Patent Publication Number: US-RE47449-E

Title: Vehicle sound absorption structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vehicle sound absorption structure. 
     2. Description of Related Art 
     Japanese Utility Model No. 3048125 (JP-U-3048125) discloses an acoustic insulation material that is intended to prevent noise, such as engine sound generated in the engine compartment, road noise generated during driving, and wind whistle sound, from entering the car cabin through the door seal. The acoustic insulation material, when in use, is fitted onto the cross section of the front fender of the car. The acoustic insulation material uses a cushion material that has a predetermined density. The cushion material is either coated with a film or not coated with any film, but subjected to waterproof treatment. 
     As described in JP-U-3048125, the special cushion material that has a predetermined density is coated with a film or is subjected to waterproof treatment on the surface of the cushion material. However, this causes an increase in costs of the cushion material, an increase in number of parts, and an increase in processing costs and management costs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low-cost vehicle sound absorption structure. 
     A first aspect of the present invention is related to a vehicle sound absorption structure that has: a recessed cell that has an opening on one end thereof; a divider that is provided within the cell and that divides an acoustic wave entering the cell from the opening into two acoustic waves; reflectors that reflect one of the two acoustic waves divided at the divider and reflect the other acoustic wave respectively toward the opening, and that generate a phase difference between the one acoustic wave and the other acoustic wave; and an interfering portion that is provided within the cell and that causes the one acoustic wave and the other acoustic wave, which have been reflected respectively at the reflectors, to interfere with each other. 
     According to the first aspect of the invention, an acoustic wave enters the cell from the opening of the cell and is divided at the divider into two acoustic waves. When the two acoustic waves are reflected respectively at the reflectors, a phase difference is generated between the one acoustic wave and the other acoustic wave. The one acoustic wave and the other acoustic wave interfere with each other at the interfering portion, while having the phase difference, so that a sound absorption effect is produced. This allows the vehicle sound absorption structure to be provided at low cost without increasing the number of parts. 
     According to the first aspect of the invention, an end of the divider on the side of the opening may be positioned more inward in the cell than the opening and positioned closer to the opening than one of the reflectors. The one of the reflectors may be positioned closer to the opening than the other reflector. The interfering portion may be positioned between the end and the opening. 
     As described above, the vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of providing the low-cost vehicle sound absorption structure. 
     In the above aspect, the divider may have an end on the side of the opening and positioned more inward in the cell than the opening, while being positioned closer to the opening than one of the reflectors, the one of the reflectors being positioned closer to the opening than the other reflector. And, the interfering portion may be positioned between the opening and the end of the divider. This allows an acoustic wave that enters the cell to be absorbed in the cell. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of producing the sound absorption effect in the cell in a stable manner. 
     In the first aspect of the invention, the vehicle sound absorption structure may further include a cover member that covers the opening. The cover member may have a slit that is provided on an extension of the divider. 
     According to the first aspect of the invention, when incident acoustic waves obliquely to the depth direction of the cell enter the cell through the slit, the acoustic waves become in phase due to a diffraction phenomenon through the slit. This ensures that the one acoustic wave and the other acoustic wave that are divided at the divider have a desired phase difference therebetween. Therefore, the vehicle sound absorption structure has improved sound absorption performance with respect to the acoustic wave frequency of the sound to be absorbed. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of improving the sound absorption performance with respect to the acoustic wave frequency of the sound to be absorbed. 
     In the first aspect of the invention, the slit may extend in a direction along the end of the divider. 
     As described above, the slit extends in a direction along the end of the divider. Thus, when acoustic waves enter the cell through the slit, the acoustic waves become in phase over a wide frequency range due to a diffraction phenomenon. Therefore, the vehicle sound absorption structure has further improved sound absorption performance with respect to the acoustic wave frequency of the sound to be absorbed. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of further improving the sound absorption performance with respect to the acoustic wave frequency of the sound to be absorbed. 
     In the first aspect of the invention, the cell may be partitioned by a wall that includes a base part and the other part, and the base part may be thinner than the other part. 
     As described above, the cell is partitioned by a wall that includes a base part and the other part, and the base part is thinner than the other part. Thus, upon the entry of an acoustic wave into the cell, film resonance occurs on the wall that partitions the cell. Due to the film resonance, energy of the acoustic wave is converted into kinetic energy, so that the sound is absorbed. This further improves the sound absorption performance. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of further improving the sound absorption performance. 
     In the first aspect of the invention, the vehicle sound absorption structure may also serve as a fender protector that shields a gap between a vehicle body framework and a rear end of a front fender panel, the rear end being positioned inner side of the vehicle. The opening may be formed toward the front side of the vehicle. 
     The vehicle sound absorption structure according to the first aspect of the invention also serves as a fender protector, and the opening is formed toward the front side of the vehicle. This allows noise from a front tyre to be absorbed without increasing the number of parts. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of absorbing the noise from the front tyre without increasing the number of parts. 
     In the first aspect of the invention, the vehicle sound absorption structure may be provided on a front surface of the fender protector that shields the gap between the vehicle body framework and the rear end of the front fender panel, in which the front surface faces toward the front side of the vehicle, and the rear end is positioned inner side of the vehicle. 
     As described above, the vehicle sound absorption structure is provided on the front surface of the fender protector, the front surface facing toward the front side of the vehicle. This enables the noise from the front tyre to be absorbed. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of absorbing road noise from the front tyre. 
     In the first aspect of the invention, the vehicle sound absorption structure may be formed in combination with the fender protector. The front surface of the fender protector may be utilised as the reflectors, and the front surface faces toward the front side of the vehicle. 
     As described above, the front surface of the fender protector is utilised as the reflectors. This enables reductions in material costs of and in weight of the vehicle-sound absorption structure, compared to the case when the reflector is continuously formed with a wall of the cell. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of reducing the material costs and the weight, compared to the case when the reflector is continuously formed with a wall of the cell. 
     In the first aspect of the invention, the vehicle sound absorption structure may also serve as at least one of an engine cover and a floor under cover. 
     In the first aspect of the invention, the vehicle sound absorption structure may also serve as a fender liner that is formed on an inside of the front fender panel to cover the front tyre. The opening may be formed toward the front side of the vehicle. 
     In the first aspect of the invention, a plurality of the cells may be arranged in parallel in a vertical direction with the opening of each of the cells facing toward the front side of the vehicle. Communicating holes communicate with a lower adjacent one of the cells may be formed on lower walls of the cells. 
     As described above, a plurality of the cells are arranged in parallel in a vertical direction with the opening of each of the cells facing toward the front side of the vehicle. This allows the sound absorption effect to be produced in a wide region. In addition, the cells each have lower walls through which communication holes are formed. The communication holes communicate with a lower adjacent one of the cells. This provides an additional function as an expansion silencer. Further, water in the cell can be drained through the communication holes. This further improves the sound absorption performance, while preventing the water from remaining in the cell to maintain the sound absorption performance. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of further improving the sound absorption performance, while preventing the water from remaining in the cell to maintain the sound absorption performance. 
     In the first aspect of the invention, the lower walls may be angled downwardly to the front side of the vehicle. 
     As described above, the lower walls of the cell are angled downwardly to the front side of the vehicle. This makes it difficult for the water to remain in the cell. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of making it difficult for the water to remain in the cell. 
     In the first aspect of the invention, the communication holes may be provided on the lower walls at respective ends that face toward the rear side of the vehicle. 
     As described above, the communication holes are provided on the lower walls at respective ends that face toward the rear side of the vehicle. Thus, when the vehicle is accelerating, the water in the cell flows toward the rear side of the vehicle to be efficiently discharged out of the cell through the communication holes. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of efficiently discharging the water flowing in the cell toward the rear side of the vehicle out of the cell through the communication holes when the vehicle is accelerating. 
     In the first aspect of the invention, the vehicle sound absorption structure may further include a Helmholtz resonator that is provided adjacent to the cells and that has a neck and a cavity, and the neck may be open in the same direction as the opening. The neck may be an opening of the Helmholtz resonator. This opening may be defined by the walls. 
     According to the first aspect of the invention, the Helmholtz resonator is provided adjacent to the cells to further improve the sound absorption performance. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of further improving the sound absorption performance by using the Helmholtz resonator that is provided adjacent to the cells. 
     In the first aspect of the invention, a volume of the cavity, a diameter of the neck or a length of the neck may be varied in order to adjust sound-absorption characteristics of the Helmholtz resonator. 
     In the first aspect of the invention, the cavity may be formed into a trapezium or trapezoid shape. 
     In the first aspect of the invention, the vehicle sound absorption structure may further has: a primary tubular passage that is provided adjacent to the cells and is open toward the same direction as the opening; and a secondary tubular passage that is branched from the primary tubular passage at a midsection of the primary tubular passage and that rejoins the primary tubular passage at a point where the primary tubular passage and the secondary tubular passage join together. The primary tubular passage and the secondary tubular passage may respectively have predetermined lengths such that an acoustic wave passing through the primary tubular passage alone and another acoustic wave passing through the primary tubular passage via the secondary tubular passage have a phase difference therebetween at the point where the primary tubular passage and the secondary tubular passage join together. 
     As described above, the acoustic wave passing through the primary tubular passage alone and the another acoustic wave passing through the primary tubular passage via the secondary tubular passage have a phase difference therebetween, and then interfere with each other at the point where the primary tubular passage and the secondary tubular passage join together. Consequently, the sound absorption effect is produced. This still further improves the sound absorption performance. 
     The vehicle sound absorption structure according to the first aspect of the invention has an excellent effect of still further improving the sound absorption performance. 
     A second aspect of the present invention is related to a front fender structure that has: a front fender panel; a fender protector that shields a gap between a vehicle body framework and a rear end of the front fender panel, the rear end facing toward the inner side of the vehicle; and a vehicle sound absorption structure according to the first aspect of the invention, the vehicle sound absorption structure being mounted onto a front surface of the fender protector with the opening facing toward the front side of the vehicle, the front surface facing toward the front side of the vehicle. 
     According to the second aspect of the invention, noise from the front tyre is absorbed at low cost. 
     The vehicle sound absorption structure according to the second aspect of the invention has an excellent effect of absorbing noise from the front tyre at low cost. 
     In the second aspect of the invention, a plurality of the cells may be arranged in parallel in the vertical direction, the cells may each have lower walls through which communication holes are formed, and the communication holes may communicate with a lower adjacent one of the cells. 
     As described above, a plurality of the cells are arranged in parallel in the vertical direction. This allows the sound absorption effect to be produced in a wide region. In addition, the cells each have lower walls through which communication holes are formed. The communication holes communicate with a lower adjacent one of the cells. This provides an additional function as an expansion silencer. Further, water in the cell can be drained through the communication holes. This further improves the sound absorption performance, while preventing the water from remaining in the cell to maintain the sound absorption performance. 
     The vehicle sound absorption structure according to the second aspect of the invention has an excellent effect of further improving the sound absorption performance, while preventing the water from remaining in the cell to maintain the sound absorption performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a perspective view of a vehicle sound absorption structure according to a first embodiment of the invention, when viewed from the front side of the vehicle; 
         FIG. 2  is a perspective view of the vehicle sound absorption structure according to the first embodiment of the invention, when viewed from the rear side of the vehicle; 
         FIG. 3  is an enlarged sectional view of the vehicle sound absorption structure according to the first embodiment of the invention; 
         FIG. 4  is an enlarged sectional view of the vehicle sound absorption structure that illustrates a sound absorption effect within a cell; 
         FIG. 5  is an enlarged sectional view of a modification of the vehicle sound absorption structure according to the first embodiment of the invention; 
         FIG. 6  is a chart that illustrates the relationship between a frequency and a sound absorption coefficient; 
         FIG. 7  is a perspective view of a front fender structure that uses the vehicle sound absorption structure according to the first embodiment of the invention; 
         FIG. 8  is a sectional view of the front fender structure; 
         FIG. 9  is an enlarged perspective sectional view of the vehicle sound absorption structure according to a second embodiment of the invention; 
         FIG. 10  is a chart that illustrates the relationship between a frequency and a sound absorption coefficient; 
         FIG. 11  is an enlarged sectional view of the vehicle sound absorption structure according to a third embodiment of the invention; 
         FIG. 12  is a sectional view that illustrates film resonance that occurs on walls; 
         FIG. 13  is an enlarged sectional view of the vehicle sound absorption structure according to a fourth embodiment of the invention; 
         FIG. 14  is an enlarged sectional view of a modification of the vehicle sound absorption structure according to the fourth embodiment of the invention; 
         FIG. 15  is an enlarged sectional view of another modification of the vehicle sound absorption structure according to the fourth embodiment of the invention; 
         FIG. 16  is a chart that illustrates the relationship between a frequency and a sound absorption coefficient; 
         FIG. 17  is an enlarged sectional view of the vehicle sound absorption structure according to a fifth embodiment of the invention; 
         FIG. 18  is a perspective sectional view of the vehicle sound absorption structure according to a sixth embodiment of the invention; 
         FIG. 19  is a schematic view of an expansion silencer; 
         FIG. 20  is a sectional view of the front fender structure in which the vehicle sound absorption structure according to the other another embodiment of the invention is provided on a front surface of a fender protector; 
         FIG. 21  is an enlarged sectional view that illustrates an example in which the front surface of the fender protector is utilised as a reflector; and 
         FIG. 22  is a perspective sectional view of a modification of the vehicle sound absorption structure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     [First Embodiment] As illustrated in  FIG. 1  to  FIG. 4 , a vehicle sound absorption structure  10  according to the first embodiment of the invention is a single molded piece of, for example, synthetic resin. The vehicle sound absorption structure  10  has recessed cells  12 , dividers  14 , reflectors  16  and  18 , and interfering portions  22 . 
     As illustrated in  FIG. 3  and  FIG. 4 , the cells  12  are partitioned from each other by walls  26 . The cells  12  each have an opening  24  on one end of the cell  12 . In the vehicle sound absorption structure  10 , the plural cells  12  are arranged in parallel, for example, in the vertical and lateral directions with the respective openings  24  facing toward the front side of the vehicle. 
     The dividers  14  each are provided within the cell  12  to divide an acoustic wave that enters the cell  12  from the opening  24  into two acoustic waves. The divider  14  extends is positioned, for example, in the vehicle width direction between the left and right walls  26 . The divider  14  has an end  14 A on the side of the opening  24  and positioned more inward in the cell  12  than the opening  24 . In other words, the end  14 A is positioned more inward than a plane (not illustrated) that connects the walls  26  at their ends on the side of the openings  24 . Also, the end  14 A is positioned closer to the opening  24  than the reflector  16 . The reflector  16  is positioned closer to the opening  24  than the reflector  18 . 
     The divider  14  divides an acoustic wave into one acoustic wave  32  and the other acoustic wave  34 . The reflectors  16  and  18  are designed respectively to reflect the one acoustic wave  32  and the other acoustic wave  34  toward the opening  24 . The reflectors  16  and  18  are also designed to generate a phase difference between the one acoustic wave  32  and the other acoustic wave  34 . A depth from the opening  24  to the reflector  18  is greater than a depth from the opening  24  to the reflector  16 . 
     As illustrated in  FIG. 3  and  FIG. 4 , the interfering portions  22  each are provided within the cell  12 . The interfering portion  22  is designed to cause the one acoustic wave  32  reflected at the reflector  16  and the other acoustic wave  34  reflected at the reflector  18  to interfere with each other. The interfering portion  22  is positioned between the opening  24  and the end  14 A of the divider  14 . The end  14 A of the divider  14  is located more inward than the opening  24 , thus to define a space or the interfering portion  22  within the cell  12 . 
     It should be understood that, as illustrated in FIG,  5 , a wall  27  may extend to the opening  24  in a fashion similar to the wall  26 , and may have a window  27 A that serves as the interfering portion  22 . Then, an innermost one of the edges of the window  27 A in the cell  12  may be the end  14 A of the divider  14 . 
     As illustrated in  FIG. 4 , when λ is the wavelength of the acoustic wave frequency of the sound to be absorbed, a difference between the depth from the opening  24  to the reflector  16  and the depth from the opening  24  to the reflector  18  may be set to λ/4. In other words, the distance from the reflector  16  to the reflector  18  is defined as λ/4. An acoustic wave enters the cell  12  from the opening  24  and is divided at the divider  14  into the two acoustic waves  32  and  34 . Then, these acoustic waves  32  and  34  are reflected at the respective reflectors  16  and  18 . This results in a phase difference of 2×λ/4=λ/2 (a half wavelength) between the acoustic waves  32  and  34  when the acoustic waves  32  and  34  interfere with each other at the interfering portion  22  after the reflection. Then, the acoustic waves  32  and  34  interfere with each other and cancel each other out. This improves the sound absorption coefficient. 
     It should be understood that in the case the sound to be absorbed has plural frequencies, the cells  12  can have different structures that correspond to various wavelengths λ of the plural frequencies, and these cells  12  can be used in combination. 
     As illustrated in  FIG. 7  and  FIG. 8 , a front fender structure S has a front fender panel  36  and the vehicle sound absorption structure  10 . The vehicle sound absorption structure  10  also serves as a separate fender protector  28 . The fender protector  28  will be described later. The front fender panel  36  includes a rear end  36 A that is positioned on an inner side of the vehicle. The fender protector is typically a member designed to shield a gap  48  between the rear end  36 A and a vehicle body framework  46 . The vehicle sound absorption structure  10  according to the first embodiment of the invention is the fender protector with a sound absorption function. 
     The front fender panel  36  is a member that forms a front outside surface of the vehicle. An arch-shaped fender liner  52  is mounted to the inside of the front fender panel  36 . The fender liner  52  covers a front tyre  44 . 
     As illustrated in FIG,  8 , the vehicle body framework  46  is, for example, a front pillar on which a door  56  is mounted through a hinge  58 . The vehicle sound absorption structure  10  also serves as the fender protector. This prevents water from entering through the gap  48 , adhering to the hinge  58 , and freezing in cold climates. 
     The vehicle sound absorption structure  10  is mounted using, for example, a clip (not illustrated) such that the vehicle sound absorption structure  10  shields the gap  48  between the vehicle body framework  46  and the rear end  36 A of the front fender panel  36 . The rear end  36 A is positioned on an inner side of the vehicle. This mounting condition ensures a gap  62  between the fender liner  52  and the vehicle sound absorption structure  10 . As illustrated in  FIG. 1  and  FIG. 2 , the vehicle sound absorption structure  10  has, for example, three securing points  42  through which the clips (not illustrated) are installed. 
     The vehicle sound absorption structure  10  according to the first embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  10  will be described below. As illustrated in  FIG. 4 , in the vehicle sound absorption structure  10  according to the first embodiment of the invention, an acoustic wave enters the cell  12  from the opening  24  of the cell  12 , and is divided at the divider  14  into the two acoustic waves  32  and  34 . It should be understood that although the two acoustic waves  32  and  34  are separately illustrated in the drawings for simplification, the acoustic waves  32  and  34  are derived from an acoustic wave (not illustrated) that enters the cell  12  and that is then divided at the divider  14 . The one acoustic wave  32  is reflected at the reflector  16 , while the other acoustic wave  34  is reflected at the reflector  18 . The reflector  18  lies in a deeper position of the cell  12 , compared to the reflector  16 . This results in a phase difference between the reflected acoustic waves  32  and  34 . Then, these reflected acoustic waves  32  and  34  interfere with each other at the interfering portion  22 , while having the phase difference therebetween. This produces a sound absorption effect. 
     Specifically, when λ is the wavelength of the acoustic wave frequency of the sound to be absorbed and a difference between the depth from the opening  24  to the reflector  16  and the depth from the opening  24  to the reflector  18  (the distance from the reflector  16  to the reflector  18 ) is equal to λ/4, an acoustic wave that enters the cell  12  from the opening  24  is divided at the divider  14  into the two acoustic waves  32  and  34 . Then, these acoustic waves  32  and  34  are reflected at the respective reflectors  16  and  18 . This results in a phase difference of 2×λ/4=λ/2 (a half wavelength) between the acoustic waves  32  and  34  after the reflection, where the acoustic waves  32  and  34  interfere with each other at the interfering portion  22 . 
     The one acoustic wave  32  and the other acoustic wave  34  that have the phase difference of a half wavelength therebetween interfere with each other, and cancel each other out, at the interfering portion  22 . This results in the maximum sound absorption coefficient with respect to the acoustic wave frequency of the sound to be absorbed, as illustrated in  FIG. 6 . The end  14 A of the divider  14  is on the side of the opening  24  and is positioned more inward in the cell  12  than the opening  24 , while being positioned closer to the opening  24  than the reflector  16 . The reflector  16  is positioned closer to the opening  24  than the reflector  18 . The interfering portion  22  is positioned between the end  14 A and the opening  24 . This allows an acoustic wave that enters the cell  12  to be absorbed in the cell  12 . Consequently, a sound absorption effect is produced in the cell  12  in a stable manner. 
     As described above, according to the first embodiment of the invention, the vehicle sound absorption structure  10  is provided at low cost without increasing the number of parts. 
     As illustrated in  FIG. 8 , in the front fender structure S, the vehicle sound absorption structure  10  also serves as a fender protector, and is provided such that the plural cells  12  are arranged in parallel in the vertical direction with their respective openings  24  facing toward the front side of the vehicle. The front fender structure S allows the sound absorption effect to be produced in a wide region, and thus allows noise from the front tyre  44  to be absorbed efficiently at low cost. 
     It should be understood that the vehicle sound absorption structure  10  is not limited to the one that is used as a fender protector, but may also be applied to various components of the vehicle, such as an engine cover (not illustrated), a floor under cover (not illustrated), and the fender liner  52 . It should also be understood that the vehicle sound absorption structure  10  may be formed on at least one of the fender protector, the engine cover, the floor undercover, and the fender liner  52 . 
     [Second Embodiment] As illustrated in  FIG. 9 , a vehicle sound absorption structure  20  according to the second embodiment of the invention includes a film member  66  over the opening  24 . The film member  66  is an example of a cover member. The film member  66  is formed over a plane (not illustrated) that connects the walls  26  at their ends on the side of the openings  24 . The film member  66  has a slit  64  that is provided on an extension of the divider  14  or at a position where an imaginary line (not illustrated) that extends from the divider  14  toward the opening  24  intersects the film member  66 . The slit  64  extends in the direction along the end  14 A of the divider  14  or in the vehicle width direction. 
     Other portions of the vehicle sound absorption structure  20  are the same as those according to the first embodiment. Therefore, in the drawings, like numerals are used to represent like elements, and the description of the like elements is not repeated. 
     The vehicle sound absorption structure  20  according to the second embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  20  will be described below. An acoustic wave can be incident from any direction relative to the cell  12  in a random manner. In the structure according to the first embodiment, an acoustic wave is obliquely incident to the depth direction of the cell  12 , and accordingly, peaks  68  and troughs  70  (wavefronts) of the acoustic wave enter the cell  12  in the oblique direction. This does not always result in a distance difference of a half wavelength between the distance from the wavefronts to the reflector  16  and the distance from the wavefronts to the reflector  18 . 
     However, as illustrated in  FIG. 9 , in the vehicle sound absorption structure  20  according to the second embodiment, when an acoustic wave incident obliquely to the depth direction of the cell  12  enters the cell  12  through the slit  64 , the phases of the wavefronts  72  of the acoustic wave are matched due to a diffraction phenomenon through the slit  64 . This allows the one acoustic wave  32  and the other acoustic wave  34  that are divided at the divider  14  to be in phase, and thus ensures that the one acoustic wave  32  and the other acoustic wave  34  have a particular phase difference therebetween after the acoustic waves  32  and  34  are reflected. Therefore, the vehicle sound absorption structure  20  has improved sound absorption performance with respect to the acoustic wave frequency of the sound to be absorbed. 
     According to the second embodiment of the invention, the slit  64  extends in the direction along the end  14 A of the divider  14  or in the vehicle width direction. Thus, when an acoustic wave enters the cell  12  through the slit  64 , the phases of the acoustic waves  32  and  34  are matched over a wide frequency range. Therefore, as illustrated by a solid line in  FIG. 10 , the sound absorption coefficient with respect to the acoustic wave frequency of the sound to be absorbed further improves, compared to the case when no slit is provided (illustrated by a dotted line). 
     [Third Embodiment] As illustrated in  FIG. 11 , a vehicle sound absorption structure  30  according to the third embodiment of the invention includes the walls  26  that partition the cells  12  from each other. The walls  26  each include a base part  26 B and the other part  26 A. The base part  26 B is thinner than the other part  26 A. This, causes film resonance to occur on the walls  26 . Therefore, the vehicle sound absorption structure  30  has a sound absorption function that is provided by the walls  26  on which the film resonance occurs, in addition to the sound absorption function that is provided by the cells  12 . 
     The walls  26  each connect to the reflector  16  or  18  at the base part  2613  26B. The base part  26 B is positioned deeper in the cell  12 . The sound absorption performance obtained from the film resonance on the walls  26  can be controlled by varying a thickness t and a length L of the base part  26 B. Specifically, as the thickness t of the base part  26 B decreases or as the length L of the base part  2613  26B increases, the rigidity of the base part  26 B decreases, so that a lower resonance frequency may be predetermined. In contrast, as the thickness t of the base part  26 B increases or as the length L of the base part  26 B decreases, the rigidity of the base part  26 B increases, so that a higher resonance frequency may be predetermined. 
     As illustrated in  FIG. 12 , cuts  74  are provided at an intersection of the walls  26  to help the respective walls  26  vibrate more easily. A columnar portion  76  is formed in a square intersection surrounded by the cuts  74 . It should be understood that the divider  14  is not illustrated in  FIG. 12  for the sake of simplicity. 
     Other portions of the vehicle sound absorption structure  30  are the same as those according to the first embodiment. Therefore, in the drawings, like numerals are used to represent like elements, and the description of the like elements is not repeated. In addition, the third embodiment of the invention may be combined with at least one of the first and the second embodiments of the invention. 
     The vehicle sound absorption structure  30  according to the third embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  30  will be described below. As illustrated in  FIG. 11  and  FIG. 12 , in the vehicle sound absorption structure  30  according to the third embodiment of the invention, the walls  26  partition the cells  12  from each other. The walls  26  each have the base part  26 B and the other portion  26 A. The base part  26 B is thinner than the other part  26 A. Therefore, upon the entry of an acoustic wave into the cell  12 , the film resonance occurs on the walls  26  in an arrow A-B direction and in an arrow C-D direction. Due to the film resonance, energy of the acoustic wave is converted into kinetic energy, so that the sound is absorbed. Particularly, the cuts  74  are provided at the intersection of the walls  26  to help the walls  26  vibrate more easily. Thus, upon the entry of an acoustic wave into the cells  12 , the film resonance occurs efficiently on the walls  26 . This further improves the sound absorption performance. 
     In addition, as the rigidity of the base part  26 B of the wall  26  decreases, the vehicle sound absorption structure  30  deforms more easily upon a collision of the vehicle. This ensures shock absorbing performance upon a collision of the vehicle. 
     Fourth Embodiment 
     As illustrated in  FIG. 13 , a vehicle sound absorption structure  40  according to the fourth embodiment of the invention includes a Helmholtz resonator  80  adjacent to the cells  12 . The Helmholtz resonator  80  has a neck  78  that is open toward the same direction as the opening  24 . The neck  78  is an opening of the Helmholtz resonator  80 . This opening is defined by the walls  26 . A cavity  82  is defined more inward than the neck  78 . The Helmholtz resonator  80  is a spring-mass resonance system in which the air in the neck  78  acts as a mass, while the air in the cavity  82  acts as a spring. 
     The Helmholtz resonator  80  is provided between the two adjacent cells  12 . The two adjacent cells  12  are arranged symmetrically such that their respective reflectors  16  are adjacent to each other to form a space on the backside of the reflectors  16 . This space is closed by a wall  38  to define the cavity  82 . The wall  38  is continuously formed, for example, with the reflectors  18 . 
     The Helmholtz resonator  80  has sound-absorption characteristics that can be adjusted, for example, by varying a volume of the cavity  82  or a diameter ND of the neck  78  as illustrated in  FIG. 14 , or by forming the cavity  82  into a trapezium or trapezoid (having two parallel edges) shape or varying a length NL of the neck  78  as illustrated in  FIG. 15 . In the example illustrated in  FIG. 15 , the wall  38  includes: an edge  38 A that corresponds to an upper edge of the trapezium or trapezoid; and a pair of oblique edges  38 B that extend respectively from opposite ends of the edge  38 A. The oblique edges  38 B each have an end on the side of the opening  24  and that is connected nearly to the intersection of the divider  14  and the reflector  16 . 
     Other portions of the vehicle sound absorption structure  40  are the same as those according to the first embodiment. Therefore, in the drawings, like numerals are used to represent like elements, and the description of the like elements is not repeated. In addition, the fourth embodiment of the invention may be combined with at least one of the first, the second, and the third embodiments of the invention. 
     The vehicle sound absorption structure  40  according to the fourth embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  40  will be described below. As illustrated in  FIG. 13 , as in the first embodiment, in a vehicle sound absorption structure  40  according to the fourth embodiment, an acoustic wave enters the cells&#39;  12  to be divided into the one acoustic wave  32  and the other acoustic wave  34 , and the one acoustic wave  32  is reflected at the reflector  16 , while the other acoustic wave  34  is reflected at the reflector  18 . Then, the one acoustic wave  32  and the other acoustic wave  34  interfere with each other and cancel each other out at the interfering portion  22 . This produces a sound absorption effect, as illustrated by a solid line LC in  FIG. 16 . 
     In addition, according to the fourth embodiment of the invention, the Helmholtz resonator  80  is provided adjacent to the cells  12 . This further improves the sound absorption performance. Specifically, the wall  38  or the bottom of the cavity  82  can be positioned as illustrated by a dot-and-dash line in  FIG. 14  so that the volume of the cavity  82  increases. This improves the sound absorption coefficient in the low-frequency range, as illustrated in a dot-and-dash line L 1  in  FIG. 16 . 
     Secondly, the wall  38  or the bottom of the cavity  82  can be positioned as illustrated by a dotted line in  FIG. 14  so that the volume of the cavity  82  is slightly smaller than the aforementioned volume. This improves the sound absorption coefficient in the mid-frequency range, as illustrated in a dotted line L 2  in  FIG. 16 . 
     Thirdly, the wall  38  or the bottom of the cavity  82  can be positioned as illustrated by a chain double-dashed line in  FIG. 14  so that the volume of the cavity  82  is further smaller than the aforementioned volumes. This improves the sound absorption coefficient in the high-frequency range, as illustrated in a chain double-dashed tine L 3  in  FIG. 16 . 
     [Fifth Embodiment] As illustrated in  FIG. 17 , a vehicle sound absorption structure  50  according to the fifth embodiment of the invention includes: primary tubular passages  84 ; and secondary tubular passages  86 , in addition to the cells  12 . The primary tubular passage  84  and the secondary tubular passage  86  respectively have predetermined lengths such that acoustic waves  92  and  94  have a phase difference therebetween at a point  88  where the primary tubular passage  84  and the secondary tubular passage  86  join together. The acoustic wave  92  passes through the primary tubular passage  84  alone. The acoustic wave  94  rejoins the primary tubular passage  84  via the secondary tubular passage  86 . 
     The primary tubular passage  84  is provided adjacent to the cells  12  and is open toward the same direction as the opening  24 . The secondary tubular passage  86  is branched from the primary tubular passage  84  at its midsection and rejoins the primary tubular passage  84 . The secondary tubular passage  86  is formed, for example, along the backside of the reflector  16 , along a wall  96 , and then along the wall  38 . The wall  96  is continuously formed with the divider  14 . The wall  38  is continuously formed with the reflector  18 . Additionally, isolated portions  98  are provided to define the primary tubular passages  84  and the secondary tubular passages  86 . The isolated portions  98  are each spaced apart from the wall  26 , the backside of the reflector  16 , the wall  96 , and the wall  38 . The isolated portions  98  are each formed into, for example, a hollow profile. 
     The length of the secondary tubular passage  86  may be set to λ/2 (a half wavelength) relative to the wavelength λ of the acoustic wave frequency of the sound to be absorbed. Because this results in the maxim sound absorption coefficient when the acoustic waves  92  and  94  interfere with each other and cancel each other out at the point  88  after the acoustic wave  92  passes through the primary tubular passage  84  alone, while the acoustic wave  94  rejoins the primary tubular passage  84  via the secondary tubular passage  86 . 
     Other portions of the vehicle sound absorption structure  50  are the same as those according to the first embodiment. Therefore, in the drawings, like numerals are used to represent like elements, and the description of the like elements is not repeated. In addition, the fifth embodiment of the invention may be combined with at least one of the first, the second, and the third embodiments of the invention. 
     The vehicle sound absorption structure  50  according to the fifth embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  50  will be described below. As illustrated in  FIG. 17 , the vehicle sound absorption structure  50  according to the fifth embodiment of the invention produces another sound absorption effect in addition to the sound absorption effect provided by the cells  12 . The other sound absorption effect is produced in the following manner: The acoustic wave  92  passes through the primary tubular passage  84  alone, while the acoustic wave  94  rejoins the primary tubular passage  84  via the secondary tubular passage  86 , so that these acoustic waves  92  and  94  have a phase difference of, for example, a half wavelength therebetween, and then interfere with each other at the point  88  where the primary tubular passage  84  and the secondary tubular passage  86  join together. The damped acoustic waves by the interference pass through between the vehicle sound absorption structure  50  and, for example, the separate fender protector  28 . The fender protector  28  will be described below. According to the fifth embodiment of the invention, the space defined on the backside of the reflector  16  is utilized to efficiently improve the sound absorption performance. 
     It should be understood that the structures of the primary tubular passage  84  and the secondary tubular passage  86  are not limited to the illustrated examples. 
     [Sixth Embodiment] As illustrated in  FIG. 18 , a vehicle sound absorption structure  60  according to the sixth embodiment of the invention includes the plural cells  12  that are arranged in parallel in the vertical direction with their openings  24  facing toward the front side of the vehicle. The cells  12  each have the walls  26  and  96 . The walls  26  and  96  are an example of the lower walls. The wall  96  is continuously formed with the divider  14 . The walls  26  and  96  have respective communication holes  100  that communicate with the immediately lower adjacent one of the cells  12 . These communication holes  100  are provided respectively, for example, on the walls  26  and  96  at their rear ends. The reason for this is that in the case the cells  12  contain water, when the vehicle is accelerating, an inertia force causes the water to flow toward the rear side of the vehicle to be discharged through the communication holes  100 . The backside of the reflector  16 , the walls  26  and  96 , and the wall  38  define a cavity  102 . 
     It should be understood that the walls  26  and  96  may be angled downwardly to the front side of the vehicle. The walls  26  and  96  are an example of the lower walls. The wall  96  is continuously formed with the divider  14 . These angled walls  26  and  96  make it difficult for the water to remain in the cells  12 . In the case the communication holes  100  are formed through the lower walls, a similar communication hole (not illustrated) may also be formed through an upper wall (for example, the divider  14 ). The reason for this is that it is considered relatively easy to form the communication holes through the individual walls. 
     The arrangement direction of the vehicle sound absorption structure  60  may be varied such that, for example, the openings  24  can be positioned on the upper side of the vehicle, while the reflectors  16  and  18  can be positioned on the lower side of the vehicle. In this case, the communication holes  100  may be formed through the reflectors  16  and  18 . 
     Other portions of the vehicle sound absorption structure  60  are the same as those according to the first embodiment. Therefore, in the drawings, like numerals are used to represent like elements, and the description of the like elements is not repeated. In addition, the sixth embodiment of the invention may be combined with at least one of the first, the second, the third, the fourth, and the fifth embodiments of the invention. 
     The vehicle sound absorption structure  60  according to the sixth embodiment of the invention has the aforementioned configuration, and the effect of vehicle sound absorption structure  60  will be described below. As illustrated in  FIG. 18 , in the vehicle sound absorption structure  60  according to the sixth embodiment of the invention, the communication holes  100  are formed respectively through the lower walls of the cell  12  or the walls  26  and  96  at their rear ends. The communication holes  100  each communicate with the immediately lower adjacent one of the cells  12 . This provides an additional function as an expansion silencer in addition to the sound absorption function provided by the cells  12 . Specifically, the cavity  102  and the communication holes  100  that are formed above and below the cavity  102  act as the expansion silencer illustrated in  FIG. 19 . This further improves the sound absorption performance. 
     When S 1  is a cross-sectional area of a smaller-diameter portion  104  that corresponds to the communication hole  100 , S 2  is a cross-sectional area of a larger-diameter portion  106  that corresponds to the cavity  102 , and LS is a length of the larger diameter portion  106 , transmission loss TL is expressed as the following equation 1, where: m=S 2 /S 1 , f is frequency and c is velocity of sound. 
     
       
         
           
             
               
                 
                   TL 
                   = 
                   
                     10 
                     ⁢ 
                     
                       log 
                       [ 
                       
                         1 
                         + 
                         
                           
                             1 
                             4 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 m 
                                 - 
                                 
                                   1 
                                   m 
                                 
                               
                               ) 
                             
                             2 
                           
                           ⁢ 
                           
                             sin 
                             2 
                           
                           ⁢ 
                           
                             
                               2 
                               ⁢ 
                               π 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               f 
                             
                             c 
                           
                           ⁢ 
                           LS 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     The water in the cells  12  can be drained through the communication holes  100 . Specifically, in the case the water (not illustrated) enters the cells  12 , when the vehicle is accelerating, an inertial force causes the water to flow toward the rear side of the vehicle. Thus, the water drops through the communication holes  100  to the lower cells  12  sequentially. This allows the water in the cells  12  to be efficiently discharged out of the cells  12 . This prevents the water from remaining in the cells  12 , and thus prevents a decrease in sound absorption performance due to freezing of the water, thereby maintains the sound absorption performance. It should be understood that when the vehicle is decelerating, the water is discharged from the openings  24  of the cells  12 . In addition, the walls  26  and  96  may be angled downwardly to the front side of the vehicle to make it difficult for the water to remain in the cells  12 . The walls  26  and  96  are an example of the lower walls. The wall  96  is continuously formed with the divider  14 . 
     (Other Embodiment) As illustrated in  FIG. 20 , the vehicle sound absorption structure  10  may be separated from the fender protector  28 . The fender protector  28  includes a front surface  28 A. The vehicle sound absorption structure  10  may be mounted onto the front surface  28 A, using, for example, a clip (not illustrated), such that the openings  24  of the cells  12  face toward the front side of the vehicle. 
     The fender protector  28  is a member designed to shield the gap  48  between the vehicle body framework  46  and the rear end  36 A of the front fender panel  36 . The rear end  36 A is positioned on an inner side of the vehicle. The fender protector  28  is secured to the front fender panel  36  at its rear end  36 A and to the vehicle body framework  46  on its forward side of the vehicle, using, for example, an adhesive  54 . 
     The vehicle sound absorption structure  10  is provided on the front surface  28 A of the fender protector  28  such that the plural cells  12  are arranged in parallel in the vertical direction with their respective openings  24  facing toward the front side of the vehicle. This allows the sound absorption effect to be produced in a wide region, thus to efficiently absorb noise from the front tyre  44  at low cost. 
     (Further Other Embodiment) illustrated Illustrated as a further modification of the vehicle sound absorption structure  10  in  FIG. 21 , the cell  12  may be open on its rear end side to be shielded by a separate member such that the separate member may be utilised as the reflector  18 . The separate member is, for example, the fender protector  28 . In other words, the vehicle sound absorption structure  60  according to the sixth embodiment is not limited to the configuration illustrated in  FIG. 18  in which the rear part of the vehicle sound absorption structure  60  is shielded by the wall  38  that is continuously formed with the wall  26  of the cell  12 . 
     According to the further modification, the front surface  28 A of the fender protector  28  is utilised as the reflector  18 . This enables reductions in material costs of and in weight of the vehicle sound absorption structure  10 , compared to the case when the reflector  18  is continuously formed with the wall  26  of the cell  12 . As illustrated in  FIG. 22 , the walls  26  and  96  may have notches at their respective rear ends, and these rear ends may be shielded by a separate member (fender protector  28 ). This facilitates forming of the communication holes  100 . Therefore, the vehicle sound absorption structure  60  has improved formability. 
     While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.