Sound absorption device and parameter estimation method

Abstract

To provide a sound absorption device of simple constitution capable of absorbing noises of two frequencies in a low-frequency band, and a method for estimating parameters of the sound absorption device.SOLUTION: A sound absorption device 10 comprises a hollow member 12, a surface plate 11 as a first plate, a back plate 13 as a second plate, a suspension plate 15 as a third plate, and a connection member 14. The surface plate 11 is connected to the hollow member 12 and can vibrate. The back plate 13 faces the surface plate 11 and is connected to the hollow member 12. The suspension plate 15 is arranged between the surface plate 11 and back plate 13 and can vibrate. The connection member 14 connects the suspension plate 15 to the surface plate 11. The surface plate 11, back plate 13 and hollow member 12 form an internal space 30, and the suspension plate 15 is suspended from the surface plate 11 in the internal space 30 by the connection member 14.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a sound absorption device and a parameter estimation method.

Background Art

[0002] An acoustic metamaterial having a configuration in which Helmholtz resonance and plate vibration are coupled is known. The coupling of Helmholtz resonance and plate vibration enables broadband sound absorption characteristics. However, the production of a perforated plate, also called a Helmholtz sound hole plate, requires high costs.

[0003] By the way, there is a social need to be able to reduce noise at two frequencies in the frequency band below 200 Hz. As the noise at two frequencies, for example, noise at the fundamental frequency and noise at a frequency twice the fundamental frequency are assumed. For example, when the noise is derived from a power supply, the fundamental frequency can be 50 Hz or 60 Hz.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

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 simple-structured sound absorption device capable of absorbing noise at two frequencies in the low frequency band. Further, a method for estimating parameters of the sound absorption device is provided.

Means for Solving the Problems

[0006] The sound absorption device according to one embodiment includes a hollow member, a first plate, a second plate, a third plate, and a connecting member. The first plate is connected to the hollow member and is capable of vibrating. The second plate faces the first plate and is connected to the hollow member. The third plate is disposed between the first plate and the second plate and is capable of vibrating. The connecting member connects the third plate to the first plate. The first plate, the second plate, and the hollow member form an internal space, and the third plate is suspended from the first plate by the connecting member in the internal space.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

Figure 24

Figure 25

Figure 26

Figure 27

Figure 28

Figure 29

Figure 30

Figure 31

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

Figure 37

Figure 38

Figure 39

Figure 40

Mode for Carrying Out the Invention

[0008] Hereinafter, embodiments will be described with reference to the drawings.

[0009] FIG. 1 schematically shows a cross section of a sound absorption device 10 according to an embodiment. As shown in FIG. 1, the sound absorption device 10 includes a top plate 11, a hollow member 12, a back plate 13, a connecting member 14, a suspension plate 15, and a plurality of weights 16.

[0010] The hollow member 12 is, for example, a cylindrical frame. The hollow member 12 has a first opening and a second opening facing the first opening. The front panel 11 is, for example, a flat plate. The front panel 11 is, for example, circular in shape. The front panel 11 is connected to the hollow member 12 so as to close the first opening of the hollow member 12. The outer edge portion of the front panel 11 is supported by the hollow member 12 so that the front panel 11 can vibrate in the axial direction as indicated by the double arrows. The axial direction is the direction along the virtual axis of the sound absorption device 10 and is perpendicular to the front panel 11. The rear panel 13 is, for example, a flat plate. The rear panel 13 is, for example, circular in shape. The rear panel 13 is connected to the hollow member 12 so as to close the second opening of the hollow member 12. The rear panel 13 faces the front panel 11 at a distance of the length L1 of the hollow member 12. The length L1 of the hollow member 12 is the dimension along the axis of the sound absorption device 10. The hollow member 12, the front panel 11, and the rear panel 13 form an internal space 30. The length L1 of the hollow member 12 corresponds to the thickness of the internal space 30.

[0011] The connection between the front panel 11 and the rear panel 13 and the hollow member 12 can be implemented using an adhesive, a fixture, etc., but is not limited thereto. In the example described later, the front panel 11 and the rear panel 13 are pressed against the hollow member 12 using the tension of the string member, whereby the front panel 11 and the rear panel 13 are connected to the hollow member 12.

[0012] The connecting member 14 and the hanging plate 15 are arranged in the internal space 30 between the front panel 11 and the rear panel 13. The connecting member 14 connects the hanging plate 15 to the front panel 11. Specifically, the hanging plate 15 is suspended from the front panel 11 by the connecting member 14 in the internal space 30. One end of the connecting member 14 is connected to the central portion of the front panel 11, and the other end of the connecting member 14 is connected to the central portion of the hanging plate 15. The hanging plate 15 is, for example, a circular flat plate. The central portion of the hanging plate 15 is supported by the connecting member 14 so that the hanging plate 15 can vibrate in the axial direction as indicated by the double arrows. The weights 16 are dispersedly arranged on the outer edge portion of the hanging plate 15.

[0013] Instead of the weight 16, a weight having a shape that matches the outer edge portion of the suspension plate 15, that is, an annular weight, may be used. The annular weight is disposed on the outer edge portion of the suspension plate 15. When a plurality of annular weights are used, the annular weights may be stacked. The weight 16 may be disposed between the outer edge portion of the suspension plate 15 and the surface plate 11, between the outer edge portion of the suspension plate 15 and the rear plate 13, and between the outer edge portion of the suspension plate 15 and the hollow member 12.

[0014] The sound absorption device 10 having the above-described configuration can absorb or reduce noise at two frequencies in the low frequency band of 200 Hz or less. In one example, the sound absorption device 10 is designed to be able to absorb or reduce the noise of the fundamental frequency generated from a noise source such as an electronic device and its harmonic (specifically, the noise having a frequency twice the fundamental frequency). When the noise is derived from the power supply, the fundamental frequency can be 50 Hz or 60 Hz. When the noise is derived from a device having a fan, the fundamental frequency is the product of the rotational frequency of the fan and the number of fan blades. For example, when the rotational speed is 600 rpm and the fan has 5 blades, the fundamental frequency is 50 Hz. Further, since the Helmholtz sound hole plate is not required and the number of components is small, the manufacturing cost is low and the assembly is easy.

[0015] The vibration system having the structure shown in FIG. 1 can be represented by the two-degree-of-freedom model shown in FIG. 2. In FIG. 2, m 1 , k 1 , c 1 are the mass, spring constant, and damping coefficient of the first-degree-of-freedom system, and m 2 , k 2 , c 2 are the mass, spring constant, and damping coefficient of the second-degree-of-freedom system. The mass m 1 represents the mass of the connecting member 14 and the equivalent mass of the surface plate 11. The spring constant k 1 represents the equivalent rigidity of the surface plate 11 and the air spring of the internal space 30. The damping coefficient c 1 represents the equivalent viscosity of the surface plate 11. The mass m 2 represents the mass of the weight 16 and the equivalent mass of the suspension plate 15. The spring constant k 2represents the equivalent rigidity of the suspension plate 15. The damping coefficient c 2 represents the equivalent viscosity of the suspension plate 15.

[0016] In the sound absorption device 10, the surface plate 11 vibrates upon receiving sound waves, and the suspension plate 15 vibrates along with the vibration of the surface plate 11. As vibration modes, there occur a in-phase drive mode in which the surface plate 11 and the suspension plate 15 vibrate in the same direction and an anti-phase drive mode in which the surface plate 11 and the suspension plate 15 vibrate in opposite directions. As a result, the frequency at which the peak value of the vibration velocity is taken is separated into two frequencies. Since the sound absorption effect is correlated with the kinetic energy, two frequencies at which the peak value of the sound absorption rate is taken consequently occur.

[0017] The sound absorption frequency, which is the frequency to be reduced by the sound absorption device 10, can be adjusted by, for example, the following five parameters. (1) The length L1 of the hollow member 12 The air spring of the internal space 30 depends on the length L1 of the hollow member 12. Specifically, the smaller the length L1, the stronger the air spring of the internal space 30. Therefore, by changing the length L1, the spring constant k 1 can be adjusted. When the length L1 is increased, the spring constant k 1 decreases, whereby the frequency at which the peak value of the sound absorption rate is taken decreases. When the length L1 is decreased, the spring constant k 1 increases, whereby the frequency at which the peak value of the sound absorption rate is taken increases. (2) The radius of the suspension plate 15 The rigidity of the suspension plate 15 depends on the radius of the suspension plate 15. Specifically, the smaller the radius of the suspension plate 15, the higher the rigidity of the suspension plate 15. Therefore, by changing the radius of the suspension plate 15, the spring constant k 2 can be adjusted. When the radius is enlarged, the spring constant k 2 decreases, whereby the interval between the frequencies at which the peak value of the sound absorption rate is taken is reduced. When the radius is reduced, the spring constant k 2 increases, whereby the interval between the frequencies at which the peak value of the sound absorption rate is taken is enlarged. (3) The number of weights 16 By changing the number of weights 16, the mass m2 can be adjusted. Specifically, the larger the number of weights 16, the greater the mass m 2 will be. When the mass m 2 is increased, the mass ratio (m 2 / m 1 ) of the two-degree-of-freedom system increases, and thereby the frequency at which the sound absorption rate peak value is taken decreases. When the mass m 2 is decreased, the mass ratio (m 2 / m 1 ) decreases, and thereby the frequency at which the sound absorption rate peak value is taken increases. In addition, changing the number of weights 16 brings about a change in the spring constant k 2 . Specifically, when the number of weights 16 is increased, the spring constant k 2 decreases, and when the number of weights 16 is decreased, the spring constant k 2 increases. Without changing the number of weights 16, the weight of the weights 16 may be changed. In this case, a change in the spring constant k 2 can be suppressed. (4) Thickness of the suspension plate 15 The rigidity of the suspension plate 15 depends on the thickness of the suspension plate 15. Specifically, the thicker the suspension plate 15, the higher the rigidity of the suspension plate 15. Therefore, by changing the thickness of the suspension plate 15, the spring constant k 2 can be adjusted. When the suspension plate 15 is made thinner, the spring constant k 2 decreases, and thereby the interval between the frequencies at which the sound absorption rate peak values are taken is reduced. When the suspension plate 15 is made thicker, the spring constant k 2 increases, and thereby the interval between the frequencies at which the sound absorption rate peak values are taken is expanded. (5) Mass of the connecting member 14 By changing the mass of the connecting member 14, the mass m 1 can be adjusted. The change in the mass m 1 brings about a change in the natural frequency in the state where the suspension plate 15 is not present. Therefore, by changing the mass of the connecting member 14, the center frequency can be adjusted.

[0018] Hereinafter, the frequency at which the sound absorption rate peak value is taken is also referred to as the peak frequency.

[0019] By adjusting these parameters, the sound absorption device 10 can be adjusted so as to obtain, for example, sound absorption effects at 60 Hz and 120 Hz.

[0020] Note that when the natural frequencies of the degree-of-freedom system (m 1 , k 1 ) are made closer to the natural frequencies of the degree-of-freedom system (m 2 , k 2 ), eigenvalue matching occurs and the values of the two sound absorption rate peaks become closer. Although it is desirable that the values of the two sound absorption rate peaks become closer, they may be separated as long as the values of the two sound absorption rate peaks are equal to or greater than a predetermined value (for example, 0.6).

[0021] A specific example of the sound absorption device 10 and an example of the assembly procedure of the sound absorption device 10 according to the specific example will be described.

[0022] FIG. 3 schematically shows an example of the connecting member 14. As shown in FIG. 3, the connecting member 14 includes resin bolts 141, resin nuts 142, 143, 144, and resin washers 145, 146, 147, 148. In the example shown in FIG. 3, the bolt 141 is a half bolt. Openings for inserting the resin bolt 141 are provided in the central portions of the surface plate 11 and the suspension plate 15. A resin nut 142 is screwed onto the resin bolt 141, and the resin bolt 141 is inserted through the resin washer 145, the surface plate 11, and the resin washer 146. A resin nut 143 is screwed onto the resin bolt 141, and the resin bolt 141 is inserted through the resin washer 147, the suspension plate 15, and the resin washer 148. A resin nut 144 is screwed onto the resin bolt 141. The surface plate 11 is fixed to the resin bolt 141 by being clamped by the resin nuts 142 and 143, and the suspension plate 15 is fixed to the resin bolt 141 by being clamped by the resin nuts 143 and 144. Additional washers such as stainless steel washers may be provided between the resin bolt and the resin washer. The addition of washers means continuously providing a plurality of washers between at least any of the resin nut 142, the surface plate 11, the resin nut 143, the suspension plate 15, and the resin nut 144 inserted in the order described for the resin bolt 141, or providing a washer between the head of the resin bolt 141 and the resin nut 142. The washers are not limited to being made of resin or stainless steel, and a plurality of types with different materials can be mixed and used. By adding washers, it is possible to adjust the weight of the connecting member 14. The washers through which the bolt is inserted are clamped by nuts. The resin nut 143 may be divided into two nuts, one for clamping the surface plate 11 and the other for clamping the suspension plate 15.

[0023] Figure 4 shows a state in which twelve weights 16 are attached to a suspension plate 15 at equal intervals. In the example shown in Figure 4, each weight 16 includes a resin screw, a resin collar, and a resin nut. An opening for inserting the resin screw is provided at the outer edge portion of the suspension plate 15. The plurality of openings are provided, for example, at positions that are rotationally symmetric a plurality of times with respect to the center of the suspension plate 15. By fastening the resin screw and the resin nut, the weight 16 is fixed to the suspension plate 15. The plurality of weights 16 are arranged, for example, such that their centers of gravity overlap the center of the suspension plate 15. The attachment of the weights 16 to the suspension plate 15 may be performed before the attachment of the suspension plate 15 to the connecting member 14, or may be performed after the attachment of the suspension plate 15 to the connecting member 14.

[0024] Figure 5 shows a fixing member 50 and a band 56 as fixing tools. As shown in Figure 5, the fixing member 50 includes a cylindrical member 51, a fixing plate 52, and a guide 53. The cylindrical member 51 and the guide 53 are fixed to the fixing plate 52. The fixing plate 52 has an opening at a portion facing the surface plate 11. Further, two slits 54, 55 for passing the band 56 are provided at each of the four corners of the fixing plate 52. Each band 56 is passed through the corresponding two slits 54, 55. A hook-and-loop fastener 57 is provided on one surface of the band 56.

[0025] As shown in Fig. 6, the surface plate 11 to which the connecting member 14 and the suspension plate 15 are attached is placed on the fixing member 50. The surface plate 11 is positioned by the guide 53. Then, as shown in Fig. 7, the hollow member 12 is placed on the surface plate 11. The hollow member 12 is positioned by the guide 53. The outer edge portion of the surface plate 11 is sandwiched between the cylindrical member 51 of the fixing member 50 and the hollow member 12. Subsequently, as shown in Fig. 8, the rear plate 13 is placed on the hollow member 12. In this example, the rear plate 13 is a square flat plate and also functions as a part of the fixture. The rear plate 13 is provided with a guide for positioning the rear plate 13 with respect to the hollow member 12. Also, two slits 63, 64 for passing the band 56 are provided at each of the four corners of the rear plate 13. The surface of the rear plate 13 facing the surface plate 11 is expressed as the front surface. Hook-and-loop fasteners 62 are provided at the four corners of the front surface of the rear plate 13, and hook-and-loop fasteners 61 are provided on the back surface of the rear plate 13. The hook-and-loop fastener 62 is located on the outer edge side of the rear plate 13 with respect to the slit 64. The hook-and-loop fastener 61 is located on the central side of the rear plate 13 with respect to the slit 63. As shown in Fig. 9, with one end of each band 56 passed through one slit 63 of the rear plate 13, the hook-and-loop fastener 57 of the band 56 is pressed against the hook-and-loop fastener 61 of the rear plate 13, whereby one end of the band 56 is fixed to the rear plate 13. Further, as shown in Fig. 10, the other end of the band 56 is passed through the other slit 64 of the rear plate 13, and while pulling the band 56, the hook-and-loop fastener 57 of the band 56 is pressed against the hook-and-loop fastener 62 of the rear plate 13, whereby the other end of the band 56 is also fixed to the rear plate 13. A force in the axial direction (specifically, a force acting such that the rear plate 13 and the fixing member 50 approach each other) is generated by the tension of the band 56, whereby the surface plate 11 and the rear plate 13 are fixed to the hollow member 12 as shown in Fig. 11. In order to increase the axial force, as shown in Fig. 12, the band 58 may be wound in the circumferential direction.

[0026] When using the fixture described with reference to Figs. 5 to 12, the sound absorption device 10 can be easily disassembled. For this reason, parameter adjustment becomes easy.

[0027] Next, the change in the sound absorption characteristics of the sound absorption device 10 due to parameter adjustment will be described. Here, as the sound absorption characteristics, the results of measuring the normal incidence sound absorption rate are shown.

[0028] As a common matter of each embodiment, the hollow member 12 has an outer diameter of 196 mm and an inner diameter of 190 mm. The surface plate 11 is an aluminum plate with a diameter of 196 mm and a thickness of 0.5 mm. The connecting member 14 includes, as shown in FIG. 3, an M10 resin bolt, three M10 resin nuts, and four resin washers. The weight 16 includes an M3 resin bolt, a resin collar, and a resin nut, and has a weight of 0.5 g. PVC (polyvinyl chloride) tape is attached to both sides of the suspension plate 15 to prevent fluttering.

[0029] In the first embodiment, the suspension plate 15 is an aluminum plate with a diameter of 115 mm and a thickness of 0.3 mm. Four weights 16 are attached to the outer edge portion of the suspension plate 15 at equal intervals.

[0030] FIG. 13 shows the sound absorption characteristics of the sound absorption device 10 according to the first embodiment under the condition that the length L1 is 80 mm. In FIG. 13, the solid line indicates the sound absorption characteristics of the sound absorption device 10 according to the first embodiment, and the broken line indicates the sound absorption characteristics of the sound absorption device according to the comparative example. The sound absorption device according to the comparative example is obtained by removing the suspension plate 15 from the sound absorption device 10 according to the first comparative example. From FIG. 13, it can be confirmed that the sound absorption device according to the comparative example has a sound absorption rate peak value at 100 Hz, and the sound absorption device 10 according to the first embodiment has two sound absorption rate peak values near 60 Hz and 120 Hz. The peak near 60 Hz is the sound absorption performance generated in the mode in which the surface plate 11 and the suspension plate 15 vibrate in the same phase, and the peak near 120 Hz is the sound absorption performance generated in the mode in which the surface plate 11 and the suspension plate 15 vibrate in the opposite phase.

[0031] Therefore, it can be confirmed that the sound absorption device 10 having the configuration shown in FIG. 1 exhibits high sound absorption performance at two frequencies.

[0032] Figure 14 shows the sound absorption characteristics of the sound absorption device 10 according to the first embodiment under the condition that the length L1 is 120 mm. Comparing Fig. 13 and Fig. 14, it can be confirmed that when the length L1 is increased, the peak frequency decreases.

[0033] In the second embodiment, the suspension plate 15 is an aluminum plate with a diameter of 95 mm and a thickness of 0.3 mm. Four weights 16 are attached to the outer edge portion of the suspension plate 15 at equal intervals.

[0034] Figure 15 shows the sound absorption characteristics of the sound absorption device 10 according to the second embodiment under the condition that the length L1 is 80 mm. Comparing Fig. 13 and Fig. 15, it can be confirmed that when the radius of the suspension plate 15 is reduced (i.e., the rigidity of the suspension plate 15 is increased), the interval between the peak frequencies expands.

[0035] Figure 16 shows the sound absorption characteristics of the sound absorption device 10 according to the second embodiment under the condition that the length L1 is 120 mm. Comparing Fig. 15 and Fig. 16, it can be confirmed that when the length L1 is increased, the peak frequency decreases.

[0036] In the third embodiment, the suspension plate 15 is an aluminum plate with a diameter of 95 mm and a thickness of 0.3 mm. Twelve weights 16 are attached to the outer edge portion of the suspension plate 15 at equal intervals.

[0037] Figure 17 shows the sound absorption characteristics of the sound absorption device 10 according to the third embodiment under the condition that the length L1 is 80 mm. Comparing Fig. 15 and Fig. 17, it can be confirmed that when the mass m 2 is increased, the peak frequency decreases.

[0038] Figure 18 shows the sound absorption characteristics of the sound absorption device 10 according to the third embodiment under the condition that the length L1 is 120 mm. Comparing Fig. 17 and Fig. 18, it can be confirmed that when the length L1 is increased, the peak frequency decreases. Also, in Fig. 18, since the peak frequency of the sound absorption device according to the comparative example is approximately in the middle of the two peak frequencies of the sound absorption device 10 according to the third embodiment, it can be said that the condition of eigenvalue matching is achieved.

[0039] Figure 19 shows the sound absorption characteristics of the sound absorption device 10 according to the third embodiment under the condition that the length L1 is 160 mm. Comparing FIGS. 17, 18, and 19, it can be confirmed that the peak frequency decreases as the length L1 increases. Further, as shown in FIG. 19, under the condition that the length L1 is 160 mm, the two peak frequencies are in the vicinity of 60 Hz and 120 Hz.

[0040] In the fourth embodiment, the suspension plate 15 is an aluminum plate with a diameter of 95 mm and a thickness of 0.2 mm. Twelve weights 16 are attached to the outer edge portion of the suspension plate 15 at equal intervals of 12.

[0041] Figure 20 shows the sound absorption characteristics of the sound absorption device 10 according to the fourth embodiment under the condition that the length L1 is 80 mm. Comparing FIG. 17 and FIG. 20, it can be confirmed that when the suspension plate 15 is thinned (that is, the rigidity of the suspension plate 15 is decreased), the interval between the peak frequencies is reduced.

[0042] From FIGS. 13 to 20, it can be confirmed that the sound absorption device 10 having the configuration shown in FIG. 1 has a high sound absorption rate at two frequencies, and further, the validity of the above-described parameter adjustment method can also be confirmed. By parameter adjustment, as shown in FIGS. 13 and 19, a sound absorption device 10 that absorbs noise at the fundamental frequency such as 60 Hz and 120 Hz and noise at twice that frequency can be provided.

[0043] Next, a parameter estimation method will be described. Each parameter estimation method described below is for estimating parameter values that can realize desired sound absorption characteristics, and facilitates parameter adjustment.

[0044] The configuration of the sound absorption device 10 shown in FIG. 1 corresponds to the two-degree-of-freedom system model shown in FIG. 2, and the equation of motion can be described as follows.

Equation

[0045] (m 1 ,k 1 ) consisting of the natural frequency ω n1 is ω n1 2 =k 1 / m 1 and, (m 2 ,k 2 ) consisting of the natural frequency ω n2 is ω n2 2 =k 2 / m 2 and, setting the mass ratio as α = m 2 / m 1 , then the natural frequency ω r1、 ω r2 can be obtained by the following formulas (1) and (2).

Equation

[0046] The first parameter estimation method will be described. In the first parameter estimation method, the sound absorption rate is measured without the suspension plate 15 provided to identify the frequency at which the sound absorption rate peak value is taken, and the natural frequency ω n1 is obtained from the identified frequency. Further, the sound absorption rate is measured with the suspension plate 15 attached to identify two frequencies at which the sound absorption rate peak value is taken, and the natural frequencies ω r1、 ω r2 are obtained from the identified frequencies. The first parameter estimation method performs parameter estimation of the model shown in FIG. 2 using these natural frequencies ω n1 , ω r1、 ω r2 . However, the first parameter estimation method does not estimate the parameter values themselves, but obtains a group of parameters with m 1 normalized to 1.

[0047] When the following formulas (3) and (4) are used, the following formula (5) is derived from the above formulas (1) and (2).

Equation

[0048] Therefore, the natural frequency ω n2 and the corresponding wavelength λ 2 are expressed by the following formula (6).

Equation

[0049] However, since m 1 is normalized to 1, each parameter is the value obtained by dividing the true value by m 1 . The damping coefficients c 1 , c 2 are set to be similar to the measurement results.

[0050] Since each parameter is obtained by the above procedure, the transfer characteristics dx1 and dx2 shown in the following (8) and (9) can be obtained by the following formula (7).

Equation

[0051] Since the sound absorption effect is correlated with kinetic energy, the tendency of the sound absorption rate can be estimated from the transmission characteristics dx1 and dx2 shown in the above (8) and (9).

[0052] Since the kinetic energy consumed by the surface plate 11 and the suspension plate 15 is as follows, the transmission characteristics of the vibration velocity are correlated with the sound absorption rate.

Number

[0053] In the first embodiment (L1 = 80 mm), as shown in FIG. 13, ω n1 / 2π = 100 Hz, ω r1 / 2π = 61 Hz, ω r2 / 2π = 119.8 Hz. By repeating the following operations while changing the mass ratio α, a graph with the horizontal axis α and the vertical axis ω r2 -ω r1 is created. The operations include setting the value of the mass ratio α, substituting the natural frequencies ω n1 , ω r1 , ω r2 and the mass ratio α into the above formula (6) to obtain the natural frequency ω n2 , obtaining the natural frequencies ω n1 , ω n2 and the natural frequencies ω r1 , ω r2 from the above formulas (1) and (2).

[0054] As measurement values, ω r1 / 2π = 61 Hz and ω r2 / 2π = 119.8 Hz are obtained. Therefore, the mass ratio α when (ω r1 -ω r2 ) / 2π becomes approximately 60 Hz is determined from FIG. 21. In this example, α = 0.54. The natural frequency ω n1 / 2π = 100 Hz, ω r1 / 2π = 61 Hz, ω r2 / 2π = 119.8 Hz, α = 0.54, and the natural frequency ω n2 is calculated from formulas (4) and (6).

[0055] By the above-described processing, the natural frequencies ω n1 , ω n2 , the mass ratio α, and the mass m 1 are determined. From these values, the mass m 2 , the spring constant k 1 , k 2 are calculated. The damping coefficients c 1 , c 2 are set to be similar to the measurement results.

[0056] FIG. 22 shows the parameter estimation results and the estimated results of the vibration transmission characteristics for the first embodiment under the condition that the length L1 is 80 mm. In FIG. 22, the solid line indicates the estimated result of the transmission characteristic dx1, the broken line indicates the estimated result of the transmission characteristic dx2, and the alternate long and short dash line indicates the estimated result of the transmission characteristic when the hanging plate 15 is not present. From FIG. 22, it can be confirmed that the transmission characteristic dx1 is similar to the tendency of the sound absorption rate.

[0057] FIG. 23 shows the parameter estimation results and the estimated results of the vibration transmission characteristics for the first embodiment under the condition that the length L1 is 120 mm. Assuming that the rigidity of the air spring decreases as the length L1 increases from 80 mm to 120 m, the spring constant k 1 is set to a smaller value. The value of the spring constant k 1 is determined so that the natural frequency when the hanging plate 15 is not present is similar to the measurement results. From FIG. 23, it can be confirmed that the transmission characteristic dx1 is similar to the tendency of the sound absorption rate. Therefore, it can be seen that it is reasonable that the peak frequency decreases when the length L1 is increased.

[0058] FIG. 24 shows the parameter estimation results and the estimated results of the vibration transmission characteristics for the second embodiment under the condition that the length L1 is 80 mm. Assuming that the spring constant k 2 increases as the diameter of the hanging plate 15 decreases from 115 mm to 95 mm, the spring constant k 2 is set to a larger value. From FIG. 24, it can be confirmed that the interval between the peak frequencies increases. Therefore, it can be seen that it is reasonable that the interval between the peak frequencies expands when the radius of the hanging plate 15 is reduced.

[0059] Figure 25 shows the parameter estimation results and the estimated results of the vibration transmission characteristics for the second embodiment under the condition that the length L1 is 120 mm. Assuming that the stiffness of the air spring decreases as the length L1 increases from 80 mm to 120 m, the spring constant k 1 is set to a smaller value. From Figure 25, it can be seen that it is reasonable that the peak frequency decreases when the length L1 is increased.

[0060] With reference to Figures 26, 27, and 28, the tendency of the change in the vibration transmission characteristics due to the parameter change will be described.

[0061] Figure 26 shows the change in the vibration transmission characteristics when the mass m 1 is increased. From Figure 26, it can be confirmed that when the mass m 1 is increased, the natural frequency ω n1 decreases, and furthermore, the frequencies at which the vibration transmission characteristics dx1 and dx2 peak tend to decrease.

[0062] Figure 27 shows the change in the vibration transmission characteristics when the mass m 2 is increased. From Figure 27, it can be confirmed that when the mass m 2 is increased, the natural frequency ω n2 decreases, and furthermore, the frequencies at which the vibration transmission characteristics dx1 and dx2 peak tend to decrease.

[0063] Figure 28 shows the change in the vibration transmission characteristics when the spring constant k 2 is increased. From Figure 28, it can be confirmed that when the spring constant k 2 is increased, the natural frequency ω n2 increases, and furthermore, the interval between the frequencies at which the vibration transmission characteristics dx1 and dx2 peak tends to expand.

[0064] As described above, since the measurement results can be reproduced by the first parameter estimation method, it becomes possible to grasp in advance the tendency change due to parameter adjustment through simulation and to conduct a parameter tuning test. That is, parameter tuning can be carried out without trial and error.

[0065] However, in the first parameter estimation method, since the mass m 1 is normalized to 1, it can only grasp the tendency. That is, it is impossible to determine how many grams the mass m 2 should be increased. In order to obtain the desired sound absorption characteristics, it can only be used to grasp whether the mass m 2 should be increased or decreased.

[0066] The second parameter estimation method will be described. The second parameter estimation method estimates the parameters as physical property values using the first parameter estimation method. Briefly, the second parameter estimation method measures the sound absorption rate under conditions where the weights of the weights 16 are different, and determines the mass m 2 from the measurement results. According to the second parameter estimation method, in order to obtain the desired sound absorption characteristics, it becomes possible to grasp to some extent the determination of how many grams the mass m 2 should be increased.

[0067] Procedure 0) Measure the sound absorption rate under two conditions with different weights of the weights 16 provided on the suspension plate 15. Case 1: m 2 (reference) Case 2: m 2 (reference) + m 2s Here, m 2s represents the mass difference between the two conditions. The mass difference m 2s is known because it is the mass of the added weight 16. Procedure 1) For each of Case 1 and Case 2, the mass ratio α and the natural frequency ω n2Derive it. Let the mass ratio α in Case 1 be α 1 and the mass ratio α in Case 2 be α 2 . Step 2) Since the mass m 1 is common to both Case 1 and Case 2, calculate the mass m 2 from the following formula.

Equation

[0068] Figure 29 shows the parameter estimation results in the second and third embodiments under the condition that the length L1 is 80 mm. The second embodiment corresponds to Case 1, the third embodiment corresponds to Case 2, and the mass difference m 2s is 4 grams. As shown in Figure 29, the value of α increases in the third embodiment where the number of weights 16 increases. Also, since it changes from 4 equal divisions to 12 equal divisions, the spring constant k 2 decreases in the third embodiment.

[0069] Figure 30 shows the vibration transmission characteristics based on the parameter estimation results shown in Figure 29. It can be confirmed from Figure 30 that the vibration transmission characteristic dx1 has a similar tendency to the sound absorption characteristic shown in the measurement results.

[0070] Figure 31 shows the parameter estimation results in the second and third embodiments under the condition that the length L1 is 120 mm. The second embodiment corresponds to Case 1, the third embodiment corresponds to Case 2, and the mass difference m 2s is 4 grams. Similar to the explanation with reference to Figure 29, in the third embodiment, the value of the mass ratio α increases and the spring constant k 2 decreases.

[0071] Figure 32 shows the vibration transmission characteristics based on the parameter estimation results shown in Figure 31. From Figure 32, it can be confirmed that the vibration transmission characteristic dx1 is similar in tendency to the sound absorption characteristic shown in the measurement results.

[0072] Comparing Figure 29 and Figure 31, it can be confirmed that as the length L1 increases, the mass ratio α decreases. For example, in the second embodiment, when the length L1 is 80 mm, α = 0.46, and when the length L1 is 120 mm, α = 0.39. In the third embodiment, when the length L1 is 80 mm, α = 0.6, and when the length L1 is 120 mm, α = 0.54. Since such differences are considered to be due to measurement errors, it can be said that it is desirable to average α in order to further improve the parameter estimation accuracy.

[0073] The third parameter estimation method will be described. The third parameter estimation method utilizes the second parameter estimation method. Specifically, the third parameter estimation method applies the second parameter estimation method to two sound absorption devices with different lengths L1 of the hollow member 12 to obtain two mass ratios, and obtains their average as the mass ratio α.

[0074] Procedure 0) Measure the sound absorption rate under two conditions with different numbers of weights 16. Case1: m 2 (reference), L1(reference) Case2: m 2 (reference)+m 2s , L1(reference) Change the length L1 and measure the sound absorption rate under two conditions with different numbers of weights 16. Case1′: m 2 (reference), L1(changed) Case2′: m 2 (reference)+m 2s , L1(changed) Procedure 1) For each of Case1, Case2, Case1′, and Case2′, derive the mass ratio α and the natural frequency ω n2 by the first parameter estimation method. Procedure 2) Average the α of Case 1 and the α of Case 1' to obtain α 1 . Average the α of Case 2 and the α of Case 2' to obtain α 2 . Procedure 3) Based on the correspondence relationships for each case between the α and ω obtained in Procedure 1 n2 find ω n2 . Procedure 4) Since the mass m 1 is common to both Case 1 and Case 2, find the mass m 2 (reference) from the following formula. [Number] Procedure 5) From ω n1 , ω n2 , α, and m 2 find m 1 , k 1 , k 2 . Specifically, for each of Case 1 and Case 1', from ω n1 , ω n2 , α 1 , and m 2 find m 1 , k 1 , k 2 . For each of Case 2 and Case 2', from ω n1 , ω n2 , α 2 , and m 2 find m 1 , k 1 , k 2 . Note that ω n1 is obtained from the measured values.

[0075] Figure 33 shows the parameter estimation results in the second and third embodiments under the condition that the length L1 is 80 mm and the second and third embodiments under the condition that the length L1 is 120 mm. As shown in Figure 29, in the second embodiment under the condition that the length L1 is 80 mm, α = 0.46. As shown in Figure 31, in the second embodiment under the condition that the length L1 is 120 mm, α = 0.39. In the parameter estimation results shown in Figure 33, the mass ratio α is 0.425, which is the average of these values. Also, as shown in Figure 29, in the third embodiment under the condition that the length L1 is 80 mm, α = 0.6. As shown in Figure 31, in the third embodiment under the condition that the length L1 is 120 mm, α = 0.54. In the parameter estimation results shown in Figure 33, the mass ratio α is 0.57, which is the average of these values. Thus, in the third parameter estimation method, the mass ratio α is the same regardless of the length L1.

[0076] Figure 34 shows the vibration transmission characteristics based on the parameter estimation results in the second and third embodiments under the condition that the length L1 shown in Figure 33 is 80 mm, and Figure 35 shows the vibration transmission characteristics based on the parameter estimation results in the second and third embodiments under the condition that the length L1 shown in Figure 33 is 120 mm. As shown in Figures 34 and 35, since the mass ratio α is averaged, there is a slight difference between the vibration transmission characteristic dx1 and the sound absorption characteristic shown in the measurement results. However, it is possible to sufficiently grasp the tendency of the sound absorption characteristic from the vibration transmission characteristics dx1 shown in Figures 34 and 35.

[0077] So far, the first parameter estimation method, the second parameter estimation method, and the third parameter estimation method have been described. The first parameter estimation method can be estimated from one measurement data. The mass m 1 is normalized, so a specific mass change cannot be obtained, but a guideline for the increase or decrease direction can be established. The second parameter estimation method can be estimated from two measurement data with different masses m 2 Specific mass changes and the like can be obtained. However, the estimation results include the influence of measurement errors. The third parameter estimation method is for the mass m2 It can be estimated from four measurement data with different lengths L1. Specific mass changes and the like can be obtained. The influence of measurement errors can be suppressed by using averaging.

[0078] The design objectives of the sound absorption device 10 are as follows. · Reduce the noise at the fundamental frequency and its double frequency. · Achieve a high sound absorption rate (e.g., 0.6) for the above two frequencies. · Compact size

[0079] Therefore, it is necessary to minimize the length L1, maintain the relationship between the two frequencies, and perform parameter tuning to have high vibration transmission characteristics at the two frequencies.

[0080] Based on this, an example of parameter tuning using the third parameter estimation method will be described.

[0081] Figure 36 shows the parameter estimation results when the mass m is increased by 6 g for the third embodiment with a length L1 of 80 mm, and Figure 37 shows the vibration transmission characteristics based on the parameter estimation results shown in Figure 36. As shown in Figure 37, the vibration transmission characteristics peak at the target frequencies of 60 Hz and 120 Hz. Since the length L1 is 80 mm, it has a compact size. 1 As described above, according to the above-described parameter estimation method, parameter tuning can be performed without trial and error.

[0082] As described above, according to the parameter estimation method described above, parameter tuning can be performed without trial and error.

[0083] The front panel 11 and the suspension plate 15 vibrate upon receiving sound waves. It is possible to generate electricity by utilizing the vibrations of the front panel 11 and the suspension plate 15. For example, as shown in FIG. 38, a piezoelectric film 81 as a piezoelectric element may be attached to the front panel 11, or as shown in FIG. 39, a piezoelectric film 82 as a piezoelectric element may be attached to the suspension plate 15. For example, the piezoelectric film 81 and / or the piezoelectric film 82 are connected via a conducting wire to an electric circuit including a full-wave rectifier, and the alternating current power generated by the piezoelectric film 81 and / or the piezoelectric film 82 is converted into direct current power by the electric circuit and supplied to a load such as a sensor.

[0084] The sound absorption device 10 can be used as each of a plurality of units constituting the acoustic metamaterial.

[0085] FIG. 40 schematically shows an acoustic metamaterial 20 according to an embodiment. As shown in FIG. 40, the acoustic metamaterial 20 includes a plurality of units 21 and a plate member 22. The units 21 are arranged periodically (in a matrix in this example) and fixed to the plate member 22. As each unit 21, the sound absorption device 10 shown in FIG. 1 is used.

[0086] As described above, the sound absorption device 10 according to the present embodiment includes a front panel 11, a hollow member 12, a rear panel 13, a connecting member 14, and a suspension plate 15. The front panel 11 is connected to the hollow member 12 and configured to vibrate. The rear panel 13 is connected to the hollow member 12 facing the front panel 11. The front panel 11, the rear panel 13, and the hollow member 12 form an internal space 30. The suspension plate 15 is suspended from the front panel 11 by the connecting member 14 in the internal space 30.

[0087] In the above configuration, the front panel 11 vibrates upon receiving sound waves, and the suspension plate 15 vibrates along with the vibration of the front panel 11. Thereby, it is possible to absorb sounds of two frequencies in the low frequency band. Further, since there are few components, the configuration is simple. Furthermore, a Helmholtz sound hole plate is not required. Therefore, it can be manufactured at low cost.

[0088] In one example, a bolt can be used as the connecting member 14. In this case, the weight of the connecting member 14 can be easily adjusted using a nut and a washer. Therefore, it becomes easy to adjust the frequency at which the sound absorption device 10 can absorb sound.

[0089] A plurality of weights 16 may be dispersedly arranged on the outer edge portion of the suspension plate 15, or an annular weight may be arranged. By attaching a weight to the suspension plate 15, it becomes easy to adjust the frequency at which the sound absorption device 10 can absorb sound. In one example, a bolt can be used as the weight 16. In this case, the weight of the weight 16 can be easily adjusted using a nut or the like. Therefore, it becomes even easier to adjust the frequency at which the sound absorption device 10 can absorb sound.

[0090] The sound absorption device 10 may further include a band 56 as a string member, and a fixture for connecting the front plate 11 and the rear plate 13 to the hollow member 12 using the tension of the band 56. In such a configuration, it is possible to easily disassemble the sound absorption device 10, and it is easy to adjust the parameters for adjusting the frequency at which the sound absorption device 10 can absorb sound.

[0091] A piezoelectric film may be provided on the front plate 11 and / or the suspension plate 15. In such a configuration, it becomes possible to generate electricity while absorbing sound.

[0092] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

Description of Reference Numerals

[0093] 10…Sound absorption device, 11…Front panel, 12…Hollow member, 13…Rear panel, 14…Connecting member, 15…Suspension plate, 20…Acoustic metamaterial, 21…Unit, 22…Plate member, 30…Internal space, 50…Fixing member, 51…Cylindrical member, 52…Fixing plate, 53…Guide, 54, 55…Slit, 56…Band, 57…Hook-and-loop fastener, 58…Band, 61, 62…Hook-and-loop fastener, 63, 64…Slit, 81, 82…Piezoelectric film, 141…Resin bolt, 142, 143, 144…Resin nut, 145, 146, 147, 148…Resin washer.

Claims

1. A hollow member, a first vibratable plate connected to the hollow member, a second plate facing the first plate and connected to the hollow member, a third vibratable plate disposed between the first plate and the second plate, a connecting member connecting the third plate to the first plate, comprising: the first plate, the second plate, and the hollow member form an internal space, the third plate is suspended from the first plate by the connecting member in the internal space, a sound absorption device.

2. The sound absorption device according to claim 1, wherein the connecting member includes bolts.

3. The connecting member further includes a plurality of nuts inserted through the bolts, the third plate has an opening through which the bolts are inserted at a central portion and is fixed by the plurality of nuts, the first plate has an opening through which the bolts are inserted at a central portion and is fixed by the plurality of nuts, the sound absorption device according to claim 2.

4. The sound absorption device according to claim 3, wherein the connecting member further includes a plurality of washers inserted through the bolts.

5. The plurality of washers include a plurality of washers continuously positioned either between the first plate and the nut fixing the first plate or between the third plate and the nut fixing the third plate, the sound absorption device according to claim 4.

6. The sound absorption device according to claim 4, wherein the plurality of washers include at least two types of washers having different materials.

7. The third plate has an opening provided at a peripheral portion, the bolts pass through the opening, the sound absorption device according to claim 4.

8. The sound absorption device according to claim 1, further comprising a plurality of weights dispersedly arranged at a peripheral portion of the third plate.

9. The sound absorption device according to claim 8, wherein the plurality of weights include screws.

10. The sound absorption device according to claim 1, further comprising an annular weight provided at a peripheral portion of the third plate.

11. The sound absorption device according to claim 1, further comprising a fixing tool including a string member and connecting the first plate and the second plate to the hollow member using the tension of the string member.

12. The sound absorption device according to claim 1, further comprising a piezoelectric element provided on the first plate.

13. The sound absorption device according to claim 1, further comprising a piezoelectric element provided on the third plate.

14. A hollow member, a first vibratable plate connected to the hollow member, a second plate facing the first plate and connected to the hollow member, a third vibratable plate disposed between the first plate and the second plate, a connecting member connecting the third plate to the first plate, and a weight provided on the third plate, wherein the first plate, the second plate, and the hollow member form an internal space, and the third plate is suspended from the first plate by the connecting member in the internal space, and a method for estimating parameters in a sound absorption device, By measuring the sound absorption characteristics of the sound absorption device, determining a first natural frequency and a second natural frequency corresponding to two frequencies at which two sound absorption rate peak values are obtained; By measuring the sound absorption characteristics of the sound absorption device in a state where the third plate is not provided, determining a third natural frequency corresponding to a frequency at which a sound absorption rate peak value is obtained; Based on the first natural frequency, the second natural frequency, and the third natural frequency, determining a mass ratio that is the ratio of the mass of the weight and the equivalent mass of the third plate to the first mass representing the mass of the connecting member and the equivalent mass of the first plate; Based on the first natural frequency, the second natural frequency, the third natural frequency, and the mass ratio, determining a fourth natural frequency that is the natural frequency of a degree-of-freedom system composed of the third plate and the weight; By normalizing the first mass to 1, based on the mass ratio, the third natural frequency, and the fourth natural frequency, determining the second mass, a first spring constant representing the equivalent rigidity of the first plate and the air spring of the internal space, and a second spring constant representing the equivalent rigidity of the third plate; A method comprising the steps of.

15. A hollow member, a first vibratable plate connected to the hollow member, a second plate facing the first plate and connected to the hollow member, a third vibratable plate disposed between the first plate and the second plate, a connecting member connecting the third plate to the first plate, and a weight provided on the third plate, wherein the first plate, the second plate, and the hollow member form an internal space, and the third plate is suspended from the first plate by the connecting member in the internal space, and a method for estimating parameters in a sound absorption device, By measuring the sound absorption characteristics of the sound absorption device, determining a first natural frequency and a second natural frequency corresponding to two frequencies at which two sound absorption rate peak values are obtained; By measuring the sound absorption characteristics of the sound absorption device in a state where the third plate is not provided, determining a third natural frequency corresponding to a frequency at which a sound absorption rate peak value is obtained; Based on the first natural frequency, the second natural frequency, and the third natural frequency, determining a mass ratio that is the ratio of the mass of the weight and the equivalent mass of the third plate to the first mass representing the mass of the connecting member and the equivalent mass of the first plate; Based on the first natural frequency, the second natural frequency, the third natural frequency, and the mass ratio, determining a fourth natural frequency that is the natural frequency of the degree-of-freedom system composed of the third plate and the weight; Performing the process including the above for each of a first condition where the second mass is a first value and a second condition where the second mass is a second value greater than the first value; Determining the first value based on the difference between the first mass and the second mass, the mass ratio corresponding to the first condition, and the mass ratio corresponding to the second condition; For each of the first condition and the second condition, based on the third natural frequency, the fourth natural frequency, the mass ratio, and the first value, determining the first mass, a first spring constant representing the equivalent rigidity of the first plate and the air spring of the internal space, and a second spring constant representing the equivalent rigidity of the third plate; A method comprising the above.

16. A method for estimating parameters in a sound absorption device, comprising: a hollow member; a first plate that is vibratable and connected to the hollow member; a second plate that faces the first plate and is connected to the hollow member; a third plate that is vibratable and disposed between the first plate and the second plate; a connecting member that connects the third plate to the first plate; and a weight provided on the third plate, wherein the first plate, the second plate, and the hollow member form an internal space, and the third plate is suspended from the first plate by the connecting member in the internal space, the method including: By measuring the sound absorption characteristics of the sound absorption device, determining a first natural frequency and a second natural frequency corresponding to two frequencies at which two sound absorption rate peak values are obtained; By measuring the sound absorption characteristics of the sound absorption device in a state where the third plate is not provided, determining a third natural frequency corresponding to the frequency at which the sound absorption rate peak value is taken; Based on the first natural frequency, the second natural frequency, and the third natural frequency, determining a mass ratio that is the ratio of the mass of the weight to the mass of the first plate representing the mass of the connecting member and the equivalent mass of the third plate to the first mass; Based on the first natural frequency, the second natural frequency, the third natural frequency, and the mass ratio, determining a fourth natural frequency that is the natural frequency of the degree-of-freedom system composed of the third plate and the weight; Performing a process including: for each of a first condition where the second mass is a first value and the length of the hollow member is a second value, a second condition where the second mass is a third value greater than the first value and the length of the hollow member is the second value, a third condition where the second mass is the first value and the length of the hollow member is a fourth value greater than the second value, and a fourth condition where the second mass is the third value and the length of the hollow member is the fourth value; Calculating an average of the mass ratio corresponding to the first condition and the mass ratio corresponding to the third condition as a first average mass ratio; Calculating an average of the mass ratio corresponding to the second condition and the mass ratio corresponding to the fourth condition as a second average mass ratio; Based on the difference between the first mass and the second mass, the first average mass ratio, and the second average mass ratio, determining the first value; For each of the first condition and the third condition, based on the third natural frequency, the fourth natural frequency, the first average mass ratio, and the first value, determining the first mass, a first spring constant representing the equivalent rigidity of the first plate and the air spring of the internal space, and a second spring constant representing the equivalent rigidity of the third plate; For each of the second condition and the fourth condition, based on the third natural frequency, the fourth natural frequency, the second average mass ratio, and the first value, determining the first mass, the first spring constant, and the second spring constant; A method comprising the above.

Images