JP5493378B2 - Sound absorbing structure - Google Patents

Description

The present invention relates to a sound absorbing structure for absorbing sound.

  As the sound absorbing structure, a plate / membrane vibration comprising a housing having a bottom portion and an opening, and a plate-like or membrane-like vibrating body provided in the opening and defining an air layer in the housing. There is a type (Patent Document 1). In this type of sound absorbing structure, the vibrating body elastically vibrates due to the difference between the sound pressure applied from the outside of the vibrating body and the sound pressure on the air layer side (that is, the sound pressure difference before and after the vibrating body). Thereby, the energy of the sound wave that reaches the sound absorbing structure is consumed by the vibration of the vibrating body, and the sound is absorbed.

In this type of sound absorbing structure, a spring mass system is formed by the mass component of the vibrating body and the spring component of the air layer. Here, the density of air is ρ ₀ [kg / m ³ ], the speed of sound is c ₀ [m / s], the density of the vibrating body is ρ [kg / m ³ ], the thickness of the vibrating body is t [m], Assuming that the thickness of the air layer is L [m], the resonance frequency f [Hz] of the spring mass system is expressed by Equation 1.

In addition, in the sound absorbing structure, when a vibrating body having elasticity elastically vibrates, a bending system property due to elastic vibration is added.
The shape of the vibrator is rectangular, the length of one side is a [m], the length of the other side is b [m], the Young's modulus of the vibrator is E [N / m ² ], and the Poisson's ratio of the vibrator is σ [ −], P and q are positive integers, the resonance frequency of the plate / membrane vibration type sound absorbing structure is obtained as shown in the following formula 2. In the field of architectural acoustics, the obtained resonance frequency is used for acoustic design (for example, see Non-Patent Document 2).

上記数式２において、共振周波数ｆは、バネマス系に係る項（ρ_０ｃ_０ ^２／ρｔＬ）と屈曲系に係る項（バネマス系の項の後に直列に加えられている項）とを加算した値となっている。この数式２に示すように、吸音構造体においては、振動体のバネマス系と、弾性振動による屈曲系とが、吸音条件を決める重要な要素となっている。

In Equation 2, the resonance frequency f is a value obtained by adding a term related to the spring mass system (ρ ₀ c ₀ ² / ρtL) and a term related to the bending system (a term added in series after the term of the spring mass system). It has become. As shown in Formula 2, in the sound absorbing structure, the spring mass system of the vibrating body and the bending system due to elastic vibration are important factors that determine the sound absorbing conditions.

  Thus, the sound absorption characteristic (resonance frequency f) of the sound absorbing structure depends on the spring mass system of the vibrating body and the flexural vibrating body due to elastic vibration. Therefore, when the thickness L of the air layer formed by the vibrating body and the casing is determined, the sound absorption characteristics of the sound absorbing structure are determined, and the characteristics cannot be changed.

  On the other hand, in order to increase the sound absorption efficiency, an air layer is defined on the back surface of the fine perforated plate with the surface facing the sound field, and the incident sound from the sound field is separated into the air layer by a partition perpendicular to the fine perforated plate. There is a sound-absorbing structure that has a structure that partitions a plurality of cylindrical voids having a diameter smaller than the wavelength (Patent Document 2).

JP 2006-11412 A JP 2007-11034 A

Sho Kimura “Architectural Acoustics and Noise Prevention Project” Shokokusha Co., Ltd., February 20, 1981, p.150

It is an object of the present invention to provide a sound absorbing structure that can enhance sound absorbing efficiency in the sound absorbing structure of the plate / membrane vibration type.

In order to solve the above-described problems, the structure of the sound absorbing structure adopted by the present invention includes a housing having an opening and a plate that is provided in the opening and defines an air layer sealed in the housing. Or a film-like vibrating body, and a flow path that is provided outside the vibrating body and that has a component that is incident on the vibrating body from a substantially vertical direction to the sound that is incident on the vibrating body A plurality of flow path forming means, and a resonance frequency of a spring mass system formed by a fundamental frequency of a bending system due to elastic vibration of the vibrating body, a mass of the vibrating body, and a spring component of the air layer Satisfies the following equation .
0.05 ≦ fa / fb ≦ 0.65
Here, fa represents the fundamental frequency of the bending system, and fb represents the resonance frequency of the spring mass system.

  In the above configuration, the flow path is preferably formed such that the opening on the vibrating body side is smaller than the opening through which sound enters.

  According to the present invention, since the sound that has passed through each flow path of the flow path forming means is incident on the vibrating body from the vertical direction, compared to the case where the flow path forming means is not provided, Increase sound energy by increasing the sound coming from the vertical direction. Thereby, the sound absorption efficiency in the sound absorption structure can be increased. In particular, the sound absorbing structure according to the present invention can be expected to be highly effective when installed in a place where sound waves propagate in a plurality of directions.

It is a disassembled perspective view of the sound absorption structure by embodiment. It is the top view which looked at the sound absorption structure by an embodiment from the upper part. It is the longitudinal cross-sectional view seen from the arrow III-III direction of FIG. It is a top view which shows the sound absorption structure by a modification (1-1). It is a top view which shows the sound absorption structure by a modification (1-2). It is a top view which shows the sound absorption structure by a modification (1-3). It is a disassembled perspective view of the sound absorption structure by a modification (1-4). It is a longitudinal cross-sectional view of the sound absorption structure by a modification (1-4). It is a longitudinal cross-sectional view of the sound absorption structure by a modification (1-5). It is a longitudinal cross-sectional view of the sound absorption structure by a modification (1-6). It is a longitudinal cross-sectional view of the sound absorption structure by a modification (1-7). It is a characteristic diagram which shows the characteristic by a modification (3).

<Configuration of sound absorbing structure>
1 is an exploded perspective view of a sound absorbing structure 10 according to an embodiment of the present invention, FIG. 2 is a plan view of the sound absorbing structure 10 viewed from above, and FIG. 3 is viewed from the direction of arrows III-III in FIG. It is a longitudinal cross-sectional view. In the drawing, in order to easily illustrate the configuration of the present embodiment, the drawing is drawn with dimensions different from the actual dimensions of the sound absorbing structure 10.
As shown in the figure, the sound absorbing structure 10 includes a casing 20 that forms the base of the sound absorbing structure 10, a vibrating body 30 that covers the opening 23 of the casing 20, and the casing 20 and the vibrating body 30. And an acoustic filter 50 provided on the outside of the vibrating body 30 (opposite to the surface on which the housing 20 is provided) and serving as a flow path forming means. .

  The housing 20 is formed of a synthetic resin (for example, ABS resin) into a rectangular and shallow bottomed cylindrical shape, and includes a bottom plate 21, a side plate 22, and an opening 23. The bottom plate 21 is disposed on the surface facing the opening 23, and the side plate 22 is disposed around the opening 23. The vibrating body 30 is formed in a square plate shape from an elastic polymer compound (for example, an olefin copolymer containing an inorganic filler), and the periphery thereof is bonded and fixed to the opening 23 of the housing 20. Inside the sound absorbing structure 10 (behind the vibrating body 30), the vibrating body 30 is fixed to the opening 23 of the housing 20, whereby a sealed air layer 40 is defined.

  In the present embodiment, the material of the vibrating body 30 is a synthetic resin. However, the material of the vibrating body 30 is not limited to a synthetic resin, and other materials such as paper, metal, and fiberboard can be used as long as they generate elastic vibration. It may be. Further, the shape of the vibrating body 30 is not limited to a plate shape, and may be a film shape. In short, the vibrating body 30 may be any shape or member that deforms when a force is applied and vibrates by generating a restoring force by elasticity or tension.

  Here, the plate shape is a shape having a relatively thin thickness and a two-dimensional extension with respect to a rectangular parallelepiped (solid), and the film shape (film shape, sheet shape) is more than the plate shape. It is relatively thin and generates a restoring force by tension.

Further, the vibrating body 30 has relatively low rigidity with respect to the casing 20 other than the vibrating body 30 (low Young's modulus, thin thickness, small cross-sectional second moment), or mechanical impedance (8 × (bending rigidity × surface density) ^1/2 ) is formed with a relatively low shape / member. That is, the vibration body 30 makes the sound absorption structure 10 exhibit a sound absorption action by the vibration body 30 by making the vibration body 30 easily cause elastic vibration.

The acoustic filter 50 is disposed outside the vibrating body 30 and has a rectangular frame 51 having an outer shape, and a plurality of sheets that are arranged in a lattice shape in the frame 51 and form a plurality of flow paths 53. Plate bodies 52, 52,... The extending direction of each flow channel 53 is substantially equal to the direction perpendicular to the vibrating body 30. Each plate 52 is shorter than the height of the frame 51, and when the frame 51 of the acoustic filter 50 is provided around the vibrating body 30, a gap 54 ( 3) is formed. The gap 54 ensures a region where the vibrating body 30 vibrates so that the vibrating body 30 and each plate 52 do not come into contact with each other.
In addition, the acoustic filter 50 gives the sound that has passed through each flow path 53 a component in a substantially vertical direction. Thereby, the sound is incident on the vibrating body 30 from a substantially vertical direction.

<Operation of sound absorbing structure>
In the sound absorbing structure 10 configured as described above, the vibrating body 30 is caused by the difference between the sound pressure applied from the outside of the vibrating body 30 and the sound pressure on the air layer 40 side (that is, the sound pressure difference before and after the vibrating body 30). Elastically vibrates. As a result, the energy of the sound wave that reaches the sound absorbing structure 10 is consumed by the vibration of the vibrating body 30 and the sound is absorbed. At this time, the vibrating body 30 absorbs a frequency centered on the resonance frequency f set as shown in the formula 2.

<Operation and Effect of Sound Absorbing Structure in Embodiment>
In the sound absorbing structure 10 according to the present embodiment, the sound that has passed through each flow path 53 of the acoustic filter 50 has a component that is substantially perpendicular to the vibrating body 30. Incident light is incident from the vertical direction. Thereby, the propagation direction of the sound incident from the oblique direction including the vertical direction with respect to the vibrating body 30 is changed to a substantially vertical direction with respect to the vibrating body 30 by the acoustic filter 50, thereby Energy is transmitted efficiently. As a result, since the sound transmitted efficiently is absorbed by the vibration of the vibrating body 30, the sound absorbing structure 10 having the acoustic filter 50 does not have the acoustic filter 50 even in the same sound field. The sound absorption efficiency can be increased compared to the body.

  In particular, the sound absorbing structure 10 according to the present embodiment causes the sound that has passed through each flow path 53 of the acoustic filter 50 to the vibrating body 30 even when the sound absorbing structure 10 is installed in a place where sound waves propagate in a plurality of directions. Since a component in a substantially vertical direction is given, a high effect can be expected.

<Modification>
Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and can be implemented in various other forms. For example, the present invention may be implemented by modifying the above-described embodiment as follows.
<Modification 1>
In the embodiment, the case where the plate body 52 of the acoustic filter 50 is formed in a lattice shape is illustrated, but the present invention is not limited to this, and various acoustic filters (flow path forming means) can be used.

<Modification 1-1>
An acoustic filter 50A shown in FIG. 4 is configured such that a plate body 52A is formed in a honeycomb shape with respect to a rectangular frame body 51A, and a hexagonal channel 53A is formed between the plate bodies 52A.

<Modification 1-2>
An acoustic filter 50B shown in FIG. 5 is configured such that cylindrical bodies 52B having different diameters are arranged concentrically with respect to a rectangular frame body 51B, and an annular flow path 53B is formed between the cylindrical bodies 52B. is there.

<Modification 1-3>
An acoustic filter 50C shown in FIG. 6 is configured such that a plate body 52C is arranged in parallel with a rectangular frame body 51C, and a slit-like flow path 53C is formed between the plate bodies 52C.

<Modification 1-4>
As shown in FIGS. 7 and 8, the sound absorbing structure 10D described in this modification includes a bottomed cylindrical casing 20D that forms the base of the sound absorbing structure 10D, and an opening 23D of the casing 20D. A disc-shaped vibrating body 30D to which the lid is applied, an air layer 40D defined in the casing 20D by the casing 20D and the vibrating body 30D, and provided outside the vibrating body 30D to serve as a flow path forming means. And an acoustic filter 50D.
The acoustic filter 50D has a cylindrical body 52D in which a plurality of cylindrical bodies 52D having the same shape and different diameters are concentrically arranged with respect to a truncated cone-shaped frame body 51D obtained by cutting a cone head. Annular channels 53D, 53D... Are formed between them. As shown in FIG. 8, beams 55D and 55D for fixing each cylinder 52D are installed in both openings of the frame 51D. Further, a gap 54D for securing a region where the vibrating body 30D vibrates is formed between each cylindrical body 52D and the vibrating body 30D.

  The acoustic filter 50D of the sound absorbing structure 10D configured as described above has a cylindrical body 52D whose diameter is gradually reduced toward the vibrating body 30D, and is closer to the vibrating body 30D than the opening on the sound incident side. These openings form a channel 53D with a smaller area. Thereby, the collected sound can be given to the vibrating body 30D.

<Modification 1-5>
The sound absorbing structure 10E shown in FIG. 9 has a configuration in which the frame 51D of the acoustic filter 50E is formed as a part of the housing 20E, thereby ensuring a larger capacity of the air layer 40E than that of the modified example 1-4. ing. Note that the same constituent elements as those in Modification 1-4 are given the suffix “E”, and the description thereof is omitted. In the sound absorbing structure 10E, the opening 23E of the housing 20E to which the vibrating body 30E is attached is a reduced diameter portion of the frame 51E.
In the sound absorbing structure 10E configured as described above, the air layer 40E has the same size as the maximum size (diameter dimension of the frame 51D) of the sound absorbing structure 10D described in Modification 1-4. A large capacity can be secured. In the sound absorbing structure 10E, the amplitude of the vibrating body 30E is increased by reducing the coefficient of the air spring formed by the air layer 40E, and the sound absorbing efficiency can be increased.

<Modification 1-6>
The sound absorbing structure 10F shown in FIG. 10 forms the frame 51F of the acoustic filter 50F as a part of the housing 20F, and the vibration body 30F is provided in the housing 20F, so that the capacity of the air layer 40F is increased. Compared with the modified example 1-4, the configuration is ensured. Note that the same constituent elements as those in Modification 1-4 are given the suffix “F” and description thereof is omitted.
In the sound absorbing structure 10F, when the diameter of the opening 23F of the housing 20F to which the vibrating body 30F is attached is L1, and the diameter of the reduced diameter portion of the frame 51F is L2, L1> L2. And the acoustic filter 50F injects a sound toward the center part of the vibrating body 30F which became the diameter dimension L2.

  In the sound absorbing structure 10F configured as described above, the air layer 40F has the same size as the maximum size of the sound absorbing structure 10D described in Modification 1-4 (the diameter of the frame 51D). A large capacity can be secured. The sound absorbing structure 10F can increase the vibration of the vibrating body 30F and increase the sound absorption efficiency by lowering the coefficient of the air spring formed by the air layer 40F.

<Modification 1-7>
A sound absorbing structure 10G shown in FIG. 11 is filled with a porous material 56G (for example, cotton-like fibers such as foamed resin, felt, polyester wool) in each flow path 53G of the acoustic filter 50G. Note that the same constituent elements as those of the modification 1-5 are given the suffix “G”, and the description thereof is omitted.
In the sound absorbing structure 10G configured as described above, the sound absorbing frequency band is widened by providing the acoustic filter 50G with a structure that absorbs a frequency different from the frequency absorbed by the vibration of the vibrating body 30G. Is possible.

  Note that the sound absorption structure provided to the acoustic filter is not limited to the porous material, but is formed into a tubular shape like the flow path shown in the embodiment and the modified example 1-1, and the flow path surface of the flow path is provided with resistance. By generating the path resistance, other sound absorbing structures may be incorporated in the acoustic filter, either as tube sound absorption.

  Furthermore, the porous material is used not only for the sound absorbing structure 10E described in Modification 1-5, but also for the sound absorbing structure 10 described in the embodiment and the sound absorbing structures 10A to 10D, 10F, 10G described in the modification. Of course, it may be used.

<Modification 2>
In the sound absorbing structure configured as described above, the relationship between the resonance frequency of the spring mass system and the resonance frequency of the bending system due to the elastic vibration due to the elasticity of the plate is uniquely determined by Equation 2, but actually In fact, the structure of a sound-absorbing structure that exhibits high sound-absorbing power in the low sound range has not been established.

Therefore, the inventors conducted intensive experiments, and as a result, when the value of the fundamental vibration frequency of the bending system is fa and the value of the resonance frequency of the spring mass system is fb, the above parameters are satisfied so as to satisfy the relationship of Equation 3 below. Set. As a result, the fundamental vibration of the bending system is coupled with the spring component of the air layer behind, and a vibration having a large amplitude is excited in the band between the resonance frequency of the spring mass system and the fundamental frequency of the bending system (the bending system). The fact that the resonance frequency fa <the sound absorption peak frequency f <the spring mass system fundamental frequency fb) and the sound absorption rate is high was verified.
(Equation 3)
0.05 ≦ fa / fb ≦ 0.65

Furthermore, when the following Expression 4 is set, the frequency of the sound absorption peak is sufficiently smaller than the resonance frequency of the spring mass system. In this case, it was also verified that the fundamental frequency of the bending system is sufficiently smaller than the resonance frequency of the spring mass system due to the mode of low-order elastic vibration, and is suitable as a sound absorbing structure that absorbs sound having a frequency of 300 [Hz] or less.
(Equation 4)
0.05 ≦ fa / fb ≦ 0.40
As described above, by setting various parameters so as to satisfy the conditions of the above-described Expressions 3 and 4, it is possible to configure a sound absorbing structure in which the frequency at which the sound absorption is peaked is lowered.

The various parameters are parameters for setting the resonance frequency f shown in Equation 2, and include gas density ρ _0, sound velocity c _0, vibration body density ρ, vibration body thickness t, and gas layer thickness L. , The casing length a, the casing length b, the Young's modulus E of the vibrating body, the Poisson's ratio σ, the mode orders p, q, and the like.

<Modification 3>
In the above example, the vibrating body has been described as having a uniform configuration. However, a part of the vibrating body 30 is formed so as to have a different density from the other parts, or a part thereof has a different thickness than the other parts. Alternatively, the vibrating body 30 may be formed such that a part of the vibrating body 30 has a mass different from that of the other part. By forming the vibrating body 30 in this way, it is possible to change the vibration conditions for the vibrating body 30.

  As described above, the sound absorbing structure 10 includes a sound absorbing mechanism including a spring mass system and a bending system. Here, the inventors conducted an experiment of the sound absorption coefficient at the resonance frequency when the surface density of the vibrating body 30 was changed.

  FIG. 12 shows that a vibrating body 30 (size: 100 mm × 100 mm, thickness: 0.85 mm) is fixed to a casing 20 having a vertical and horizontal size of 100 mm × 100 mm and a thickness of 10 mm. It is the figure which showed the simulation result of the normal incidence sound absorption coefficient of the sound-absorbing structure 10 at the time of changing the surface density of a part (a magnitude | size is 20 mm x 20 mm, thickness 0.85mm). In addition, the simulation method is based on JIS 1405-2 (measurement of sound absorption coefficient and impedance by an acoustic tube—part 2: transfer function method). The sound absorption characteristics were calculated from the transfer function.

Specifically, the surface density of the central part is set to (1) 399.5 [g / m ² ], (2) 799 [g / m ² ], (3) 1199 [g / m ² ], (4) 1598 [g / m ² ], (5) 2297 [g / m ² ], the surface density of the peripheral member is 799 [g / m ² ], and the average density of the vibrating body 30 is (1) 783 [g / m ² ]. m ² ], (2) 799 [g / m ² ], (3) 815 [g / m ² ], (4) 831 [g / m ² ], (5) 863 [g / m ² ] This is a simulation result.
Looking at the simulation results, the sound absorption rate is high between 300 and 500 [Hz] and in the vicinity of 700 [Hz].

  The high sound absorption rate near 700 [Hz] is due to resonance of the spring mass system formed by the mass of the vibrating body 30 and the spring component of the air layer 40. In the sound absorbing structure 10, sound is absorbed with the sound absorption coefficient at the resonance frequency of the spring mass system as a peak, and even if the surface density of the central portion is increased, the mass of the entire vibrating body 30 does not change greatly. It can be seen that the resonance frequency does not change significantly.

  Further, the sound absorption coefficient between 300 and 500 [Hz] is high due to the resonance of the bending system formed by the bending vibration of the vibrating body 30. In the sound absorbing structure 10, the sound absorption coefficient at the resonance frequency of the bending system appears as a peak on the low frequency range side, and the surface density of the central portion corresponding to the region that becomes an antinode when the vibrating body 30 undergoes bending vibration is obtained. It can be seen that only the resonance frequency of the bending system decreases as the value increases.

  In general, the resonance frequency of the bending system is determined by an equation of motion governing the elastic vibration of the vibrating body 30 and is inversely proportional to the density (surface density) of the vibrating body 30. The resonance frequency is greatly influenced by the density of the antinodes of natural vibration (when the amplitude is a maximum value). For this reason, in the simulation described above, the region that becomes the antinode of the 1 × 1 eigenmode is formed at different surface densities in the central portion, so that the resonance frequency of the bending system is changed.

  Thus, the simulation results show that when the surface density of the central part is made larger than the surface density of the peripheral part, the peak of the sound absorption coefficient on the low frequency side of the frequency that becomes the peak of sound absorption moves further to the low frequency side. Represents. Therefore, it is shown that by changing the surface density of the central portion, a part of the frequency at which the sound absorption is peaked can be moved (shifted) further to the low sound region side or the high sound region side.

  In the sound absorbing structure 10 described above, the frequency of the sound absorption sound can be changed (shifted) simply by changing the surface density at the center of the vibrating body. Compared to the case where the sound absorbing structure 10 is made of the same material and the sound absorbing structure 10 is made heavy to increase the sound absorbing sound, the sound absorbing sound can be lowered without greatly changing the sound absorbing structure 10 overall mass. .

<Modification 4>
In addition, the configuration of the sound absorbing structure 10 in the embodiment includes a rectangular housing 20, a vibrating body 30 that closes the opening 23 of the housing 20, an air layer 40 defined in the housing 20, and However, the shape of the housing according to the present invention is not limited to a rectangular shape, and may be a circular shape or a polygonal shape.

  Furthermore, the sound absorption coefficient peak value may be increased by filling the air layer 40 of the sound absorbing structure 10 with a porous sound absorbing material (for example, cotton-like fibers such as foamed resin, felt, polyester wool). .

<Modification 5>
Further, in the present invention, when the sound absorbing structure group is formed, not only the sound absorbing structure group is formed by combining a plurality of sound absorbing structures of any one of the above-described embodiments or modifications, but for example, a sound absorbing characteristic Sound absorbing structures having different sound absorbing characteristics may be combined to form a sound absorbing structure group, such as combining sound absorbing structures having different sound absorption characteristics, or combining sound absorbing structures having three or more different sound absorbing characteristics.

  Further, the sound absorbing structure according to the present invention and the sound absorbing structure group obtained by combining the sound absorbing structures can be disposed in various acoustic chambers that control acoustic characteristics. Here, the various acoustic rooms include soundproof rooms, halls, theaters, listening rooms for audio equipment, living rooms such as conference rooms, spaces for various transport equipment such as vehicles, and housings for speakers and musical instruments.

10, 10D to 10G ... sound absorbing structure, 20, 20D to 20G ... casing, 21 ... bottom plate, 22 ... side plate, 23, 23D-23G ... opening, 30, 30D ... 30G ... vibrating body, 40, 40D-40G ... air layer, 50, 50A-50G ... acoustic filter, 51, 51A-51G ... frame, 52, 52A, 52C ... plate , 52B, 52D, 52E, 52F, 52G ... cylindrical body, 53, 53A-53G ... flow path.

Claims (2)

A housing having an opening;
A plate-like or membrane-like vibrator provided in the opening and defining an air layer sealed in the housing;
A flow path forming means provided apart from the vibrating body and forming a plurality of flow paths for giving the sound incident on the vibrating body a component that is incident from a substantially vertical direction toward the vibrating body; ,
Equipped with,
A sound-absorbing structure characterized in that a fundamental frequency of a bending system due to elastic vibration of the vibrating body and a resonance frequency of a spring mass system formed by a mass of the vibrating body and a spring component of the air layer satisfy the following expression: .
0.05 ≦ fa / fb ≦ 0.65
Here, fa represents the fundamental frequency of the bending system, and fb represents the resonance frequency of the spring mass system. The sound absorbing structure according to claim 1,
In the sound absorbing structure, the flow path is formed such that an opening on the vibrating body side is smaller than an opening on a sound incident side.

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