U.S. patent application number 12/537929 was filed with the patent office on 2010-02-25 for sound absorbing structure using closed-cell porous medium.
Invention is credited to Rento TANASE.
Application Number | 20100044148 12/537929 |
Document ID | / |
Family ID | 41395109 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100044148 |
Kind Code |
A1 |
TANASE; Rento |
February 25, 2010 |
SOUND ABSORBING STRUCTURE USING CLOSED-CELL POROUS MEDIUM
Abstract
A sound absorbing structure is constituted of a housing, a
vibration member composed of a closed-cell porous material whose
airflow rate is less than 0.1 dm.sup.3/s, and an air cavity formed
inside the housing in the rear side of the vibration member.
Alternatively, the vibration member is formed by laminating an
open-cell porous material or an air-permeable member with the
closed-cell porous material. The sound absorbing structure
demonstrates a high sound absorption in a low frequency range due
to the closed-cell porous material.
Inventors: |
TANASE; Rento; (Iwata-shi,
JP) |
Correspondence
Address: |
SMITH PATENT OFFICE
1901 PENNSYLVANIA AVENUE N W, SUITE 901
WASHINGTON
DC
20006
US
|
Family ID: |
41395109 |
Appl. No.: |
12/537929 |
Filed: |
August 7, 2009 |
Current U.S.
Class: |
181/198 ;
181/284; 181/290 |
Current CPC
Class: |
G10K 11/172
20130101 |
Class at
Publication: |
181/198 ;
181/284; 181/290 |
International
Class: |
E04B 1/82 20060101
E04B001/82 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2008 |
JP |
2008-211972 |
Claims
1. A sound absorbing structure comprising: a vibration member
composed of a closed-cell porous material; and an air cavity formed
in a rear side of the vibration member.
2. A sound absorbing structure comprising: a vibration member
composed of a closed-cell porous material and an open-cell porous
material which are laminated together; and an air cavity formed in
a rear side of the closed-cell porous material.
3. A sound absorbing structure comprising: a vibration member
composed of a closed-cell porous material and an air-permeable
member which are laminated together; and an air cavity formed in a
rear side of the closed-cell porous material.
4. The sound absorbing structure according to claim 1, wherein an
airflow rate of the closed-cell porous material is less than 0.1
dm.sup.3/s.
5. The sound absorbing structure according to claim 2, wherein an
airflow rate of the closed-cell porous material is less than 0.1
dm.sup.3/s.
6. The sound absorbing structure according to claim 3, wherein an
airflow rate of the closed-cell porous material is less than 0.1
dm.sup.3/s.
7. A sound absorbent group including a plurality of sound absorbing
structures, each of which is constituted of a vibration member
composed of a closed-cell porous material, and an air cavity formed
in a rear side of the vibration member.
8. A sound absorbent group including a plurality of sound absorbing
structures, each of which is constituted of a vibration member
composed of a closed-cell porous material and an open-cell porous
material which are laminated together, and an air cavity formed in
a rear side of the closed-cell porous material.
9. A sound absorbent group including a plurality of sound absorbing
structures, each of which is constituted of a vibration member
composed of a closed-cell porous material and an air-permeable
member which are laminated together, and an air cavity formed in a
rear side of the closed-cell porous material.
10. A sound chamber including at least one sound absorbing
structure, which is constituted of a vibration member composed of a
closed-cell porous material, and an air cavity formed in a rear
side of the vibration member.
11. A sound chamber including at least one sound absorbing
structure, which is constituted of a vibration member composed of a
closed-cell porous material and an open-cell porous material which
are laminated together, and an air cavity formed in a rear side of
the closed-cell porous material.
12. A sound chamber including at least one sound absorbing
structure, which is constituted of a vibration member composed of a
closed-cell porous material and an air-permeable member which are
laminated together, and an air cavity formed in a rear side of the
closed-cell porous material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to sound absorbing structures
using closed-cell porous media. The present invention also relates
to sound chambers using sound absorbing structures.
[0003] The present application claims priority on Japanese Patent
Application No. 2008-211972, the content of which is incorporated
herein by reference.
[0004] 2. Description of the Related Art
[0005] In generally-known sound absorbing structures using
open-cell porous media (e.g. glass wools), the sound absorption
increases proportionally to the particle velocity of sound waves so
that it becomes high in a high frequency range but it becomes low
in a low frequency range. Generating high sound absorption in a low
frequency range requires sound absorbing structures having a
thickness of about .lamda./4 (e.g. 34 cm for 250 Hz), which are
difficult to be installed in a small space.
[0006] It is possible to generate a high sound absorption in a low
frequency range by use of a sound absorbing structure which absorbs
sound by way of a plate or membrane vibration member and its rear
air cavity, wherein a laminated board having a thickness of 4 mm is
equipped with a rear air cavity having a thickness of 45 mm which
is filled with glass wools therein, so that the sound absorption
coefficient thereof peaks at 0.6 in a low frequency of 250 Hz, for
example. [0007] Patent Document 1: Japanese Unexamined Patent
Application Publication No. 2003-316364 [0008] Patent Document 2:
Japanese Unexamined Patent Application Publication No.
2006-11412
[0009] Patent Document 1 discloses sound absorbing media using
open-cell porous materials (or cellular porous materials), which
are well known in the fields of sound absorbing technology.
[0010] Patent Document 2 discloses sound absorbing media using
open-cell porous materials and closed-cell porous materials with an
airflow rate of 0.1 dm.sup.3/s or more. A high airflow rate does
not cause a sound pressure difference between the surface and the
backside of the porous material, which in turn makes it difficult
for a plate vibration member to vibrate, thus degrading a sound
absorbing effect of a plate-vibration sound absorbing
structure.
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide a sound
absorbing structure using a closed-cell porous material having
elasticity but not having air permeability. Specifically, the
present invention aims at demonstrating a high sound absorbing
effect in a low frequency range with a thin sound absorbing
structure whose total thickness (i.e. the sum of the thickness of a
porous material and the thickness of a rear air cavity) is about 50
mm.
[0012] A sound absorbing structure of the present invention is
constituted of a vibration member composed of a closed-cell porous
material, and an air cavity formed in the rear side of the
vibration member.
[0013] Alternatively, the vibration member is formed by laminating
an open-cell porous material with the closed-cell porous material
or by laminating an air-permeable member with the closed-cell
porous material.
[0014] In the above, it is preferable that an airflow rate of the
closed-cell porous material be less than 0.1 dm.sup.3/s.
[0015] A sound absorbent group is formed using a plurality of sound
absorbing structures, each of which is constituted of the vibration
member and the air cavity.
[0016] A sound chamber is formed using at least one sound absorbing
structure including the vibration member and the air cavity.
[0017] As described above, the sound absorbing structure of the
present invention is a plate/film-vibration sound absorbing
structure in which the air cavity formed inside the housing is
closed with the vibration member composed of the closed-cell porous
material, wherein it is possible to prevent the degradation of the
vibration member while securing high sound absorption
characteristics, thus improving the reliability in sound
absorption.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other objects, aspects, and embodiments of the
present invention will be described in more detail with reference
to the following drawings.
[0019] FIG. 1 is a perspective view showing the constitution of a
sound absorbing structure according to a preferred embodiment of
the present invention.
[0020] FIG. 2 is an exploded perspective view of the sound
absorbing structure which is constituted of a housing, a vibration
member, and an air cavity.
[0021] FIG. 3A is a sectional view taken along line III-III in FIG.
1 showing that the housing is covered with the vibration member
composed of a closed-cell porous material.
[0022] FIG. 3B is a sectional view taken along line III-III in FIG.
1 showing that the housing is covered with the vibration member
composed of a closed-cell porous material and an open-cell porous
material.
[0023] FIG. 3C is a sectional view taken along line III-III in FIG.
1 showing that the housing is covered with the vibration member
composed of a closed-cell porous material and an air-permeable
member.
[0024] FIG. 4A is a sectional view diagrammatically showing the
closed-cell porous material including a plurality of closed
cells.
[0025] FIG. 4B is a sectional view diagrammatically showing the
open-cell porous material including a plurality of open cells.
[0026] FIG. 5 is a graph showing open-cell and closed-cell
characteristic curves based on experimental results with a
10-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0027] FIG. 6 is a graph showing open-cell and closed-cell
characteristic curves based on experimental results with a
20-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0028] FIG. 7 is a graph-showing open-cell and closed-cell
characteristic curves based on experimental results with a
30-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0029] FIG. 8 is a graph showing open-cell and closed-cell
characteristic curves based on experimental results with a
10-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0030] FIG. 9 is a graph showing open-cell and closed-cell
characteristic curves based on experimental results with a
20-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0031] FIG. 10 is a graph showing open-cell and closed-cell
characteristic curves based on experimental results with a
30-mm-thickness air cavity formed in the rear side of the vibration
member in the sound absorbing structure.
[0032] FIG. 11 is a graph showing simulation results of normal
incidence sound absorption coefficients on a sound absorbing
structure according to a third variation of the present embodiment,
wherein five characteristic curves are plotted with respect to
various surface densities at the center of the vibration
member.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0033] The present invention will be described in further detail by
way of examples with reference to the accompanying drawings.
1. Constitution of Sound Absorbing Structure
[0034] FIG. 1 is a perspective view showing the constitution of a
sound absorbing structure 10 according to a preferred embodiment of
the present invention. FIG. 2 is an exploded perspective view of
the sound absorbing structure 10. FIGS. 3A to 3C are sectional
views taken along line III-III in FIG. 1. For the sake of
convenience, FIGS. 1 and 2 and FIGS. 3A to 3C are illustrated with
prescribed dimensions, which do not precisely match the actual
design dimensions, in order to distinctively show the constituent
elements of the sound absorbing structure 10.
[0035] The sound absorbing structure 10 is constituted of a housing
20 (serving as the base of the sound absorbing structure 10), a
vibration member 30 for covering an opening 23 of the housing 20,
and an air cavity 40 which is formed inside the housing 20 equipped
with the vibration member 30.
[0036] The housing 20 is formed in a closed-bottom rectangular
prismatic shape composed of a synthetic resin (e.g. an ABS resin),
which is constituted of a base 21 and a side wall 22 as well as the
opening 23. The base 21 is disposed opposite to the opening 23,
while the side wall 22 is disposed to encompass the opening 23. The
vibration member 30 is a squared board composed of a high polymer
compound (e.g. a silicon foam, a urethane foam, a polyethylene
foam, an ethylene-propylene rubber foam, etc.). The periphery of
the vibration member 30 is bonded to the edge of the opening 23.
Since the vibration member 30 is fixed upon the opening 23 of the
housing 20, a tightly-closed air cavity 40 is formed inside the
sound absorbing structure 10 (or in the rear side of the vibration
member 30).
[0037] The vibration member 30 is not necessarily formed in a plate
(or board) shape but is formed in a film (or membrane) shape. In
short, the present embodiment requires that the vibration member 30
be formed of any type of material which is deformable upon
receiving an external force and is restorable in shape due to
elasticity.
[0038] In this connection, the plate shape is defined as a thin
three-dimensional shape (or a rectangular parallelepiped shape)
which is reduced in thickness and is enlarged in a two-dimensional
area, while the film shape (or sheet shape) is farther reduced in
thickness compared to the plate shape and is restorable in shape
due to tension.
[0039] The vibration member 30 is formed in a prescribed shape and
of a prescribed material which is reduced in terms of a rigidity
(i.e. a Young's modulus, a thickness, and a geometrical moment of
inertia) and/or a mechanical impedance, i.e. 8.times.{(bending
rigidity).times.(surface density)}.sup.1/2, in comparison with the
housing 20. That is, the vibration member 30 possesses
elastic-vibration ability relative to the housing 20, so that the
sound absorbing structure 10 demonstrates the sound absorbing
operation by means of the vibration member 30.
[0040] The sound absorbing structure 10 having the above basic
constitution is characterized in that the vibration member 30 is
formed using a closed-cell porous material 50 shown in FIG. 3A. The
airflow rate of the closed-cell porous material 50 is less than 0.1
dm.sup.3/s, thus shutting off an airflow therethrough. As the
closed-cell porous material, it is possible to use a silicon foam
and an ethylene-propylene rubber foam (or EPDM, i.e.
ethylene-propylene-diene-methylene rubber), for example.
[0041] FIGS. 4A and 4B illustrate the cross-sectional comparison
between the closed-cell porous material 50 and an open-cell porous
material 60.
[0042] In the closed-cell porous material 50 shown in FIG. 4A, a
plurality of closed cells 51 do not communicate with each other and
overlap with each other so that they are independent of each other.
The closed-cell porous material 51 having elasticity serves as an
integrally vibrating board, in other words, the closed-cell porous
material 51 has elasticity but does not have air permeability.
[0043] FIG. 4A diagrammatically shows that the closed cells 51 are
regularly aligned, but they may be aligned in a random manner; that
is, the closed-cell porous material 50 includes the closed cells
51, which do not overlap with each other, so as to prevent an
airflow occurring between the surface and the backside thereof.
[0044] In the open-cell porous material 60 shown in FIG. 4B, a
plurality of open cells 61 partially overlap with each other and
communicate with each other; hence, the open-cell porous material
60 has a sponge-like texture dependent upon the material and the
size of the cell 61. FIG. 4B diagrammatically shows that the open
cells 61 are regularly aligned, but they may be aligned in a random
manner; that is, the open-cell porous material 60 includes the open
cells 61, which adjoin together to partially overlap with each
other, so as to establish an air flow occurring between the surface
and the backside thereof.
2. Operation of Sound Absorbing Structure
[0045] Generally speaking, the sound absorbing structure 10 serves
as a spring-mass system composed of the mass of the vibration
member 30 and the spring component of the air cavity 40.
[0046] A resonance frequency f [Hz] of the spring-mass system is
given by equation (1) using an air density .rho..sub.0
[kg/m.sup.3], the speed of sound c.sub.0 [m/s], a density .rho.
[kg/m.sup.3], a thickness t [m] of the vibration member 30, and a
thickness L [m] of the air cavity 40.
f = 1 2 .pi. ( .rho. 0 c 0 2 .rho. tL ) 1 / 2 ( 1 )
##EQU00001##
[0047] When the sound absorbing structure 10 includes the vibration
member 30 having elasticity subjected to elastic vibration, a
bending system (representing the elastic vibration) is applied to
the spring-mass system.
[0048] A resonance frequency f [Hz] of a plate/film-vibrating sound
absorbing structure is given by equation (2) using a one-side
length "a" [m] and another-side length "b" [m] of the rectangular
shape of the vibration member 30, a Poisson ratio .sigma. [-] of
the vibration member 30, and integral numbers p, q. In the field of
architectural acoustics, the calculation result of the above
resonance frequency f is used for architectural acoustic
designs.
f = 1 2 .pi. [ .rho. 0 c 0 2 .rho. tL + { ( p a ) 2 + ( q b ) 2 } 2
{ .pi. 4 Et 3 12 .rho. t ( 1 - .sigma. 2 ) } ] 1 / 2 ( 2 )
##EQU00002##
[0049] According to equation (2), the resonance frequency f
represents the sum of the term of the spring-mass system
".rho..sub.0c.sub.0.sup.2/.rho.tL" and the term of the bending
system (i.e. the term directly subsequent to the term of the
spring-mass system). According to equation (2), the spring-mass
system of the vibration member 30 and the bending system
representing the elastic vibration form important factors
determining the sound absorbing condition for the sound absorbing
structure 10.
[0050] In the sound absorbing structure 10 of the present
embodiment, the vibration member 30 is subjected to elastic
vibration dependent upon the difference between the external sound
pressure applied to the exterior surface of the vibration member 30
and the internal sound pressure occurring inside the air cavity 40,
in other words, the sound-pressure difference between the surface
and the backside of the vibration member 30. Sound is absorbed in
such a way that energy of sound waves reaching the sound absorbing
structure 10 is consumed by way of the vibration of the vibration
member 30. The vibration member 30 absorbs sound in a certain
frequency range whose center frequency corresponds to the resonance
frequency f according to equation (2).
3. Sound Absorbing Effect
[0051] The sound absorbing effect of the sound absorbing structure
10 will be described with reference to FIGS. 5 to 7. FIGS. 5 to 7
are graphs of characteristic curves representing results of
experiments in which sounds having various frequencies are applied
to sound absorbing structures (i.e. experimental subjects) so as to
measure normal incidence sound absorbing coefficients.
[0052] Specifically, FIGS. 5 to 7 show experimental results with
respect to two types of sound absorbing structures, one of which
includes an open-cell type vibration member composed of a
10-mm-thickness open-cell urethane foam and the other of which
includes a closed-cell type vibration member composed of a
10-mm-thickness closed-cell silicon foam. That is, an open-cell
characteristic curve A represents the sound absorption
characteristic regarding the open-cell type vibration member, while
a closed-cell characteristic curve B represents the sound
absorption characteristic regarding the closed-cell type vibration
member.
[0053] In addition, FIGS. 5 to 7 differ from each other in terms of
the thickness of an air cavity formed in the rear side of the
vibration member; that is, FIG. 5 shows the experimental result
with regard to a 10-mm-thickness air cavity; FIG. 6 shows the
experimental result with regard to a 20-mm-thickness air cavity;
and FIG. 7 shows the experimental result with regard to a
30-mm-thickness air cavity.
[0054] The open-cell characteristic curves A of FIGS. 5 to 7 show
that sound absorption coefficients decrease in a low frequency
range but increase in a high frequency range, while the closed-cell
characteristic curves B show that sound absorption coefficients
peak at maximum values in a further low frequency range. This
proves that the sound absorbing structure 10 including the
vibration member 30 composed of a closed-cell porous material
demonstrates an adequate sound absorbing effect. In the above, the
density of the closed-cell porous material is set to 250
kg/m.sup.3, while the density of the open-cell porous material is
set to 35 kg/m.sup.3.
[0055] FIGS. 8 to 10 show experimental results with respect to two
types of sound absorbing structures, one of which includes an
open-cell type vibration member composed of a 10-mm-thickness
open-cell urethane foam and the other of which includes a
closed-cell type vibration member composed of a 10-mm-thickness
closed-cell EPDM, i.e. an ethylene-propylene-diene-methylene
rubber. Herein, an open-cell characteristic curve A represents the
sound absorption characteristic regarding the open-cell type
vibration member, while a closed-cell characteristic curve B
represents the sound absorption characteristic regarding the
closed-cell type vibration member.
[0056] In addition, FIGS. 8 to 10 differ from each other in terms
of the thickness of an air cavity formed in the rear side of the
vibration member; that is, FIG. 8 shows the experimental result
with regard to a 10-mm-thickness air cavity; FIG. 9 shows the
experimental result with regard to a 20-mm-thickness air cavity;
and FIG. 10 shows the experimental result with regard to a
30-mm-thickness air cavity.
[0057] Similar to the experimental results of FIGS. 5 to 7, the
experimental results of FIGS. 8 to 10, which are measured using the
closed-cell vibration member composed of EPDM, sound absorption
coefficients peak at maximum values in a low frequency range.
[0058] According to the above experimental results, the sound
absorbing structure 10 including the vibration member 30 composed
of the closed-cell porous material 50 is capable of absorbing sound
in a low frequency range regardless of the "slim" thickness of the
vibration member 30 and the air cavity 40 in total which is 50 mm
or less.
[0059] Since the closed-cell porous material 50 shuts off an
airflow therethrough, it is possible to prevent external air from
entering into the air cavity 40 via the vibration member 30 even
when the sound absorbing structure 10 is positioned in a dusty
sound field or environment. That is, it is possible to prevent the
air cavity 40 from being contaminated with dust or foreign
matter.
[0060] Since the closed-cell porous material 50 inherently blocks
air or humidity entering therein, it is possible to enhance the
durability of the vibration member 30 and to thereby improve the
reliability of the sound absorbing structure 10.
[0061] Since the closed-cell porous material 50 is lower in
manufacturing cost than the open-cell porous material 60, it is
possible to manufacture the sound absorbing structure 10 at a
relatively low cost. Since it is easier to perform cutting on the
closed-cell porous material 50 rather than the open-cell porous
material 60, it is possible to improve the productivity. As
described above, the present embodiment demonstrates various
outstanding effects.
4. Variations
[0062] The present invention is not necessarily limited to the
present embodiment, which can be modified in various ways.
(1) First Variation
[0063] The present embodiment exemplifies the sound absorbing
structure 10 including the vibration member 30 composed of the
closed-cell porous material 50, which can be modified in various
ways.
[0064] FIG. 3B is a sectional view of a vibration member 31 in
which the open-cell porous material 60 is laminated on the surface
(i.e. the sound-incidence side) of the closed-cell porous material
50. The vibration member 31 is fixed to the housing 20 in such a
way that the air cavity 40 is formed in the rear side of the
closed-cell porous material 50.
[0065] FIG. 3C is a sectional view of a vibration member 32 in
which an air-permeable member 70 composed of a fabric material such
as a mesh, cloth, and flocked fabric is laminated on the surface
(i.e. the sound-incidence side) of the closed-cell porous material
50. The vibration member 32 is fixed to the housing 20 in such a
way that the air cavity 40 is formed in the rear side of the
closed-cell porous material 50.
[0066] It is possible to demonstrate the foregoing effect of the
present embodiment by use of the vibration members 31 and 32. Due
to the arrangement of the open-cell porous material 60 or the
air-permeability member 70 on the surface of the closed-cell porous
material 50, it is possible to demonstrate an additional effect
that sound is easily absorbed by the material 60 or 70.
[0067] It is possible to further modify the vibration member 31
such that three or more layers of the open-cell porous material 60
are laminated on the closed-cell porous material 50. Alternatively,
it is possible to further laminate the air-permeability member 70
on the open-cell porous material 60 above the closed-cell porous
material 50. In short, the first variation requires that the
vibration member be formed using the closed-cell porous material 50
so as to reliably shut off the airflow occurring between the air
cavity 40 and the external air.
(2) Second Variation
[0068] Although the relevancy between the resonance frequency of
the spring-mass system and the resonance frequency of the bending
system based on the elastic vibration of the plate is univocally
defined by equation (2), the actual behavior of the sound absorbing
structure has not been fully clarified, hence, the actual working
model of the sound absorbing structure demonstrating a high sound
absorption in a low frequency range has not been established.
[0069] For this reason, the present inventor performed various
detailed experiments so as to determine inequality (3) regarding
the relationship between a fundamental frequency fa of the bending
system and a resonance frequency fb of the spring-mass system. By
setting parameters to suit inequality (3), the present inventor
actually verified an improvement of the sound absorption, since the
fundamental vibration of the bending system cooperates with the
spring component of the rear air cavity so that a relatively high
amplitude of vibration occurs in a frequency band between the
resonance frequency of the spring-mass system and the fundamental
frequency of the bending system, i.e. (resonance frequency fa of
bending system)<(peak sound-absorption frequency
f)<(fundamental frequency fb of spring-mass system).
0.05.ltoreq.fa/fb.ltoreq.0.65 (3)
[0070] By setting parameters to suit inequality (4), it is possible
to substantially make the peak sound-absorption frequency lower
than the resonance frequency of the spring-mass system. Herein, the
present inventor verified that the sound absorbing structure
including parameters according to inequality (4) is suitable for
absorbing sound in a low frequency range which is 300 Hz or less,
since the fundamental frequency of the bending system is
sufficiently lowered due to a low-degree elastic vibration
mode.
0.05.ltoreq.fa/fb.ltoreq.0.40 (4)
[0071] By setting parameters to suit inequalities (3) and (4), it
is possible to design the sound absorbing structure whose peak
sound-absorption frequency is lowered in a low frequency range.
(3) Third Variation
[0072] The present embodiment exemplifies the sound absorbing
structure 10 which is constituted of the rectangular housing 20,
the vibration member 30 for closing the opening 23 of the housing
20, and the air cavity 40 formed inside the housing 20. The housing
20 is not necessarily formed in a rectangular shape, which can be
changed to other shapes such as a circular shape and a polygonal
shape. In addition, it is possible to dispose the concentrated
mass, which is an important factor for changing the vibration
condition with respect to the vibration member 30, at the center of
the vibration member 30.
[0073] As described above, the sound absorbing structure 10
possesses a sound absorption mechanism composed of the spring-mass
system and the bending system. The present inventor performed
experiments on sound absorption coefficients at resonance
frequencies with various surface densities applied to the vibration
member 30.
[0074] FIG. 11 show simulation results on normal incidence sound
absorption coefficients with respect to the sound absorbing
structure 10, in which the vibration member 30 having the length
and breadth of 100 mm.times.100 mm and the thickness of 0.85 mm is
fixed to the housing 20 containing the air cavity 40 having the
length and breadth of 100 mm.times.100 mm and the thickness of 10
mm and in which the surface density is changed with respect to the
center portion having the length and breadth of 20 mm.times.20 mm
and the thickness of 0.85 mm. The simulation is performed based on
JIS A 1405-2 (titled "Acoustics--Determination of sound absorption
coefficient and impedance in impedance tubes--Part 2:
Transfer-function method") so as to determine a sound field of a
sound chamber for arranging the sound absorbing structure 10 in
accordance with the finite element method, thus calculating sound
absorption characteristics by way of transfer functions.
[0075] Specifically, FIG. 11 shows five characteristic curves D1 to
D5 which are plotted using the same surface density of the
periphery of the vibration member 30 of 799 g/m.sup.2 while
changing the surface density of the center portion of the vibration
member 30 as 399.5 g/m.sup.2, 799 g/m.sup.2, 1199 g/m.sup.2, 1598
g/m.sup.2, and 2297 g/m.sup.2 in D1, D2, D3, D4, and D5
respectively. Thus, the average density of the vibration member 30
is set to 783 g/m.sup.2, 799 g/m.sup.2, 815 g/m.sup.2, 831
g/m.sup.2, and 863 g/m.sup.2 in D1, D2, D3, D4, and D5
respectively.
[0076] The simulation results of FIG. 11 clarify that sound
absorption coefficients peak in a frequency range between 300 Hz
and 500 Hz and at a frequency around 700 Hz.
[0077] Sound absorption coefficients peak around 700 Hz due to the
resonance of the spring-mass system composed of the mass of the
vibration member 30 and the spring component of the air cavity 40.
The sound absorbing structure 10 absorbs sound in such a way that
the sound absorption coefficient peaks at the resonance frequency
of the bending system in a low frequency range, wherein the
resonance frequency of the bending system gradually decreases as
the surface density of the center portion of the vibration member
30 increases.
[0078] Generally speaking, the resonance frequency of the bending
system is determined by the equation of motion directing the
elastic vibration of the vibration member 30 and varies in inverse
proportion to the surface density of the vibration member 30. The
resonance frequency is greatly affected by the density of the
antinode of the characteristic vibration (at which the amplitude
becomes maximum). The simulation is performed by changing the
surface density of the center portion with respect to the antinode
region of 1.times.1 characteristic mode, thus causing variations of
the resonance frequency of the bending system.
[0079] The simulation result clarifies that, by increasing the
surface density of the center portion to be higher than the surface
density of the periphery, the prescribed frequencies causing peak
sound absorption coefficients are further lowered in a low
frequency range. In other words, by changing the surface density of
the center portion, it is possible to partially shift prescribed
frequencies causing peak sound absorption coefficients to a further
low frequency range or a further high frequency range.
[0080] Since the sound absorbing structure 10 is capable of
shifting the prescribed frequency causing a peak sound absorption
coefficient by simply changing the surface density of the center
portion of the vibration member 30, it is possible to lower the
sound absorption frequency without greatly changing the overall
weight of the sound absorbing structure 10 in contrast to a typical
example of the sound absorbing structure whose sound absorption
frequency is changed by increasing the overall weight.
[0081] In this connection, it is possible to further increase the
peak sound absorption coefficient by filling other porous
sound-absorbent materials (e.g. resin foams, felts, cottony fibers
such as polyester wools) inside the air cavity 40 of the sound
absorbing structure 10.
(4) Fourth Variation
[0082] It is possible to form a sound absorbent group including a
plurality of sound absorbing structures according to one of the
present embodiment and variations. Alternatively, it is possible to
form a sound absorbent group including a plurality of sound
absorbing structures having different sound absorption
characteristics or a plurality of sound absorbing structures having
three or more different sound absorption characteristics.
[0083] The sound absorbing structure and the sound absorbent group
are applicable to various types of sound chambers having controlled
acoustic characteristics such as soundproof chambers, halls,
theaters, listening rooms of audio devices, conference rooms,
compartment spaces of transportation such as vehicles, and housings
of speakers and musical instruments.
[0084] Lastly, the present invention is not necessarily limited to
the present invention and variations, which can be further modified
in a variety of ways within the scope of the invention defined by
the appended claims.
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