of sound waves on a large area in a target frequency band, so as to achieve the purpose of underwater acoustic stealth. Moreover, the multi-layer composite structure arranged in an array splicing manner can also achieve frequency band splicing and collaborative regulation through the size and parameter differentiation between the sound absorption units of each layer, thereby further improving the sound absorption performance.

[0051] Figure 6 The flow chart of the preparation method provided by the embodiment of the present application is shown in FIG. 1. Figure 6 The embodiment of the present application also provides a preparation method for preparing the microslit and spiral coupling based sound absorption unit shown in FIG. 1, which comprises the following steps: Figures 1 to 2

[0052] S1, determining the size parameters of the shell 1, the microslit incident channel 12 and the spiral winding channel 13, establishing a three-axis coordinate system of x, y and z axes, constructing a two-dimensional shape in the zx direction and generating a three-dimensional structure along the y axis.

[0053] S2, establishing a finite element simulation model, constructing an air domain and applying an incident sound pressure on the incident side, calculating the transfer function, the reflection coefficient and the sound absorption coefficient and performing parameterized optimization to obtain the performance in the target frequency band.

[0054] Specifically, when the simulation simulation model of the finite element simulation model is established, due to the small overall size, the boundary effect of the microslit incident channel 12 and the spiral winding channel 13 cannot be ignored, and a thermal viscous boundary layer must be set to simulate the wall loss generated when the sound wave enters the sound absorption structure. The microslit incident channel 12 and the spiral winding channel 13 are provided with a thermal viscous boundary layer impedance, and the remaining boundaries in the rectangular inner cavity 11 are provided with a rigid boundary.

[0055] S3, exporting the model and adopting 3D printing forming.

[0056] The microslit and spiral coupling based sound absorption unit prepared by the above preparation method is directed towards the sound wave through the incident side of the shell 1. The transmission path of the sound wave contacting the sound absorption unit is: after entering the sound absorption unit from the entrance of the microslit incident channel 12, first entering the rectangular inner cavity 11, then propagating to the outside end of the spiral winding channel 13 to enter the spiral winding channel 13, transmitting along the spiral winding channel 13 from the outside to the inside, and finally reaching the inside end of the spiral winding channel 13, forming a complete sound wave dissipation path. This design optimizes the sound wave incidence efficiency through the microslit incident channel 12, combines the lengthening and variable pitch characteristics of the spiral path of the spiral winding channel 13, enhances the multiple reflection and energy dissipation of the sound wave, and thus realizes wideband and high-efficiency sound absorption.​

[0057] Unless otherwise defined, technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein and the claims that follow is not intended to be limiting of the scope of the application. Similarly, the terms "first", "second", and the like, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms "a" and "an" and "the" and similar referents are used, by themselves, to indicate that at least one of the item is present. The terms "comprises", "comprising", "includes", "including", and the like can be used to mean either "consisting of" or "consisting essentially of" or "including" the elements listed after such term and not excluding additional elements. The term "connected" and "coupled" are used in reference to the direct and / or indirect coupling between two elements. The term "upstream" and "downstream" are used in reference to the relative position of elements in a fluid flow path.

[0058] The foregoing is merely illustrative of the principles of this application and various modifications can be made by those skilled in the art without departing from the scope of the application.

Claims

1. A sound-absorbing unit based on micro-slit and helical coupling, characterized in that, The device includes a housing (1) having a rectangular inner cavity (11). The housing (1) has a micro-slit injection channel (12) communicating with the rectangular inner cavity (11) on its incident side. The rectangular inner cavity (11) has a spiral-curved channel (13) whose outer end communicates with the micro-slit injection channel (12). The spiral-curved channel (13) is sealed and enclosed by a partition (131) and the opposite side walls of the rectangular inner cavity (11). The partition (131) is spiral-curved on a cross-sectional plane perpendicular to the incident direction of the spiral-curved channel (13) and extends along the incident direction.

2. The sound-absorbing unit based on micro-slit and helical coupling according to claim 1, characterized in that, The spiral curve is a logarithmic spiral curve and satisfies the following formula: In the formula: x and y are the rectangular coordinates in the plane containing the spiral curve, a is the initial radius of the spiral curve relative to the pole, b is the spiral growth rate, and θ is the polar angle.

3. The sound-absorbing unit based on micro-slit and helical coupling according to claim 2, characterized in that, The initial radius a = 0.5 mm, the spiral growth rate b = 0.098, the spiral curve thickness is 0.25 mm, and the polar angle θ ranges from 0 to 10π.

4. The sound-absorbing unit based on micro-slit and helical coupling according to claim 2, characterized in that, The cover (1) is rectangular. The total length of the sound-absorbing unit is a0=25mm, the width is b0=20mm, the height is h0=20mm, the length of the micro-slit injection channel is a1=5mm, the height is h1=0.4mm, and the length of the rectangular inner cavity (11) is a2=20mm.

5. The sound-absorbing unit based on micro-slit and helical coupling according to claim 4, characterized in that, The inner end of the spiral channel (13) is closed at the geometric center of the rectangular cavity (11).

6. The sound-absorbing unit based on micro-slit and helical coupling according to claim 1, characterized in that, The sound-absorbing unit is a photosensitive resin structural component.

7. The sound-absorbing unit based on micro-slit and helical coupling according to claim 6, characterized in that, The sound-absorbing unit is manufactured using 3D printing.

8. A sound-absorbing structure, comprising a sound-absorbing unit based on micro-slit and helical coupling as described in any one of claims 1 to 7, characterized in that, The sound-absorbing structure includes multiple sound-absorbing units arranged in an array, and the entrances of the micro-slit incident channels (12) of the multiple sound-absorbing units are located on the same side.

9. A preparation method for preparing a sound-absorbing unit based on microslit and helical coupling as described in any one of claims 1 to 7, characterized in that, include: Step 1: Determine the size parameters of the cover (1), the micro-slit incident channel (12) and the spiral curled channel (13), establish a three-axis coordinate system of x, y and z axes, construct a two-dimensional shape in the zx direction and stretch it along the y axis to generate a three-dimensional structure; Step 2: Establish a finite element simulation model, construct an air domain and apply incident sound pressure on the incident side, calculate the transfer function, reflection coefficient and absorption coefficient, and perform parameterized optimization to obtain the target frequency band performance; Step 3: Export the model and 3D print it.

10. The preparation method according to claim 9, characterized in that, Step 2 includes: The microslit incident channel (12) and the spiral coiled channel (13) are provided with thermoviscous boundary layer impedance, and the remaining boundaries in the rectangular inner cavity (11) are provided with rigid boundaries.

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