Underwater sound-absorbing structure

Abstract

To provide a new underwater sound absorption structure which improves sound absorption effect in a low frequency region without increasing thickness of a sound absorption material.SOLUTION: In an underwater sound absorption structure (10) for absorbing sound transmitting in water being a medium (11), a block (B) constituted by combining a first member (M1) having first acoustic impedance (ZL) lower than the medium (11) and a second member (M2) having second acoustic impedance (ZH) higher than the medium (11) is arranged on a face of the underwater sound absorption structure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an underwater sound-absorbing structure that absorbs sound propagating through water. [Background technology]

[0002] Underwater sound-absorbing materials generally consist of a multilayer structure of sound-absorbing sheets with a viscoelastic base material such as rubber or urethane, with adjusted density, sound speed, and loss coefficient. For example, Patent Document 1 discloses an underwater sound-absorbing material in which sound-absorbing sheets are stacked so that the specific acoustic impedance Z (=ρ×c), defined by the density ρ and sound speed c, gradually decreases, and layers of water are provided between the sound-absorbing sheets to prevent a decrease in sound-absorbing performance. Furthermore, Patent Document 2 discloses a sound-absorbing structure in which multiple porous sound-absorbing materials with different absorption frequency ranges are stacked to improve sound-absorbing characteristics over a wide frequency range. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-178266 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-017787 Summary of the Invention [Problem to be solved by the invention]

[0004] However, because the number of layers and thickness of the underwater sound-absorbing material described above are set according to the frequency of the sound waves to be absorbed and the required sound absorption performance, good characteristics are shown in the frequency range where the total thickness of the sound-absorbing material is 1 / 4 of the wavelength or more, but the thickness of the sound-absorbing material must be increased on the low-frequency side. Increasing the thickness of the sound-absorbing material is not a realistic solution because it increases the mass and makes installation more difficult.

[0005] Furthermore, Patent Document 2 describes a sound-absorbing structure in which a layer with a high sound absorption coefficient in the high frequency range and a layer with a high sound absorption coefficient in the low frequency range are laminated together. However, because it is a laminated structure, the problems of increased mass and difficulty in installation remain, and there is no specific description as to whether it is effective in the low frequency range.

[0006] The present invention has been devised in view of the above circumstances, and an object of the present invention is to provide a new underwater sound-absorbing structure that improves the sound-absorbing effect in the low frequency range without increasing the thickness of the sound-absorbing material. [Means for solving the problem]

[0007] To achieve the above object, one embodiment of the present invention is an underwater sound-absorbing structure that absorbs sound propagating through water, which is a medium, characterized in that a block formed by combining a first member having a first acoustic impedance lower than that of the medium and a second member having a second acoustic impedance higher than that of the medium is arranged on a surface. Furthermore, according to one embodiment of the present invention, the block has a configuration in which first sub-blocks and second sub-blocks are arranged alternately, the first sub-block is configured by arranging at least one first member element having the first acoustic impedance at regular intervals in the base material, and the second sub-block is configured by arranging at least one second member element having the second acoustic impedance at regular intervals in the base material. Furthermore, according to one embodiment of the present invention, the first sub-block and the second sub-block have a rectangular shape, and the length of one side is equal to or greater than 1 / 4 of the wavelength corresponding to the lower limit frequency of the effective sound absorption frequency. Moreover, according to one embodiment of the present invention, the block is disposed on a plate member having an acoustic impedance equal to or greater than the second acoustic impedance. According to one embodiment of the present invention, the first acoustic impedance is Z L , the acoustic impedance of the medium is Zw, and the second acoustic impedance is Z H Then, Z H / Zw>10 and ZL / Zw<0.1. According to one embodiment of the present invention, the acoustic impedance of the base material of the block is set to Z M , where Zw is the acoustic impedance of the medium, <Z M / Zw<3. [Effects of the Invention]

[0008] According to one embodiment of the present invention, the sound waves reflected by the first member and the sound waves reflected by the second member cancel each other out in intensity, so that when an underwater sound-absorbing structure is installed in a medium, the sound absorption effect can be improved in the low frequency range regardless of the thickness of the underwater sound-absorbing structure. Furthermore, according to one embodiment of the present invention, the first sub-block and the second sub-block are respectively formed by arranging first members and second members at regular intervals, so that it is possible to adjust the frequency characteristics of each sub-block using parameters such as the material, size, and arrangement interval of the first and second members, and furthermore, it is possible to reduce the weight of the underwater sound-absorbing structure. According to one embodiment of the present invention, the acoustic impedance of the first member and the second member is set to Z. H / Zw>10 and Z L / Zw<0.1, and the acoustic impedance of the base material is set to 0.3 <Z M By setting / Zw<3, the sound absorption effect in the low frequency range can be further improved. Furthermore, according to one embodiment of the present invention, by making the length of one side of the sub-block at least 1 / 4 of the wavelength corresponding to the lower limit frequency of the effective sound absorption frequency, the sound absorption effect in the low frequency range can be improved regardless of the thickness of the sub-block. Furthermore, according to one embodiment of the present invention, the block can be easily arranged on a plane by placing the block on a plate member having an acoustic impedance equal to or greater than the second acoustic impedance. [Brief explanation of the drawings]

[0009] [Figure 1]1 is a perspective view illustrating a schematic configuration of an underwater sound-absorbing structure according to an embodiment of the present invention; [Figure 2] 1A to 1C are schematic diagrams for explaining the sound absorbing principle of the underwater sound absorbing structure according to the present embodiment. [Figure 3] 3 is a diagram showing an example of the application range of the relative reflectance and the relative phase difference, illustrating the simulation results in FIG. 2. FIG. [Figure 4] 1 is a perspective view showing an example of the configuration of a block in an underwater sound-absorbing structure according to the present embodiment. FIG. [Figure 5] 5 is a graph showing frequency characteristics of the reflection reduction amount of the block shown in FIG. 4. [Figure 6] 1A is a plan view showing the configuration of a block of an underwater sound-absorbing structure according to a first embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along line II thereof. [Figure 7] FIG. 10 is a plan view showing the configuration of a block of an underwater sound-absorbing structure according to a second embodiment of the present invention. [Figure 8] 7 is a graph showing frequency characteristics of reflection reduction amounts in the first embodiment shown in FIG. 6 and the second embodiment shown in FIG. [Figure 9] FIG. 10 is a diagram showing the configuration of Pareto solutions and the realizable area ratios when N=3, with the area ratio as a parameter. [Figure 10] FIG. 10 is a plan view illustrating the schematic configuration of a block in an underwater sound-absorbing structure according to another embodiment of the present invention. [Figure 11] 11 is a plan view showing an example of the configuration of the block shown in FIG. 10. FIG. [Figure 12] 10 is a graph showing frequency characteristics of the reflection reduction amount at the area ratios obtained in the Pareto solutions and the area ratios of the feasible solutions when N=3. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in the following embodiments are merely examples and are not intended to limit the technical scope of the present invention to those. For example, the underwater sound-absorbing structure of the present invention can be used in any environment as long as it is in a liquid that propagates sound, and "underwater" includes underwater environments such as the sea, rivers, lakes, and anechoic tanks.

[0011] 1. One embodiment As shown in Fig. 1, an underwater sound-absorbing structure 10 according to an embodiment of the present invention has a structure in which a plurality of blocks B are arranged on a plane, and can effectively suppress the reflection of sound incident on the plane through a medium 11. Each block B has a low acoustic impedance Z L The first member M1 is made of a medium with high acoustic impedance Z H The second member M2 is made of a medium.

[0012] In Figure 1, block B has a square shape and is divided into four squares of equal area. L The first member M1 and the high Z H The above example illustrates a configuration in which the first and second members M1 and M2 are alternately arranged, but the present invention is not limited to this. As will be described later, the block B may be divided into two equal areas or into three unequal areas, and the planar shapes of the first and second members M2 and M3 may be circular instead of rectangular. As will be described later, the underwater sound-absorbing structure 10 as a whole may have a low Z L The first member M1 and the high Z H The second member M2 may have any structure as long as it is arranged so that the intensities of the reflected waves cancel each other out.

[0013] Here, the acoustic impedance Z of a medium is a specific quantity defined by the density ρ of the medium and the sound speed c, and is calculated as Z = ρ × c. Hereinafter, if the acoustic impedance of the medium 11 is Zw, then a low acoustic impedance Z L is smaller than Zw and has high acoustic impedance Z H is greater than Zw (Z L <Zw<Z H ). More preferably, ZH / Zw>10, Z L / Zw<0.1.

[0014] In Figure 2, Z L <Zw<Z H The first member M1 and the second member M2 are arranged adjacent to each other on the surface of the complete reflection layer, and sound P is transmitted to each of them through the medium 11. i is assumed to be incident.

[0015] Sound P from medium 11 to first member M1 i When incident perpendicularly, Zw>Z L Therefore, the incident wave P i is reflected at the boundary. r1 On the other hand, the phase of sound P i When incident perpendicularly, Zw <Z H Therefore, the incident wave P i is reflected at the boundary. r2 The phase of the light is inverted and shifted by 1 / 2 wavelength.

[0016] Therefore, the reflected wave P r1 and P r2 and cancel each other out, and the reflected wave P r1 and P r2 The amplitude of the composite wave of the incident wave P i The intensity of the reflected wave is the reflected wave P r1 and P r2 The reflected wave P is attenuated according to the relative intensity and phase shift between r1 and P r2 If the reflected wave has the same intensity and the phase difference is 1 / 2 wavelength (180°), the intensity of the reflected wave will be 0. The conditions for obtaining sufficient attenuation for practical purposes are as follows:

[0017] Low Z L The reflection coefficient of the first member M1 is Γ1, and the high frequency Z H The reflection coefficient at the second member M2 is Γ2, and the reflected wave P r1 and P r2 If the relative phase difference between the two is θ, it is desirable to satisfy the following formula:

[0018]

number

[0019] The above formula is the reflected wave P r1 and P r2 The amplitude of the composite wave is the reflected wave P r1 and the reflected wave P at the second member M2 r2 This means that the value is smaller than that of

[0020] In Figure 3, the range in which the amplitude of the composite wave between the relative reflectance Γ1 / Γ2 and the relative phase difference θ is smaller than the relative reflectance Γ1 / Γ2 is shown in bold. For example, when the relative reflectance Γ1 / Γ2 is 0.6, the above formula is satisfied if the relative phase difference θ is 90° or more.

[0021] The principle of reflection reduction shown in Figure 2 does not depend on the sound absorption characteristics of the medium as in the past, but is based on the difference in reflection phase that depends on the difference in acoustic impedance between media at the boundary, and there is no need to change the thickness of the medium to manipulate the reflection phase. Therefore, it is possible to manipulate the reflection phase over a wide frequency range, and a high sound absorption effect can be expected, especially in the low frequency range. Simulation results are shown below with reference to Figures 4 and 5.

[0022] As shown in FIG. 4, a 4×4 array model 100 is used for the simulation. Specifically, a plate-shaped base material 101 is provided with a low Z L The first sub-block SB1 is composed of a first member M1 and a low Z member element 102. H The first sub-block SB1 is formed by periodically arranging 2×2 low-Z member elements 102 at regular intervals, and the second sub-block SB2 is formed by arranging 2×2 low-Z member elements 102 at regular intervals, and the high-Z member elements 103 are arranged alternately. H The second sub-block SB2 is formed by periodically arranging 2×2 high Z member elements 103 at regular intervals. Assume that this array model 100 is placed on the back structure 12 of a steel plate.

[0023] As an example, the periodically arranged low-Z member elements 102 and high-Z member elements 103 are shaped as cylinders, with air being used as the material for the low-Z member elements 102 and steel being used as the material for the high-Z member elements 103. In other words, the low-Z member elements 102 are hollow cylinders, and the high-Z member elements 103 are steel cylinders. Needless to say, these are just examples, and the low-Z member elements 102 and high-Z member elements 103 can also be shaped other than cylinders, and can be made of low-Z and high-Z materials other than air and steel. In the following figures, the low-Z member elements (hollow cylinders) 102 are shown without hatching, and the high-Z member elements (steel cylinders) 103 are shown with hatching.

[0024] The base material 101 can be made of resin, viscoelastic material, wood, etc., whose acoustic impedance is close to that of the medium 11. M So, 0.3 <Z M It is desirable that / Zw<3. As an example, the low Z member element 102 containing air has a density ρ=1.23 [kg / m 3 ] and sound velocity c = 341 [m / s], and the high Z member element 103 containing steel has a density ρ = 7800 [kg / m 3 ] and sound velocity c = 5400 [m / s], and the base material 101 is a lossless medium that matches water with density ρ = 1000 [kg / m 3 ] and the speed of sound c=1500[m / s]. In this case, Z H / Zw>10, Z L / Zw<0.0001.

[0025] The 4×4 array model 100 corresponds to the quartered block B in Fig. 1, and the array model 100 is periodically repeated to form the underwater sound-absorbing structure 10, so that in the simulation, a periodic boundary condition is set around the array model 100 in Fig. 4. In the incident-side medium 11_i, the incident wave P i is incident perpendicularly, and the reflected wave P r In the medium 11_o on the exit side, a transmitted wave P t is emitted vertically.

[0026] In Fig. 5, curve 201 shows the reflection reduction characteristics of a 4x4 array model 100 (composite cylinder model) configured by arranging two first sub-blocks SB1 each consisting of 2x2 low-Z material elements 102 (hole cylinders) and two second sub-blocks SB2 each consisting of 2x2 high-Z material elements 103 (steel cylinders). As comparative examples, curve 202 shows the reflection reduction characteristics when the 4x4 array model 100 is configured entirely of hole cylinders, and curve 203 shows the characteristics when it is configured entirely of steel cylinders. As shown in curve 201, it can be seen that with the composite cylinder model, a large reflection reduction ER can be obtained in the frequency range of 10 kHz or less.

[0027] As shown in Figure 4, the first sub-block SB1 and the second sub-block SB2 are formed by arranging low-Z member elements 102 and high-Z member elements 103 in a matrix, which has the advantage of allowing for easy modification of the shape, area, and sound-absorbing properties of the sub-blocks. For example, if 4 x 4 cylindrical openings are pre-formed in the base material 101, the first sub-block SB1 and the second sub-block SB2 can be formed simultaneously by simply embedding steel cylinders 103 in the openings that form the second sub-block SB2. Furthermore, sound-absorbing properties can be easily modified by changing the material embedded in the openings and the diameter of the openings.

[0028] 2. Working Example <First Example> As illustrated in Figure 6, each block 300 in the underwater sound-absorbing structure 10 according to the first embodiment of the present invention is configured by two first sub-blocks SB1, each of which has 4 x 4 hollow cylinders 302 arranged at equal intervals in a base material 301, and two second sub-blocks SB2, each of which has 4 x 4 steel cylinders 303 arranged at equal intervals, being alternately and periodically arranged like block B in Figure 1. The hollow cylinders 302 correspond to the cylindrical low-Z member elements 102 described above, and the steel cylinders 303 correspond to the cylindrical high-Z member elements 103.

[0029] The plate-shaped base material 301 is made of a resin, such as PLA (polylactic acid) resin, having an acoustic impedance similar to that of the medium 11, and has eight vertical and eight horizontal cylindrical openings of the same size arranged at equal intervals. Such base material 301 may be formed using a 3D printer, or the 8 × 8 openings may be machined into the plate-shaped base material.

[0030] The 8x8 openings are divided into four equal parts to form four sub-blocks, and 4x4 hollow cylinders 302 and 4x4 steel cylinders 303 are formed so that the sub-blocks are adjacent vertically and horizontally. In this example, the steel cylinders 303 are embedded only in the openings of the two diagonal sub-blocks, thereby forming adjacent low Z L The first subblock SB1 and high Z H In the figure, the base material 301 is shown with thin hatching, the hollow cylinder 302 is shown without hatching, and the steel cylinder 303 is shown with thick hatching. The acoustic impedances Zw, Z M , Z L and Z H is as mentioned above, and Zw ≒ Z M , Z H / Zw>10, Z L / Zw<0.0001.

[0031] <Second Example> 7, each block 400 in the underwater sound-absorbing structure 10 according to the second embodiment of the present invention is divided into two halves, and a first sub-block SB1 having 8 × 4 hollow cylinders 402 arranged at equal intervals in a base material 401 and a second sub-block SB2 having 8 × 4 steel cylinders 403 arranged at equal intervals are arranged adjacent to each other. The hollow cylinders 402 correspond to the cylindrical low-Z member elements 102 described above, and the steel cylinders 403 correspond to the cylindrical high-Z member elements 103.

[0032] The plate-shaped base material 401 is made of the same material and in the same manner as in the first embodiment, and has eight vertical and eight horizontal cylindrical openings of the same size arranged at equal intervals. In this example, a steel cylinder 403 is embedded in one of the 8 × 4 openings, thereby forming a low Z L The first sub-block SB1 and the high Z H In the figure, the base material 401 is shown with thin hatching, the hollow cylinder 402 is shown without hatching, and the steel cylinder 403 is shown with thick hatching. The acoustic impedances Zw, Z M , Z L and Z H is as mentioned above, and Zw ≒ Z M , Z H / Zw>10, Z L / Zw<0.0001.

[0033] <Frequency characteristics of reflection reduction> FIG. 8 is a graph showing actual measurements of the change in reflection reduction amount with respect to frequency in the block 300 of FIG. 6 and the block 400 of FIG.

[0034] In Figure 8, the reflection reduction characteristics of block 300 in Figure 6 (black circles) and the reflection reduction characteristics of block 400 in Figure 7 (+) show a higher reflection reduction in the frequency band below 100 kHz compared to the characteristics when PLA resin alone is used for block B (white circles).

[0035] 3. Other Embodiments In the present invention, as shown in FIGS. 6 and 7, the block B is equally divided into low Z L The first subblock SB1 and high Z H The configuration is not limited to the alternating arrangement of the first and second sub-blocks SB1 and SB2, but may be divided into N sub-blocks with different areas, with hollow cylinders or steel spheres arranged at different intervals in each sub-block. Below, we will explain the Pareto solutions for N=3, where the area ratio is used as a parameter, and the configuration of the sub-blocks formed based on them.

[0036] As shown in the example of Figure 9, if block B is divided into three, the optimal structural combination (Pareto solution) is determined using the area ratio as a parameter. In the Pareto solution, the cylindrical array structures of the three sub-blocks are arranged in descending order of area (steel, hollow, hollow), and if the array periods of these structures are respectively (0.02 [m], 0.1 [m], 0.08 [m]), the feasible area ratio of the three sub-blocks according to the Pareto solution can be determined as follows. Note that in Figure 9, the product of the array period and the value of kh1 (0.8) is the diameter of each cylinder.

[0037] As shown in Figure 10, block B is divided into three sub-blocks, and the sub-blocks are sorted in descending order of area. L ,S M and S S The sizes of the sub-blocks are H1 × W1, H2 × W2, and H3 × W3. From the geometric relationship shown in FIG. 10, H1 = H2 + H3 and W2 = W3. L ,S M and S S Using the area ratio of 1.427:1.0:0.769 and approximating the fractions to integers, the following dimension ratios can be determined: W1:W2:W3=4:5:5 H1:H2:H3=9:5:4

[0038] Furthermore, sub-block S L ,S M and S S The arrangement periods of the sub-blocks are (0.02[m], 0.1[m], 0.08[m]), so taking this into consideration, the number of cylinders arranged in each sub-block is set to M 1W ×M 1H , M 2W ×M 2H , M 3W ×M 3H This leads to the following relationship:

[0039] (1) Sub-block S in Figure 10 M and S S From the relationship, the following equation is obtained:

[0040]

number

[0041]

number

[0042] (2) Sub-block S in Figure 10 L and S M From the relationship, the following equation is obtained:

[0043]

number

[0044]

number

[0045] An integer M that satisfies the above relationship 1W , M 1H , M 2W , M 2H , M 3W and M 3H The minimum combination of (the number of cylindrical arrays in each subblock) is M 2W must be a multiple of 4, so [M 1W , M 2W , M 3W ] = [16, 4, 5] [M 1H , M 2H , M 3H ] = [ 9, 1, 1] This becomes:

[0046] Here, to obtain a complex reflectivity equivalent to that of an infinitely large array, the dimensions of the sub-blocks must be λ / 2 or more. For example, the wavelength of a 1 kHz sound wave is 1.5 m, so the side of one sub-block must be 0.75 m or more. Taking this into consideration, the minimum combination of the number of cylinders in an array is determined as follows: [M 1W , M 2W , M 3W ] = [32, 8,10] [M 1H , M 2H , M 3H ] = [90,10,10]

[0047] Sub-Block S L ,S M and S S are (0.02 [m], 0.1 [m], 0.08 [m]), respectively, and the size of each sub-block is determined as follows from the combination of the minimum number of cylindrical arrangements mentioned above. [W1, W2, W3] =[0.64[m], 0.8[m], 0.8[m]] [H1, H2, H3] =[1.8 [m], 1 [m], 0.8[m]] In this case, the minimum size of block B is width W = 1.44 [m], height H = 1.8 [m]. In addition, since the cylinder diameter of the low-Z member elements and high-Z member elements arranged in each sub-block is determined by the size of the sub-block and the number of cylinders arranged in each sub-block, the cylinder diameter may differ depending on the sub-block even if materials with the same acoustic impedance are used.

[0048] From the above, sub-block S L ,S M and S S The realizable area ratio of the sub-blocks S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14, S15, S16, S17, S18, S19, S20, S21, S22, S23, S24, S25, S30, S41, S42, S43, S44, S45, S L ,S M and S S An example is shown in Figure 11.

[0049] In FIG. 11, each block 500 in the underwater sound-absorbing structure 10 according to this embodiment is divided into three sub-blocks, each of which is a first sub-block S in which 8×10 hollow cylinders 502 a are arranged at equal intervals in a base material 501. M and a second sub-block S in which 10×10 hollow cylinders 502b are arranged at equal intervals. S and the third sub-block S, which is made of 32 x 90 steel cylinders 503 arranged at equal intervals. L and are arranged adjacent to each other.

[0050] The base material 501, the hollow cylinders 502a and 502b, and the steel cylinder 503 are made of the same materials and in the same manner as in the first embodiment, and each has an acoustic impedance of Z M , Z L and Z H If the acoustic impedance of the medium 11 is Zw, then as described above, Zw≒Z M , Z H / Zw>10, Z L / Zw<0.0001.

[0051] <Frequency characteristics of reflection reduction> Fig. 12 shows the sub-block S in the Pareto solution for N=3 shown in Fig. 10. L ,S M and S S 12 is a graph showing the change in reflection reduction amount ER with respect to frequency when the area ratio is 1.427:1.0:0.769 in the above example and when the area ratio is 1.44:1:0.8 in the feasible solution shown in FIG.

[0052] As shown in FIG. 12, the difference between the ER characteristics of the feasible solutions (black circles) and the ER characteristics of the Pareto solutions (+) is very small, and it can be seen that the ER characteristics are high in the low frequency range below 10 kHz.

[0053] 4.Other In the configuration of the blocks used in the underwater sound-absorbing structure 10 according to the above-described embodiment and example, as shown in FIGS. 6, 7 and 11, L The material is air, high Z HThe material is steel, periodically arranged Z L / Z H Although the shape of the member is a cylinder, the present invention is not limited to this. As already mentioned, when the acoustic impedance of the medium 11 is Zw, the low-Z material and the high-Z material are L is smaller than Zw and has high acoustic impedance Z H is larger than Zw (Z L <Zw<Z H ), preferably Z H / Zw>10, Z L All you need to do is select a material that satisfies the relationship / Zw<0.1.

[0054] Z L / Z H The shape of the member may be a polygonal column, a cone, a polygonal pyramid, a truncated cone, a truncated polygonal pyramid, a sphere, a partial sphere, or the like, in addition to a cylinder. L Materials and Z H The members may have different shapes or dimensions. L ,S M and S S The thickness of these sub-blocks is basically the same, but if the thickness is different, a high Z material such as steel material should be attached to the back side of the thin sub-block. H A plate-shaped member of material may be provided. [Explanation of symbols]

[0055] 10 Underwater sound absorbing structure 11 Medium 12 Fully reflective layer 100 4x4 array model 300 blocks 101, 301, 401, 501 Base material 102, 302, 402, 502a, 502b Low Z member element / hollow cylinder 103, 303, 403, 503 High Z member element / steel column B Sub-block SB1 1st Sub-Block SB2 Second Sub-Block SL Large sub-blocks S M Middle sub-block S S Small sub-blocks

Claims

1. An underwater sound-absorbing structure that absorbs sound propagating through water, which is a medium, a block formed by combining a first member having a first acoustic impedance lower than the medium and a second member having a second acoustic impedance higher than the medium is placed on the surface; The block has a configuration in which first sub-blocks and second sub-blocks are alternately arranged, the first sub-block is configured by arranging at least one first member element having the first acoustic impedance at regular intervals in a base material, The second sub-block is configured by arranging at least one second member element having the second acoustic impedance at regular intervals in the base material. An underwater sound-absorbing structure characterized by:

2. 2. The underwater sound-absorbing structure according to claim 1, wherein the first sub-block and the second sub-block have a rectangular shape, and the length of one side is equal to or greater than 1 / 4 of the wavelength corresponding to the lower limit frequency of the effective sound absorption frequency.

3. 3. The underwater sound-absorbing structure according to claim 1, wherein the block is disposed on a plate member having an acoustic impedance equal to or greater than the second acoustic impedance.

4. The first acoustic impedance is Z Ｌ , the acoustic impedance of the medium is Zw, and the second acoustic impedance is Z Ｈ Then, Z Ｈ / Zw>10 and Z Ｌ 4. The underwater sound-absorbing structure according to claim 1, wherein Zw<0.

1.

5. The acoustic impedance of the base material of the block is Z Ｍ , where Zw is the acoustic impedance of the medium, 0.3<Z Ｍ 5. The underwater sound-absorbing structure according to claim 1, wherein Zw<3.

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