Patent Publication Number: US-2019176900-A1

Title: Vehicle lower structure

Description:
BACKGROUND 
     The present disclosure relates to a vehicle lower structure. 
     Japanese Laid-Open Patent Publication No. 2005-186825 describes a vehicle lower structure that includes an under cover covering the internal combustion engine and the transmission from below. This under cover includes a lower plate section, which is located below the internal combustion engine and the transmission, and two side plate sections, which extend upward from the opposite edges in the vehicle width direction of the lower plate section. The lower surface of the lower plate section of the under cover forms the underside of the vehicle. An air-permeable sound-absorbing member is attached to the upper surface of the lower plate section and the inner surfaces of the two side plate sections, which face each other. 
     The vehicle lower structure of the above-described publication has a space below the internal combustion engine and the transmission and above the under cover. If the acoustic waves caused by vibration of the internal combustion engine or the transmission resonate in the space above the under cover, they may cause noise. The vehicle lower structure of the above-described publication fails to sufficiently prevent this noise, even though its sound-absorbing member is capable of reducing noises to some extent. There is a need for further reducing the noise occurring in the space above the under cover in a vehicle lower structure. 
     SUMMARY 
     In accordance with one aspect of the present disclosure, a vehicle lower structure is provided that includes a plate-shaped under cover and an air-permeable sound-absorbing member. The under cover is configured to be attached to a lower section of a vehicle to form a part of an underside of the vehicle. The sound-absorbing member is attached to an upper surface of the under cover. The under cover includes one or more vent holes extending through the under cover in a thickness direction. The sound-absorbing member is attached to the under cover so as to close the one or more vent holes. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a bottom view showing a vehicle lower structure of an embodiment arranged in the front section of a vehicle; 
         FIG. 2  is an exploded perspective view of the vehicle lower structure of  FIG. 1  as viewed obliquely from below; 
         FIG. 3  is a top view of the vehicle lower structure of  FIG. 1 ; 
         FIG. 4  is a top view of a side covering member of the vehicle lower structure of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  in  FIG. 3 ; 
         FIG. 6  is a top view of a center covering member of the vehicle lower structure of  FIG. 1 ; 
         FIG. 7  is a top view of a center sound-absorbing member of the vehicle lower structure of  FIG. 1 ; and 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     A vehicle C that includes a vehicle lower structure according to the present disclosure is now described. In the following description, the front-rear direction, the up-down direction, and the width direction (left-right direction) of the vehicle C are used to describe directions. 
     As shown in  FIG. 1 , the front section of the vehicle C includes an internal combustion engine E, which is a driving source of the vehicle C. The internal combustion engine E is located between the two front wheels FW in the vehicle width direction. The rear side of the internal combustion engine E is coupled to a transmission TM, to which rotation of the crankshaft of the internal combustion engine E is transmitted. The transmission TM converts the torque and the rotational speed of the transmitted rotational force. 
     The vehicle lower structure of the present embodiment includes an under cover  10 , a center covering member  31 , a center sound-absorbing member  40 , a plurality of side covering members  21 , and a plurality of side sound-absorbing members  50 . The vehicle lower structure is a structural member arranged at the underside of the vehicle C. 
     The plate-shaped under cover  10  is attached to the lower section of the vehicle C to cover the internal combustion engine E and the transmission TM from below. The under cover  10  is located at the center in the width direction of the vehicle C and covers the region including the rear section of the internal combustion engine E and the front section of the transmission TM from below. The lower surface of the under cover  10  forms a part of the underside of the vehicle C. 
     As shown in  FIGS. 2 and 3 , the under cover  10  includes a plate-shaped under cover body  11 . As viewed from above, the under cover body  11  has a trapezoidal shape that is generally symmetrical. Of the short side and the long side that are parallel to each other in the trapezoid, the long side is located rearward of the short side. From the left and right sides of the under cover body  11 , side ridges  12  extend downward (toward the rear side of the drawing sheet of  FIG. 3 ). The two side ridges  12  each extend substantially along the entire length of the corresponding one of the left and right side edges  11   a  of the under cover body  11  (the oblique sides of the trapezoid). The two side ridges  12  are formed by partially deforming the plate-shaped under cover body  11  to bulge. 
     A rear ridge  13  extends downward from the rear section of the under cover body  11 . The rear ridge  13  extends along the rear edge  11   b  (the long side of the trapezoid) of the under cover body  11 . The rear ridge  13  has a length of about 60% of the length of the rear edge  11   b  of the under cover body  11 , and the center in the extending direction of the rear ridge  13  is located at the center in the vehicle width direction of the under cover body  11 . 
     The under cover body  11  includes a plurality of bolt holes  14  extending through the under cover body  11  in the thickness direction. The bolt holes  14  are located at two positions near the front edge  11   c  (the short side of the trapezoid) of the under cover body  11  and at four positions near the rear edge  11   b  of the under cover body  11 . Bolts (not shown) are inserted into the bolt holes  14  to fix the under cover body  11  (the under cover  10 ) to the bodywork (frame) or other members of the vehicle C. 
     As shown in  FIGS. 2 and 3 , the under cover body  11  includes at its center a center service hole  15  extending through the under cover body  11  in the thickness direction. As viewed from above, the center service hole  15  substantially has the shape of a droplet. Specifically, the center service hole  15  has a straight front edge, a tapered section, in which the dimension in the vehicle width direction decreases toward the front edge from the center in the front-rear direction, and a semicircular section extending rearward from the center in the front-rear direction. The center service hole  15  has an opening that is sized so that fingers or tools are insertable from below the under cover body  11  toward the components above the under cover body  11 , such as the internal combustion engine E and the transmission TM. Specifically, the dimensions of the center service hole  15  in the front-rear direction and vehicle width direction are greater than or equal to 10 cm. The rim of the opening of the center service hole  15  in the under cover body  11  is bent upward so as to form a step. The height of this step of the rim is about the same as the thickness of the under cover body  11 , for example. In this embodiment, when the under cover  10  is attached to the lower section of the vehicle C, the center service hole  15  is located directly below the internal combustion engine E in the up-down direction of the vehicle C. 
     The under cover body  11  includes two first attachment holes  16 , which extend through the under cover body  11  in the thickness direction near the rim of the opening of the center service hole  15 . The two first attachment holes  16  are located separately in the vehicle width direction at positions along the semicircular arc of the rim of the opening of the center service hole  15 . 
     The under cover body  11  includes two side service holes  17  extending through the under cover body  11  in the thickness direction. The side service holes  17  are located outward of the center service hole  15  and inward of the two side ridges  12  in the vehicle width direction. The side service holes  17  are located on the opposite sides of the center service hole  15  in the vehicle width direction. As viewed from above, each side service hole  17  is elliptical and has a dimension in the vehicle width direction that is slightly longer than the dimension in the front-rear direction. Each side service hole  17  includes a straight front edge and a straight section, which extends relative to the front edge such that the side service hole  17  is tapered down toward the inner side in the vehicle width direction. As is the case with the center service hole  15 , each side service hole  17  has an opening that is sized so that fingers or tools are insertable from below the under cover body  11  toward the components above the under cover body  11 , such as the internal combustion engine E and the transmission TM. The rim of the opening of each side service hole  17  in the under cover body  11  is bent upward so as to form a step. The height of this step of the rim is about the same as the thickness of the under cover body  11 , for example. In this embodiment, when the under cover  10  is attached to the lower section of the vehicle C, the side service holes  17  are located directly below the internal combustion engine E in the up-down direction of the vehicle C. 
     The under cover body  11  includes two second attachment holes  18  near the rim of the opening of each of the two side service holes  17 . The second attachment holes  18  extend through the under cover body  11  in the thickness direction. That is, two second attachment holes  18  are provided for each side service hole  17 . One of the two second attachment holes  18  near each side service hole  17  is located beside the section of the rim of the opening of the side service hole  17  that is closest to the rear ridge  13 . The other one of the two second attachment holes  18  is located outward of the side service hole  17  in the vehicle width direction. 
     As shown in  FIG. 2 , the under cover body  11  includes a plurality of third attachment holes  19  between the rear edge  11   b  and the two side service holes  17 . The third attachment holes  19  extend through the under cover body  11  in the thickness direction. The under cover body  11  includes six third attachment holes  19  in total, three on the inner side of each of the left and right side edges  11   a  in the vehicle width direction. 
     As shown in  FIGS. 2 and 3 , side covering members  21  are attached to the under cover body  11  to close the side service holes  17 . Two side covering members  21  are provided, each attached to the corresponding one of the left and right side service holes  17 . 
     As shown in  FIG. 4 , each side covering member  21  includes a plate section  22 , which has a similar shape as the side service hole  17  and has a larger size than the side service hole  17  as viewed from above. In this embodiment, the outer periphery of the plate section  22  is sized to be fitted to the step of the rim of the opening of the side service hole  17  in the under cover body  11 . 
     A plate-shaped locking section  23  extends from the front edge of the plate section  22 . The locking section  23  is rectangular as viewed from above. As shown in  FIG. 5 , the locking section  23  is bent so as to be located above the plate section  22 , and the boundary section between the locking section  23  and the plate section  22  includes a step. As shown in  FIG. 4 , attachment sections  24  extend from the rear edge and the outer edge in the vehicle width direction of the plate section  22 . Each attachment section  24  includes one through hole  25  extending through the attachment section  24  in the thickness direction. The two attachment sections  24  (the through holes  25 ) have the same positional relationship as the corresponding two second attachment holes  18 .  FIG. 4  is a top view of the right one of the two side covering members  21 . The left side covering member  21  is symmetrical to the right side covering member  21 . 
     As shown in  FIG. 5 , the locking section  23  of the side covering member  21  is engaged with the front edge of the side service hole  17  and placed over the upper surface of the under cover body  11 . Bolts B 1  are inserted into the through holes  25  of the side covering members  21  and the second attachment holes  18  of the under cover body  11 . The bolts B 1  fasten the side covering member  21  to the under cover body  11  with the side service hole  17  of the under cover body  11  closed by the plate section  22  of the side covering member  21 .  FIGS. 2 and 3  do not show the bolts B 1 . 
     As shown in  FIGS. 2 and 3 , a center covering member  31  is attached to the under cover body  11  to close the center service hole  15 . As shown in  FIG. 6 , the center covering member  31  includes a plate section  32 . The plate section  32  has a similar shape as the center service hole  15  as viewed from above and has a larger size than the center service hole  15 . In this embodiment, the outer periphery of the plate section  32  is sized to be fitted to the step of the rim of the opening of the center service hole  15  in the under cover body  11 . 
     A plate-shaped locking section  33  extends from the front edge of the plate section  32 . The locking section  33  extends along the front edge of the plate section  32  and the two edge sections extending from opposite ends of the front edge. As shown in  FIG. 8 , the locking section  33  is bent so as to be positioned above the plate section  32 , and the boundary section between the locking section  33  and the plate section  32  includes a step. As shown in  FIG. 6 , attachment sections  34  extend from the rear edge of the plate section  32 . One attachment section  34  is formed on each of the opposite sides in the vehicle width direction of the plate section  32 . Each attachment section  34  includes one through hole  35  extending through the attachment section  34  in the thickness direction. The positions of the two attachment sections  34  (the through holes  35 ) corresponds to the positions of the two first attachment holes  16  in the under cover body  11 . 
     The plate section  32  includes two slits  36  at the opposite ends of the locking section  33 . Each slit  36  extends toward the center from the edge. The slits  36  are formed adjacent to the opposite ends in the extending direction of the locking section  33 . 
     The plate section  32  includes a protrusion  37  extending downward between the two slits  36  in the vehicle width direction. The protrusion  37  is trapezoidal as viewed from above. Of the short side and the long side parallel to each other in the trapezoid, the long side is located rearward of the short side. As shown in  FIG. 8 , the protrusion  37  is formed by deforming a part of the plate section  32  to bulge. 
     As shown in  FIG. 6 , the plate section  32  includes a plurality of vent holes  38  located rearward of the protrusion  37 . The vent holes  38  extend through the plate section  32  in the thickness direction. This embodiment has forty-one vent holes  38 , which are arranged at substantially equal intervals in the front-rear direction and the vehicle width direction. Each vent hole  38  is circular as viewed from above. The inner diameter of each vent hole  38  may be between 5 mm and 15 mm inclusive. In this embodiment, it is 8 mm. All vent holes  38  of this embodiment are identical in shape and size (inner diameter). In  FIG. 6 , the reference numeral is indicated for only some of the forty-one vent holes  38 . 
     As shown in  FIG. 8 , the locking section  33  of the center covering member  31  is engaged with the front edge of the center service hole  15  and placed over the upper surface of the under cover body  11 . As shown in  FIGS. 3 and 6 , bolts B 2  (see  FIG. 8 ) are inserted in the through holes  35  in the center covering member  31  and the first attachment holes  16  in the under cover body  11 . The bolts B 2  fasten the center covering member  31  to the under cover body  11  with the center service hole  15  in the under cover body  11  closed by the plate section  32  of the center covering member  31 . In this embodiment, the under cover  10  includes the under cover body  11 , the two side covering members  21 , and the center covering member  31 .  FIGS. 2 and 3  do not show the bolts B 2 . 
     As shown in  FIGS. 2 and 3 , a plate-shaped center sound-absorbing member  40  is attached to the upper surface of the plate section  32  of the center covering member  31 . The center sound-absorbing member  40  has a laminated structure including a cotton felt material and polyester nonwoven fabric laminated on the opposite sides in the thickness direction of the cotton felt material. There are gaps between the cotton fibers in the cotton felt material and between the polyester fibers in the nonwoven fabric, and adjacent gaps connect to each other. The center sound-absorbing member  40  thus has air permeability. The figures do not show the laminated structure of the center sound-absorbing member  40 . 
     As shown in  FIGS. 3 and 7 , the center sound-absorbing member  40  has, as viewed from above, the shape that corresponds to the region of the center covering member  31  that has the vent holes  38 . In other words, the center sound-absorbing member  40  is shaped so as to cover all vent holes  38  in the center covering member  31 . The dimensions in the front-rear direction and the vehicle width direction of the center sound-absorbing member  40  are less than those of the center service hole  15  in the under cover body  11 . As such, the center sound-absorbing member  40  attached to the upper surface of the center covering member  31  does not extend over the region where the under cover body  11  is present. 
     As shown in  FIG. 7 , the upper surface of the center sound-absorbing member  40  includes recesses  41 , which are circular as viewed from above. The center sound-absorbing member  40  has three recesses  41  in total, two in the front section and one in the rear section. The bottom surface of each recess  41  includes a hole  42  extending through the center sound-absorbing member  40  in the thickness direction. The hole  42  has a smaller inner diameter than the recess  41 . The positions of the three recesses  41  (the holes  42 ) in the center sound-absorbing member  40  correspond to the positions of three of the forty-one vent holes  38  in the center covering member  31 . These three vent holes  38  function as clip holes  39  used to attach the center sound-absorbing member  40 . 
     As shown in  FIG. 8 , the center sound-absorbing member  40  is fixed to the plate section  32  of the center covering member  31  with clips  45 , which serve as fixing members. Each clip  45  includes a rod-shaped shaft  46 . The shaft  46  has an outer diameter that is smaller than the inner diameter of the holes  42  in the center sound-absorbing member  40  and the inner diameter of the clip holes  39  (the vent holes  38 ) in the center covering member  31 . Two flanges  47  extend radially outward from the outer circumferential surface of the shaft  46 . Each flange  47  is annular as viewed from above and has an outer diameter that is larger than the inner diameter of the holes  42  in the center sound-absorbing member  40  and the inner diameter of the clip holes  39  (the vent holes  38 ) in the center covering member  31 . The two flanges  47  are spaced apart from each other in the axial direction of the shaft  46 . The clip  45  is made of plastic and is thus elastically deformable to some extent. 
     To fix the center sound-absorbing member  40  to the plate section  32  of the center covering member  31 , one of the two flanges  47  of each clip  45  is elastically deformed so that the shaft  46  is inserted into a clip hole  39  (a vent hole  38 ) in the center covering member  31  and a hole  42  in the center sound-absorbing member  40 . This places one of the two flanges  47  within a recess  41  of the center sound-absorbing member  40  and places the other below the center covering member  31 . Accordingly, the center sound-absorbing member  40  and the plate section  32  of the center covering member  31  are sandwiched between the two flanges  47  of the clip  45  and thus fixed. 
     As shown in  FIG. 2 , plate-shaped side sound-absorbing members  50  are attached to the sections of the upper surface of the under cover body  11  rearward of the two side service holes  17 . In the same manner as the center sound-absorbing member  40 , each side sound-absorbing member  50  has a laminated structure including a cotton felt material and polyester nonwoven fabric laminated on the opposite sides of the cotton felt material. As shown in  FIG. 3 , the side sound-absorbing members  50  are rectangular as viewed from above. Each of the two side sound-absorbing members  50  is arranged such that its longitudinal axis extends along the corresponding one of the two side edges  11   a  of the under cover body  11 . 
     The upper surface of each side sound-absorbing member  50  includes three recesses  51 , which are circular as viewed from above. The bottom surface of each recess  51  includes a hole  52  extending through the side sound-absorbing member  50  in the thickness direction. The hole  52  has a smaller inner diameter than the recess  51 . The positions of the three recesses  51  (the holes  52 ) correspond to the positions of the three third attachment holes  19  on the corresponding side in the vehicle width direction of the under cover body  11 . Although not shown, the side sound-absorbing members  50  are fixed to the under cover body  11  with clips, which have a similar structure as the clips  45  that fix the center sound-absorbing member  40  to the center covering member  31 . 
     An operation and advantages of the present embodiment will now be described. 
     In the lower section of the vehicle C, the presence of the under cover  10  forms a space below the internal combustion engine E and the transmission TM of the vehicle and above the under cover  10 . If the acoustic waves caused by vibration of the internal combustion engine E or the transmission TM and vibration from the road surface are transmitted to the components defining this space, the acoustic waves may resonate in the space and create noise. In particular, when the vibration of the internal combustion engine E and the transmission TM and the vibration from the road surface are close to the natural frequency of the space or its multiples, the resonance in the space increases. 
     In this embodiment, the side sound-absorbing members  50  are attached to the upper surface of the under cover body  11 , and the center sound-absorbing member  40  is attached to the upper surface of the center covering member  31 . Inside the center and side sound-absorbing members  40  and  50 , minute gaps are formed between fibers. This allows the center and side sound-absorbing members  40  and  50  to reduce the noise of a relatively high frequency range. 
     In the present embodiment, the plate section  32  of the center covering member  31  includes a plurality of vent holes  38 , which provides communication between the space above the under cover body  11  and the outside space under the vehicle C. This helps to guide the acoustic waves of relatively low frequencies, which are not effectively absorbed by the side sound-absorbing members  50  or the center sound-absorbing member  40 , to the outside. This limits resonance in the space above the under cover body  11 , thereby reducing low-frequency noise occurring in the space above the under cover body  11 . 
     In this embodiment, the position of the center service hole  15  in the under cover body  11 , that is, the position of the vent holes  38  in the center covering member  31 , is directly below the internal combustion engine E. Among the noises occurring in the space above the under cover body  11 , a noise of a relatively low frequency range is likely to result from the vibration of the internal combustion engine E. The present embodiment helps to guide the low-frequency noise, which is caused by the vibration of the internal combustion engine E, to the outside through the vent holes  38  in the center covering member  31 , which are positioned directly below the internal combustion engine E. This maximizes the noise reduction effect of the vent holes  38 . 
     In the present embodiment, the center sound-absorbing member  40  closes all the vent holes  38  in the center covering member  31 . The center sound-absorbing member  40  blocks entry of foreign matter, which may be sand or stones thrown from the road surface while the vehicle C travels, into the space above the under cover body  11  through the vent holes  38 . In the present embodiment, the inner diameter of the vent holes  38  is 8 mm, so that foreign matter larger than this diameter cannot pass through the vent holes  38 . This reduces the possibility that excessively large foreign matter hits and breaks the center sound-absorbing member  40 . The center sound-absorbing member  40  has air permeability. As such, even though the center sound-absorbing member  40  closes the vent holes  38 , this does not unduly hinder the transmission of acoustic waves through the vent holes  38 . 
     If the center covering member  31  does not have the center sound-absorbing member  40 , the air flowing above the under cover  10  would move to the space below the under cover  10  through the vent holes  38  in the center covering member  31 . Such a flow of air would disturb the flow of air in the space below the under cover  10 . This may prevent the under cover  10  from improving the aerodynamic characteristics as expected. Although the center sound-absorbing member  40  has air permeability, the center sound-absorbing member  40  still reasonably limits the flow of air through it. As such, the air flowing above the under cover  10  does not easily flow to the space below the under cover  10  through the vent holes  38  and the center sound-absorbing member  40 . The under cover  10  thus maintains the effect of improving the aerodynamic characteristics. 
     As for the under cover  10  described above, the same under cover bodies  11  may be used for various types of vehicles. However, different vehicle types may require different noise frequencies to be reduced because of the variations in type of the internal combustion engine E used and the variations in volume or shape of the space above the under cover  10 , for example. Thus, to effectively reduce noise in various types of vehicles, it is preferable that the shape, number, and layout of the vent holes  38  in the center covering member  31  be appropriately designed for each vehicle type. 
     In the present embodiment, the vent holes  38  are provided in the center covering member  31 . As such, preparing various center covering members that differ in shape, number, and layout of vent holes accommodates the need for different vehicle types, without changing the design of the under cover body  11 . That is, the vehicle lower structure of the present embodiment can be used for various vehicle types with minimum specification changes and is therefore highly versatile. 
     The present embodiment uses some of the vent holes  38  in the center covering member  31  as the clip holes  39 , which receive clips  45 . This eliminates the need for separately forming, in the center covering member  31 , holes for clips  45  that differ from the vent holes  38  in structure. This simplifies the process of manufacturing the center covering member  31 . 
     In this embodiment, the center sound-absorbing member  40  is attached to the upper surface of the center covering member  31  and does not extend over the under cover body  11 . Removing the center covering member  31  from the under cover body  11  also removes the center sound-absorbing member  40  from the under cover body  11 . Thus, the center sound-absorbing member  40  may be replaced without removing the under cover body  11  from the frame of the vehicle C, for example, and is thus easy to replace when deteriorated or damaged. 
     The present embodiment may be modified as follows. The present embodiment and the following modifications can be combined as long as the combined modifications remain technically consistent with each other. 
     The under cover  10  does not have to be positioned below the internal combustion engine E and the transmission TM. For example, the under cover  10  may be positioned under the passenger compartment (the driver&#39;s seat, the front passenger seat, and the rear seat) or under the trunk compartment (luggage space). Even when the position of the under cover  10  is changed such that the vent holes  38  in the center covering member  31  are no longer positioned directly below the internal combustion engine E and the transmission TM, the vent holes  38  still reduce noise. 
     The overall shape of the under cover body  11  as viewed from above does not have to be trapezoidal and may be modified. The shape of the under cover body  11  may be designed according to the attachment position of the under cover  10  and the shapes of other structures (other under covers), for example. 
     The positions and shapes of the center service hole  15  and the side service holes  17  are not limited to those of the present embodiment. As long as the services holes  15  and  17  are sized and shaped so as to allow insertion of fingers or tools, their shapes and positions may be modified according to the position and the shape of the target to be accessed (the internal combustion engine E and the transmission TM) through the service holes. 
     The shape of the two side service holes  17  does not have to be symmetrical. For example, the suitable position and shape of the side service holes  17  may vary among different vehicle types. Thus, one of the side service holes  17  may have a shape that is suitable for a certain vehicle type, and the other side service hole  17  may have a shape that is suitable for another vehicle type. This configuration allows the under cover body  11  of the common specification to be used for various vehicle types. 
     The center service hole  15 , the side service holes  17 , the center covering member  31 , and the side covering member  21  may be omitted. In this case, vent holes may be formed in the under cover body  11 , and a sound-absorbing member may be attached to the under cover body  11  so as to close the vent holes. 
     The manner in which the center covering member  31  is attached to the under cover body  11  may be modified. For example, the locking section  33  of the center covering member  31  may be omitted, and the center covering member  31  may be attached to the under cover body  11  only with bolts. Nevertheless, since the center service hole  15  may be used for maintenance of the vehicle C, the center service hole  15  is preferably attached in a manner that enables easy removal of the center service hole  15 . The same applies to the side service holes  17 . 
     Instead of or in place of the vent holes  38  in the center covering member  31 , the side covering members  21  may include vent holes. For example, when the characteristics of the noise occurring in the space above the under cover  10  are such that the noise is effectively reduced by vent holes that coincide with the side service holes  17 , the structure of the vent holes  38  in the center covering member  31  may be used for the side covering members  21 . Tests or simulations may be performed to determine which covering member should include vent holes. When a side covering member  21  includes vent holes, a sound-absorbing member is preferably attached to the upper surface of this side covering member  21  to close the vent holes. 
     The shape and size of the vent holes  38  in the center covering member  31  may be modified. For example, as viewed from above, each vent hole  38  may be non-circular, that is, it may be polygonal or has the shape of an elongated ellipse (a groove). However, to reduce the noise of a relatively low frequency range, the dimension of the smallest dimension section of the vent holes  38  is preferably greater than or equal to 5 mm. The dimension of the smallest dimension section may be the dimension of the shorter sides when the vent hole  38  is rectangular (has the shape of a groove), or the dimension of the minor axis when the vent hole  38  is elliptical. Further, to reduce damage of the center sound-absorbing member  40 , the dimension of the largest dimension section of the vent hole  38  is preferably less than or equal to 15 mm. Consequently, the size of any foreign matter entering through such vent holes  38  will be less than 15 mm. This reduces the possibility that the sound-absorbing member  40  is damaged when hit by any foreign matter. The dimension of the largest dimension section may be the dimension of a diagonal line when the vent hole  38  is rectangular, or the dimension of the major axis when the vent hole  38  is elliptical. The number of the vent holes  38  may be modified according to the shape and size of the vent holes  38 . 
     Depending on the frequency range of the noise to be reduced, the dimension of the smallest dimension section of the vent hole  38  may be less than 5 mm. 
     Depending on the structure of the sound-absorbing member, the dimension of the largest dimension section of the vent hole  38  may be greater than 15 mm. For example, the center covering member  31  may be omitted, and the center service hole  15  may be used as a vent hole. In this case, a sound-absorbing member may be attached so as to close the center service hole  15 . 
     The center and side sound-absorbing members  40  and  50  may be made of any material with air permeability. For example, a foamed plastic having continuous pores may be used. When the center sound-absorbing member  40  is made of a relatively hard material that resists damage when hit by some foreign matter, the vent holes  38  may be enlarged without causing problems. 
     The center covering member  31  may have clip holes that are formed separately from the vent holes  38  to attach the center sound-absorbing member  40 . Further, some of the clip holes for attachment of the center sound-absorbing member  40  may differ from the vent holes  38  in shape. Furthermore, the center sound-absorbing member  40  may be attached to the center covering member  31  with an adhesive and without using clip holes or clips  45 . 
     Provided that the vent holes  38  in the center covering member  31  are closed, the center sound-absorbing member  40  may be attached to the under cover body  11  instead of the center covering member  31 . 
     The center sound-absorbing member  40  does not necessarily have to close all the vent holes  38 , and may close only some of the vent holes  38 . In addition, the center sound-absorbing member  40  may be divided into a plurality of parts.