Low-frequency broadband sound absorption covering layer embedded with diagonal gradient cylindrical cavities

Abstract

The invention discloses a low-frequency broadband sound absorption covering layer embedded with diagonal gradient cylindrical cavities, and belongs to the technical field of low-frequency broadband sound absorption covering layers. The problem of contradiction between light weight and sound absorption performance of the sound absorption covering layer is solved. The sealing layer is connected with the perforation layer and the base layer and is subjected to integrated cold press molding, and the sealing layer and the perforation layer are both prepared from a PDMS polydimethylsiloxane silicone rubber material. The problem that the low-frequency sound absorption performance is poor due to the fact that a single material is adopted is solved; the radiuses and the intervals of the four layers of cylindrical cavities are designed in a gradient change mode, so that the sound absorption frequency band width is effectively increased, meanwhile, the thickness of the perforated layer is remarkably reduced, the practical application requirement of light weight can be met, and a better underwater low-frequency broadband sound absorption effect is achieved. The invention has the advantages of simple structure and manufacturing process.

Description

Technical Field

[0001] The invention relates to a low-frequency broadband sound-absorbing covering layer with distributed cylindrical cavities, in particular to a low-frequency broadband sound-absorbing covering layer with embedded diagonal gradient cylindrical cavities. Background Art

[0002] Underwater unmanned submersibles can realize all-weather unmanned cruising, unmanned underwater target detection, and accurate seabed exploration. They are an important guarantee for protecting my country's "underwater national gate" and maintaining marine national defense security. Acoustic stealth technology with sound-absorbing covering layer as the core is a key component for underwater unmanned submersibles to achieve their concealment and safety. Lightweight and miniaturization are its important development directions. However, traditional sound-absorbing covering layers have the problems of large volume and weight. This seriously restricts the development of underwater unmanned submersibles towards miniaturization. In this regard, existing solutions include: 1. Combining cavity structures of different sizes (Zhao Honggang, Wen Jihong, Yang Haibin, Lu Linmei, Wen Xisen. Sound absorption mechanism and optimization of a rubber layer containing a cylindrical cavity structure. Acta Physica Sinica 2014, 63(13):134303). The design concept of this structure is: adopting a parallel design, forming a combined cavity with cavity structures of different sizes in the same thickness layer, and optimizing the low-frequency sound absorption characteristics using a genetic algorithm to achieve the best absorption bandwidth effect. But the disadvantage is that with the emergence of large cavities, the peak value of the sound absorption coefficient gradually decreases, and the sound absorption performance is poor; second, a soft elastic material with an embedded transverse cylindrical cavity is used (Sharma GS, Skvortsov A, MacGillivray I, Kessissoglou N. Sound absorption by rubber coatings with periodic voids and hard inclusions. Appl. Acoust. 2019, 143: 200-210.), the density of which is only 1000 kg / m 3 It is very light and similar to water, forming a higher absorption peak at 200Hz, which can effectively absorb the low-frequency line spectrum, but its disadvantage is that the bandwidth is narrow. Although the sound absorption frequency band can be widened by embedding multiple layers of cavities, the multi-layer cavities will increase the thickness of the low-frequency broadband sound absorption covering layer, which cannot achieve the purpose of lightweight. Summary of the invention

[0003] The present invention aims at the above-mentioned prior art. The present invention aims to solve the problem of how to break the contradiction between the lightweight sound-absorbing covering layer and the sound absorption performance. It provides a lightweight sound-absorbing covering layer with lightweight materials and thin structure, and can effectively absorb low-frequency sound waves. It provides a low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity. The specific technical solution to solve the above-mentioned problem is as follows:

[0004] The low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity of the present invention is composed of a sealing layer, a perforated layer and a base layer, wherein the sealing layer is connected to the perforated layer and the base layer and is integrally cold-pressed;

[0005] The sealing layer, perforated layer and base layer are respectively composed of a number of identical rectangular parallelepiped units arranged periodically, the front and rear inner surfaces of each sealing layer unit are mutually bonded with the front and rear end surfaces of the long sides of the perforated layer unit, the lower surface of the sealing layer unit is mutually bonded with the upper surface of the base layer unit, and the sealing layer and perforated layer are made of PDMS polydimethylsiloxane silicone rubber material;

[0006] The perforated layer is rectangular, and is composed of a first layer of cylindrical cavities, a second layer of cylindrical cavities, a third layer of cylindrical cavities and a fourth layer of cylindrical cavities. The first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are obliquely embedded from bottom to top on the diagonal line of the perforated layer. The cavity radii of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are different and are arranged in a gradient. The cavity center distances of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are arranged in a gradient. The perforated layer is made of PDMS polydimethylsiloxane silicone rubber material.

[0007] It consists of a sealing layer, a perforated layer and a base layer, wherein the sealing layer is connected to the perforated layer and the base layer and is integrally cold-pressed;

[0008] The sealing layer is composed of a number of identical rectangular units arranged periodically, the front and rear inner surfaces of each sealing layer are bonded to the front and rear end surfaces of the long side of the perforated layer, the lower surface of the sealing layer is bonded to the upper surface of the edge of the base layer, and the sealing layer is made of PDMS polydimethylsiloxane silicone rubber material;

[0009] The perforated layer is rectangular, and is composed of a first layer of cylindrical cavities, a second layer of cylindrical cavities, a third layer of cylindrical cavities and a fourth layer of cylindrical cavities. The first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are obliquely embedded from bottom to top on the diagonal line of the perforated layer. The cavity radii of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are different and are arranged in a gradient. The cavity center distances of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are arranged in a gradient. The perforated layer is made of PDMS polydimethylsiloxane silicone rubber material.

[0010] Beneficial effects of the present invention: The present invention overcomes the problem of poor low-frequency sound absorption performance caused by the use of a single material; the radius and spacing of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are designed with gradient changes to effectively increase the width of the sound absorption frequency band; the four layers of cylindrical cavities are distributed on the diagonal of the perforated layer unit, which can effectively reduce the thickness of the perforated layer, and the thickness of the perforated layer is reduced by 86.6%, which is significantly thinned. Then, the sound absorption performance of the low-frequency broadband layer is obtained through a thinner size, which can not only meet the practical application requirements of lightweight, but also obtain better underwater low-frequency broadband sound absorption effect. The present invention has the characteristics of simple structure, simple manufacturing process, and strong designability, and is suitable for underwater low-frequency broadband vibration reduction and noise reduction applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The structure schematic diagram of the sound absorbing covering layer of the present invention is a section of the sound absorbing covering layer of the present invention, which includes three units of a base layer, a sealing layer and a perforated layer;

[0012] Figure 2 yes Figure 1 Schematic diagram of the unit structure of the perforated layer in the middle sound absorbing cover layer;

[0013] Figure 3 yes Figure 2 The main view;

[0014] Figure 4 yes Figure 2 Left view of;

[0015] Figure 5 yes Figure 2 A top view of

[0016] Figure 6 A comparison diagram of the sound absorption coefficient curves of the sound absorbing covering layer of the present invention and the sound absorbing covering layer with gradient in the central axis direction in Example 11;

[0017] Figure 7 It is a comparison diagram of the sound absorption coefficient curves of the sound absorption covering layer of the present invention in Example 12 and the sound absorption covering layer with the same radius and spacing of the cylindrical cavities on the diagonal line. DETAILED DESCRIPTION

[0018] Embodiment 1: Combination Figure 1 , Figure 2 and Figure 3 Describe this embodiment, this embodiment is composed of a sealing layer 1, a perforated layer 2 and a base layer 3, the sealing layer 1 is connected with the perforated layer 2 and the base layer 3, and is integrally cold-pressed;

[0019] The sealing layer, perforated layer and base layer are respectively composed of a number of identical rectangular parallelepiped units arranged periodically, the front and rear inner surfaces of each sealing layer unit are mutually bonded with the front and rear end surfaces of the long sides of the perforated layer unit, the lower surface of the sealing layer unit is mutually bonded with the upper surface of the base layer unit, and the sealing layer and perforated layer are made of PDMS polydimethylsiloxane silicone rubber material;

[0020] The perforated layer 2 is rectangular and consists of a first layer of cylindrical cavity 2-1, a second layer of cylindrical cavity 2-2, a third layer of cylindrical cavity 2-3 and a fourth layer of cylindrical cavity 2-4. The first layer of cylindrical cavity 2-1, the second layer of cylindrical cavity 2-2, the third layer of cylindrical cavity 2-3 and the fourth layer of cylindrical cavity 2-4 are obliquely embedded from bottom to top on the diagonal line of the perforated layer 2. The cavity radii of the first layer of cylindrical cavity 2-1, the second layer of cylindrical cavity 2-2, the third layer of cylindrical cavity 2-3 and the fourth layer of cylindrical cavity 2-4 are different and arranged in a gradient. The cavity center distances of the first layer of cylindrical cavity 2-1, the second layer of cylindrical cavity 2-2, the third layer of cylindrical cavity 2-3 and the fourth layer of cylindrical cavity 2-4 are arranged in a gradient. The perforated layer 2 is made of PDMS polydimethylsiloxane silicone rubber material.

[0021] Embodiment 2: Combination Figure 1 , Figure 2 and Figure 3 In this embodiment, the sealing layer 1 and the perforated layer 2 are both made of PDMS polydimethylsiloxane silicone rubber material, and the density is 1000 kg / m 3 , the complex Young's modulus is (1.879+0.540i)MPa, and the Poisson's ratio is 0.4997; the base layer 3 is made of steel material, and its density is 7890kg / m 3 , Young's modulus is 210GPa, and Poisson's ratio is 0.3.

[0022] Embodiment 3: Combination Figure 2 , Figure 3 , Figure 4 and Figure 5 Describing this embodiment,

[0023] The distance between the upper and lower end faces of each unit in the perforated layer described in this embodiment is its thickness direction, and its thickness d=20mm; the distance between the left and right side surfaces of each unit is its length direction, and its length a=200mm; the distance between the front and rear end surfaces of each unit in the perforated layer is its width direction, and its width is 200mm; the distance between the front and rear end faces of each unit in the sealing layer is its thickness direction, and its thickness is 30mm; the distance between the left and right end faces of each unit in the sealing layer is its length direction, and its length is 30mm; the distance between the upper and lower end faces of each unit in the sealing layer is its height direction, and its height is 20mm; the distance between the upper and lower end faces of the base layer is its thickness direction, and its thickness is 20mm; the distance between the front and rear end faces of the base layer is its width direction, and its width is 260mm; the distance between the left and right end faces of the base layer is its length direction, and its length is 200mm. The sealing layer, the perforated layer and the base layer are bonded by an adhesive and then cold-pressed as a whole.

[0024] Embodiment 4: Combination Figure 1 , Figure 2 and Figure 3 Describing this embodiment, each unit in the perforated layer 2 described in this embodiment is distributed with a first layer of cylindrical cavity 2-1, a second layer of cylindrical cavity 2-2 and a third layer of cylindrical cavity 2-3 from bottom to top, and the axes of the first layer of cylindrical cavity 2-1, the second layer of cylindrical cavity 2-2 and the third layer of cylindrical cavity 2-3 are respectively parallel to the upper and lower surfaces and the left and right side surfaces of the perforated layer 2; the lengths of the first layer of cylindrical cavity 2-1, the second layer of cylindrical cavity 2-2 and the third layer of cylindrical cavity 2-3 are equal, which is 200 mm.

[0025] Embodiment 5: Combination Figure 1 , Figure 2 Describing this embodiment, in each unit of the perforated layer 2 described in this embodiment, the centers of the first layer cylindrical cavity 2-1, the second layer cylindrical cavity 2-2, the third layer cylindrical cavity 2-3 and the fourth layer cylindrical cavity 2-4 are distributed on a diagonal line of the unit, and the diagonal line is determined by the upper right vertex and the lower left point of the front surface of the perforated layer 2 unit.

[0026] Embodiment 6: Combination Figure 1 , Figure 2 and Figure 3 Describe this embodiment. In this embodiment, the cavity radius of each unit of the perforated layer 2 is r 1 , the radius of the second cylindrical cavity is r 2 , the radius of the third cylindrical cavity is r 3 , the radius of the fourth cylindrical cavity is r 4 , that is, the radius of the four-layer cylindrical cavity changes from top to bottom with a positive gradient, that is, r 1 >r 2 >r3 >r 4 , and there is r 1 :r 4 ≤3:1.

[0027] Embodiment 7, combination Figure 1 Describing this embodiment, the centers of the first-layer cylindrical cavities 2-1 and the first-layer cylindrical cavities 2-1 of adjacent units in the perforated layer 2 described in this embodiment are on the same horizontal line, and the horizontal distance between the centers of the two circles is 200 mm; similarly, the centers of the second-layer cylindrical cavities 2-2 and the second-layer cylindrical cavities 2-2, the third-layer cylindrical cavities 2-3 and the third-layer cylindrical cavities 2-3, and the fourth-layer cylindrical cavities 2-4 and the fourth-layer cylindrical cavities 2-4 of adjacent units are also on the same horizontal line, and the average horizontal distance between the centers of the two circles is 200 mm.

[0028] Embodiment 8, combination Figure 2 and Figure 3 Describe this embodiment. In each unit of the perforated layer 2 described in this embodiment, the distance between the lower left point and the center of the first layer cylindrical cavity 2-1 is d 1 The distance between the centers of the first cylindrical cavity 2-1 and the second cylindrical cavity 2-2 is d 2 The distance between the centers of the second cylindrical cavity 2-2 and the third cylindrical cavity 2-3 is d 3 The distance between the centers of the third cylindrical cavity 2-3 and the fourth cylindrical cavity 2-4 is d 4 , the distance from the fourth layer cylindrical cavity 2-4 to the upper right vertex of the unit is d 5 , d 2 >d 3 >d 4 , that is, the spacing between the four layers of cylindrical cavities changes gradually, and there is d 3 :d 2 >d 4 :d 3 ,d 1 =d 5 .

[0029] Embodiment 9: Combination Figure 2 and Figure 3 Describe this embodiment, the d in each unit of the perforated layer 2 described in this embodiment 1 =35.7mm, d 2 =12.5mm,d 3 =37.5mm, d 4 =87.5mm, d 5 =35.7mm.

[0030] Embodiment 10, combination Figure 1 , Figure 2 and Figure 3Describe this embodiment, the cross-sectional shape of the four-layer cylindrical cavity described in this embodiment is circular. In order to satisfy the relationship that the spacing between the four-layer cylindrical cavities changes in a positive gradient from bottom to top, the lower left corner of the perforated layer 2 unit in the sound-absorbing covering layer is 35.7 mm away from the center of the first-layer cylindrical cavity 2-1, the center distance between the first-layer cylindrical cavity 2-1 and the second-layer cylindrical cavity 2-2 is 12.5 mm, the center distance between the second-layer cylindrical cavity 2-2 and the third-layer cylindrical cavity 2-3 is 37.5 mm, the center distance between the third-layer cylindrical cavity 2-3 and the fourth-layer cylindrical cavity 2-4 is 87.5 mm, and the center of the fourth-layer cylindrical cavity 2-4 is 35.7 mm away from the upper right corner of the unit in the sound-absorbing covering layer. In order to satisfy the relationship that the radius of each layer of cylindrical cavity changes with a positive gradient from top to bottom, the radius of the first layer of cylindrical cavity 2-1 is 2mm, the radius of the second layer of cylindrical cavity 2-2 is 1.5mm, the radius of the third layer of cylindrical cavity 2-3 is 1.3mm, and the radius of the fourth layer of cylindrical cavity 2-4 is 1mm. The lengths of the four cylindrical cavities are the same.

[0031] Embodiment 11: Combination Figure 1 , Figure 2 , Figure 3 and Figure 6 Describe this embodiment, this example uses the sound-absorbing covering layer of the present invention of embodiment ten. The sound-absorbing covering layer of this embodiment is compared with the gradient sound-absorbing covering layer in the direction of the central axis. Among them, the gradient sound-absorbing covering layer in the direction of the central axis is a sound-absorbing covering layer in which the radius and spacing of the cylindrical cavities are gradiently distributed on the central axis of the perforated layer 2 unit. The perforated layer 2 of the gradient sound-absorbing covering layer in the direction of the central axis is similar to the perforated layer 2 of the gradient sound-absorbing covering layer in the diagonal direction, and both are composed of several identical rectangular units arranged in a periodic manner. For the perforated layer 2 unit of the gradient sound-absorbing covering layer in the direction of the central axis, the radius of the first layer cylindrical cavity 2-1 is 6.5mm, the radius of the second layer cylindrical cavity 2-2 is 4.7mm, the radius of the third layer cylindrical cavity 2-3 is 2mm, and the radius of the fourth layer cylindrical cavity 2-4 is 1mm. The distance between the center of the first layer cylindrical cavity 2-1 and the bottom of the unit is 11.5mm, the distance between the center of the first layer cylindrical cavity 2-1 and the second layer cylindrical cavity 2-2 is 25mm, the distance between the center of the second layer cylindrical cavity 2-2 and the third layer cylindrical cavity 2-3 is 47mm, and the distance between the center of the third layer cylindrical cavity 2-3 and the fourth layer cylindrical cavity 2-4 is 118.5mm. The thickness of each perforated layer 2 unit is equal, which is 150mm, and the width is equal, which is 40mm.

[0032] In addition to the above description, the parameters of the sound-absorbing covering layer of this embodiment are the same as those of the sound-absorbing covering layer with gradient in the central axis direction; for example, the average density of the PDMS silicone rubber is 1000 kg / m 3The complex Young's modulus is (1.879+0.540i)MPa, the Poisson's ratio is 0.4997; the thickness of the base layer 3 is 20mm, and the density of the base layer 3 is 7890kg / m 3 , Young’s modulus is 210 GPa, and Poisson’s ratio is 0.3.

[0033] Depend on Figure 6 The comparison results of the sound absorption characteristics of the two sound-absorbing covering layers show that the sound-absorbing covering layer with a gradient distribution of cavity structure parameters on the central axis has only one absorption peak in the frequency band, and the starting frequency of the sound absorption coefficient above 0.6 is 850Hz, the cut-off frequency is 2200Hz, the bandwidth is 1350Hz, and the peak value is 0.98. However, the low-frequency broadband sound-absorbing covering layer (i.e., the diagonal gradient sound-absorbing covering layer) with an embedded diagonal gradient cylindrical cavity of the present invention has two absorption peaks in the sound absorption coefficient curve, the peak sound absorption coefficient of the first absorption peak is 1, and the peak sound absorption coefficient of the second absorption peak is 0.9. These two absorption peaks together form a broadband absorption peak, and the lower frequency limit of the broadband absorption peak with a sound absorption coefficient above 0.6 is 600Hz, the upper frequency limit is 2600Hz, and the bandwidth is 2000Hz. By comparing the average sound absorption coefficient and the sound absorption bandwidth, it can be seen that the average sound absorption coefficient of the sound absorption covering layer with a gradient distribution of cavity structure parameters on the diagonal line is about 0.72, and the average sound absorption coefficient of the sound absorption covering layer with a gradient distribution of cavity structure parameters on the central axis is about 0.63. The bandwidth of the sound absorption covering layer with a gradient distribution of cavity structure parameters on the diagonal line is about 48% higher than that of the sound absorption covering layer with a gradient distribution of cavity structure parameters on the central axis; the thickness is reduced by 86.6%, which is significantly thinner. Therefore, the use of a sound absorption covering layer with a gradient distribution of cavity structure parameters on the diagonal line can obtain better low-frequency broadband sound absorption effect, and the sound absorption covering layer of the present invention achieves the goal of lightweight design.

[0034] Embodiment 12: Combination Figure 1 , Figure 2 , Figure 3 and Figure 7 This embodiment is described. This embodiment uses the sound-absorbing covering layer of the present invention of embodiment 10. The sound-absorbing covering layer of the present invention (i.e., gradient parameter structure) is compared with the sound-absorbing covering layer (i.e., equal parameter structure) in which the radius and spacing of the cylindrical cavities on the diagonal are respectively equal. The radius of the four cylindrical cavities in the sound-absorbing covering layer in which the radius and spacing of the cylindrical cavities on the diagonal are respectively equal are 1.5 mm. In each perforated layer 2 unit, the center distance between adjacent cylindrical cavities is 42.2 mm. The distance between the center of the first layer cylindrical cavity 2-1 and the lower left corner of the perforated layer 2 unit is 42.2 mm. The distance between the center of the fourth layer cylindrical cavity 2-4 and the upper right corner of the unit is 42.2 mm. The thickness of each perforated layer 2 unit is equal, which is 20 mm, and the width is equal, which is 200 mm.

[0035] In addition to the above description, the parameters of the sound-absorbing covering layer of this embodiment are the same as those of the sound-absorbing covering layer with the same radius and spacing of the cylindrical cavities on the unit diagonal line; the density of the PDMS silicone rubber is 1000kg / m 3 The complex Young's modulus is (1.879+0.540i)MPa, the Poisson's ratio is 0.4997; the thickness of the base layer 3 is 20mm, and the density of the base layer 3 is 7890kg / m 3 , Young’s modulus is 210 GPa, and Poisson’s ratio is 0.3.

[0036] Figure 7 The comparison results of the sound absorption characteristics of two sound-absorbing covering layers. It can be seen that there is an absorption peak in the sound absorption coefficient curve of the sound-absorbing covering layer with equal cylindrical cavity radius and spacing on the diagonal line, the starting frequency of the sound absorption coefficient above 0.6 is 600Hz, the cut-off frequency is 1200Hz, the bandwidth is 600Hz, and the peak value of the sound absorption coefficient is 0.95. The sound absorption coefficient curve of the low-frequency broadband sound-absorbing covering layer with embedded diagonal gradient cylindrical cavities of the present invention has two absorption peaks, the peak sound absorption coefficient of the first absorption peak is 1, and the peak sound absorption coefficient of the second absorption peak is 0.9. These two absorption peaks together form a broadband absorption peak, the starting frequency of the broadband absorption peak with a sound absorption coefficient above 0.6 is 600Hz, the cut-off frequency is 2600Hz, and the bandwidth is 2000Hz. Its sound absorption coefficient and sound absorption bandwidth are much larger than those of the sound-absorbing covering layer with equal cylindrical cavity radius and spacing on the diagonal line. Therefore, a better low-frequency broadband sound absorption effect can be obtained by using a sound-absorbing covering layer with a gradient distribution of the cylindrical cavity radius and spacing on the diagonal line.

[0037] The above embodiments are merely exemplary and do not limit the present invention. It should be pointed out that for those skilled in the art, under the inspiration of the technical solution provided by the present invention, it should be understood that ordinary technicians in the field can make many modifications and changes based on the concept of the present invention without creative work. Therefore, all other equivalent changes, modifications, substitutions and variations made by technicians in this technical field based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be regarded as the protection scope of the present invention.

Claims

1. A low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity, which consists of a sealing layer, a perforated layer and a base layer, characterized in that: The sealing layer is connected with the perforated layer and the base layer and is integrally cold-pressed; The sealing layer, perforated layer and base layer are respectively composed of a number of identical rectangular parallelepiped units arranged periodically, the front and rear inner surfaces of each sealing layer unit are mutually bonded with the front and rear end surfaces of the long sides of the perforated layer unit, the lower surface of the sealing layer unit is mutually bonded with the upper surface of the base layer unit, and the sealing layer and perforated layer are made of PDMS polydimethylsiloxane silicone rubber material; The perforated layer is rectangular, and is composed of a first layer of cylindrical cavities, a second layer of cylindrical cavities, a third layer of cylindrical cavities and a fourth layer of cylindrical cavities. The first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are obliquely embedded from bottom to top on the diagonal line of the perforated layer. The cavity radii of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are different and are arranged in a gradient. The cavity center distances of the first layer of cylindrical cavities, the second layer of cylindrical cavities, the third layer of cylindrical cavities and the fourth layer of cylindrical cavities are arranged in a gradient. The perforated layer is made of PDMS polydimethylsiloxane silicone rubber material.

2. A low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 1, characterized in that: The sealing layer and the perforation layer are both made of PDMS polydimethylsiloxane silicone rubber with a density of 1000kg / m 3 , the complex Young's modulus is (1.879+0.540i)MPa, and the Poisson's ratio is 0.4997; the base layer is made of steel material, and its density is 7890kg / m 3 , Young's modulus is 210GPa, and Poisson's ratio is 0.

3.

3. The low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 1, characterized in that: The distance between the upper and lower end faces of each unit in the perforated layer is in the thickness direction, and its thickness d=20mm; the distance between the left and right side surfaces of each unit is in the length direction, and its length a=200mm; the distance between the front and rear end surfaces of each unit in the perforated layer is in the width direction, and its width is 200mm; the distance between the front and rear end faces of each unit in the sealing layer is in the thickness direction, and its thickness is 30mm; the distance between the left and right end faces of each unit in the sealing layer is in the length direction, and its length is 30mm; the distance between the upper and lower end faces of each unit in the sealing layer is in the height direction, and its height is 20mm; the distance between the upper and lower end faces of the base layer is in the thickness direction, and its thickness is 20mm; the distance between the front and rear end faces of the base layer is in the width direction, and its width is 260mm; the distance between the left and right end faces of the base layer is in the length direction, and its length is 200mm. The sealing layer, the perforated layer and the base layer are bonded by an adhesive and then integrally cold-pressed.

4. The low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 1, characterized in that: Each unit in the perforated layer has a first layer of cylindrical cavity, a second layer of cylindrical cavity and a third layer of cylindrical cavity distributed from bottom to top, and the axes of the first layer of cylindrical cavity, the second layer of cylindrical cavity and the third layer of cylindrical cavity are respectively parallel to the upper and lower surfaces and the left and right sides of the perforated layer; the lengths of the first layer of cylindrical cavity, the second layer of cylindrical cavity and the third layer of cylindrical cavity are equal, all of which are 200 mm.

5. The low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 1, characterized in that: In each unit of the perforated layer, the centers of the first cylindrical cavity, the second cylindrical cavity, the third cylindrical cavity and the fourth cylindrical cavity are distributed on a diagonal line of the unit, which is the line connecting the upper right vertex and the lower left point of the front surface of the perforated layer unit.

6. A low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 5, characterized in that: For each unit in the perforated layer, the cavity radius of the first layer of cylindrical cavity is r1, the cavity radius of the second layer of cylindrical cavity is r2, the cavity radius of the third layer of cylindrical cavity is r3, and the cavity radius of the fourth layer of cylindrical cavity is r4, that is, the radii of the four layers of cylindrical cavities change with a positive gradient from top to bottom, that is, r1>r2>r3>r4, and r1:r4≤3:

1.

7. A low-frequency broadband sound-absorbing covering layer with an embedded diagonally gradient cylindrical cavity according to claim 6, characterized in that: The center distances of the first cylindrical cavity, the second cylindrical cavity, the third cylindrical cavity and the fourth cylindrical cavity of adjacent units in the perforated layer are all 200 mm.

8. The low-frequency broadband sound-absorbing covering layer with embedded diagonally gradient cylindrical cavities according to claim 1, characterized in that: In each unit of the perforated layer, the distance between the lower left point of the front end face and the center of the first cylindrical cavity is d1, the distance between the centers of the first cylindrical cavity and the second cylindrical cavity is d2, the distance between the centers of the second cylindrical cavity and the third cylindrical cavity is d3, the distance between the centers of the third cylindrical cavity and the fourth cylindrical cavity is d4, and the distance from the fourth cylindrical cavity to the upper right vertex of the unit is d5, d2>d3>d4, that is, the distances between the four layers of cylindrical cavities vary gradually, and d3:d2>d4:d3, d1=d5.

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