Patent Publication Number: US-2009223738-A1

Title: Sound absorbing structure and vehicle component having sound absorption property

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to sound absorbing structures adapted to sound chambers, and in particular to vehicle components having sound absorbing properties. 
     The present application claims priority on Japanese Patent Application No. 2008-41772, Japanese Patent Application No. 2008-55367, Japanese Patent Application No. 2008-69794, Japanese Patent Application No. 2008-104965, Japanese Patent Application No. 2008-69795, Japanese Patent Application No. 2008-111481, Japanese Patent Application No. 2008-223442, Japanese Patent Application No. 2008-221316, and Japanese Patent Application No. 2008-219129, the contents of which are incorporated herein by reference in their entirety. 
     2. Description of the Related Art 
     Conventionally, various types of sound absorbing structures have been developed and disclosed in various documents such as Patent Document 1. 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-11412 
     Patent Document 1 discloses a sound absorbing structure (hereinafter, referred to as a panel/membrane-vibration sound absorbing structure) which absorbs sound by a vibration member composed of a panel or membrane and an air cavity formed on the backside of the vibration member. The panel/membrane-vibration sound absorbing structure is recognized as a spring-mass system which is constituted of a mass of the vibration member and a spring component of the air cavity. When the vibration member having elasticity performs bending vibration, the property of a bending system due to bending vibration is added to the property of the spring-mass system. 
     By increasing the density of the vibration member, it is possible for the panel/membrane-vibration sound absorbing structure to decrease the frequency of absorbed sound, thus decreasing the pitch of absorbed sound. However, the total mass of the vibration member becomes large as the density of the vibration member increases, thus increasing the overall weight of the sound absorbing structure. It becomes difficult to apply the sound absorbing structure having a heavy weight to the existing field which requires weight reductions. In addition, when the sound absorbing structure having a heavy weight is disposed on a wall surface, it is necessary to arrange a high-strength support structure bearing the weight of the sound absorbing structure, which is thus difficult to be simply disposed on the wall surface. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a technology for changing the property of absorbed sound without substantially changing the overall weight of a sound absorbing structure having a vibration member. 
     In one embodiment of the present invention, a sound absorbing structure is constituted of a housing having a hollow portion and an opening, and a vibration member composed of a panel or membrane. The opening of the housing is closed with the vibration member so as to form an air cavity inside the housing. The density of at least a part of the vibration member except for a first area causing a node or minimum amplitude of bending vibration differs from the density of the first area of the vibration member. Alternatively, the density of the vibration member at a second area causing the maximum amplitude of bending vibration differs from the density of the vibration member except for the second area. 
     It is possible to modify the sound absorbing structure in such a way that the thickness of at least a part of the vibration member except for the first area causing a node or minimum amplitude of bending vibration differs from the thickness of the first area of the vibration member. Alternatively, the thickness of the vibration member at the second area causing the maximum amplitude of bending vibration differs from the thickness of the vibration member except for the second area 
     It is possible to modify the sound absorbing structure in such a way that a secondary member is attached to a part of the vibration member except for the first area causing the node or minimum amplitude of bending vibration. Alternatively, the secondary member is attached to the vibration member at the second area causing the maximum amplitude of bending vibration. In this connection, the secondary member is attached to the surface of the vibration member or incorporated into the vibration member. 
     In another embodiment of the present invention, a grouped sound absorbing structure is composed of a plurality of sound absorbing structures. Herein, the weights of the secondary members attached to the vibration members differ from each other with respect to the respective sound absorbing structures. Alternatively, the sizes or thicknesses of the air cavities formed in the housings differ from each other with respect to the respective sound absorbing structures. 
     A sound chamber can be formed using the above sound absorbing structure or the above grouped sound absorbing structure. 
     In a further embodiment of the present invention, an adjustment method is adapted to the sound absorbing structure so as to change the density or thickness of the vibration member except for the first area, thus adjusting the resonance frequency of the sound absorbing structure. Alternatively, an adjustment method is adapted to the sound absorbing structure so as to change the secondary member, thus adjusting the resonance frequency of the sound absorbing structure. 
     In a further embodiment of the present invention, a noise reduction method is adapted to the sound absorbing structure so as to reduce noise by the vibration member. 
     The present invention demonstrates the outstanding effect for arbitrarily changing or adjusting the frequency of absorbed sound without substantially changing the overall weight of the sound absorbing structure and its vibration member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the external appearance of a sound absorbing structure according to a first embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the sound absorbing structure. 
         FIG. 3  is a cross-sectional view of the sound absorbing structure. 
         FIG. 4  is a graph showing the simulation result of the sound absorbing structure. 
         FIG. 5  is a cross-sectional view of a sound absorbing structure according to a first variation of the first embodiment. 
         FIG. 6  is a graph showing the measurement result regarding sound absorption coefficients of sound absorbing structures according to variations of the first embodiment. 
         FIG. 7  is a cross-sectional view of a sound absorbing structure according to a second variation of the first embodiment. 
         FIG. 8  is a cross-sectional view of a sound absorbing structure according to a third variation of the first embodiment. 
         FIG. 9  is a perspective view showing the external appearance of a grouped sound absorbing structure. 
         FIG. 10  is an exploded perspective view of a sound absorbing structure according to a fourth variation of the first embodiment. 
         FIG. 11  is a perspective view showing the external appearance of a vehicle adopting sound absorbers according to a second embodiment of the present invention. 
         FIG. 12  is a side view showing a chassis of the vehicle. 
         FIG. 13  is an enlarged sectional view of a position Pa in  FIG. 12 . 
         FIG. 14  is an exploded perspective view related to  FIG. 13 . 
         FIG. 15  is a perspective view showing the external appearance of a vehicle adopting sound absorbers according to a third embodiment of the present invention. 
         FIG. 16  is a graph showing a noise reduction effect in a rear seat by a sound absorber installed in a roof of the vehicle. 
         FIG. 17  is a development illustration of a sun visor adopting a sound absorber according to a fourth embodiment of the present invention. 
         FIG. 18  is a sectional view taken along line A-A in  FIG. 17 . 
         FIG. 19  is a sectional view showing a sound absorber according to a fifth embodiment of the present invention, which is installed in a rear pillar of a vehicle. 
         FIG. 20  is a sectional view showing a variation of the sound absorber shown in  FIG. 19 . 
         FIG. 21  is a sectional view showing a sound absorber according to a sixth embodiment of the present invention, which is installed in a door of a vehicle. 
         FIG. 22  is a sectional view showing a modified example of the sound absorber shown in  FIG. 21 . 
         FIG. 23  is a partly cut plan view showing a sound absorber according to a seventh embodiment of the present invention, which is installed in a floor of a vehicle. 
         FIG. 24  is an illustration used for explaining the sound absorption principle of a sound absorber composed of plural pipes. 
         FIG. 25A  is a perspective view showing a modified example of the seventh embodiment. 
         FIG. 25B  is an illustration showing a side sill of the floor viewed in an X-direction of  FIG. 25A . 
         FIG. 26  is a perspective view showing the external appearance of an instrument panel of a vehicle adopting a sound absorber according to an eighth embodiment of the present invention. 
         FIG. 27  is a sectional view taken along line X-X in  FIG. 26 , which shows the internal structure of the instrument panel arranging plural sound absorbers. 
         FIG. 28  is an illustration viewed in an I-direction in  FIG. 27 , which shows the arrangement of plural sound absorbers. 
         FIG. 29  is a perspective view showing the external appearance of an instrument panel adopting a sound absorber according to a modified example of the eighth embodiment. 
         FIG. 30  is a sectional view taken along line Y-Y in  FIG. 29 , which shows the arrangement of plural sound absorbers according to the modified example. 
         FIG. 31A  is a sectional view showing an example in which a panel-vibration sound absorbing structure according to a ninth embodiment of the present invention is installed inside the instrument panel. 
         FIG. 31B  is a plan view of the upper side of the instrument panel shown in  FIG. 31A . 
         FIG. 31C  is a plan view showing an example in which plural sound absorbers forming the panel-vibration sound absorbing structure installed inside the instrument panel are aligned in parallel with left-right directions of a vehicle. 
         FIG. 31D  is a sectional view showing an example in which the panel-vibration sound absorbing structure is installed in a tray beneath a rear glass of a vehicle. 
         FIG. 31E  is a sectional view showing an example in which the panel-vibration sound absorbing structure is installed in the lower portion of a floor of a vehicle. 
         FIG. 32A  is a sectional view showing an example in which a panel-vibration sound absorbing structure composed of plural housings each aligning plural sound absorbers is installed inside a front seat of a vehicle. 
         FIG. 32B  is a sectional view showing an example in which a panel-vibration sound absorbing structure composed of plural housings each aligning plural sound absorbers is installed inside a rear seat of a vehicle. 
         FIG. 33A  is a sectional view showing a panel-vibration sound absorbing structure according to a first modified example of the ninth embodiment. 
         FIG. 33B  is a sectional view showing a panel-vibration sound absorbing structure according to a second modified example of the ninth embodiment. 
         FIG. 33C  is a sectional view showing a panel-vibration sound absorbing structure according to a third modified example of the ninth embodiment. 
         FIG. 33D  is a sectional view showing a panel-vibration sound absorbing structure according to a fourth modified example of the ninth embodiment. 
         FIG. 33E  is a sectional view showing a panel-vibration sound absorbing structure according to a fifth modified example of the ninth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. First Embodiment 
     (A) Sound Absorbing Structure 
       FIG. 1  shows the external appearance of a sound absorbing structure  1  according to a first embodiment of the present invention.  FIG. 2  is an exploded perspective view of the sound absorbing structure  1 , and  FIG. 3  is a cross-sectional view taken along line A-A in  FIG. 2 . In these drawings, the illustrated shape and dimensions of the sound absorbing structure  1  do not precisely match those of an actual product of the sound absorbing structure  1  in order to simply illustrate the present embodiment in an easy-to-understand manner. 
     The sound absorbing structure  1  is constituted of a housing  10  and a vibration member  20 . The housing  10  composed of a synthetic resin is formed in a rectangular parallelepiped shape whose cross section is shaped in a square and which has an opening at one end thereof while the other end thereof is closed, so that the housing  10  has a bottom portion  11  surrounded by a side wall  12 . 
     The vibration member  20  is constituted of a first member  21  which is a square-shaped small board composed of a synthetic resin having elasticity, and a second member  22 . When a force is applied to the vibration member  20 , the vibration member  20  is temporarily deformed but is restored in shape due to elasticity so as to cause a vibration. The second member  22  is composed of a synthetic resin having elasticity such that the surface density thereof is smaller than that of the first member  21 . The second member  22  has a square hole at the center thereof. The thickness of the first member  21  is identical to the thickness of the second member  22 . The first member  21  is fixed in the square-shaped hole of the second member  22  so as to form the vibration member  20  as an integrally unified board. 
     The material of the vibration member  20  is not necessarily limited to the synthetic resin; hence, the vibration member  20  can be composed of other materials having elasticity and causing panel vibration, such as paper, metals, and fibered boards. 
     The area of the first member  21  within the plane of the vibration member  20  includes a prescribed position at which an amplitude of the vibration member  20  subjected to bending vibration becomes maximum. In this connection, the area of the first member  21  is not necessarily limited to the illustrated position and area and can be changed arbitrarily as long as it contains the prescribed position having the maximum amplitude of the vibration member  20  subjected to bending vibration. 
     The bottom portion  11  is fixed to the side wall  12  so as to form the housing  10 ; then, the vibration member  20  is bonded to the rectangular opening of the housing  10  so as to form an air cavity  30  defined inside the sound absorbing structure  1  (or on the backside of the vibration member  20 ). A sound absorbing mechanism of a spring-mass system is formed using a mass of the vibration member  20  and a spring component of the air cavity  30  in the sound absorbing structure  1 . Since the vibration member  20  having elasticity causes bending vibration in the sound absorbing structure  1 , a sound absorbing structure of a bending system due to bending vibration is added to the property of the sound absorbing structure  1 . The air cavity  30  is not necessarily closed so that few holes are formed in the housing  10  so as to allow the air cavity  30  to communicate with the external space. 
     In the sound absorbing structure  1 , when sound waves reach the vibration member  20 , the vibration member  20  vibrates due to the difference between the sound pressure of sound waves and the internal pressure of the air cavity  30 , so that energy of sound waves is consumed due to vibration of the vibration member  20 . Since the sound absorbing structure  1  adopts both of the sound absorbing mechanisms of the spring-mass system and bending system, the sound absorption coefficient becomes high at the resonance frequency of the spring-mass system and the resonance frequency of the bending system in connection with the relationship between the frequency of absorbed sound and the sound absorption coefficient. 
       FIG. 4  is a graph showing the simulation result of the normal incidence sound absorption coefficient of the sound absorbing structure  1  in which the vibration member  20  (having longitudinal/lateral dimensions of 100 mm×100 mm and a thickness of 0.85 mm) is attached to the housing  10  (containing the air cavity  30  having longitudinal/lateral dimensions of 100 mm×100 mm and a thickness of 10 mm) and in which the first member  21  (having longitudinal/lateral dimensions of 20 mm×20 mm and a thickness of 0.85 mm) is varied in surface density. Herein, 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”), wherein the sound field of an acoustic tube disposing the sound absorbing structure is calculated in accordance with the finite element method and boundary element method, wherein sound absorption characteristics are calculated based on the transfer function. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Condition 
                 SD1 [g/m 2 ] 
                 ASD [g/m 2 ] 
                 F RB  [Hz] 
                 F RSM  [Hz] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 (1) 
                 399.5 
                 783 
                 440 
                 690 
               
               
                 (2) 
                 799 
                 799 
                 400 
                 680 
               
               
                 (3) 
                 1,199 
                 815 
                 365 
                 670 
               
               
                 (4) 
                 1,598 
                 831 
                 337 
                 665 
               
               
                 (5) 
                 2,379 
                 862.9 
                 295 
                 660 
               
               
                   
               
            
           
         
       
     
     Table 1 shows the simulation result regarding a resonance frequency F RB  [Hz] of the bending system and a resonance frequency F RSM  [Hz] of the spring-mass system based on the conditions (1) to (5), in which a surface density SD 2  [g/m 2 ] of the second member  22  is fixed to “799” while a surface density SD 1  [g/m 2 ] of the first member  21  is varied at “399.5” in (1), “799” in (2), “1,199” in (3), “1,598” in (4), and “2,397” in (5), and an average surface density ASD [g/m 2 ] of the vibration member  20  is varied at “783” in (1), “799” in (2), “815” in (3), “831” in (4), and “862.9” in (5). 
     The condition (2) is directed to the simulation result in which the vibration member  20  is entirely composed of the same material so that the surface density SD 1  of the first member  21  is identical to the surface density SD 2  of the second member  22 , wherein the resonance frequency F RB  becomes a peak at 400 Hz in response to a 1×1 mode of natural vibration. 
     According to the simulation result shown in  FIG. 4 , the sound absorption coefficient rapidly increases in the frequency range between 300 Hz and 500 Hz and in proximity to 700 Hz. The peak of the sound absorption coefficient occurs around 700 Hz due to the resonance of the spring-mass system composed of the mass of the vibration member  20  and the spring component of the air cavity  30 . The sound absorbing structure  1  absorbs sound with a peak sound absorption coefficient at the resonance frequency F RSM  of the spring-mass system, wherein the mass of the vibration member  20  does not vary irrespective of an increase of the surface density SD 1  of the first member  21 , so that the resonance frequency F RSM  of the spring-mass system does not vary substantially. 
     The sound absorption coefficient spikes in the frequency range between 300 Hz and 500 Hz due to the resonance of the bending system caused by the bending vibration of the vibration member  20 . In the sound absorbing structure  1 , a peak sound absorption coefficient in a low frequency range appears at the resonance frequency F RB  of the bending system, wherein the simulation result clearly shows that only the resonance frequency F RB  of the bending system decreases as the surface density SD 1  of the first member  21  increases. In general, the resonance frequency F RB  of the bending system is determined by the equation of motion dominating elastic vibration of the vibration member and is inversely proportional to the surface density of the vibration member. In addition, the resonance frequency F RB  of the bending system is greatly affected by the density at the antinode of natural vibration (whose amplitude becomes maximum). In the simulation, the first member  21  is changed in the surface density SD 1  in connection with the antinode of the 1×1 mode of natural vibration, thus varying the resonance frequency F RB  of the bending system. 
     As described above, a peak sound absorption coefficient in the lower frequency range moves further into the lower frequency range when the surface density SD 1  of the first member  21  becomes higher than the surface density SD 2  of the second member  22 . This indicates that the peak sound absorption coefficient shifts (or moves) further into the lower frequency range or to a higher frequency range by varying the surface density SD 1  of the first member  21 . 
     The sound absorbing structure  1  allows the peak sound absorption coefficient to be shifted in the frequency range by simply varying the surface density SD 1  of the first member  21 . Compared with the foregoing sound absorbing structure in which the vibration member is entirely composed of the same material and is increased in weight so as to change the frequency of absorbed sound, it is possible for the present embodiment to decrease the frequency of absorbed sound without substantially changing the overall weight of the sound absorbing structure  1 . 
     (B) Variations 
     The present embodiment is not necessarily limited to the sound absorbing structure  1  but can be modified in various ways. 
     The vibration member  20  having elasticity can be formed in other shapes such as membranes (e.g. films and sheets) other than panels. Herein, panels have two-dimensional areas of three-dimensional (rectangular parallelepiped) shapes having small thicknesses, while membranes are further reduced in thickness compared with panels so as to gain restoration force by way of tension force. 
     In the present embodiment, the first member  21  has a square shape in plan view, which can be changed with other shapes such as rectangular shapes, trapezoidal shapes, polygonal shapes, circular shapes, and elliptical shapes. Even when the first member  21  does not have a square shape in plan view, it is possible to lower the frequency of absorbed sound compared with the foregoing sound absorbing structure whose vibration member is entirely composed of the same material in the condition in which the surface density of the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20  is higher than the surface density of the second member  22 . 
     In the present embodiment, the first member  21  whose surface density is higher than the surface density of the second member  22  is arranged in the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20 ; but this is not a restriction. That is, it is possible to design a sound absorbing structure  1 A shown in  FIG. 5  in which the vibration member  20  is entirely composed of the same material and in which a first region  23  including the area causing the maximum amplitude of bending vibration (corresponding to approximately the center of the vibration member  20 ) is increased in thickness compared with the peripheral portion of the vibration member  20 . 
       FIG. 6  is a graph regarding the measurement result of the normal incidence sound absorption coefficient of the sound absorbing structure  1 A based on JIS A 1405-2 (titled “Acoustics—Determination of sound absorption coefficient and impedance in impedance tubes—Part 2: Transfer-function method”), in which the vibration member  20  (having longitudinal/lateral dimensions of 100 mm×100 mm) having a surface density of 800 [g/m 2 ] is fixed to the housing  10  (having longitudinal/lateral dimensions of 100 mm×100 mm and thickness of 10 mm) and in which the thickness of the first region  23  is changed in conditions (1) to (5) such that it is identical to the thickness of the peripheral portion of the vibration member  20  (i.e. 0.85 mm) in (1), it is double the thickness of the peripheral portion in (2), it is triple the thickness of the peripheral portion in (3), it is four times the thickness of the peripheral portion in (4), and it is five times the thickness of the peripheral portion in (5). 
     The graph of  FIG. 6  clearly shows that a peak sound absorption coefficient occurs in the frequency range between 200 Hz and 500 Hz at the resonance frequency F RB  of the bending system corresponding to the vibration member  20  in the sound absorbing structure  1 A, wherein the resonance frequency F RB  decreases as the thickness of the first region  23  increases. 
     The above measurement result indicates that the frequency of absorbed sound decreases as the thickness of the first region  23  (including the prescribed area causing the maximum amplitude of bending vibration) increases. In addition, it also indicates that the frequency of absorbed sound can be varied by varying the thickness of the first region  23 . 
     Since the sound absorbing structure  1 A is designed to change the frequency of absorbed sound by changing the thickness of the first region  23 , it is possible to decrease the frequency of absorbed sound without substantially changing the overall weight of the sound absorbing structure  1 A compared to the foregoing sound absorbing structure whose vibration member is increased in weight so as to change the frequency of absorbed sound. In this connection, it is possible to vary the thickness of the first region  23  in such a way that the first region  23  is gradually increased in thickness from the peripheral portion of the vibration member  20 . In addition, it is possible to arbitrarily change the shape and dimensions of the first region  23  as long as the first region  23  includes the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20 . 
     It is possible to provide a sound absorbing structure  1 B shown in  FIG. 7  in which the vibration member  20  is constituted of a primary member  24  (having a rectangular shape in plan view) and a secondary member  25 . The primary member  24  is a square-shaped member composed of an elastic material, while the secondary member  25  is a rectangular material which is integrally fixed to the primary member  24 . 
     In the vibration member  20  shown in  FIG. 7 , the secondary member  25  is adhered to the prescribed region (i.e. the first region  23  shown in  FIG. 5 ) including the prescribed area causing the maximum amplitude of bending vibration of the primary member  24 . In the sound absorbing structure  1 B, the secondary member  25  can be attached to the interior surface of the vibration member  20  attached to the housing  10  so as to directly face the air cavity  30 . Alternatively, the secondary member  25  can be attached to the exterior surface of the vibration member  20  opposite to the air cavity  30 . 
     In the above constitution, the weight of the center portion of the vibration member  20  included in the sound absorbing structure  1 B is heavier than the weight of the center portion of the foregoing vibration member which is entirely composed of the same material. That is, it is possible to decrease the resonance frequency of the bending system in the sound absorbing structure  1 B compared to the foregoing sound absorbing structure whose vibration member is entirely composed of the same material; this makes it possible to change the frequency of absorbed sound by changing the weight of the secondary member  25 . 
     It is possible to modify the sound absorbing structure  1 B as shown in  FIG. 8  such that the secondary member  25  is incorporated into the prescribed region of the primary member  24  including the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20 . In the sound absorbing structure  1 B, the secondary member  25 , which is incorporated into the prescribed region of the primary member  24  including the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20 , is not necessarily formed in a rectangular shape but can be replaced with a plurality of grains whose density is higher than the density of the primary member  24 . Alternatively, the secondary member  25  can be replaced with a plurality of linear members whose density is higher than the density of the primary member  24 . 
     The above sound absorbing structures  1 ,  1 A, and  1 B according to the first embodiment and its variations can be each installed in sound chambers whose acoustic characteristics are controlled, such as soundproof chambers, halls, theaters, listening rooms of audio devices, and conference rooms as well as spaces of transportation systems and housings or casings of speakers and musical instruments. 
     It is possible to assemble a plurality of sound absorbing structures (e.g. sound absorbing structures  1 ,  1 A, and  1 B) having the same dimensions and shape to form a grouped sound absorbing structure as shown in  FIG. 9 . When a plurality of sound absorbing structures  1  show in  FIG. 1  is assembled together, it is possible to change the surface density of the first member  21  with respect to each of the sound absorbing structures  1 , thus achieving sound absorption at various frequencies. 
     When a plurality of sound absorbing structures  1 A shown in  FIG. 5  is assembled together, it is possible to change the thickness of the first region  23  with respect to each of the sound absorbing structures  1 A, thus achieving sound absorption at various frequencies. When a plurality of sound absorbing structures  1 B shown in  FIGS. 7 and 8  is assembled together, it is possible to change the as of the secondary member  25  with respect to each of the sound absorbing structures  1 B, thus achieving sound absorption at various frequencies. A plurality of sound absorbing structures can be assembled together by changing the thickness of the air cavity  30  while fixing longitudinal/lateral dimensions of the air cavity  30  with respect to each sound absorbing structure. Alternatively, it is possible to change longitudinal/lateral dimensions of the air cavity  30  while fixing the thickness of the air cavity  30  with respect to each sound absorbing structure. Furthermore, it is possible to change both the longitudinal/lateral dimensions and the thickness of the air cavity  30  with respect to each sound absorbing structure. 
     It is possible to provide a sound absorbing structure shown in  FIG. 10 , in which the inside space of the housing  10  is partitioned using a grid-shaped partition member  13  so as to form plural sections of the air cavity  30 , which are covered with the vibration member  20 . A plurality of secondary members  25  is adhered onto the exterior surface of the primary member  24  of the vibration member  20  at regions which are opposite to the plural sections of the air cavity  30  and each of which includes the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20 . In this constitution, it is possible to change the weight of each secondary member  25 . Thus, it is possible to achieve sound absorption at various frequencies. 
     It is possible to arrange each of the first member  21 , the secondary member  25 , and the first region  23  at another position each including the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20  other than the center portion of the vibration member  20 . 
     Alternatively, it is possible to arrange each of the first member  21  and the secondary member  25  at the periphery of the prescribed area causing the maximum amplitude of bending vibration in the vibration member  20 . Herein, the thickness of the periphery of the prescribed area causing the maximum amplitude of bending vibration of the vibration member  20  can be increased to be larger than the thickness of the prescribed area of the vibration member  20 . 
     It is possible to arrange each of the first member  21  and the secondary member  25  on at least a part of the vibration member  20  except for the prescribed area causing the node or minimum amplitude of bending vibration. Herein, the thickness of the periphery of the prescribed area causing the node or minimum amplitude of bending vibration can be increased to be larger than the thickness of the prescribed area of the vibration member  20 . 
     In the present embodiment, the vibration member  20  is fixed to the housing  10 , thus limiting the displacement (or movement) and rotation at the fixed point. Alternatively, the vibration member  20  can be simply supported by the housing  10  so as to limit the displacement thereof relative to the housing  10  but to allow the rotation thereof. 
     It is possible to establish a simply supported state (limiting the displacement) or a freely supported state between the vibration member  20  and the housing  10 . Alternatively, it is possible to form a complex vibration structure combining the aforementioned vibration members. 
     It is possible to realize the constitution in which the density of a part of the vibration member  20  other than the prescribed area causing the node or minimum amplitude of bending vibration differs from the density of the prescribed area of the vibration member  20  by adopting different densities to the first member  21  and the second member  22 . Herein, a plurality of first members  21  having different densities is prepared in advance and is each selected for use in the second member  22 . Thus, it is possible to adjust the resonance frequency of the spring-mass system and the resonance frequency of the bending system, thus adjusting the frequency causing the peak sound absorption coefficient. 
     In the constitution in which the thickness of a part of the vibration member  20  other than the prescribed area causing the node or minimum amplitude of bending vibration differs from the thickness of the prescribed area of the vibration member  20 , it is possible to adjust the resonance frequency of the spring-mass system and the resonance frequency of the bending system by reducing the thickness of the first region  23  via cutting or by increasing the thickness of the first region  23  using an additional member (which is composed of the same material as the vibration member  20 ), thus adjusting the frequency causing the peak sound absorption coefficient. 
     It is possible to realize the constitution in which the secondary member  25  is added to a part of the vibration member  20  except for the prescribed area causing the node or minimum amplitude of bending vibration. Herein, a plurality of secondary members  25  having different densities is prepared in advance and is each selected for use in the primary member  24 . Thus, it is possible to adjust the resonance frequency of the spring-mass system and the resonance frequency of the bending system, thus adjusting the frequency causing the peak sound absorption coefficient. 
     According to the above adjustment method applied to the sound absorbing structure, it is possible to easily adjust the resonance frequency of the spring-mass system and the resonance frequency of the bending system, thus adjusting the frequency causing the peak sound absorption coefficient with ease. 
     It is possible to locate the sound absorbing structure, in which the density of a part of the vibration member  20  (constituted of the first member  21  and the second member  22 ) except for the prescribed area causing the node or minimum amplitude of bending vibration differs from the density of the prescribed area of the vibration member  20 , at the place causing noise whose frequency matches the frequency causing the peak sound absorption coefficient. 
     It is possible to locate the sound absorbing structure, in which the vibration member  20  does not have uniform thickness so that the thickness of a part of the vibration member  20  except for the prescribed area causing the node or minimum amplitude of bending vibration differs from the thickness of the prescribed area of the vibration member  20 , at the place causing noise whose frequency matches the frequency of the peak sound absorption coefficient. 
     It is possible to locate the sound absorbing structure, in which the secondary member  25  is disposed in a part of the vibration member  20  (constituted of the primary member  24  and the secondary member  25 ) except for the prescribed area causing the node or minimum amplitude of bending vibration, at the place causing noise whose frequency matches the frequency causing the peak sound absorption coefficient. 
     According to the above noise reduction method in which the sound absorbing structure is located at the place causing noise so as to reduce noise, the vibration member  20  vibrates so as to consume energy of noise, thus reducing noise. 
     As the places causing noise, it is possible to list the internal spaces of transportation systems such as vehicles and airplanes, factories, and machines operated at construction sites. 
     2. Second Embodiment 
       FIG. 11  is a perspective view showing the external appearance of a four-door sedan vehicle  100  adopting a sound absorber SA_ 1  according to a second embodiment of the present invention. In the vehicle  100 , a hood (or a bonnet)  101 , four doors  102 , and a trunk door  103  are each attached to a chassis  110  corresponding to a base of a vehicle structure in an open/close manner. 
       FIG. 12  is a side view showing the chassis  110  of the vehicle  100 . The chassis  110  is equipped with a floor  111 , a front pillar  112  extending upwardly from the floor  111 , a center pillar  113 , a rear pillar  114 , a roof  115  (which is supported by the pillars  112 ,  113 , and  114 ), an engine partition  116  for partitioning the internal space of the vehicle  100  into a compartment  105  and an engine room  106 , and a trunk partition  120  for partitioning between the compartment  105  and a luggage space  107 . The trunk partition  120  is equipped with a rear package tray  130 . 
     As shown in  FIG. 12 , the trunk partition  120  includes a back support of a rear seat and is thus bent in an L-shape in cross section. 
     The following description is based on the premise that the trunk partition  120  partitions between the compartment  105  and the luggage space  107 . 
     The second embodiment is characterized in that the box-shaped sound absorber SA_ 1  is attached to the trunk partition  120  of the chassis  110 .  FIG. 13  is a cross-sectional view of a position Pa in  FIG. 10 , and  FIG. 12  is an exploded sectional view for assembling the sound absorber SA_ 1  with the trunk partition  120 .  FIGS. 13 and 14  show a single sound absorber SA_ 1 ; in actuality, a plurality of sound absorbers SA_ 1  having different shapes is installed in the trunk partition  120  as show in  FIG. 11 . In this connection, the shape of the sound absorber SA_ 1  is similar to or identical to the shape of the trunk partition  120  for partitioning between the compartment  105  and the luggage space  107 . 
     As shown in  FIG. 13 , the rear package tray  130  is attached to the trunk partition  120  so as to form a trunk board  140 . 
     The rear package tray  130  is constituted of a core material  131  composed of a wooden fiber board and a fabric having acoustic transmissivity. The surface of the core material  131  is covered with a surface material  135 . A through-hole  132  having a rectangular opening is formed in a part of the core material  131  positioned opposite to the sound absorber SA_ 1 . That is, the through-hole  132  of the surface material  135  forms an acoustic transmitter  136  which transmits sound pressure occurring in the compartment  105  toward the sound absorber SA_ 1 . The opening shape of the through-hole  132  is not necessarily limited to the rectangular shape, which can be changed to a circular shape. That is, the opening shape of the through-hole  132  is determined to transmit air of the compartment  105  to the sound absorber SA_ 1 . 
     3. Third Embodiment 
     A third embodiment of the present invention will be described with reference to  FIGS. 15 and 16 . In  FIG. 15 , the constituent elements identical to those shown in  FIGS. 11 and 12  are designated by the same reference numerals. 
       FIG. 15  is a perspective view showing the external appearance of the four-door sedan vehicle  100  adopting a sound absorber SA_ 2  according to the third embodiment of the present invention. The hood  101 , the four doors  102 , and the trunk door  103  are each attached to the chassis  110  corresponding to the base of the vehicle structure in an open/close manner. The chassis  110  of the vehicle  100  is formed as shown in  FIG. 12 . Compared to the second embodiment in which the sound absorber SA_ 1  is attached to the rear package tray  130 , the third embodiment is designed to attach the sound absorber SA_ 2  to a roof  240 . The roof  240  is constituted of a roof outer panel (corresponding to the roof  115  in  FIG. 10 ) and a roof inner panel  230 . 
     The third embodiment is characterized in that the box-shaped sound absorber SA_ 2  is attached to the roof  240  of the vehicle  100 . In  FIG. 15 , the sound absorber SA_ 2  includes four sound absorbers SA_ 2   a  and SA_ 2   b  having different sizes in total. 
     In the roof  240 , the roof inner panel  230  is clipped to the roof outer panel forming a part of the chassis  110 . 
     In the roof inner panel  230 , the surface of a core material  231  composed of a wooden fiber board is covered with a surface material  238  composed of a fabric having acoustic transmissivity. A rectangular through-hole  232 A is formed in the core material  231  in proximity to the rear seat, wherein a part of the surface material  238  positioned opposite to the through-hole  232 A forms an acoustic transmitter  239 A. The sound absorber SA_ 2  communicates with the compartment  105  via the acoustic transmitter  239 A. The acoustic transmitter  239 A is not necessarily attached to the roof  240  in proximity to the rear seat, which can be changed to the front seat.  FIG. 16  is a graph showing a noise reduction effect at the rear seat. 
     4. Fourth Embodiment 
     A fourth embodiment is characterized in that a box-shaped sound absorber SA_ 3  is attached to a sun visor  330  of the vehicle  100 .  FIG. 17  is a development of the sun visor  330  attached to the upper portion of the roof  115  of the vehicle  100 , and  FIG. 18  is a cross-sectional view taken along line A-A in  FIG. 17 . 
     The sun visor  330  is constituted of a panel-shaped light insulation portion  340  and an L-shaped support shaft  350  for supporting the light insulation portion  340  in a rotatable manner. 
     The light insulation portion  340  is constituted of a core material  341  composed of an ABC resin (or engineering plastic) and a surface material  360  composed of a nonwoven fabric having acoustic transmissivity. The core material  341  is covered with the surface material  360  in such a way that respective sides of the surface material  360  are bonded together so as to cover the surface and backside of the core material  341 . 
     A bracket  351  used for attaching the sun visor  330  to the roof  115  is unified with one end of the support shaft  350 . A pair of screw holes  352  is formed in the bracket  351 . The sun visor  330  is fixed to the roof  115  by screwing the bracket  351  to a predetermined position of the roof  115 . 
     A rectangular through-hole  342  used for attaching the sound absorber SA_ 3  is formed in the core material  341 . The through-hole  342  of the surface material  360  serves as an acoustic transmitter  361 . 
     5. Fifth Embodiment 
     A fifth embodiment is characterized in that a box-shaped sound absorber SA_ 4  is attached to the rear pillar  114 . In actuality, it is possible to attach a plurality of sound absorbers SA_ 4  having different shapes to the rear pillar  114 . 
       FIG. 19  is a cross-sectional view of the sound absorber SA_ 4  attached to the rear pillar  114 . The rear pillar  114  is equipped with a rear outer panel  420  (which forms a part of the chassis  110 ) and a rear inner panel  430  (which is attached to the rear outer panel  420 ). 
     The rear outer panel  420  is formed using a planar portion  421  of a rectangular parallelepiped shape having a trapezoidal cross section. Fitting holes  422  fitted with the rear inner panel  430  and fitting holes  423  fitted with projections of the sound absorber SA_ 4  are formed in the planar portion  421 . A rear glass  117  is disposed at one end of the rear outer panel  420  via a seal (not shown), and a door glass  118  is disposed at the other end of the rear outer panel  420  via a seal (not shown). 
     The rear inner panel  430  is constituted of a core material  431  composed of a polypropylene resin and a surface material  439  composed of a fabric having acoustic transmissivity, wherein the surface of the core material  431  is covered with the surface material  439 . 
     The core material  431  is constituted of a circular portion  432  and an incline portion  433  (which extends outside of the circular portion  432 ). A plurality of through-holes  434  is formed in the circular portion  432 . The rear pillar  114  communicates with the compartment  105  via the through-holes  434 . 
       FIG. 20  shows a variation of the fifth embodiment in which the sound absorber SA_ 4  is inserted into a rectangular recess  436  of the core material  431 , which is opened in the compartment  105 . Fitting holes  436 A are formed in the bottom portion of the recess  436 . The sound absorber SA_ 4  is fixed inside the recess  436  while the projections thereof are inserted into the fitting holes  436 A. 
     The present embodiment is designed to attach the sound absorber SA_ 4  to the rear pillar  114 ; but this is not a restriction. For instance, it is possible to attach the sound absorber SA_ 4  to the front pillar  112  or the center pillar  113 . 
     6. Sixth Embodiment 
     A sixth embodiment is characterized in that a box-shaped sound absorber SA_ 5  is attached to the door  102  of the vehicle  100 . 
     The interior of the door  102  includes a door-trim base  520 , an interior material  530 , an armrest  540 , and a door pocket  550 . The interior material  530  is constituted of the door-trim base  520  composed of a synthetic resin and a surface material  535  composed of a nonwoven fabric having acoustic transmissivity. The surface of the door-trim base  520  is covered with the surface material  535 . 
       FIG. 21  shows that the sound absorber SA_ 5  is installed inside the armrest  540  in communication with a plurality of through-holes  520 A formed in the door-trim base  520 . 
       FIG. 22  shows that a plurality of sound absorbers SA_ 5  is installed inside the interior material  530  in communication with a plurality of through-holes  520 A, while another sound absorber SA_ 5  is used for the door pocket  550 . 
     7. Seventh Embodiment 
     A seventh embodiment is characterized in that a sound absorber SA_ 6  composed of a plurality of sound absorbing pipes is installed in the floor  111  of the vehicle  100 . As shown in  FIG. 23 , a sound absorber  630  (i.e., the sound absorber SA_ 6 ) is installed in a recess  600  formed in the floor  111 . 
     The sound absorber  630  is formed by interconnecting and unifying a plurality of pipes  631  (e.g.  631 - 1  to  631 - 9 ) having different lengths which are linearly aligned. Each pipe  631  is a linear rigid pipe which is composed of a synthetic resin and whose cross section has a circular shape. One end of each pipe  631  is closed in the form of a closed portion  632 , while the other end is opened in the form of an opening (serving as an acoustic transmitter)  633 , wherein the inside of each pipe  631  is a hollow portion  634 . The opening  633  of each pipe  631  communicates with the compartment  105  via a gap which is formed when the door  102  is closed. 
       FIG. 24  shows the relationship between adjacent pipes  631 - i  and  631 - j  whose hollow portions have different lengths L 1  and L 2 . Sound waves of wavelengths λ 1  and λ 2  (where L 1 =¼, L 2 = 2/4), which are four times longer than the lengths L 1  and L 2 , create standing waves S 1  and S 2 , which in turn cause vibrations repeatedly propagating in the pipes  631 - i  and  631 - j  so as to consume acoustic energy, thus achieving sound absorption about the wavelengths λ 1  and λ 2 . 
       FIG. 25A  shows a variation of the seventh embodiment, wherein the pipe  631  is disposed in a side-sill  601  of the floor  111  such that the hollow portion  634  thereof extends in the front-back direction of the vehicle  100 .  FIG. 25B  is an illustration of the side-sill  601  viewed in the X-direction of  FIG. 25A . 
     8. Eighth Embodiment 
     An eighth embodiment is characterized in that a sound absorber SA_ 8  is installed in an instrument panel  700  disposed below a front glass  105 F in the compartment  105  of the vehicle  100 . 
       FIG. 26  is a perspective view showing the external appearance of the instrument panel  700 . The sound absorber SA_ 8  is disposed in a space S between the instrument panel  700  and the engine partition  116 . 
     The instrument panel  700  is equipped with various instruments, speakers  701  and  702  of an audio device, and warm/cool air outlets  703 . A plurality of defroster outlets  704  is formed in the upper surface of the instrument panel  700  so as to output a warm air supplied from an air-conditioner unit  705 . A glove box  707  is arranged in the lower-left position of the instrument panel  700  and is closed by a cover  708 . 
       FIG. 27  shows the internal structure of the instrument panel  700  and is a cross-sectional view taken along line X-X in  FIG. 24 . The air-conditioner unit  705 , a defrost duct  706 , and a plurality of sound absorbers SA_ 8 A are arranged in the internal space S of the instrument panel  700 . The internal space S of the instrument panel  700  communicates with the compartment  105  via a hole H. 
       FIG. 28  is an illustration of the instrument panel  700  viewed in the I-direction in  FIG. 27 , which shows the arrangement of the sound absorbers SA_ 8 A in the upper view. A plurality of sound absorbers SA_ 8 A is disposed in a wide range of area on the upper side of the interior wall of the instrument panel  700 . In addition, the sound absorbers SA_ 8 A are disposed in proximity to the defrost duct  706  and the other portion of the interior wall of the instrument panel  700 . 
       FIG. 29  is a perspective view showing the external appearance of the instrument panel  700  adopting sound absorbers SA_ 8 B according to a variation of the eighth embodiment. A speaker SP together with two sound absorbers SA_ 8 B are disposed on each of the right and left sides of the upper surface of the instrument panel  700 .  FIG. 30  is a cross-sectional view taken along line Y-Y in  FIG. 27 , which shows the internal structure of the instrument panel  700 . A recess  730  is formed in each of the right and left sides of the upper surface of the instrument panel  700 . One speaker SP and two sound absorbers SA_ 8 B are disposed inside the recess  730 , the opening of which is covered with a net N. The other sound absorbers SA_ 8 B are disposed on the interior wall of the instrument panel  700  as well. In this constitution, the sound absorbers SA_ 8 B consume acoustic energy propagated from the compartment  105  and energy of an engine sound emitted from the engine room  106  via the engine partition  116 , thus achieving sound absorption. 
     In the above, the sound absorbers SA_ 8 B are not necessarily disposed in the recess  730  holding the speaker SP; hence, they can be disposed in another space for arranging instruments and the like. The sound absorbers SA_ 8 B are not necessarily covered with the net N; hence, they can be rearranged to communicate with the compartment  105  via a grill, mesh, and slits. 
     9. Ninth Embodiment 
     A ninth embodiment is characterized in that a three-dimensional sound absorbing structure is formed by combining a plurality of sound absorbers. 
     Specifically, a panel-vibration sound absorbing structure  800  according to the ninth embodiment includes a plurality of sound absorbers  820  in a housing  810  thereof. 
     Examples for attaching the present embodiment to various positions of the vehicle  100  will be described with reference to  FIGS. 31A to 31E .  FIG. 31A  is a cross-sectional view of the instrument panel  700  equipped with the panel-vibration sound absorbing structure  800 , and  FIG. 31B  is an upper plan view of the instrument panel  700 . 
     As shown in  FIGS. 31A and 31B , the housing  810  of the panel-vibration sound absorbing structure  800  is attached to a lower position of the instrument panel  700 , wherein an elongated hole  733  which is elongated in the longitudinal direction is formed in the instrument panel  700  in proximity to the boundary of a front glass  105 F and is covered with a grill G 1 . The housing  810  is curved in the longitudinal direction, and the opening thereof has substantially the same dimensions as the elongated hole  733  of the instrument panel  700 . That is, the panel-vibration sound absorbing structure  800  is attached to the lower position of the instrument panel  700  in such a way that the opening of the housing  810  is positioned opposite to the elongated hole  733  of the instrument panel  700 . 
     A plurality of sound absorbers  820  is disposed in the housing  810  such that the vibration surfaces thereof are perpendicular to a virtual opening plane encompassed by the opening edge of the housing  810 . Specifically, the vibration surfaces of the sound absorbers  820  are disposed in parallel with the front-back direction of the vehicle  100 , wherein the sound absorbers  820  are disposed in the housing  810  along the elongated hole  733  of the instrument panel  700  in the right-left direction of the vehicle  100 . 
     By arranging two or more sound absorbers  820  per unit area corresponding to the surface area of the sound absorber  820  in the housing  810 , it is possible to achieve the panel-vibration sound absorbing structure  800  having a high sound absorption coefficient. It is preferable that the panel-vibration sound absorbing structure  800  of the present embodiment be disposed at a predetermined position at which sound pressure tends to increase in the vehicle  100 . Since the sound absorbers  820  are disposed in the housing  810  such that the vibration surfaces thereof cross the opening plane of the housing  810 , it is possible to appropriately change the directions of disposing the sound absorbers  820 . In  FIG. 31C , a plurality of sound absorbers  830  is disposed in the housing  810  of the panel-vibration sound absorbing structure  800  such that the vibration surfaces thereof are aligned in parallel with the left-right direction of the vehicle  100 . Of course, it is possible to align the sound absorbers  820  and  830  such that their vibration surfaces are not perpendicular to the opening plane of the housing  810 . 
       FIG. 31D  shows an example in which a tray  117 T beneath the rear glass  117  of the vehicle  100  serves as a housing  811  of the panel-vibration sound absorbing structure  800 . The opening of the housing  811  is covered with a grill G 2 . A plurality of sound absorbers  840  is disposed in the housing  811  so as to effectively reduce noise in the rear seat of the vehicle  100 . 
       FIG. 31E  shows an example in which a housing  812  of the panel-vibration sound absorbing structure  800  is disposed beneath the floor  111  of the vehicle  100 . The floor  111  is equipped with a perforated metal so as to achieve acoustic transmissivity, wherein a floor carpet  111 C is attached to the upper surface of the floor  111 . The housing  812  is attached beneath the floor  111  such that the opening thereof is directed to the floor  111 . In order to increase a sound absorption effect, a felt F is adhered to the bottom of the housing  812  and is covered with a sound insulation layer SP composed of a rubber sheet, so that a plurality of sound absorbers  850  is aligned on the sound insulation layer SP. In this constitution, it is possible to effectively reduce road noise entering into the compartment  105  from below the vehicle  100 . 
       FIG. 32A  shows that a panel-vibration sound absorbing structure  800 A having a plurality of housings  815   a ,  815   b , and  815   c  is installed in a front seat  100 F of the vehicle  100 . Grill-shaped openings (drawn with dotted lines) are formed in the front seat  100 F in proximity to the openings of the housings  815   a ,  815   b , and  815   c . A plurality of sound absorbers  860   a  is disposed in the housing  815   a ; a plurality of sound absorbers  860   b  is disposed in the housing  815   b ; and a plurality of sound absorbers  860   c  is disposed in the housing  815   c . In this constitution, it is possible to absorb noise in the compartment  105 , and it is possible to reduce acoustic energy transmitted to a human body from the front seat  100 F. 
       FIG. 32B  shows an example in which sound waves such as noise are guided to a panel-vibration sound absorbing structure  800 B installed in a rear seat  100 R so as to effectively absorb sound. The overall constitution of the panel-vibration sound absorbing structure  800 B is roughly identical to that of the panel-vibration sound absorbing structure  800 A. An opening  800 P is formed in the upper section of a space formed in the backside of a back support of the rear seat  100 R, wherein the space communicates with the opening of the housing  815   b . When sound waves enter into the backside of the rear seat  100 R via the opening  800 P in proximity to the rear seat  100 R, it is possible to effectively suppress them. 
     Next, variations of the present embodiment will be described with respect to the alignment of sound absorbers  920  in a housing  910  of a panel-vibration sound absorbing structure  900  in conjunction with  FIGS. 33A to 33E . 
       FIG. 33A  shows that a plurality of sound absorbers  920 A is disposed in a housing  910 A of a panel-vibration sound absorbing structure  900 A. The sound absorbers  920 A have support members  940 A, each of which has a hexahedron shape whose two opposite sides are removed so as to leave four sides, wherein a single surface is formed perpendicular to the center of each of the four sides. When the support member  940 A is subjected to cutting in a direction which is perpendicular to one pair of opposite sides within the four sides and in a direction which is parallel to the other pair of opposite sides, the cross-sectional shape thereof is roughly H-shaped. Due to the above constitution of the support member  940 A, openings are formed on opposite ends of each side, wherein the sound absorber  920 A is assembled in such a way that each opening joins each vibration member  930 A. 
     An opening is formed on one side of the housing  910 A. The vibration surfaces of the vibration members  930 A are aligned to cross the virtual opening plane encompassed by the edge of the opening of the housing  910 A. This makes it possible to easily adjust the number of the sound absorbers  920 A disposed in the housing  910 A of the panel-vibration sound absorbing structure  900 A, thus improving the sound absorption coefficient. 
     It is possible to incline the positions of the sound absorbers  920 A linearly aligned in the panel-vibration sound absorbing structure  900 A shown in  FIG. 33A .  FIG. 33B  shows a panel-vibration sound absorbing structure  900 B enclosed in a housing  910 B in which a plurality of sound absorbers  920 B is disposed and inclined in position. This makes it possible to reduce the height without reducing the overall area of the vibration surfaces of the sound absorbers  920 B. Thus, it is possible to achieve the panel-vibration sound absorbing structure  900 B having a small height and a high sound absorption coefficient. 
     A plurality of vibration members can be formed using one sheet. Similar to the panel-vibration sound absorbing structure  900 A shown in  FIG. 33A , a plurality of support members  940 C is disposed in a housing  900 C of a panel-vibration sound absorbing structure  900 C, wherein the support members  940 C join together while closing openings thereof by bending one sheet. This produces a panel-shaped structure which is limited in position by the openings of the support members  940 C and which is used to form vibration members  930 C so as to absorb sound. This constitution allows one sheet to form a plurality of sound absorbers  920 C equipped with a plurality of vibration members  930 C; hence, it is possible to easily produce the panel-vibration sound absorbing structure  900 C. 
     It is possible to provide different shapes to the support members  940 A of the sound absorbers  920 A shown in  FIG. 33A . In a panel-vibration sound absorbing structure  900 D shown in  FIG. 33D , panel-shaped support members  940 D are attached to the bottom of a housing  910 D so as to direct toward the upper opening. A bent sheet is attached to the ends of the support members  940 D and the bottom of the housing  910 D, thus forming vibration members  930 D supported by the support members  940 D. This constitution allows one sheet to form a plurality of sound absorbers  920 D equipped with a plurality of vibration members  930 D inside the housing  910 D; hence, it is possible to easily produce the panel-vibration sound absorbing structure  900 D. 
     Since the support member of the sound absorber is used to support the vibration member and to form an air cavity on one side thereof, it is unnecessary to form the air cavity in the surrounding area of the support member.  FIG. 33E  shows a panel-vibration sound absorbing structure  900 E in which sound absorbers  920 E are subjected to cutting in a direction perpendicular to each side and the bottom of a housing  910 E. 
       FIG. 33E  shows that a pair of opposite sides of the sound absorber  920 E is positioned opposite to a support member  940 E and that in one side within the opposite sides, the support member  940 E is partially cut out in the range from the position which comes in contact with a plane perpendicular to the center of each side to one vibration member  930 E, while in the other side, the support member  940 E is partially cut out in the range from the position which comes in contact with the plane to the other vibration member  930 E. That is, the sound absorber  920 E whose support member  940 E is partially cut out is integrally unified with the vibration member  930 E and is fixed to the center of the side wall of the housing  910 E. In the panel-vibration sound absorbing structure  900 E of  FIG. 33E , the sound absorber  920 E is constituted of the vibration member  930 E and the support member  940 E. 
     In  FIG. 33E , the support member  940 E is fixed to the center of the side wall of the housing  910 E so that an air cavity is formed between the vibration member  930 E and the support member  940 E while a relatively large air cavity is also formed beneath the vibration member  930 E and the support member  940 E (i.e. above the bottom of the housing  910 E). This constitution allows the total volume of the air cavities to be easily adjusted, thus easily adjusting the frequency band subjected to sound absorption. 
     The shape of the vibration member of the sound absorber in the panel-vibration sound absorbing structure is not necessarily limited to the square shape, which can be changed to various shapes such as polygonal shapes, circular shapes, and elliptic shapes. In addition, it is possible to control the frequency band of sound absorption by additionally forming holes in the vibration member and the support member. 
     Lastly, the present invention is not necessarily limited to the above embodiments and variations, which can be further modified within the scope of the invention as defined in the appended claims.