Gradient impedance matching layer

Abstract

The application belongs to the technical field of ultrasonic transducer and acoustic detection, and particularly relates to a gradient impedance matching layer, which comprises a first matching layer and a second matching layer; a first background medium and a second background medium are arranged at the interface between the first matching layer and the second matching layer; the first matching layer is composed of solid fillers or hollow fillers arranged in a discrete manner in the first background medium; the second matching layer is composed of hollow fillers arranged in a discrete manner in the second background medium; the impedance of the gradient matching layer presents an exponential distribution from low to high, the frequency of a quarter wavelength corresponding to the thickness of each layer is consistent, and wide-frequency transmission is realized through multi-layer resonance superposition. The gradient impedance matching layer can realize wide-frequency transmission across the interface of different background media.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic transducer and acoustic detection technology, specifically a gradient impedance matching layer, and more specifically a gradient impedance matching layer that can achieve wideband liquid-gas sound transmission. Background Technology

[0002] With the development and utilization of marine resources, underwater and air communication has become crucial. Since sound waves attenuate far less in seawater than electromagnetic waves, they serve as the primary carrier of information and energy in the ocean. However, due to the significant impedance difference between water and air, only 0.1% of the sound energy can propagate through the water-air interface when a sound wave is incident on it, posing a significant challenge to sound-based water-air communication.

[0003] Currently, gradient impedance matching layers for broadband acoustic transmission are widely used in fields such as ultrasonic testing and medical ultrasound. However, due to the significant impedance difference between water and air, it is difficult to implement a corresponding gradient impedance matching layer. Research on water-air acoustic transmission mainly focuses on single-frequency transmission achieved through resonance, without broadband transmission. However, single-frequency water-air acoustic transmission can only carry a limited amount of information and has low transmission efficiency, which cannot meet the needs of practical communication. Summary of the Invention

[0004] The purpose of this invention is to provide a gradient impedance matching layer that can not only achieve broadband transmission across the gas-liquid interface, but also provide gradient impedance matching layers for different target frequency bands and bandwidths and different numbers of layers according to the actual application scenario requirements.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A gradient impedance matching layer, the gradient impedance matching layer comprising a first matching layer and a second matching layer; the first matching layer and the second matching layer are separated by a first background medium and a second background medium;

[0007] The first matching layer consists of solid or hollow fillers arranged discretely in a first background medium;

[0008] The second matching layer consists of hollow fillers arranged discretely in the second background medium;

[0009] The impedance of the gradient matching layer exhibits an exponential distribution from low to high, and the frequency of each quarter wavelength corresponding to the thickness of each layer remains consistent. Through the superposition of multiple layers of resonance, broadband transmission is achieved.

[0010] Preferably, the equivalent sound velocity c of the first matching layer eff1 and equivalent impedance Z eff1 It is calculated using the following formula:

[0011]

[0012] Wherein, the filling ratio f is the proportion of the filler in the first background medium, which is a dimensionless parameter, and c a Z is the velocity of sound in the first background medium, expressed in m / s. a The impedance of the first background medium is expressed in Rayles.

[0013] Preferably, the equivalent impedance Z of the second matching layer eff2 and equivalent speed of sound c eff2 From the formula Calculated;

[0014] Wherein, the equivalent density ρ eff and bulk modulus K eff It is calculated using the following formula:

[0015] ρ eff =ρ w f w +ρ s f s +ρ a f a

[0016]

[0017] f w ,f s ,f a These represent the filling rates of the second background medium, the hollow-filled frame, and the first background medium within the hollow-filled frame, respectively, in the discrete element.

[0018] ρ w ,ρ s ,ρ a Densities of the second background medium, the hollow filling frame, and the first background medium, respectively, in kg / m³. 3 ;

[0019] S u The area of ​​a discrete element, in meters. 2 ;

[0020] S w The area occupied by the second background medium in a single discrete cell, in meters. 2 K w The bulk modulus of the second background medium, in Pa;

[0021] dS al It is the deformation of hollow filler caused by external pressure, in meters (m). 2 / Pa.

[0022] Preferably, the solid filler has a cross-sectional shape of a regular n-gon or a circle, where n ≥ 4; the hollow filler has a cross-sectional shape of a regular n-gon or a circle, where n ≥ 4. Those skilled in the art can also choose other cross-sectional shapes as needed, such as rectangles or irregular polygons like trapezoids.

[0023] Preferably, the solid and hollow fillers are made of wood, plastic, or metal. More preferably, the metal is aluminum, copper, or steel. Those skilled in the art may also choose other materials not described in this invention, such as polyethylene, polypropylene, or epoxy resin among plastics.

[0024] Preferably, the first matching layer includes several fillers with different structural geometric parameters, wherein the cross-sectional side length of the discrete unit is b = 1-100 mm, the cross-sectional side length of the filler is a = 0.5-99.5 mm, and b is greater than a.

[0025] More preferably, the first matching layer includes two types of square solid fillers with different structural geometric parameters. The structural geometric parameters of the first type of square solid filler are: the side length of the discrete unit cross-section is b = 10 mm, the side length of the square solid filler cross-section is a = 7.5 mm, and the number of columns is 6. The structural geometric parameters of the second type of square solid filler are: the side length of the discrete unit cross-section is b = 30 mm, the side length of the square solid filler cross-section is a = 29 mm, and the number of columns is 2. In this first matching layer, the two types of fillers with different structural geometric parameters are arranged in an array shape. For example, the first type of square solid filler has 6 columns and 18 rows. Of course, those skilled in the art can choose other numbers of columns and rows as needed. The second type of square solid filler has 2 columns and 6 rows. Of course, those skilled in the art can choose other numbers of columns and rows as needed. The first and second types are arranged adjacent to each other, and the filler with the second type of structure is adjacent to the second matching layer.

[0026] Preferably, the second matching layer includes several hollow fillers with different structural geometric parameters, wherein the cross-sectional side length of the discrete unit is x = 5-100 mm, the cross-sectional side length of the hollow filler is y = 4-99 mm, and the wall thickness is t = 0.5-98 mm; and x is greater than y, and y is greater than t.

[0027] More preferably, the second matching layer includes two types of square hollow fillers with different structural geometric parameters. The structural geometric parameters of the first type of square hollow filler are: the side length of the discrete unit cross-section is x = 36 mm, the side length of the square hollow filler cross-section is y = 28 mm, the wall thickness is t = 0.5 mm, and the number of columns is 1. The structural geometric parameters of the second type of square solid filler are: the side length of the discrete unit cross-section is x = 20 mm, the side length of the square hollow filler cross-section is y = 17 mm, the wall thickness is t = 0.5 mm, and the number of columns is 9. In this second matching layer, the two types of fillers with different structural geometric parameters are arranged in an array shape. For example, the first type of square hollow filler has 1 column and 5 rows. Of course, those skilled in the art can choose other column and row numbers as needed. The second type of square hollow filler has 9 columns and 9 rows. Of course, those skilled in the art can choose other column and row numbers as needed. The first and second types are arranged adjacent to each other, and the filler with the first type of structure is adjacent to the first matching layer.

[0028] Preferably, the cross-sectional shape of the discrete unit in the discrete arrangement is square or rectangular.

[0029] Preferably, the first background medium is air, and the second background medium is water. Those skilled in the art may also select other background media as needed, such as other gases or liquids.

[0030] When the cross-sectional shapes of the discrete unit, solid filler, and hollow filler are all square, the preferred discrete arrangement is a periodic arrangement.

[0031] In this case:

[0032] The first matching layer consists of square solid fillers arranged in a discrete periodic pattern. The side length of the cross section of the discrete unit is b, and the side length of the cross section of the solid filler is a.

[0033] The second matching layer consists of hollow fillers arranged in a discrete periodic pattern. The side length of the discrete unit is x, the side length of the hollow filler is y, and the wall thickness is t.

[0034] Fill rate of the first matching layer

[0035] f in the second matching layer w ,f s ,f a The calculation formulas are as follows:

[0036] S u =x 2 ;

[0037] The filling rate of the filler in this invention can be calculated according to existing technology, such as the filling rate of regular n-gons other than square cross-sections, the filling rate of circles, and even the filling rate of other irregular shapes.

[0038] In this invention, the length of the filler can be selected as needed, and the length will not affect the equivalent acoustic parameters of the structure.

[0039] According to a preferred embodiment of the present invention, the impedance of the gradient matching layer exhibits an exponential distribution from low to high, and the frequency of each quarter wavelength corresponding to the length of each layer is 1100Hz (±100Hz). Through multi-layer resonant superposition, broadband transmission is achieved.

[0040] In this invention, a discrete unit refers to a lattice unit formed by a solid or hollow filler and its surrounding background medium. Discrete periodic arrangement refers to the arrangement of discrete units according to a certain pattern (such as an array), which in this invention can be a square array or a rectangular array, etc.

[0041] Compared with the prior art, the present invention has the following technical effects:

[0042] The impedance range achievable by this invention completely covers the impedance gap from air to water, enabling the implementation of water-air gradient impedance matching layers. It can also achieve broadband transmission across other different media interfaces. Attached Figure Description

[0043] Figure 1 This is a cross-sectional view of the first matching layer;

[0044] Figure 2 The curve showing the effect of filler filler ratio on the equivalent impedance of the first matching layer;

[0045] Figure 3 This is a structural diagram showing that the filler in the first matching layer is hollow.

[0046] Figure 4 This is a non-periodic arrangement of structures in the first matching layer;

[0047] Figure 5 This is a cross-sectional view of the second matching layer;

[0048] Figure 6 The equivalent impedance control curve of the second matching layer is given by the type of solid material used in the hollow filler.

[0049] Figure 7 The equivalent impedance modulation curve of the second matching layer on the structural shape of the hollow filler;

[0050] Figure 8 It is a broadband water-air acoustic gradient impedance matching layer;

[0051] Figure 9 The comparison curves show the acoustic transmission coefficients with and without a gradient matching layer.

[0052] Figure 10 This is a test structure diagram of the water vapor gradient impedance matching layer. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0054] Example 1

[0055] A gradient impedance matching layer that enables broadband water-air acoustic transmission includes a first matching layer and a second matching layer, with the interface between the first matching layer and the second matching layer being water and air.

[0056] The first matching layer consists of solid square cells arranged in a discrete periodic pattern in the air. The side length of each discrete cell is represented by b (in mm), and the side length of the cross-section of each solid square cell is represented by a (in mm). The structure of the first matching layer is as follows: Figure 1 As shown, the lattice unit is composed of Figure 1 The area is outlined in dark blue. The acoustic parameters of the first matching layer are primarily controlled by the filler content of the solid filler, with minimal influence from the solid material and shape of the filler, which can be disregarded here. Therefore, the equivalent acoustic parameters of the first matching layer can be adjusted by changing the filler content of the solid filler. To adjust for this, its equivalent sound velocity and equivalent impedance can be calculated using the following formulas:

[0057]

[0058] Where the fill rate f is a dimensionless parameter, c a Z is the speed of sound in the background medium, air, in m / s. a c represents the impedance of the background medium, air, in Rayles. eff1 With Z eff1 These are the equivalent sound velocity and equivalent impedance of the first matching layer, respectively, with units of m / s and Rayls.

[0059] Taking the solid filler as a square, the equivalent acoustic parameters were calculated to obtain the relationship between its equivalent impedance and the filler content, such as... Figure 2 The results show that its adjustable range is 0.000415MRals-0.029MRayls.

[0060] The first matching layer primarily achieves varying degrees of acoustic wave obstruction by adjusting the filling ratio of the solid filler material in the first background medium—air—thereby regulating its equivalent acoustic parameters. Since air has very low impedance, almost all solid materials can be considered hard boundaries; therefore, the influence of the solid filler material's material and geometry is disregarded. Similarly, replacing it with a hollow filler material of a certain thickness will not affect the equivalent parameters of the first matching layer. Figure 3 As shown.

[0061] In the matching layer, the filler is preferably arranged periodically, but this arrangement is the optimal arrangement of the present invention. The filler can also be arranged non-periodicly, such as... Figure 4 The diagram shows a weakly isotropic structure with a non-periodic arrangement. Its structure is not periodically arranged, and the infill exhibits weak anisotropy, not being a perfectly regular n-gon. Even with the same infill ratio around the perimeter of the infill, the desired effect can be achieved without significant differences.

[0062] The second matching layer consists of square hollow fillers arranged in a discrete periodic pattern in the water, with the following geometric configuration: Figure 5 As shown, the lattice unit is composed of Figure 5 The area outlined in dark blue is denoted by x (mm). The side length of the square hollow filler is denoted by y (mm), and the wall thickness is denoted by t (mm). The adjustment of the ratio of water, air, and the solid frame mentioned in this invention refers to adjusting... Figure 5 The proportions of water (second background medium), hollow-filled solid frame, and air within the hollow-filled solid frame are shown in the single representative unit highlighted in dark blue. The equivalent acoustic parameters of the second matching layer can be obtained by adjusting the filling ratios of water, air, and solids, and its equivalent density and bulk modulus can be calculated using the following formulas:

[0063] ρ eff =ρ w f w +ρ s f s +ρ a f a ,

[0064]

[0065] Among them, f w ,f s ,f a ρ represents the filling ratio of water, solid, and air in the lattice unit, respectively. w ,ρ s ,ρ a (Unit: kg / m³) 3 The densities of water, solid, and air are S, respectively. w(Unit: m) 2 ),K w (Unit: Pa) represents the area occupied by water in the second matched layer lattice unit and the bulk modulus of water, respectively, dS. solid (Unit: m) 2 / Pa) is the deformation of the hollow filler composed of a solid frame and air due to external pressure, which is mainly controlled by the filling ratio of the solid frame and air, as well as the solid material. The equivalent impedance and equivalent sound velocity of this second matching layer structure can be further expressed by the formula The calculation shows that the proportion of water in the unit can be determined by the fill rate. This indicates that the proportion of the solid frame portion in the element can be determined by the fill rate. This indicates that the air portion can be determined by the fill rate. To represent. The proportion in equivalent mass (ρ) eff =ρ w f w +ρ s f s +ρ a f a and equivalent modulus This is reflected in the calculations. Adjusting the wall thickness indirectly regulates the ratio of the three components, directly adjusting the equivalent density. On the other hand, it primarily affects the deformation dS of the solid hollow structure in the equivalent modulus. al .

[0066] about The detailed derivation is as follows:

[0067] For the second matching layer, pressure is transmitted to the solid hollow structure via the surrounding background fluid water. Therefore, the pressure-sensing components are mainly the second background medium fluid water and the solid hollow structure (a whole consisting of a solid frame and internal air). The equivalent bulk modulus of the element can be expressed based on the modulus of these two parts and the fill ratio (proportion) as follows:

[0068]

[0069] Here, f1 and f2 are the filling rates of the fluid water and the solid hollow structure, respectively. Here, f2 is the overall ratio formed by the solid and the internal hollow structure, which is different from the ratio of the solid frame alone mentioned above.

[0070]

[0071]

[0072] K1 and K2 are the bulk moduli of water and a solid hollow structure, respectively. The bulk modulus of water is fixed, while the bulk modulus of the solid hollow structure needs to be calculated according to the definition of bulk modulus.

[0073] K1 = K w

[0074]

[0075] V = y 2 Let dP be the total volume of the hollow solid structure, and dV be the deformation of the hollow solid structure caused by a pressure of 1 Pa. Since the length of the hollow solid structure is not considered, it is assumed to be a two-dimensional structure. al It means that dS al The form is mainly determined by the parameters of the solid frame, the shape of the solid structure, and the wall thickness of the solid frame. Therefore, by substituting f1, f2, K1, and K2 into the above process, we can obtain the form shown in the text.

[0076] The equivalent acoustic parameters of a material or structure generally refer to the equivalent density (ρ) and equivalent sound velocity (c), while the bulk modulus (K = ρc) is... 2 The equivalent impedance (Z = ρc) is calculated from density and sound velocity, and the four quantities can be converted into each other.

[0077] To demonstrate the impedance control range of the second matching layer, the geometric dimensions of the unit were initially set to x = 40 mm and y = 38 mm. The equivalent impedance was calculated as a function of the wall thickness t of the solid filler frame structure when the solid materials were aluminum, copper, and iron. The results are shown in... Figure 6 In this invention, the equivalent impedance of the unit changes with the solid geometry when the solid filler material is aluminum, the unit geometry is set to x = 40 mm, the solid structure wall thickness is t = 1 mm, and the water filling rate is maintained at 0.75. The results are shown in [the table / document / etc.]. Figure 7 In. Figure 6 and Figure 7 In the example results, by changing the solid filler material and the geometry of the solid filler structure, the impedance range achievable by the second matching layer is 0.0026 MRayls to 3.8 MRayls. Based on the above design method, further adjustments to the filler ratio, solid material type, and geometry can broaden the achievable impedance range.

[0078] As can be seen from the above control examples, the impedance range that can be achieved by the two matching layers completely covers the impedance gap from air to water, and can be used to realize water-air gradient impedance matching layers.

[0079] Example 2

[0080] Based on the above design method, we designed a discrete gradient impedance matching layer for matching air and water using the two matching layers mentioned above. Its structure is as follows: Figure 8As shown, a gradient impedance matching layer includes a first matching layer and a second matching layer; the first matching layer and the second matching layer are separated by a first background medium (air) and a second background medium (water).

[0081] The first matching layer consists of solid fillers with isotropic geometric configurations arranged in discrete periods, with the side length of the discrete unit cross section being b and the side length of the solid filler cross section being a.

[0082] The second matching layer consists of hollow fillers with isotropic geometric configurations arranged in discrete periods. The side length of the discrete unit is x, the side length of the hollow filler is y, and the wall thickness is t.

[0083] The first matching layer includes two types of square solid fillers with different structural geometric parameters. The structural geometric parameters of the first type of square solid filler are: the side length of the discrete element's cross-section is b = 10 mm, the side length of the square solid filler's cross-section is a = 7.5 mm, and the number of columns is 6. The structural geometric parameters of the second type of square solid filler are: the side length of the discrete element's cross-section is b = 30 mm, the side length of the square solid filler's cross-section is a = 29 mm, and the number of columns is 2. In this first matching layer, the two types of fillers with different structural geometric parameters are arranged in an array shape. For example, the first type of square solid filler has 6 columns and 18 rows, and the second type of square solid filler has 2 columns and 6 rows.

[0084] The second matching layer includes two types of square hollow fillers with different structural geometric parameters. The structural geometric parameters of the first type of square hollow filler are: the side length of the discrete element's cross-section is x = 36 mm, the side length of the square hollow filler's cross-section is y = 28 mm, the wall thickness is t = 0.5 mm, and the number of columns is 1. The structural geometric parameters of the second type of square solid filler are: the side length of the discrete element's cross-section is x = 20 mm, the side length of the square hollow filler's cross-section is y = 17 mm, the wall thickness is t = 1 mm, and the number of columns is 9. In this second matching layer, the fillers with the two different structural geometric parameters are arranged in an array shape. For example, the first type of square hollow filler has 1 column and 5 rows; the second type of square hollow filler has 9 columns and 9 rows. The first and second types are arranged adjacent to each other, and the filler with the first structure is adjacent to the first matching layer.

[0085] The gradient impedance matching layer consists of four layers, connected to air and water respectively. Layers 1-2 form the first matching layer, composed of discretely periodically arranged solid aluminum fillers in air. Layers 3-4 form the second matching layer, composed of discretely periodically arranged hollow aluminum fillers in water. The water-air interface is located between layers 2 and 3. The impedance of the matching layer exhibits an exponential distribution from low to high. The frequency corresponding to the quarter-wavelength of each layer is around 1100Hz (±100Hz). Broadband transmission is achieved through multi-layer resonant superposition. The detailed geometric and acoustic parameters of each layer of the gradient impedance matching layer are shown in Table 1 below.

[0086] Table 1

[0087]

[0088] The acoustic transmission coefficients before and after adding a matching layer between air and water were simulated and calculated, and the results are as follows: Figure 9 As shown. The results show that without the matching layer, the sound transmission coefficient between air and water is only 0.001. After adding the impedance matching layer of this invention, a broadband sound transmission coefficient of over 0.8 can be achieved in the range of 700Hz-1400Hz.

[0089] This invention fabricated a gradient impedance matching layer sample and conducted qualitative transmission tests in an acrylic water bath measuring 1.5m*1m*0.75m. Figure 10 As shown, the specific matching layer structure consists of square aluminum tubes. To maintain their discrete arrangement within the background medium, they are inserted into perforated plates fixed on both sides, forming a quasi-two-dimensional matching layer in the middle of the two perforated plates. During testing, the entire support with the sample inserted is placed in a water tank, the water level is adjusted to a designated position, sound is emitted into the air, and the transmitted sound pressure is measured directly below the sample in the water. This invention uses a loudspeaker in the air to emit a swept-frequency signal, and a hydrophone in the water tank to receive the signal. The effectiveness of the gradient impedance matching layer is verified by comparing the increase in sound pressure level in the water tank before and after the installation of the gradient impedance matching layer. The results show that the gradient impedance matching layer can achieve a sound pressure level enhancement effect of more than 15dB in the 800Hz-1600Hz range.

[0090] The above results demonstrate that the present invention can completely cover impedance gaps spanning five orders of magnitude from air to water, and that broadband transmission across the water-air interface can be achieved using the gradient matching layer implemented by the present invention. The above examples are merely illustrative of the design steps and implementation effects; in specific applications, gradient matching layers with different target frequency bands, bandwidths, and numbers of layers can be designed according to the actual application scenario requirements.

[0091] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A gradient impedance matching layer, characterized in that, The gradient impedance matching layer includes a first matching layer and a second matching layer; the first matching layer and the second matching layer are separated by a first background medium and a second background medium. The first matching layer consists of solid or hollow fillers arranged discretely in a first background medium; The second matching layer consists of hollow fillers arranged discretely in the second background medium; The impedance of the gradient impedance matching layer exhibits an exponential distribution from low to high, and the frequency of the quarter wavelength corresponding to the thickness of each layer remains consistent. Through the superposition of multiple layers of resonance, broadband transmission is achieved. The first background medium is air, and the second background medium is water.

2. The gradient impedance matching layer according to claim 1, characterized in that, Equivalent speed of sound in the first matching layer and equivalent impedance It is calculated using the following formula: ； Among them, fill rate The percentage of the filler in the first background medium is denoted as , which is a dimensionless parameter. The velocity of sound in the first background medium is expressed in m / s. The impedance of the first background medium is expressed in Rayles.

3. The gradient impedance matching layer according to claim 1, characterized in that, Equivalent impedance of the second matching layer and equivalent speed of sound From the formula Calculated; Among them, equivalent density and bulk modulus It is calculated using the following formula: These represent the filling rates of the second background medium, the hollow-filled frame, and the first background medium within the hollow-filled frame, respectively, in the discrete element. The densities of the second background medium, the hollow filling frame, and the first background medium are respectively. ; Let be the area of ​​the discrete element. ; Let be the area occupied by the second background medium in a single discrete unit. , Bulk modulus of the second background medium ; It is the deformation of the hollow filler caused by external pressure. .

4. A gradient impedance matching layer according to claim 1, characterized in that, The cross-sectional shape of the solid filler is a regular n-gon or a circle, where n ≥ 4; The cross-sectional shape of the hollow filler is a regular n-gon or a circle, where n ≥ 4.

5. A gradient impedance matching layer according to claim 1, characterized in that, The solid and hollow fillers are made of wood, plastic, or metal, with the metal being aluminum, copper, or steel.

6. A gradient impedance matching layer according to claim 1, characterized in that, The first matching layer includes several fillers with different structural geometric parameters, wherein the cross-sectional side length of the discrete unit is... The side length of the cross-section of the filler is ,and Greater than .

7. A gradient impedance matching layer according to claim 6, characterized in that, The first matching layer includes two types of square solid fillers with different structural geometric parameters; the structural geometric parameters of the first type of square solid filler are: the side length of the discrete element cross section. The side length of the cross-section of the square solid filler is The number of columns is 6; the structural geometric parameters of the second type of square solid filler are: the side length of the discrete element cross section. The side length of the cross-section of the square solid filler is The number of columns is 2.

8. A gradient impedance matching layer according to claim 1, characterized in that, The second matching layer comprises hollow fillers with several different structural geometric parameters, wherein the cross-sectional side length of the discrete unit is... The cross-sectional side length of the hollow filler is The wall thickness is ;and Greater than .

9. A gradient impedance matching layer according to claim 8, characterized in that, The second matching layer includes two types of square hollow fillers with different structural geometric parameters; the structural geometric parameters of the first type of square hollow filler are: the side length of the discrete element cross-section is... The side length of the cross-section of the square hollow filler is The wall thickness is The number of columns is 1; the structural geometric parameters of the second type of square solid filler are: the side length of the discrete element's cross-section is... The side length of the cross-section of the square hollow filler is The wall thickness is The number of columns is 9.

10. A gradient impedance matching layer according to claim 1, characterized in that, The cross-sectional shape of the discrete unit in the discrete arrangement is a square, rectangle, or regular hexagon.

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