SHIBAHARA, Norihito, (33 1 Tamagawadai 2-chome, Setagaya, Tokyo, 158-8583, JP)
| 1. A sound absorbing system comprising: a breathable surface layer; a first backing airspace adjacent to said surface layer; a perforated substrate with a through hole and adjacent to said first backing airspace; a second backing airspace communicated with said first backing airspace through said through hole of said perforated substrate. 2. The sound absorbing system according to claim 1, wherein said perforated substrate has rigidity capable of supporting said surface layer and said first backing airspace while maintaining a shape of said perforated substrate. 3. The sound absorbing system according to claim 1 or 2, wherein said surface layer has a three-dimensional shape defining said first backing airspace. 4. The sound absorbing system according to any one of claims 1-3, wherein said perforated substrate has a three-dimensional shape defining said second backing airspace. 5. The sound absorbing system according to any one of claims 1-4, further comprising an air passage provided between said perforated substrate and said second backing airspace, said air passage connecting said through hole to said second backing airspace. 6. The sound absorbing system according to any one of claims 1-5, further comprising a reflecting layer adjacent to said second backing airspace. 7. A method of making a sound absorbing system, comprising: providing a sound absorbing member including a breathable surface layer and a first backing airspace adjacent to said surface layer; providing a perforated substrate with a through hole connected to a second backing airspace in a fluidic communicating manner; and attaching said sound absorbing member to said perforated substrate, and communicating said first backing airspace to said second backing airspace through said though hole of said perforated substrate. 8. A method of making a sound absorbing system, comprising: providing a breathable surface layer; providing a first backing airspace; providing a perforated substrate with a through hole connected to a second backing airspace in a fluidic communicating manner; and attaching said first backing airspace to said perforated substrate, attaching said surface layer to said first backing airspace, and communicating said first backing airspace to said second backing airspace through said though hole of said perforated substrate. 9. The method according to claim 7 or 8, further comprising disposing a reflecting layer adjacent to said second backing airspace. |
MANUFACTURING THE SAME
Technical Field
The present invention relates to a sound absorbing system. The present invention also relates to a method for manufacturing a sound absorbing system.
Background Art
There is known that sound absorbing properties (in particular, sound absorption coefficient-frequency characteristics) of a porous sound absorbing material can be controlled by arranging an airspace behind the absorbing material. Further, there is also well known a Helmholtz sound absorbing structure in which an airspace of a
predetermined volume is arranged behind a board having through holes (i.e., a perforated board).
For example, Japanese Unexamined Patent Publication (Kokai) No. 8-30274
(Patent Document 1) discloses a sound absorbing structure including a laminated body which includes at least two layers having different air permeability and a backing airspace, wherein a fiber aggregate is used in combination with a perforated board having a
Helmholtz sound absorbing form. Patent Document 1 describes that "A total thickness of the laminated body according to the present invention has to be in a range between 0.5-20 mm. When the total thickness of the laminated board is less than 0.5 mm, the laminated board as a whole appears to be a single board and the sound absorbing properties are hardly exhibited. Conversely, when the total thickness of the laminated board exceeds 20 mm, the Helmholtz sound absorbing mechanism turns into a porous sound absorbing mechanism and, as result, it becomes difficult to ensure the intended sound absorbing properties in a low frequency region" and "an average thickness of the backing airspace has to be in a range between 0-40 mm. This range results from a constraint on layout of car components. When the thickness of the backing airspace exceeds 40 mm, it becomes difficult to ensure the space for disposing the car components on the car. The thicker backing airspace has a larger effect. But, even when the backing airspace has a thickness of 0 mm, sound absorbing properties can be achieved to some extent because of the thickness of the laminated body itself." Further, Japanese Unexamined Patent Publication (Kokai) No. 2002-127836 (Patent Document 2) discloses a car interior trim component having sound absorbing properties including a substrate that is attached to an interior side of a car body panel via a backing airspace, a skin that is disposed on a surface of the component, and a low density nonwoven fabric that is arranged between the substrate and the skin and that acts as a spring for a panel vibration type sound absorbing function of the skin. Patent Document 2 describes that "Because a backing airspace 30 is provided between a roof panel 20 and a substrate 11, this trim component has sound absorbing properties that can reduce a sound level, in particular, in a low frequency region due to panel vibration (membrane vibration) of substrate 11" and "In addition to the sound absorbing properties inherent to the substrate, the sound absorbing effect due to the panel vibration of the skin and a porous sound absorbing effect by the low density nonwoven fabric are synergistically exhibited, so that the sound level can be effectively reduced in a wide frequency range from a low frequency region to a high frequency region". Further, Patent Document 2 describes that materials for the substrate are "materials with appropriate shape retention and without breathability, such as a PPO (polyphenylene oxide) resin sheet and corrugated cardboard" as well as "materials with sound absorbing properties such as urethane and resin felt."
Summary of the Invention
When a sound absorbing structure including a breathable material such as a sound absorbing material and an airspace arranged behind the breathable material is installed in a vehicle or building, it may be difficult to provide the airspace of a size necessary to achieve intended sound absorbing properties due to a dimensional constraint on an installation space.
The present invention provides a sound absorbing system including a breathable material and an airspace arranged behind the breathable material, which can ensure that the airspace has a size necessary to achieve intended sound absorbing properties even when there is a dimensional constraint on a system installation space.
The present invention also provides a method for manufacturing a sound absorbing system including a breathable material and an airspace arranged behind the breathable material, which can ensure that the airspace has a size necessary to achieve intended sound absorption coefficient-frequency characteristics even when there is a dimensional constraint on a system installation space.
In one aspect, there is provided a sound absorbing system comprising a breathable surface layer; a first backing airspace adjacent to the surface layer; a perforated substrate with a through hole and adjacent to the first backing airspace; and a second backing airspace communicated with the first backing airspace via the through hole of the perforated substrate.
In another aspect, there is provided a method for manufacturing a sound absorbing system, comprising providing a sound absorbing member including a breathable surface layer and a first backing airspace adjacent to the surface layer; providing a perforated substrate with a through hole connected to a second backing airspace in a fluidic communicating manner; and attaching the sound absorbing member to the perforated substrate, and communicating the first backing airspace to the second backing airspace via the through hole of the perforated substrate.
In yet another aspect, there is provided a method for manufacturing a sound absorbing system, comprising providing a breathable surface layer; providing a first backing airspace; providing a perforated substrate with a through hole connected to a second backing airspace in a fluidic communicating manner; and attaching the first backing airspace to the perforated substrate, attaching the surface layer to the first backing airspace, and communicating the first backing airspace to the second backing airspace via the through hole of the perforated substrate.
In the sound absorbing system according to one aspect of the present invention, even when the first backing airspace does not have a size necessary to achieve intended sound absorbing properties, the first backing airspace is communicated with the second backing airspace via the through hole of the perforated substrate and, due to the existence of the second backing airspace, intended sound absorbing properties can be achieved without increasing the size of the first backing airspace. Consequently, even when the sound absorbing system is installed in a vehicle or building that has a dimensional constraint on a system installation space, the sound absorbing system can be installed while the airspaces (first and second airspaces) of the size necessary to achieve the intended sound absorbing properties within the dimensional constraint. As a result, the acoustic characteristics of the system installation space can be improved.
In the method for manufacturing the sound absorbing system according to another aspect of the present invention, when a sound absorbing system is installed in a vehicle or building, among structural parts originally provided in the vehicle or building, the structural part that includes the perforated substrate having the through hole and the second backing airspace that is connected to the through hole in a fluidic communicating manner can be employed and the sound absorbing member (or, in other words, surface layer and the first backing airspace) that is provided separately can be attached to the structural part to install the sound absorbing system. At this time, even when the first backing airspace does not have a size necessary to achieve intended sound absorption properties, the size may be ensured by adding the second backing airspace. In this case, because the first backing airspace communicates with the second backing airspace via the through hole of the perforated substrate, the intended sound absorbing properties (in particular, the sound absorption coefficient-frequency characteristics) can be achieved due to the existence of the second backing airspace that is originally provided in the vehicle or building, without increasing the size of the first backing airspace.
Brief Description of Drawings
[Fig. 1] A cross-sectional view schematically illustrating a sound absorbing system according to an embodiment of the present invention.
[Fig. 2] A perspective view schematically illustrating the sound absorbing system of Fig. 1.
[Figs. 3(a) - 3(f)] Diagrams illustrating a sound absorbing system manufacturing method according to an embodiment of the present invention for manufacturing the sound absorbing system of Fig. 1.
[Fig. 4] A cross-sectional view schematically illustrating a sound absorbing system according to a modification.
[Fig. 5] A cross-sectional view schematically illustrating a sound absorbing system according to another modification.
[Fig. 6] A cross-sectional view schematically illustrating a sound absorbing system according to yet another modification.
[Figs. 7(a) and 7(b)] Perspective views each illustrating an appearance of a surface layer possessed by the sound absorbing system of Fig. 6. Modes for Carrying Out the Invention
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the several views, like elements are designated by like reference numerals.
In a sound absorbing system including a breathable material and an airspace arranged behind the breathable material, it is desired to ensure that the airspace has a dimension necessary to achieve intended sound-absorbing properties, even when there is a dimensional constraint on a system installation space.
Figs. 1 and 2 illustrate a sound absorbing system 10 according to an embodiment of the present invention. Sound absorbing system 10 can be installed in an interior or exterior of a vehicle or building, although the use of the sound absorbing system of the present invention is not limited to these applications.
Sound absorbing system 10 includes a breathable surface layer 12, a first backing airspace 14 adjacent to surface layer 12, a perforated substrate 16 adjacent to first backing airspace 14, and a second backing airspace 18 communicated with first backing airspace 14 through perforated substrate 16.
Surface layer 12 is an outermost layer of sound absorbing system 10. This layer propagates a sound to be absorbed between an external environment of sound absorbing system 10 and first backing airspace 14. Surface layer 12 is formed of inherently breathable materials such as fabrics, breathable resin films and open cell foams. Surface layer 12 may be formed of a porous material that singly has the sound absorbing properties or a combination of a plurality of different materials. In the illustrated embodiment, surface layer 12 is formed as a sheet- like member having a predetermined thickness and it has an outer surface 12a on which the sound to be absorbed is incident and an inner surface 12b at the side opposite thereto. The material, basis weight, shape, dimensions and the like of surface layer 12 are not particularly limited. But, when sound absorbing system 10 is installed in the interior or exterior of the vehicle or building, it is preferable that at least outer surface 12a of surface layer 12 has intended design properties or texture. From this viewpoint, surface layer 12 may have a porous surface material, for example, described in the International Publication No. WO2009/017908 (for example, a polyethylene nonwoven fabric) at least as outer surface 12a. First backing airspace 14 is arranged behind surface layer 12 and cooperates with surface layer 12 to exhibit intended sound absorbing properties (in particular, sound absorption coefficient-frequency characteristics). First backing airspace 14 is mainly comprised of air. First backing airspace 14 may act as a support structure that lies between surface layer 12 and perforated substrate 16 for supporting surface layer 12. In this case, first backing airspace 14 may be comprised of various materials such as a sponge or other open-cell foam, a honeycomb structure and the like that can support surface layer 12 while substantially uniformly containing air. In the illustrated
embodiment, first backing airspace 14 is formed as an open cell foam of a predetermined thickness that has a first surface 14a that abuts against surface 12b of surface layer 12 and a second layer 14b at the side opposite thereto. In this configuration, inner surface 12b of surface layer 12 can be secured to first surface 14a of the first backing airspace by a publicly known fastening technique such as, for example, adhesion using an adhesive, gluing agent and the like, fusion using ultrasonic wave, induction heating and the like, or thermocompression bonding. Further, surface layer 12 and first backing airspace 14 may be formed integrally in advance. The shape, dimensions and the like of first backing airspace 14 are not particularly limited.
Perforated substrate 16 is arranged behind first backing airspace 14 and lies between first backing airspace 14 and second backing airspace 18. Perforated substrate 16 is formed of various materials such as metal, plastic, wood and paper. In perforated substrate 16, at least one through hole 20 is formed to interconnect first backing airspace 14 and second backing airspace 18 in a fluidic communicating manner. Perforated substrate 16 may have such rigidity that can support surface layer 12 and first backing airspace 14 while maintaining at least its own shape. In the illustrated embodiment, perforated substrate 16 is formed as a rigid board of a predetermined thickness that has a first surface 16a that abuts against second surface 14b of first backing airspace 14 and a second surface 16b at the side opposite thereto. In this configuration, second surface 14b of first backing airspace 14 can be secured to first surface 16a of perforated substrate 16 by a publicly known fastening technique such as, for example, adhesion using an adhesive, gluing agent and the like, fusion using ultrasonic wave, induction heating and the like, or thermocompression bonding. Further, in the illustrated embodiment, perforated substrate 16 has a three-dimensional shape defining second backing airspace 18 of an intended volume and includes a tabular base 22 in which through holes 20 are provided, and one or more columnar or wall-like extensions 24 that are integrally or separately coupled with base 22 at the side of second surface 16b. The material, shape, dimensions and the like of perforated substrate 16 and the shape, dimensions, arrangement, opening ratio and the like of through holes 20 are not particularly limited.
Second backing airspace 18 is provided behind perforated substrate 16 and communicates with first backing airspace 14 so that it cooperates with surface layer 12 and first backing airspace 14 to exhibit intended sound absorbing properties (in particular, sound absorption coefficient- frequency characteristics). Second backing airspace 18 is mainly comprised of air. Second backing airspace 18 may act as a support structure for supporting perforated substrate 16. In this case, second backing airspace 18 may be comprised of various materials such as a resin foam or other open cell foam, a honeycomb structure and the like that can support perforated substrate 16 while substantially uniformly containing air. In the illustrated embodiment, second backing airspace 18 is comprised of air accommodated in a chamber having a thickness and a volume that are determined by dimensions of base 24 and extensions 24 of perforated substrate 16. In this configuration, the chamber that forms second backing airspace 18 may be a closed space or it may be opened to the extent that it does not significantly affect the sound absorbing properties. The shape, dimensions and the like of second backing airspace 18 are not particularly limited.
Sound absorbing system 10 further includes a reflecting layer adjacent to second backing airspace 18. Reflecting layer 26 is provided at a position substantially opposed to through holes 20 of perforated substrate 16 to reflect the sound incident on second backing airspace 18 toward through holes 20. Reflecting layer 26 is formed of various materials that can reflect the sound such as metal, plastic, wood and paper. Reflecting layer 26 cooperates with perforated substrate 16 to define second backing airspace of an intended volume. In the illustrated embodiment, reflecting layer 26 is formed as a rigid board of a predetermined thickness that has a first surface (reflecting surface) 26a that abuts against respective end surfaces 24a of extensions 24 of perforated substrate 16 and a second surface 26b at the side opposite thereto. In this configuration, respective end surfaces 24a of extensions 24 of perforated substrate 16 can be secured to first surface 26a of reflecting layer 26 by a publicly known fastening technique such as, for example, adhesion using an adhesive, gluing agent and the like, fusion using ultrasonic wave, induction heating and the like, or thermocompression bonding. Alternatively, perforated substrate 16 and reflecting layer 26 may be integrally formed in advance. The material, shape, dimensions and the like of reflecting layer 26 are not particularly limited. Further, when the sound absorbing function of a Helmholtz type (resonance type) is not needed, reflecting layer 26 may be omitted.
Sound absorbing system 10 configured as described above can be manufactured by a sound absorbing system manufacturing method according to an embodiment of the present invention illustrated in Fig. 3. In this method, first, a sound absorbing member 28 including breathable surface layer 12 and first backing airspace 14 adjacent to surface layer 12 is provided (Fig. 3(a)). On the other hand, perforated substrate 16 with through holes 20 connected to second backing airspace 18 in a fluidic communicating manner is provided (Fig. 3(b)). Next, sound absorbing member 28 is attached to perforated substrate 16 so that second surface 14b of first backing airspace 14 faces first surface 16a of perforated substrate 16 and second surface 14b of first backing airspace 14 is secured to first surface 16a of perforated substrate 16 by adhesion, fusion and the like (Fig. 3(c)). As a result, first backing airspace 14 communicates with second backing airspace 18 via through holes 20 of perforated substrate 16.
Further, reflecting layer 26 is arranged adjacent to second backing airspace 18 and respective end surfaces 24a of extensions 24 of perforated substrate 16 is secured to first surface 26a of reflecting layer 26 by adhesion, fusion and the like (Fig. 3(d)). This operation can be performed before or at the same time or after attaching sound absorbing member 28 to perforated substrate 16. As described above, sound absorbing system 10 is manufactured.
In the sound absorbing system manufacturing method described above, when first backing airspace 14 includes an element other than air that acts as a support structure (for example, an open cell foam), instead of providing sound absorbing member 28, surface layer 12 and first backing airspace 14 can be provided separately so that first backing airspace 14 can be attached to perforated substrate 16 and surface layer 12 can be attached to first backing airspace 14. Further, instead of attaching reflecting layer 26 to perforated substrate 16 in the subsequent operation, perforated substrate 16 can be integrally provided with reflecting layer 26 in advance (Fig. 3(e)). Further, when perforated substrate 16 is provided, new through holes 20 can be formed in addition to through holes 20 provided in advance in the subsequent operation. Or, perforated substrate 16 can be provided by first providing a non-perforated substrate 16' that does not have through holes 20 (Fig. 3(f)) and, then forming intended through holes 20 in the subsequent operation.
In sound absorbing system 10 configured as described above and its manufacturing method, when sound absorbing system 10 is installed in the vehicle or building, among structural parts originally provided in the vehicle or building, the structural part that includes perforated substrate 16 having through holes 20 and second backing airspace 18 that is connected to through holes 20 in a fluidic communicating manner can be employed and sound absorbing member 28 (or, in other words, surface layer 12 and first backing airspace 14) that is provided separately can be attached to such structural part to install sound absorbing system 10. At this time, even when first backing airspace 14 does not have a size (thickness or volume) necessary to achieve intended sound absorbing properties (in particular, sound absorption coefficient- frequency characteristics), the size (thickness or volume) may be ensured by adding second backing airspace 18. In this case, because first backing airspace 14 communicates with second backing airspace 18 via through holes 20 of perforated substrate 16, the intended sound absorbing properties (in particular, the sound absorption coefficient-frequency characteristics) can be achieved due to the existence of second backing airspace 18 that is originally provided in the vehicle or building, without increasing the size of first backing airspace 14 (and, thus, sound absorbing member 28). Here, generally speaking, as the size of second backing airspace 18 increases, a peak sound absorbing rate in the sound absorption coefficient-frequency characteristics of sound absorbing system 10 moves to a lower frequency region.
Thus, in sound absorbing system 10 and its manufacturing method, even when there is a dimensional constraint on the system installation space, the airspace of the size necessary to achieve the intended sound absorbing properties can be ensured within the dimensional constraint. Consequently, even in the place where the conventional sound absorbing structure including the breathable material and the airspace arranged behind the breathable material cannot be disposed because it is difficult to extend the airspace due to the dimensional constraint on the installation space, sound absorbing system 10 can be disposed so that the intended sound absorbing properties can be exhibited and, as a result, acoustic characteristics of a surrounding environment of the installation place can be improved.
Sound absorbing system 10 can preferably be installed, for example, in the interior of a car (a roof, instrument panel, pillar panel, door trim, console box, and the like). In this case, any tabular part of an interior component can be employed as perforated substrate 16. Then, a space behind the tabular part can be employed as second backing airspace 18 and reinforcement ribs originally provided in the tabular part can be employed as columnar or wall- like extensions 24 of perforated substrate 16. Further, a member (a portion of an interior component or car body panel) opposed to the tabular part via a space can be employed as reflecting layer 26. If the tabular part in front of the space that can be employed as second backing airspace 18 does not have through holes 20 or the opening ratio of through holes 20 is insufficient, through holes 20 of an intended configuration (shape, dimensions, arrangement, opening ratio and the like) can be formed before attaching sound absorbing member 28 (or, in other words, surface layer 12 and first backing airspace 14) to the tabular part.
In sound absorbing system 10, on the condition that insufficiency of the size
(thickness or volume) of first backing airspace 14 necessary to achieve the intended sound absorbing properties (in particular, sound absorption coefficient- frequency characteristics) can be compensated, second backing airspace 18 may not be arranged adjacent to through holes of perforated substrate 16. For example, as illustrated in Fig. 4 as a modification (sound absorbing system 101), second backing airspace 18 that is defined between perforated substrate 16 and reflecting layer 26 can be arranged at a position deviated from through holes 20 of perforated substrate 16. In sound absorbing system 101, an air passage 30 that interconnects through holes 20 and second backing airspace 18 in a fluidic communicating manner is provided between perforated substrate 16 and second backing airspace 18 so that air can evenly spread in all through holes 20 of perforated substrate 16.
Further, as illustrated in Fig. 5 as another modification (sound absorbing system 102), second backing airspace 18 can be arranged at a position apart from perforated substrate 16. In sound absorbing system 102, at a position apart from a chamber that is formed of perforated substrate 16 and reflecting layer 26, a casing 32 that forms a second chamber for accommodating second backing airspace 18 is provided and connected to reflecting layer 26 via a connecting duct 34. Then, an air hole 36 formed in reflecting layer 26 and an air hole 38 formed in casing 32 are interconnected by a line 40 of duct 34 to constitute an air passage, and this air passage is connected to air passage 30 between perforated substrate 16 and second backing airspace 18. In either of the configurations of Fig. 4 or Fig. 5, second backing airspace 18 effectively functions and exhibits an effect similar to that of sound absorbing system 10 of Fig. 1.
In sound absorbing system 10, surface layer 12 can have a three-dimensional shape to define first backing airspace 14 having an intended shape and dimensions between surface layer 12 and perforated substrate 16. For example, as illustrated in Fig. 6 as yet another modification (sound absorbing system 103), surface layer 12 can have a bowl-like shape having a top part 42 and a side part 44. In sound absorbing system 103, air is accommodated as first backing airspace 14 in a chamber that is formed of surface layer 12 and perforated substrate 16. In this configuration, for example, as illustrated in Figs. 7(a) and 7(b), according to required design properties and the like, surface layer 12 can be provided with one (Fig. 7(a)) or a plurality of (Fig. 7(b)) projections 46 each having top part 42 and side part 44. Thus, in sound absorbing system 10, due to the existence of second backing airspace 18, first backing airspace 14 can be relatively freely designed and, as a result, the requirement of the design properties and the like can be easily met.
In order to clarify effects of sound absorbing system 10 described above, following experiments about the sound absorbing properties were performed.
Sound absorbing systems 10 and 102 illustrated in Figs. 1 and 5, respectively, were manufactured as follows:
Surface layer 12 ... (A) Polyurethane nonwoven fabric (surface density: 400 g/m 2 , thickness: 0.98 mm). First backing airspace 14 ... (B) Open cell polyurethane foam (surface density: 68 g/m 2 , thickness: 5 mm, FDW manufactured by Bridgestone Kaseihin Tokyo K (Tokyo)); (C) Air (thickness: 5 mm) Perforated substrate 16 ... (D) Punching Teflon board (thickness: 1 mm, opening ratio was changed); (E) Perforated brass plate
(thickness: 1 mm, opening ratio: 20.3 %) Second backing airspace 18 ... (F) Air (thickness was changed) Connecting duct 34 ... (G) Copper tube (thickness: 0.6 mm, length: 6 mm, inner diameter: 9 mm).
While using the above materials, the material of first backing airspace 14, the materials and opening ratio of perforated substrate 16 and the thickness of second backing airspace 18 were changed to prepare embodiments and comparative examples of sound absorbing systems 10 and 102 as follows: Table 1
As the sound absorbing properties (sound absorption coefficient-frequency characteristics) of the embodiments and the comparative examples described above, normal incidence sound absorption coefficients over 250 (400)-5000 Hz (1/3 octave average) were measured by using a normal incidence sound absorption coefficient measurement system (an impedance tube WinZacMTX manufactured by Nittobo Acoustic Engineering Co., Ltd. (Tokyo) according to ISO 10534-2 and JIS 1405-2. The measurement results are as follows. Embodiments 1 , 2 and Comparative Examples 1 , 2:
In Embodiments 1 and 2, as the opening ratio of perforated substrate 16 increased, the sound absorbing properties were improved. It demonstrated that resonant absorbance of second backing airspace 18 was exhibited in addition to that of surface layer 12 and first backing airspace 14. When the opening ratio of perforated substrate 16 was about 10 % or more, the sound absorbing properties comparable to that of Comparative Examples 1 and 2 (the opening ratio was 100 %) could be achieved. Further, by increasing the thickness of second backing airspace 18, the sound absorbing properties in a low sound frequency range were improved. On the other hand, in Comparative Examples 1 and 2 (the opening ratio was 0 %), in addition to resonant absorbance of surface layer 12 and first backing airspace 14, resonant absorbance due to collective membrane vibration of surface layer 12, first backing airspace 14 and perforated substrate 16 was exhibited, though its sound absorbing effect was very low.
Embodiments 3, 4 and Comparative Examples 3, 4:
In Embodiments 3 and 4, the measurement results similar to those of Embodiments
1 and 2 could be achieved. It demonstrated that, even when second backing airspace 18 is arranged at a position apart from perforated substrate 16, the effect of second backing airspace 18 could be achieved if the appropriate connection was established by connecting duct 34. In Embodiments 3 and 4, perforated substrate 16 had a larger weight than perforated substrate 16 of Embodiments 1 and 2. It was therefore conjectured that the opening ratio of perforated substrate 16 had to be further increased to achieve the sound absorbing properties comparable to those of Embodiments 1 and 2. In Comparative Examples 3 and 4(without second backing airspace), only the resonant absorbance of mainly surface layer 12 and first backing airspace 14 was exhibited.
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