Noise reduction device

Abstract

This invention provides a passive noise reduction device that can reduce noise originating from a rotating sound source. [Solution] The noise reduction device according to the embodiment comprises a first slit member, a second slit member, a plate member, and a plurality of routing pipes. The first slit member includes a first annular member having a plurality of first openings. The second slit member includes a second annular member having a plurality of second openings. The plate member is positioned between the first slit member and the second slit member. The plurality of routing pipes connect the plurality of first openings and the plurality of second openings. Two adjacent routing pipes among the plurality of routing pipes have different lengths from each other.

Description

Technical Field

[0001] Embodiments of the present invention relate to a noise reduction device.

Background Art

[0002] As a noise reduction device for reducing noise emitted from a rotational sound source such as a fan, an active noise reduction device employing an active noise control (ANC) method is known. Such a noise reduction device includes a plurality of speakers arranged on a circumference, and drives the speakers with a phase difference so as to emit a sound that cancels out the noise.

[0003] In an active noise reduction device, since speakers are used, the weight increases and a large space is required. Furthermore, a signal processing unit for performing signal processing for ANC is required.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problem to be solved by the present invention is to provide a passive noise reduction device that can reduce noise derived from a rotational sound source.

Means for Solving the Problems

[0007] [Figure 1] A diagram showing a rotating sound source model according to an embodiment. [Figure 2] A diagram illustrating the overview of the acoustic metamaterial according to the embodiment. [Figure 3] A diagram illustrating path grouping according to this embodiment. [Figure 4] A figure showing the simulation results for an acoustic metamaterial according to the embodiment. [Figure 5] A diagram illustrating the sound pressure reduction principle according to this embodiment. [Figure 6] A figure showing the simulation results for an acoustic metamaterial according to the embodiment. [Figure 7] A figure showing the simulation results for an acoustic metamaterial according to the embodiment. [Figure 8] A figure showing the simulation results for an acoustic metamaterial according to the embodiment. [Figure 9] A figure showing the simulation results for an acoustic metamaterial according to the embodiment. [Figure 10] A diagram illustrating the sound pressure reduction principle according to this embodiment. [Figure 11] A diagram illustrating the sound pressure reduction principle according to this embodiment. [Figure 12] This figure shows the estimated sound pressure reduction effect of the acoustic metamaterial according to the embodiment. [Figure 13]A diagram showing the basic configuration of an acoustic metamaterial according to an embodiment. [Figure 14] Figure 13 is a front view showing the slit member. [Figure 15] Figure 13 is a perspective view showing the route piping. [Figure 16] A diagram showing a method for changing the length of the route piping according to the embodiment. [Figure 17] A diagram showing a method for changing the length of the route piping according to the embodiment. [Figure 18] A perspective view showing an example of an acoustic metamaterial according to the embodiment. [Figure 19] Figure 18 is a perspective view showing the slit member and rigid plate. [Figure 20] Figure 18 is a perspective view showing the route piping. [Figure 21] Figure 18 is a perspective view showing the acoustic metamaterial attached to a duct. [Figure 22] A diagram illustrating the conditions for acoustic analysis performed on the acoustic metamaterial according to the embodiment. [Figure 23] This figure shows the results of acoustic analysis performed on the acoustic metamaterial according to the embodiment. [Figure 24] This figure shows the results of acoustic analysis performed on the acoustic metamaterial according to the embodiment. [Figure 25] This figure shows the results of acoustic analysis performed on the acoustic metamaterial according to the embodiment. [Figure 26] This figure shows the results of acoustic analysis performed on the acoustic metamaterial according to the embodiment. [Figure 27] A diagram to explain the graph shown in Figure 25. [Figure 28] A diagram illustrating an example of the application of an acoustic metamaterial according to the embodiment. [Figure 29] A diagram illustrating an example of the application of an acoustic metamaterial according to the embodiment. [Figure 30] A diagram illustrating acoustic metamaterials related to the technology. [Figure 31]A diagram illustrating acoustic metamaterials related to the technology. [Modes for carrying out the invention]

[0008] The embodiments will be described below with reference to the drawings.

[0009] In recent years, acoustic metamaterials have attracted attention as materials that possess acoustic properties that cannot be expressed by homogeneous materials. Many acoustic metamaterials have been proposed, including side-branch type configurations with multiple λ / 4 tubes arranged in a fine pattern, and configurations with multiple Helmholtz resonators of different types arranged in a row. In addition, several deflection-type acoustic metamaterials that utilize the deflection principle of sound waves have also been investigated. Deflection-type acoustic metamaterials are basically based on the acoustic theory of wavefront interference.

[0010] First, with reference to Figures 30 and 31, a brief explanation of deflection-type acoustic metamaterials related to the relevant technologies will be given.

[0011] The acoustic metamaterial 30 shown in Figure 30 suppresses the propagation of sound waves, such as noise, into a target space by deflecting them. The acoustic metamaterial 30 is installed, for example, inside a duct.

[0012] The acoustic metamaterial 30 is a structure 31 in which multiple bypasses (paths) 32 through which sound waves pass are formed. The acoustic metamaterial 30 is designed such that the distance between two adjacent bypasses 32 is d, and the path difference between two adjacent bypasses 32 is d × sinθ. The path difference d × sinθ can be obtained by changing the number of bends and the distance from the entrance to the bend for each bypass. The acoustic metamaterial 30 can be fabricated, for example, using a 3D printer. The acoustic metamaterial 30 deflects the wavefront of the sound wave by an angle θ.

[0013] Figure 31 schematically shows the results of measuring the sound pressure of a sound wave passing through the acoustic metamaterial 30 when θ = 30 [deg]. In Figure 31, the Z axis is the axis along the direction of incidence of the sound wave, and the X axis is perpendicular to the Z axis. The origin in Figure 31 (X = 0 [m], Z = 0 [m]) coincides with point O shown in Figure 30. From Figure 31, it can be confirmed that the sound wave is deflected by an angle of approximately 30 degrees.

[0014] The embodiment provides a passive acoustic metamaterial (noise reduction device) that can reduce noise emitted from a rotating sound source such as a fan. The acoustic metamaterial according to the embodiment applies acoustic deflection to the noise emitted from the rotating sound source through multiple paths of different lengths, thereby reducing the acoustic power of the rotating sound source.

[0015] The rotating sound source is L, which is arranged on the circumference. p Each point source can be represented as a model having a phase difference. Here, L p is an integer greater than or equal to 2. For example, if the rotating sound source is a fan having one or more rotor blades as shown in the left portion of Figure 1, the rotating sound source can be represented as the rotating sound source model shown in the right portion of Figure 1. In Figure 1, a represents the radius of the rotating sound source, specifically the length of the rotor blades. The fan may further have stator blades (not shown).

[0016] If the rotational speed of the rotor blades is Ω, the number of rotor blades is B, the order of focus is x, and the number of stator blades is V, then the frequency ω of the noise emitted by the rotating sound source is ω = ΩBx, and the Rob mode M corresponding to the circumferential phase distribution is M = Bx + pV, where p is an integer.

[0017] The rotating sound source model is L arranged on the circumference p Individual sound sources p1, p2, ..., pL p Assume that the phase depends on the sound source placement angle φ and the Rob mode M. In this case, the volume velocity q of the sound source pi. pi This can be expressed as equation (1) below, where i ranges from 1 to L. p It is an integer up to 1.

number

[0018] As shown in FIG. 2, the acoustic metamaterial 10 according to the embodiment is provided, for example, at the outlet of a duct 60 through which noise emitted from a rotating sound source 50 propagates. The acoustic metamaterial 10 may be provided in the middle of the duct 60. In one example, the rotating sound source 50 is a fan and the duct 60 is an exhaust duct. The acoustic metamaterial 10 has air permeability, and the air flow generated by the fan is discharged from the exhaust duct through the acoustic metamaterial 10. In FIG. 2 and subsequent figures, an XYZ orthogonal coordinate system is introduced such that the Z-axis is along the direction in which noise is incident on the acoustic metamaterial 10.

[0019] The acoustic metamaterial 10 is configured to apply circumferential deflection to the sound field of the rotating sound source 50. Specifically, the acoustic metamaterial 10 has L p paths through which sound waves pass, arranged in an annular shape when viewed from the Z-axis direction, and the acoustic metamaterial 10 is configured to convert the volume velocity of the sound source pi from q p shown in Equation (1) to q pi shown in Equation (2) through the L pi paths.

Number

[0020] L pi is the additional length from the basic length of the path pi (the path corresponding to the sound source pi). L p is the number of sound sources set in the rotating sound source model and matches the number of paths. a is the radius of the rotating sound source as described above and matches the radius of the duct 60. a×2π / L p is the arc interval and represents the distance between two adjacent paths. sinθ is used following the deflection and is an example of a variable for controlling the magnitude of the deflection applied to the sound wave. The variable may be any positive value and may be greater than 1. N i is the number corresponding to the number i of the sound source pi. L pIf we do not group the individual paths, N i The following applies:

number

[0021] L p If you do not group individual paths, L p Each path has a different length. The basic length is L. Ref Therefore, the length of path pi is L Ref +L pi That is the case.

[0022] L p If we group (divide) n paths into Q groups, then N i The following applies:

number

[0023] L p When individual paths are grouped into Q groups, the Q groups must have the same configuration or pattern, and each group must contain L p The / Q paths have different lengths from each other.

[0024] Refer to Figure 3 to explain the grouping of paths. Figure 3 shows an example of grouping 24 paths into 6 groups having the same configuration. In the example shown in Figure 3, 4 paths corresponding to 4 adjacent sound sources form one group. For example, 4 paths p1 to p4 corresponding to sound sources p1 to p4 form the first group g1, 4 paths p5 to p8 corresponding to sound sources p5 to p8 form the second group g2, 4 paths p9 to p12 corresponding to sound sources p9 to p12 form the third group g3, 4 paths p13 to p16 corresponding to sound sources p13 to p16 form the fourth group g4, 4 paths p17 to p20 corresponding to sound sources p17 to p20 form the fifth group g5, and 4 paths p21 to p24 corresponding to sound sources p21 to p24 form the sixth group g6. Thus, 6 groups g1 to g6 having the same configuration are arranged circumferentially. In the example shown in Figure 3, paths p1 to p24 are located sequentially in the circumferential direction.

[0025] ΔL = (a × 2π / L) p If we let ) × sinθ, then in the example shown in Figure 3, the path lengths of paths p1, p5, p9, p13, p17, and p21 are L Ref Therefore, the path lengths of paths p2, p6, p10, p14, p18, and p22 are L Ref The length of paths p3, p7, p11, p15, p19, and p23 is +ΔL. Ref The formula is +2 × ΔL, and the path lengths of paths p4, p8, p12, p16, p20, and p24 are L. Ref It is +3 × ΔL.

[0026] The results of simulations performed on the acoustic metamaterial according to the embodiment will be described. In the simulation, L p The parameters were set as follows: =36, a=0.3[m], f=1[kHz], M=1, θ=60[deg].

[0027] Figure 4 schematically shows the simulation results without grouping. In Figure 4, the lower panel shows the simulation results for the acoustic metamaterial according to the embodiment, and the upper panel shows the simulation results for the acoustic metamaterial according to the comparative example. In the acoustic metamaterial according to the comparative example, θ = 0 [deg], and the 36 paths have the same length. Therefore, the acoustic metamaterial according to the comparative example does not apply circumferential deflection to the sound field of the rotating sound source.

[0028] The leftmost graph shows the sound pressure in the XZ plane, and the second graph from the left shows the sound pressure in the YZ plane. These graphs confirm that the acoustic metamaterial according to the embodiment significantly reduces the sound pressure radiated toward the center.

[0029] The second graph from the right shows the sound pressure in the XY plane at Z=0.3[m], and the rightmost graph shows the sound pressure in the XY plane at Z=0.7[m]. These graphs confirm that the acoustic metamaterial according to the embodiment can achieve a sound pressure reduction effect of approximately 5dB.

[0030] A brief explanation of the principle of acoustic power reduction without grouping will be provided.

[0031] If there is no grouping, N i =i-1. Therefore, L pi This can be expressed as equation (3) below.

number

[0032] Substituting equation (3) into equation (2), we obtain equation (4) below.

number

[0033] According to equation (4), the acoustic metamaterial according to the embodiment can be considered to virtually increase the lob mode M to (k × a × sinθ + M). As shown in Figure 5, the larger the value of the lob mode M, the greater the phase delay, and consequently the lower the acoustic power. Therefore, the acoustic power of sound that has passed through the acoustic metamaterial according to the embodiment decreases.

[0034] Figures 6 and 7 schematically show the simulation results with grouping. In each of Figures 6 and 7, the lower panel shows the simulation results for the acoustic metamaterial according to the embodiment, and the upper panel shows the simulation results for the acoustic metamaterial according to the comparative example. Figure 6 shows the simulation results when 36 paths are grouped into 6 groups, and Figure 7 shows the simulation results when 36 paths are grouped into 9 groups. The simulation results for the acoustic metamaterial according to the comparative example shown in each of Figures 6 and 7 are the same as those shown in Figure 4.

[0035] In Figures 6 and 7, the leftmost graph shows the sound pressure in the XZ plane, and the second graph from the left shows the sound pressure in the YZ plane. These graphs confirm that the acoustic metamaterial according to the embodiment significantly reduces the sound pressure radiated toward the center. The sound pressure reduction effect is greater when there are 6 groups (also called the number of divisions) than when there are 9 groups.

[0036] In Figures 6 and 7, the second graph from the right shows the sound pressure in the XY plane at Z=0.3[m], and the rightmost graph shows the sound pressure in the XY plane at Z=0.7[m]. These graphs confirm that a sound pressure reduction effect of approximately 10 dB can be obtained when there are 6 groups, and a sound pressure reduction effect of approximately 5 dB can be obtained when there are 9 groups.

[0037] This section explains the effects of grouping and the conditions related to the number of groups Q.

[0038] Without grouping, the length of the longest path is L ref +(a × 2π / L) p ) × sinθ × (L p -1) When grouping is enabled, the length of the longest path is L ref +(a × 2π / L) p ) × sinθ × (L p (Q-1) Thus, grouping makes it possible to reduce the maximum path length and contributes to compactness.

[0039] As described above with reference to Figures 6 and 7, a sufficient sound pressure reduction effect can be obtained when there are 6 groups, and a certain degree of sound pressure reduction can also be obtained when there are 9 groups. Increasing the number of groups reduces the number of paths (sound sources) per group, which reduces the deflection effect and, as a result, the sound pressure reduction effect decreases. A number of paths of 4 or more per group is appropriate. That is, Q ≤ L p It is desirable to satisfy / 4.

[0040] Furthermore, to suppress spatial variations in the sound pressure reduction effect, it is desirable that the number of groups Q be 2M+1 or greater. That is, it is desirable that Q ≥ 2M+1 be satisfied. Note that the number of groups Q is correlated with the lobe mode M (spatial aliasing), and more preferably, the number of groups Q is 2M+2 or greater.

[0041] Figure 8 shows the simulation results for the case where M=2 and Q=6, and Figure 9 shows the simulation results for the case where M=3 and Q=6. In Figures 8 and 9, the lower panel shows the simulation results for the acoustic metamaterial according to the embodiment, and the upper panel shows the simulation results for the acoustic metamaterial according to the comparative example.

[0042] Figures 8 and 9 confirm that the acoustic metamaterial according to the embodiment exhibits a significant sound pressure reduction effect. However, when M=3 and Q=6, the condition Q≧2M+1 is not satisfied, and as shown in Figure 9, there are spatial variations in the intensity of the sound pressure reduction effect.

[0043] As described above, from the perspective of reducing acoustic power, it is desirable that the number of groups Q satisfies the following equation. Q≦L p / 4

[0044] Furthermore, if it is desirable to suppress spatial variations in the sound pressure reduction effect, the number of groups Q should preferably satisfy the following equation. 2M+1≦Q≦L p / 4

[0045] A brief explanation of the principle of acoustic power reduction in the case of grouping will be given.

[0046] From the spherical harmonic expansion, the sound field created by a discrete ring sound source (sound pressure at point A shown in Figure 10) can be expressed by the following equation.

number

[0047] B n m As shown in Figure 11, N i Component (B1) relating to the sound source group corresponding to =0 n m , N i Component (B1) relating to the sound source group corresponding to =1 n m , , , N i =L p / Components related to the sound source group corresponding to Q-1 (B(L p / Q)) n m It is expressed as a sum.

number

[0048] B n mis the sum of a geometric series. Therefore, when θ=0 (no bias) and m=M, B n m This is maximized. When θ > 0 (with deflection), B n m This becomes smaller than when θ=0 and m=M. B n m Because this affects acoustic power, the acoustic power decreases.

[0049] Figure 12 shows the estimated reduction effect of the acoustic metamaterial according to the embodiment. Here, L p The parameters were set to =36, a=0.2 / 0.25 / 0.3[m], θ=45 / 60[deg], Q=6, M=1, and m=1. In Figure 12, the upper row shows the results for θ=60[deg], and the lower row shows the results for θ=45[deg]. From Figure 12, it can be confirmed that as the radius a increases, the noise reduction effect occurs from lower frequencies, and as the deflection angle θ increases, the noise reduction effect occurs from lower frequencies. The reduction effect estimate corresponding to Figure 6 is 12dB for θ=60[deg], a=0.3[m], and f=1[kHz], which is in good agreement with the noise reduction effect in Figure 6.

[0050] To achieve a reduction effect even at low frequencies below 1 kHz, it is necessary to increase the radius a, which is a representative dimension, or to increase the deflection angle θ (i.e., increase the path piping difference), and in either case, the size of the acoustic metamaterial 10 will need to be increased. This is because, due to the wavelength, the size will be larger when targeting low frequencies.

[0051] The noise reduction method according to this embodiment involves arranging paths with path differences circumferentially to superimpose a deflection phase characteristic onto the phase rotation originating from a rotating sound source, thereby virtually increasing the phase rotation and reducing the overall acoustic power. The acoustic metamaterial according to this embodiment is a passive noise reduction device that does not use speakers, and therefore is lighter and more compact than an active noise reduction device that uses speakers, and furthermore, does not require a signal processing unit for performing ANC signal processing.

[0052] Figure 13 schematically shows the basic configuration of the acoustic metamaterial 10. As shown in Figure 13, the acoustic metamaterial 10 consists of a slit member 11, a slit member 12, a rigid plate 13, and L p It is equipped with individual route pipes 14. Figure 13 shows one route pipe 14 as a representative example. The following explanation will use the case where the duct 60 to which the acoustic metamaterial 10 is connected is circular as an example.

[0053] As shown in Figure 14, the slit member 11 consists of an annular member 111, a cylindrical member 112 positioned inside the annular member 111, and an L connecting the annular member 111 and the cylindrical member 112. p It comprises individual partition members 113, and in the example shown in Figure 14, L p It is 12.

[0054] The partition members 113 are arranged at equal intervals, and the space between the annular member 111 and the cylindrical member 112 is L p It is divided into individual subspaces. Each subspace corresponds to a through hole (also called a slit or opening) 114 that extends from the front of the slit member 11 to the back of the slit member 11. The front of the slit member 11 is the main surface on the side of the rotating sound source 50, and the back of the slit member 11 is the main surface on the opposite side from the front of the slit member 11. In this way, the slit member 11 has L on its front p The number of openings and the back are L p It has 1 openings. The opening on the front of the slit member 11 is sometimes called the front opening, and the opening on the back of the slit member 11 is sometimes called the back opening. The slit member 11 is circular in the XY plane, and the front and back surfaces are perpendicular to the Z axis. The slit member 11 penetrates in the Z axis direction.

[0055] The annular member 111 has an L that extends from the inner circumferential surface to the outer circumferential surface. p It is equipped with 115 through holes (also called openings). p The through holes 115 are provided at equal intervals, L p Each of the through holes 114 is connected in this way. pIt has openings. These openings are sometimes called side openings. The annular member 111 is an example of an annular member. The annular member may be a polygonal annular member. For example, L p If = 36, the annular member may be a 36-sided annular ring.

[0056] The slit member 12 has the same structure as the slit member 11. Therefore, a detailed explanation of the slit member 12 is omitted. However, the slit member 12 may have a different structure from the slit member 11.

[0057] Referring again to Figure 13, the rigid plate 13 is an example of a plate member. The rigid plate 13 may be, for example, a circular plate member or a polygonal plate member. The rigid plate 13 is provided between the slit member 11 and the slit member 12, and the slit member 11 and the slit member 12 are fixed to the rigid plate 13. Specifically, the slit member 11 is attached to the front of the rigid plate 13, and the slit member 12 is attached to the back of the rigid plate 13. As a result, the rear opening of the slit member 11 and the front opening of the slit member 12 are blocked by the rigid plate 13. In other words, the rigid plate 13 prevents sound waves and airflow from being directly transmitted from the slit member 11 to the slit member 12. The outer diameter of the rigid plate 13 is larger than the diameter of the slit member 11. Note that the outer diameter of the rigid plate 13 may be the same as the diameter of the slit member 11.

[0058] The route piping 14 is arranged circumferentially at equal intervals on the outside of the slit members 11, 12 and the rigid plate 13. One end of the route piping 14 is connected to the annular member 111 of the slit member 11, and the other end is connected to the annular member of the slit member 12. Each route piping 14 connects one corresponding side opening of the slit member 11 to one corresponding side opening of the slit member 12.

[0059] In one example, the route piping 14 is a bent pipe having two bends as shown in Figure 15. Although Figure 15 shows an example where the route piping 14 is bent at a right angle at each bend, the route piping 14 may also be a bent pipe having a curved section, such as a U-shaped pipe. Furthermore, the cross-section of the route piping 14 is not limited to a square, but may be other shapes such as a circle. The connection parts of the route piping 14 with the slit member 11 and the connection parts with the slit member 12 may be arranged in parallel. The route piping 14 can be replaced by inserting and removing it, and the route length can be adjusted. The side walls of the connection parts of the route piping 14 may be in contact with the rigid plate 13 and fixed in place. This can suppress shaking, displacement, and vibration of the route piping 14.

[0060] To avoid flow constriction in the flow path, it is desirable that the cross-sectional area S' of the route piping 14 (Figure 15) is approximately equal to the area S of the front opening (Figure 14). However, the cross-sectional area S' of the route piping 14 may be smaller than the area S of the front opening. Area S is the area of ​​one front opening. For example, 0.9S <S′<1.1Sである。

[0061] Furthermore, the arc length (a × 2π) / L corresponds to the distance between two adjacent route pipes 14. p It is desirable that the arc length is (a × 2π) / L, which corresponds to the wavelength λ of the frequency to which the sound pressure is to be reduced. p It may be λ / 4 or greater.

[0062] Referring again to Figure 13, sound waves (noise) from the rotating sound source 50 propagate through the duct 60 and enter the acoustic metamaterial 10. The sound waves are blocked by the partition member 113 of the slit member 11. p It is divided into individual parts. L p Each divided sound wave is L of the slit member 11 p From the side openings L p Sound waves enter each of the individual path pipes 14. In other words, sound waves that have passed through each front opening enter the path pipe 14 from the corresponding side opening. Divided sound waves propagating within the path pipe 14 enter the slit member 12 from the side opening of the slit member 12. Divided sound waves that have entered the slit member 12 are emitted to the outside from the rear opening of the slit member 12.

[0063] Cylindrical member 112 and L p These individual partition members 113 are collectively referred to as guide members. The guide member for slit member 11 divides the sound waves incident on slit member 11 and guides them to the side opening of slit member 11. The guide member for slit member 12 guides the sound waves incident on slit member 12 from the side opening to the rear opening of slit member 12.

[0064] The structure shown in Figure 13 is an example, and the structure of the acoustic metamaterial 10 is not limited to that shown in Figure 13. In Figure 13, the slit members 11, 12 and the rigid plate 13 are separate components. In other examples, the slit members 11, 12 and the rigid plate 13 may be a single component. In other words, the slit members 11, 12 and the rigid plate 13 may be integrally molded. In further examples, the slit member 11 and the rigid plate 13 may be configured as a single component to which the slit member 12 is fixed. The rigid plate 13 may be composed of two components. For example, as shown in Figure 13, the rigid plate 13 may comprise rigid plate 13-1 and rigid plate 13-2, which is a separate component from rigid plate 13-1. Rigid plate 13-1 is attached to the slit member 11 so as to close the rear opening of the slit member 11, and rigid plate 13-2 is attached to the slit member 12 so as to close the front opening of the slit member 12. The rigid plates 13-1 and 13-2 may be connected to each other, for example, by adhesive, or they may be connected to each other via connecting members (not shown). Furthermore, the slit member 11 and rigid plate 13-1 may be configured as a single member, or the slit member 12 and rigid plate 13-2 may be configured as a single member.

[0065] The slit member 11 is configured such that when sound waves incident on the slit member 11 are L pAny structure is acceptable as long as it is guided to the individual route pipes 14. For example, the cylindrical member 112 may be removed from the slit member 11. In this case, one end of each partition member 113 may be connected to the annular member 111, and the other end may be connected to another partition member 113 at the center of the slit member 11. Alternatively, the cylindrical member 112 and partition members 113 may be removed from the slit member 11. Furthermore, the route pipes 14 may have three or more bends, as shown in Figures 16 and 17. In Figure 16, the length of the route pipes 14 in the Z-axis direction may be longer than the length of the route pipes 14 in the direction perpendicular to the Z-axis direction. Since the route pipes 14 generally follow the duct, they are less susceptible to wind in the direction along the duct. Also, the acoustic metamaterial 10 can be made smaller.

[0066] Figure 18 schematically shows an acoustic metamaterial 20, which is an example of an acoustic metamaterial 10 according to an embodiment. As shown in Figure 18, the acoustic metamaterial 20 comprises slit members 21, 22, and 36 U-shaped tubes 24 (U-shaped tubes 24-1 to 24-36). Slit member 21 corresponds to the integrally molded slit member 11 and rigid plate 13-1 shown in Figure 13, slit member 22 corresponds to the integrally molded slit member 12 and rigid plate 13-2 shown in Figure 13, and U-shaped tubes 24 correspond to the route piping 14 shown in Figure 13.

[0067] Figure 19 shows the acoustic metamaterial 20 with the U-shaped pipe 24 omitted, Figure 20 shows the acoustic metamaterial 20 with the slit members 21 and 22 omitted, and Figure 21 shows the acoustic metamaterial 20 attached to the duct 60.

[0068] As shown in Figures 18 and 19, the slit member 21 has 36 main surface openings (front openings) and 36 side openings communicating with the 36 main surface openings, and the slit member 22 has 36 main surface openings (rear openings) and 36 side openings communicating with the 36 main surface openings. Each U-shaped tube 24 connects the side openings of the slit member 21 and the side openings of the slit member 22.

[0069] As shown in Figure 20, U-tubes 24-1 to 24-6 form the first group, U-tubes 24-7 to 24-12 form the second group, U-tubes 24-13 to 24-18 form the third group, U-tubes 24-19 to 24-24 form the fourth group, U-tubes 24-25 to 24-30 form the fifth group, and U-tubes 24-31 to 24-36 form the sixth group.

[0070] U-tubes 24-1, 24-7, 24-13, 24-19, 24-25, and 24-31 have the same structure and therefore have the same path length. U-tubes 24-2, 24-8, 24-14, 24-20, 24-26, and 24-32 have the same structure and therefore have the same path length. U-tubes 24-3, 24-9, 24-15, 24-21, 24-27, and 24-33 have the same structure and therefore have the same path length. U-tubes 24-4, 24-10, 24-16, 24-22, 24-28, and 24-34 have the same structure and therefore have the same path length. U-tubes 24-5, 24-11, 24-17, 24-23, 24-29, and 24-35 have the same structure and therefore the same path length. U-tubes 24-6, 24-12, 24-18, 24-24, 24-20, and 24-36 have the same structure and therefore the same path length.

[0071] The six U-shaped pipes 24 in each group have different path lengths. Specifically, the path length of U-shaped pipe 24-2 is longer than the path length of U-shaped pipe 24-1 by a predetermined length. The path length of U-shaped pipe 24-3 is longer than the path length of U-shaped pipe 24-2 by a predetermined length. The path length of U-shaped pipe 24-4 is longer than the path length of U-shaped pipe 24-3 by a predetermined length. The path length of U-shaped pipe 24-5 is longer than the path length of U-shaped pipe 24-4 by a predetermined length. The path length of U-shaped pipe 24-6 is longer than the path length of U-shaped pipe 24-5 by a predetermined length. The path length is, for example, the length traced along the central portion of the U-shaped pipe 24 or the path piping 14.

[0072] Thus, the two adjacent U-shaped pipes 24 have different path lengths.

[0073] As shown in Figure 21, the acoustic metamaterial 20 is attached to the duct 60 such that the slit member 11 faces the duct 60. However, since the acoustic metamaterial 20 is symmetrical, it may also be attached to the duct 60 such that the slit member 12 faces the duct 60.

[0074] Referring to Figures 22 to 27, the results of acoustic analysis performed on the acoustic metamaterial 10 according to the embodiment will be described.

[0075] The acoustic metamaterial 10 subjected to acoustic analysis is similar to the acoustic metamaterial 20 shown in Figure 18, and specifically has 36 routing pipes, which are grouped into 6 groups. U-shaped pipes with a diameter of φ30-32 and a radius of R35 are used as routing pipes. The path difference between two adjacent routing pipes is approximately 30 mm. As shown in Figure 22, the acoustic metamaterial 10 is placed in a circular opening of φ420 on one face of a rectangular box with sides of 1 m. As an analysis condition, the five faces other than the face with the circular opening are treated as non-reflective boundary conditions.

[0076] Figure 23 shows the sound pressure in the YZ plane, and Figure 24 shows the sound pressure in the XY plane. In both Figure 23 and Figure 24, the left portion shows the acoustic analysis results of the acoustic metamaterial 10 according to the embodiment, and the right portion shows the acoustic analysis results of the acoustic metamaterial according to the comparative example.

[0077] Figure 23 confirms that the acoustic metamaterial 10 according to the embodiment sufficiently reduces sound pressure, and Figure 24 confirms that there is variation in the sound pressure distribution of the sound that has passed through the acoustic metamaterial 10 according to the embodiment (i.e., deflection occurs). Therefore, the validity of the acoustic metamaterial 10 according to the embodiment is demonstrated.

[0078] Figure 25 shows the radiation characteristics in 18 planes including the Z axis, and Figure 26 shows the average of the radiation characteristics in the 18 planes shown in Figure 25. As shown in Figure 27, when the angle with the XZ plane is φ, the 18 planes including the Z axis are the plane at φ=0[deg], the plane at φ=10[deg], the plane at φ=20[deg], ..., and the plane at φ=170[deg]. In Figures 25 and 26, the dashed line shows the radiation characteristics of the acoustic metamaterial 10 according to the embodiment, and the solid line shows the radiation characteristics of the acoustic metamaterial according to the comparative example. In Figures 25 and 26, plots of predetermined sound pressure levels are shown as radiation characteristics. From Figures 25 and 26, it can be confirmed that the radiation characteristics from 30deg to 60deg are sufficiently suppressed.

[0079] The acoustic metamaterial 10 according to this embodiment can be applied not only to reducing noise originating from a rotating sound source, but also to other applications.

[0080] A speaker system 70 is known in which four speakers 71 arranged on a circumference are driven with a phase difference of 90 degrees, as shown in Figure 28. Let's assume that the speaker system 70 is positioned to radiate sound upwards. In this case, because the upward radiation characteristic is large, reflections from the ceiling and other factors become a problem. Furthermore, the sound also spreads laterally.

[0081] The acoustic metamaterial 10 according to this embodiment can be used to ensure sound pressure only in the vicinity of the speaker group shown in Figure 28. For example, as shown in Figure 29, the speaker system 70 is placed on a desk 90 so that it is located inside the pipe 80 to which the acoustic metamaterial 10 is attached. This reduces the upward radiation characteristics for the same reasons as explained with reference to Figure 6, thus eliminating the problem of reflection from the ceiling. Furthermore, the sound pressure in the lateral direction is also reduced. As a result, the sound pressure emitted from the speaker system 70 is high near the outlet of the pipe 80, but decreases rapidly as it moves away from the outlet of the pipe 80.

[0082] As described above, the acoustic metamaterial according to the embodiment includes a first slit member (for example, a slit member 21 shown in Figure 18) including a first annular member having a plurality of first openings, a second slit member (for example, a slit member 22 shown in Figure 18) including a second annular member having a plurality of second openings, a plate member disposed between the first slit member and the second slit member (in the example shown in Figure 18, the plate member is included in each of the slit member 21 and the slit member 22), and a plurality of route pipes (for example, U-shaped pipes 24-1 to 24-36 shown in Figure 18) connecting the plurality of first openings and the plurality of second openings. Two adjacent route pipes among the plurality of route pipes have different lengths from each other.

[0083] The acoustic metamaterial according to the embodiment having the above-described configuration provides circumferential deflection to incident sound waves. This makes it possible to reduce noise originating from a rotating sound source. The acoustic metamaterial according to the embodiment is a passive acoustic element that does not use a speaker. Therefore, it is lightweight and compact, and does not require a signal processing unit. Furthermore, the path piping also functions as an air passage. Thus, the acoustic metamaterial according to the embodiment can reduce noise originating from a rotating sound source while ensuring an air passage.

[0084] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0085] 10...Acoustic metamaterial, 11...Slit member, 111...Ring member, 112...Cylindrical member, 113...Partition member, 114...Through hole, 115...Through hole, 12...Slit member, 13...Rigid plate, 14...Pipe routing, 20...Acoustic metamaterial, 21...Slit member, 22...Slit member, 24...U-shaped pipe, 30...Acoustic metamaterial, 31...Structure, 32...Bypass, 50...Rotating sound source, 60...Duct, 70...Speaker system, 71...Speaker, 80...Piping, 90...Desk.

Claims

1. A first slit member including a first annular member having a plurality of first openings, A second slit member including a second annular member having a plurality of second openings, A plate member disposed between the first slit member and the second slit member, A plurality of route pipes connecting the plurality of first openings and the plurality of second openings, Equipped with, Of the aforementioned plurality of route pipes, two adjacent route pipes have different lengths. Noise reduction device.

2. The plate member includes a first plate member attached to the first slit member, and a second plate member, which is a different member from the first plate member, attached to the second slit member. The noise reduction device according to claim 1.

3. The first slit member and the first plate member are integrally molded, and the second slit member and the second plate member are integrally molded. The noise reduction device according to claim 2.

4. The first plate member is attached to the second plate member. The noise reduction device according to claim 3.

5. The first slit member is provided inside the first annular member and further includes a guide member that guides incident sound waves to the plurality of first openings. The noise reduction device according to claim 1.

6. The aforementioned plurality of route pipes have different lengths from each other. The noise reduction device according to claim 1.

7. The aforementioned multiple routing pipes are grouped into multiple groups having the same configuration. The multiple routing pipes included in each of the aforementioned multiple groups have different lengths from each other. The noise reduction device according to claim 1.

8. Each of the aforementioned groups includes four or more routing pipes. The noise reduction device according to claim 7.

9. The distance between two adjacent routing pipes among the aforementioned plurality of routing pipes is less than or equal to 1 / 4 of the wavelength for the frequency to which sound pressure is to be reduced. The noise reduction device according to claim 1.

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