Patent Publication Number: US-8528692-B2

Title: Air intake duct

Description:
TECHNICAL FIELD 
     The present invention relates to an air intake duct adapted to be arranged at an air intake portion of a vehicle engine. 
     BACKGROUND ART 
     In recent years, in view of an increasing demand for traffic noise reduction, there has been a demand for reducing intake noises generated at an air intake portion of a vehicle engine. At the air intake portion of the engine, an air intake duct is provided. The intake noises are amplified by pipe resonance generated inside the air intake duct, and are radiated from an air intake port. Therefore, conventionally, in order to reduce the intake noises, a “resonant type muffler” called as a resonator or a side branch is provided. 
     However, in an engine room, a number of devices such as various auxiliary devices of the engine and electronic devices for reducing exhaust gas are provided. Therefore, available space inside the engine room is limited, and it is difficult to provide a space for installing the resonant type muffler. In view of this, Patent Document 1 proposes an air intake duct having openings in the middle of a duct body, the openings being covered with a sheet member of heat-shrinkable fibers to reduce intake noises originating from pipe resonance. 
     Patent Document 1: JP 2000-282985 A 
     SUMMARY OF THE INVENTION 
     According to the air intake duct of Patent Document 1, the duct body is inserted into a cylindrical sheet member having a slightly larger size than the duct body, and the sheet member is heated and shrunk, whereby the sheet member is attached to the duct body such that the sheet member covers the openings. That is, according to the air intake duct of Patent Document 1, when attaching the sheet member, the sheet member needs to be heated. Therefore, manufacturing facilities require a heating apparatus, and a sheet member attaching process is cumbersome. 
     Also, because the air permeability of the sheet member changes before and after the heat shrinkage, it is difficult to control the air permeability of the sheet member after the heat shrinkage to be optimum in accordance with the opening ratio of the openings. Therefore, it is difficult to achieve desired noise reduction and airtightness. There is also a downside in that the duct body may deform due to the heating of the sheet member. 
     Further, according to the air intake duct of Patent Deference 1, the entire sheet member is exposed to the outside. Therefore, when mounting the air intake duct to a vehicle, extra care is required so as not to damage the sheet member, which makes it difficult to handle the air intake duct during the assembling operation. Moreover, because the sheet member is held by its own shrinking force, the sheet member can easily be displaced from the duct body, so that it may slip off during its use. 
     The present invention has been made in view of the problems described above, and in particular, it is an object thereof to provide an air intake duct having an easily attachable cover member. 
     To solve the problem described above, the present invention provides the following. (1) An air intake duct includes a duct body having an air flow passage communicating from one end to the other end of the duct body and a vent hole communicating from the air flow passage to the outside, an air permeable cover member arranged to cover the vent hole, and a holding member extending along an outer circumferential surface or an inner circumferential surface of the duct body to secure the cover member to the duct body. The holding member holds a portion of the cover member at a location different from the vent hole of the duct body. According to this aspect of the invention, the cover member can easily be attached to the duct body with the vent hole being covered by the cover member, simply by holding by the holding member. Also, according to this air intake duct, the cover member is held by the holding member, whereby the cover member is less likely to be displaced and can be protected by the holding member. 
     (2) The air intake duct as set forth in (1), in which at least one end portion of the holding member is detachably attached to the duct body. According to this aspect of the invention, the holding member can easily be attached to the duct body. 
     (3) The air intake duct as set forth in (1) or (2), in which the outer circumferential surface or the inner circumferential surface of the duct body is formed with a pair of ribs opposed to each other across the vent hole and extending in a circumferential direction of the duct body, and the holding member is fitted between the pair of ribs. According to this aspect of the invention, because the holding member is positioned by the pair of ribs, the cover member is even less likely to be displaced. 
     (4) The air intake duct as set forth in any one of (1) to (3), in which the holding member extends in the circumferential direction of the air flow passage of the duct body, t a locking portion provided on at least one end of the holding member in the circumferential direction, and a lock receiving portion to which the locking portion is secured is provided on the duct body. According to this aspect of the invention, the holding member can be attached to the duct body with a simple structure. 
     (5) The air intake duct as set forth in (4), in which hinge portion is provided on another end of the holding member in the circumferential direction, the holding member is rotatable with respect to the duct body around the hinge portion serving as a rotation center, and the hinge portion is formed as a one-piece structure together with the duct body and the holding member. According to this aspect of the invention, the holding member can be attached to the duct body by a simple structure and with a reduced number of components. 
     (6) The air intake duct as set forth in any one of (1) to (5), in which an opening portion is provided in the holding member at a location corresponding to the vent hole of the duct body, the holding member includes a pair of holding portions opposed to each other across the opening portion, and the pair of holding portions presses the cover member against the duct body such that a tension is applied to the cover member. According to this aspect of the invention, the cover member is pressed against the duct body by the holding member with the tension being applied to the cover member. Therefore, deformation of the cover member at a portion corresponding to the vent hole can be suppressed, thereby effectively reducing noises from the air intake duct. 
     (7) The air intake duct as set forth in (6), in which the duct body is formed with holding receiving portions at locations corresponding to the holding portions. According to this aspect of the invention, because the holding member presses the cover member against the holding receiving portions, the tension can be applied to the cover member in a more reliable manner. 
     (8) The air intake duct as set forth in (6) or (7), in which an end of the holding member in the circumferential direction is rotatably mounted on the duct body, and when the holding member rotates, one of the pair of holding portions that is near a rotation center of the holding member holds the cover member earlier than the other holding portion that is far from the rotation center. According to this aspect of the invention, the holding member rotates such that one of the holding portions presses the cover member against the duct body first and such that the other holding portion applies the tension to the cover member. Therefore, simply by rotating the holding member, the cover member can be held on the duct body with the tension being applied. 
     (9) The air intake duct as set forth in (8), in which the holding portions are protrusions protruding from the holding member toward the duct body, and the holding portion that is near the rotation center of the holding member has a higher protrusion height than the holding portion that is far from the rotation center of the holding member. When the holding member rotates, the holding portion that is near the rotation center holds the cover member earlier than the holding member that is far from the rotation center. Therefore, the tension can be applied to the cover member in a more reliable manner. 
     (10) The air intake duct as set forth in any one of (1) to (5), in which an opening portion is provided in the holding member at a location corresponding to the vent hole of the duct body, a holding portion is provided on an outer periphery of the opening portion of the holding member, a holding receiving portion corresponding to the holding portion is provided on an outer periphery of the vent hole of the duct body, and the holding portion and the holding receiving portion hold the cover member between the holding portion and the holding receiving portion and apply a tension to the cover member. According to this aspect of the invention, tension can be applied to the cover member in an area corresponding to the vent hole. Therefore, deformation of the cover member at a portion corresponding to the vent hole can be suppressing, thereby effectively reducing noises from the air intake duct. 
     (11) The air intake duct as set forth in (10), in which, when the holding member secures the cover member to the duct body, an outer side portion of the holding receiving portion is pressed by an inner side portion of the holding portion via the cover member. According to this aspect of the invention, the tension can be applied to the cover member in a more reliable manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of an air intake duct according to a first embodiment of the present invention; 
         FIG. 2  is another schematic perspective view of the air intake duct shown in  FIG. 1 , with a cover member and a holding member being removed; 
         FIG. 3  is a perspective view of a main portion cut out from the air intake duct shown in  FIG. 1 , with the cover member and the holding member being removed; 
         FIG. 4  is a sectional view of the air intake duct shown in  FIG. 1 ; 
         FIG. 5  is another sectional view of the air intake duct shown in  FIG. 1 , with the cover member and the holding member being removed; 
         FIG. 6  is a schematic perspective view of an air intake duct according to a second embodiment of the present invention; 
         FIG. 7  is a sectional view of the air intake duct shown in  FIG. 6 , with a cover member and a holding member being removed; 
         FIG. 8  is a sectional view of an air intake duct according to a third embodiment of the present invention; 
         FIG. 9  is a sectional view of the air intake duct shown in  FIG. 8 , with a cover member and a holding member being removed; 
         FIG. 10  is a perspective view of an air intake duct according to a fourth embodiment of the present invention; 
         FIG. 11  is a sectional view of the air intake duct shown in  FIG. 10 ; 
         FIG. 12  is a sectional view of an air intake duct according to a first modification of the fourth embodiment of the present invention; 
         FIG. 13  is a perspective view of an air intake duct according to a second modification of the fourth embodiment of the present invention; 
         FIG. 14  is a sectional view of the air intake duct shown in  FIG. 13 ; 
         FIG. 15  is a perspective view of an air intake duct according to a fifth embodiment of the present invention; 
         FIG. 16  is an enlarged sectional view of a main portion of the air intake duct shown in  FIG. 15 ; 
         FIG. 17  is an enlarged sectional view of a main portion of an air intake duct according to a reference example; and 
         FIG. 18  is a sectional view of an air intake duct according to a sixth embodiment of the present invention. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     &lt;First Embodiment 
     An air intake duct  10  according to a first embodiment of the present invention will be described with reference to the accompanying drawings. In the following description, with respect to the air intake duct, a direction along which the air flows through the air flow passage is referred to as a longitudinal direction, a direction perpendicular to the longitudinal direction is referred to as a radial direction, and a direction around the longitudinal direction as an axis is referred to as a circumferential direction. 
     As shown in  FIGS. 1 and 2 , the air intake duct  10  includes a cylindrical duct body  12  having vent holes  28 , cover members  32  covering the vent holes  28 , and holding members  34  securing the cover members  32  to the duct body  12 . Inside the duct body  12 , there is formed an air flow passage  14  (see  FIG. 3 ) provided with an inlet port  12   a  on one end and an outlet port  12   b  on the other end. The air intake duct  10  is attached to a vehicle engine room such that the inlet port  12   a  of the duct body  12  faces the outside of the vehicle and such that the outlet port  12   b  of the duct body  12  is coupled to an apparatus such as an air cleaner. 
     As shown in  FIG. 3 , the duct body  12  is a substantially cylindrical member. The duct body  12  is formed by combining a first half body  16  having a semicircular sectional shape and a second half body  18  also having a semicircular sectional shape. The first half body  16  and the second half body  18  are made of a synthetic resin such as polypropylene (PP) or polyethylene (PE), and are airtight members formed by injection molding or the like. 
     On the respective circumferential end portions of the first half body  16  and the second half body  18 , flange portions  16   a ,  18   a  protruding outward in the radial direction and extending along the longitudinal direction are formed. The duct body  12  is assembled by joining the flange portions  16   a ,  18   a  of the first half body  16  and the second half body  18  in a face to face manner. The flange portions  16   a ,  18   a  may be joined together by vibration welding, bonding using an adhesive, or by mechanical fastening means such as a rivet, a bolt or the like. 
     The mutually joined flange portions  16   a ,  18   a  form a flange  20  extending in the longitudinal direction. As shown in  FIG. 2 , this flange  20  is formed at two locations spaced from each other in the circumferential direction. That is, the air intake duct  10  has two flanges  20 ,  20  symmetrically arranged across the air flow passage  14 . 
     The duct body  12  is provided with the vent holes  28  that communicate from the air flow passage  14  to the outside (see  FIG. 2 ). Each of the vent holes  28  has a circular shape (see  FIG. 2  or  3 ), and its diameter is preferably in the range of 5 mm to 16 mm. As shown in the drawings, all of the vent holes  28  may have the same dimension. A plurality of (four in the illustrated example) vent holes  28  are arranged in a spaced manner along the circumferential direction of the duct body  12  to form a set of openings  26 . With regard to the number of sets of openings  26 , a plurality of sets (in this example, three sets) may be provided so as to be spaced from each other in the longitudinal direction as shown in the drawings, or only one set may be provided. 
     In the example shown in  FIG. 2 , the vent holes  28  of each set of openings  26  are arranged on the circumferential surface of the duct body  12  such that the respective center line lengths from the inlet port  12   a  are the same. Specifically, the vent holes  28  are arranged such that, in each set of openings  26 , the centers (the weighted centers) of the circular vent holes  28  are aligned in the circumferential direction. Here, the center line C of the duct body  12  is a virtual line passing through the center of the air flow passage  14  along the longitudinal direction. The center line length is the length of this center line C (see  FIG. 1 ). In the illustrated example, the vent holes  28  are formed only in the first half body  16 , so that the sets of openings  26  are formed in the first half body. 
     In the example shown in  FIG. 2 , the duct body  12  is formed with three sets (at least two sets) of openings  26 A,  26 B,  26 C. Among these sets, the first set of openings  26 A is provided at a location at half of the center line length from the inlet port  12   a  to the outlet port  12   b . The second set of openings  26 B is provided at a location at half of the center line length from the inlet port  12   a  to the first set of openings  26 A, and the third set of openings  26 C is provided at a location at half of the center line length from the inlet port  12   a  to the second set of openings  26 B. 
     That is, the air intake duct  10  has at least one set of openings  26 A at a location at half of the entire center line length. Further, another set of openings  26 B ( 26 C) is arranged at the middle position between the inlet port  12   a  and the set of openings  26 A ( 26 B) nearest to the inlet port  12   a . In this manner, the sets of openings  26 A to  26 C (the vent holes  28 ) are arranged at positions, such as one half, a quarter, one-eighth and the like of the center line of the duct body  12 , at which the phases of pipe resonances are superimposed so that the sound pressure level becomes high. Therefore, the air intake duct  10  releases the pressure pulsation in the air flow passage  14  outside through the vent holes  28 , thereby suitably dampening the pipe resonance. 
     The vent holes  28  are designed such that the total opening lengths in the circumferential direction is in the range of 13% to 67% of the circumferential length of the duct body  12 . Here, the opening length of each of the vent holes  28  in the circumferential direction is the length of the vent hole  28  in circumferential direction that passes through the weighted center thereof, and in the illustrated example, it is the diameter of the vent hole  28 . The total opening lengths is a sum of the opening lengths of the vent holes  28  of the set of openings  26  in the circumferential direction. In the illustrated example, the vent holes  28  having the same shape and same dimension are arranged in the circumferential direction of the duct body  12 . Thus, the total opening length is a sum of the diameters of the vent holes  28  of the set of openings  26 . 
     If the total opening length is below 13% of the circumferential length of the duct body  12 , the pressure pulsation of the air flow passage  14  cannot be released outward through the vent holes  28  in a suitable manner, so that sufficient dampening effect of the pipe resonance cannot be obtained. On the other hand, if the total opening length exceeds 67% of the circumferential length of the duct body  12 , the air flowing in the air flow passage  14  leaks through the vent holes  28  such that the pressure loss increases, in which case the air cannot efficiently be guided to the engine. Further, if the total opening length exceeds 67% of the circumferential length of the duct body  12 , the sound leakage through the vent holes  28  is also increased, and the strength of the air intake duct  10  is lowered. 
     The cover member  32  is an air permeable member. The cover member  32  may be a foam member, such as urethane foam, rubber or the like, having an open-cell structure, a porous member, such as a resin film, having minute openings, a fiber assembly, such as nonwoven fabric, a meshed member, or the like. Among others, the nonwoven fabric having a water-repellent surface is preferred. The nonwoven fabric is preferably be made of polyester fibers or polypropylene fibers by a span bond method or a needle punch method, and the density thereof is preferably in the range of 30 g/m 2  to 300 g/m 2 . 
     The dimensions of the cover member  32  in the longitudinal and circumferential directions are larger than the dimensions of the vent holes  28  in the longitudinal and circumferential directions, so that the cover member  32  can cover the vent holes  28 . The circumferential dimension of the cover member  32  may be set substantially the same as the spacing width between the flanges  20 ,  20  in the circumferential direction (half of the circumference of the duct body  12 ). The cover member  32  is a strip of a sheet (see  FIG. 2  or  3 ). The cover member  32  is arranged on the outer circumferential surface of the duct body  12  such that the long sides thereof extend along the circumferential direction of the air intake duct  10 . The cover member  32  arranged in this manner is held on the duct body  12  by the holding member  34  in a state in which it covers the vent holes  28 . 
     The holding members  34  are attached at locations corresponding to the respective sets of openings  26  of the duct body  12  (see  FIG. 2 ). Each of the holding member  34  is made of synthetic resin, such as polypropylene (PP) or polyethylene (PE), or rubber, and is formed is a shape of a strip having the long sides extending in the circumferential direction (see  FIG. 3 ). 
     The dimension of the short sides of the holding member  34  (the dimension in the longitudinal direction of the duct body  12 ) is larger than the dimension of the vent hole  28  is the longitudinal direction, and is also larger than the dimension of the short sides of the cover member  32  (the dimension in the longitudinal direction of the duct body  12 ). The dimension of the long sides of the holding member  34  (the dimension in the circumferential direction of the duct body  12 ) is equal to or larger than the dimension of the cover member  32  in the circumferential direction. The dimension of the long sides of the holding member  34  may be substantially equal to the spacing width between the flanges  20 ,  20  in the circumferential direction (half of the circumference of the duct body  12 ). 
     In this manner, the holding member  34  is designed to have a size that is capable of covering all the vent holes  28  of the corresponding set of openings  26  and that is larger than the cover member  32  for the corresponding set of openings  26 . The holding member  34  may be formed to have a shape that fits the outer circumferential surface of the duct body  12  in advance. Alternatively, the cover member may be made of a flexible or elastic member, so that the holding member  34  is deformed along the outer circumferential surface of the duct body  12  when attached to the duct body  12 . 
     At least one end portion of the holding member  34  is detachably attached to the duct body  12 . The holding member  34  is formed with an opening portion (an opening)  36  having a size that conforms to the vent hole  28  of the duct body  12 . This opening portion  36  is formed at a location which, when the holding member  34  is secured onto the outer circumferential surface of the duct body  12 , faces the vent hole  28 . 
     Therefore, in a state in which the holding member  34  is mounted on the outer circumferential surface of the duct body  12 , portions of the cover member  32  that correspond to the vent holes  28  are exposed to the outside through the opening portions  36 , and a portion of the cover member  32  at a location different from the vent holes  28  is held by the holding member  34 . In other words, the cover member  32  is covered and protected by the holding member  34 , except the portions that correspond to the vent holes  28 . The opening portion  36  of the holding member  34  need not be formed to have the same shape as the vent hole  28 , and may be formed to have an opening that is at least larger than the vent hole  28 . 
     As shown in  FIG. 5 , the holding member  34  has a first pawl portion (a locking portion)  38  at one end portion in the circumferential direction. This first pawl portion  38  is engaged with a first pawl hole (a lock receiving portion)  22  formed in the flange  20 . The holding member  34  also has a second pawl portion  40  at the other end portion in the circumferential direction. This second pawl portion  40  is detachably engaged with a second pawl hole  24  formed in the other flange  20 , which is next to the flange  20 , having the first pawl hole  22 , in the circumferential direction. 
     The first pawl hole  22  and the second pawl hole  24  are openings formed through the flanges  20 , and are spaced from each other in the circumferential direction. On the other hand, the first pawl portion  38  and the second pawl portion  40  are formed to have hook shapes, including tongues extending in the circumferential direction from the end portions of the holding member  34  and protrusion elements  38   a ,  40   a  protruding from distal ends of the tongues in directions intersecting the direction of insertion into the pawl holes  22 ,  24 . When the first and second pawl portions  38 ,  40  are pushed into the first and second pawl holes  22 ,  24  respectively, the protrusion elements  38   a ,  40   a  of the first and second pawl portions  38 ,  40  are caught by the edges of the first and second pawl holes  22 ,  24 , whereby the holding member  34  is held by the flanges  20  as shown in  FIG. 4 . 
     The duct body  12  further includes a pair of ribs  30 ,  30  extending in the circumferential direction such that the vent holes  28  are interposed between the ribs  30 ,  30 . By placing the cover member  32  between the pair of ribs  30 ,  30  and fitting the holding member  34  such that the holding member  34  faces the vent holes  28 , the holding member  34  can hold the cover member  32  and can be secured onto the outer circumferential surface of the duct body  12  (see  FIG. 1  or  3 ). 
     More specifically, the pair of ribs  30 ,  30  are provided on the outer circumferential surface of the first half body  16  that has the vent holes  28  so as to extend from one flange portions  16   a  to the other flange portion  16   a . The pair of ribs  30 ,  30  is arranged such that the spacing distance therebetween is the same as the dimension of the short sides of the holding member  34  (the dimension in the longitudinal direction of the duct body  12 ). Each of the ribs  30 ,  30  is formed to protrude from the outer circumferential surface of the duct body  12  such that it is higher than the thickness of the cover member  32 . 
     (Functions) Next, functions of the air intake duct  10  will be described. By covering the vent holes  28  of the duct body  12  by the air permeable cover member  32 , the air intake duct  10  can release the pressure pulsation in the air flow passage outside through the vent holes  28  and the cover member  32 , and can dampen the pipe resonance. This can reduce the generation of noises caused by the pipe resonance. Further, because the vent holes  28  are covered by the cover member  32 , the noise leakage from the vent holes  28  can be suppressed as compared with a case in which the vent holes  28  are exposed directly to the outside. 
     The cover member  32  is held by the holding member  34  that is secured to the duct body  12 . This can eliminate the need of a heating device for attaching the cover member like the heat shrinkable sheet member described as the conventional example, thereby simplifying the manufacturing facilities. Also, because the heating is not necessary to attach the cover member  32 , deformations of the duct body  12  and the cover member  32  can be avoided. 
     Further, the cover member  32  can be held simply by securing the holding member  34  to the duct body  12 . Therefore, the process of attaching the cover member  32  can be simplified. Especially, the holding member  34  can be attached to the duct body  12  simply by inserting the first and second pawl portions  38 ,  40  into the first and second pawl holes  22 ,  24  of the duct body  12 . Thus, the assembling efficiency is further improved. 
     The cover member  32  is firmly held by the holding member  34 , so that its position is less likely to be shifted. Therefore, a stable noise reduction effect can be expected for a long period of time, regardless of vibrations of the vehicle or the like. The outer side of the cover member  32 , except for the portions that correspond to the vent holes  28 , is covered and protected by the holding member  34 . The portions of the cover member  32  that correspond to the vent holes  28  and exposed to the outside are also located inward in the radial direction than the outer surface of the holding member  34 . Therefore, the cover member  32  is less likely to be damaged by contacting other members or human hand during the transportation or mounting of the air intake duct  10  or the maintenance of the vehicle. That is, the holding member  34  functions not only to hold the cover member  32  but also to protect the cover member  32 , so that the position shift and damage of the cover member  32  can efficiently be suppressed. 
     The holding member  34  has the opening portions  36  at locations that correspond to the vent holes  28  when it is secured to the duct body  12 . Therefore, the air permeability of the cover member  32  is not deteriorated by the holding member  34 . Further, because the portion of the cover member  32  is held between the duct body  12  and the holding member  34  at locations different from the vent holes  28 , the portions of the cover member  32  that correspond to the vent holes  28  is not compressed by the holding member  34 . That is, a change in the air permeability of the cover member  32  before and after the attachment can be suppressed. Therefore, the cover member  32  can be attached to the duct body  12  such that optimum air permeability corresponding to the openings of the vent holes  28  is maintained, thereby achieving desired noise reduction effect and air intake performance. 
     Further, the duct body  12  includes the pair of ribs  30 ,  30  interposing the vent holes  28  therebetween, and when the cover member  32  and holding member  34  are fitted between these ribs  30 ,  30 , the positions thereof are determined accordingly. Therefore, they can easily be attached to the duct body  12 . Also, by designing the dimension of the cover member  32  in the circumferential direction in accordance with the spacing distance between the flanges  20 ,  20  in the circumferential direction, the cover member  32  can easy be positioned between the flanges  20 ,  20 . 
     In this manner, the set of openings  26  is surrounded by the ribs  30 ,  30  and the flanges  20 ,  20 , and the cover member  32  is arranged in this surrounded space. Therefore, the holding member  34  can easily be attached, and it is possible to avoid attaching failure, such as displacement of the cover member  32  from the vent holes  28 . It is also possible to avoid other attaching failures, such as a displacement of the holding member  34  from the cover member  32 , or a displacement of the holding member  34  from its proper position and resulting in covering of the vent holes  28 . 
     (Test Examples) Changes in noise reduction effect depending on the opening ratio of the vent hole were tested. Each of test examples 1 to 5 of air intake ducts was prepared by providing a straight pipe duct body having a diameter of 71 mm and a center line length of 400 mm from the inlet port to the outlet port, forming vent holes at a location at one half of the center line length of the duct body from the inlet port to the outlet port, and covering the vent holes with a nonwoven fabric having a density of 100 g/m 2 . Also prepared was a comparison example 1 having a duct body that is similar to the test examples but does not have vent holes, and a comparison example 2 in which the vent holes are not covered with nonwoven fabric. 
     With regard to the evaluation of the noise reduction effect, sound was input into the air intake duct from a sound source attached to the inlet port, and the sound volume was measured by a sound collector attached to the outlet port. It was evaluated as being effective, when there was a sound reduction amount of 20 dB or more in the frequency of 300 Hz to 600 Hz and when there was a sound reduction amount of 10 dB or more in the frequency of 600 Hz to 1200 Hz. The results are shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Test 
                 Comparative 
               
               
                   
                 Examples 
                 Examples 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 1 
                 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Vent Hole 
                 — 
                 10 
                 10 
                 16 
                 5 
                 N/A 
                 10 
               
               
                 Diameter (mm) 
               
               
                 Number of 
                 1 
                 5 
                 15 
                 2 
                 20 
                 — 
                 5 
               
               
                 Vent Holes 
               
               
                 Opening Area 
                 2375 
                 392.5 
                 1177.5 
                 401.92 
                 392.5 
                 — 
                 392.5 
               
               
                 (mm 2 ) 
               
               
                 Opening Length 
                 30 
                 50 
                 150 
                 32 
                 100 
                 — 
                 50 
               
               
                 (mm) 
               
               
                 Opening Ratio 
                 13 
                 22 
                 67 
                 14 
                 45 
                 — 
                 22 
               
               
                 (%) 
               
               
                 Effectiveness 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
               
               
                   
               
               
                 * The opening area is the total opening area of the vent holes. 
               
               
                 * The opening length is the total opening length of the vent holes in the circumferential direction. 
               
               
                 * The opening ratio is the ratio of the total opening length of the vent holes circumferential direction to the circumferential length of the duct body. 
               
            
           
         
       
     
     According to the results shown in Table 1, it was confirmed that, when the opening ratio of the vent holes to the circumferential length of the duct body is in the range of 13% to 67%, sufficient noise reduction effect can be obtained. 
     The present invention is not limited to the embodiment described above, and can also be modified as follows. 
     &lt;Second Embodiment 
     The attaching structure for the holding member  34  in the air intake duct  10  according to the first embodiment is configured such that both end portions of the holding member  34  that are spaced in the circumferential direction are detachably attached to the duct body  12 . However, only one of the end portions may be configured to be detachably attached.  FIGS. 6 and 7  are sectional views of an air intake duct  50  according to a second embodiment of the present invention. 
     As shown in  FIGS. 6 and 7 , the holding member  52  of the air intake duct  50  is rotatably attached to the flange  20  of the duct body  12  at an end portion in the circumferential direction via a hinge portion  56 . The hinge portion  56  is formed as a one-piece structure together with the first half body  16  of the duct body  12 . By forming the holding member  52  and the first half body  16  as a one-piece structure, the number of components of the air intake duct  50  can be reduced, and so that the handling thereof can be further improved. 
     The holding member  52  configured in this manner is rotated relative to the duct body  12  with the hinge portion  56  serving as the fulcrum, and is held onto the duct body  12  by the engagement of a pawl portion (a locking portion)  54  provided on a side opposite to the hinge portion  56  with respect to the circumferential direction with the pawl hole (a lock receiving portion)  22  of the flange  20 , such that the cover member  32  is held between the holding member  52  and the outer circumferential surface of the duct body  12 . 
     &lt;Third Embodiment 
     While the air intake duct  50  according to the second embodiment is configured such that the hinge portion  56  connects the holding member  52  and the first half body  16 , it may be configured such that the hinge portion  56  connects the holding member  52  and second half body  18 , like an air intake duct  60  according to a third embodiment shown in  FIGS. 8 and 9 . 
     In the air intake duct  60  according to the third embodiment, the pawl portion  54  of the holding member  52 , which is formed as a one-piece structure together with the second half body  18 , is engaged with the pawl hole  22  provided in the second half body  18 . Therefore, the holding member  52 , which is integral with the second half body  18 , can press the cover member  32  and the first half body  16  against the second half body  18 . As a result, at the same time as securing the cover member  32  to the first half body  16 , the first half body  16  and the second half body  18  can be secured together. 
     &lt;Fourth Embodiment 
       FIG. 10  is a perspective view of an air intake duct  70  according to a fourth embodiment of the present invention, and  FIG. 11  is a sectional view of the air intake duct  70 , taken along a plane perpendicular to the longitudinal direction thereof. The air intake duct  70  according to the fourth embodiment is configured such that, in the air intake duct  60  according to the third embodiment, eight vent holes  28  form one set of openings  26 , and a pair of protrusions (holding portions)  71 ,  72  are formed on the holding member  52  to hold the cover member  32 . 
     According to the air intake duct  70 , a plurality of vent holes  28  are arranged in a plurality of rows in the circumferential direction and in the longitudinal direction to form one set of openings  26 . By arranging the vent holes  28  in a plurality of rows and not in a single row, the opening area can be ensured while maintaining high strength of the duct body  12 . 
     The holding member  52  that covers the vent holes  28  is formed to extend along the circumferential direction, similarly to the first to third embodiments described above. One end of the holding member  52  in the circumferential direction is connected to the duct body  12  via the hinge portion  56 , and the pawl portion  54  is provided on the other end in the circumferential direction and is detachably engaged with the pawl hole  22  of the duct body  12 . In the vicinity of the hinge portion  56  of the holding member  52  (near the one end in the circumferential direction) and in the vicinity of the pawl portion  54  (near the other end in the circumferential direction), the pair of protrusions  71 ,  72  protruding toward the duct body  12  is provided. The pair of protrusions  71 ,  72  are opposed to each other across the opening portions  36 , and extend in the longitudinal direction of the duct body  12 . 
     According to the air intake duct  70  of the fourth embodiment, when rotating the holding member  52  around the hinge portion  56  to attach the cover member  32  to the duct body  12 , firstly, the protrusion  71  near the hinge portion  56  contacts the cover member  32  and presses the cover member  32  against the outer circumferential surface of the duct body  12 . In this state, a portion of the cover member  32  on a side of the hinge portion  56  is held between the protrusion  71  and the outer circumferential surface of the duct body  12 . 
     When the holding member  52  is further rotated, the protrusion  72  near the pawl portion  54  contacts the cover member  32  and presses the cover member  32  against the outer circumferential surface of the duct body  12 . When the holding member  52  is further rotated, the protrusion  72  moves while pulling a portion of the cover member  32  on a side of the pawl hole  22  toward the pawl hole  22 . Finally, in a state in which the pawl portion  54  is engaged with the pawl hole  22  so that the holding member  52  is secured to the duct body  12 , the holding member  52  is held on the duct body  12  such that a tension is applied to the cover member  32  by the pair of protrusions  71 ,  72 . 
     By holding the cover member  32  with the tension applied in this manner, noises can be reduced more effectively. According to the air intake duct  70  of the fourth embodiment, because the tension is applied to the cover member  32 , the deformation of the cover member  32  due to pressure variations in the inside of the duct body  12  can be suppressed. Therefore, the wear of the cover member  32  can be suppressed so that the noises can be reduced for a long period of time. Further, because no clearance is formed between the cover member  32  and duct body  12 , the noises do not leak to the outside, so that the noises can further be suppressed. 
     If the cover member  32  is held with no tension being applied, an increase in pressure inside the air flow passage due to the pipe resonance may cause the cover member  32  to bulge from the vent holes  28 , or a decrease in the pressure may cause the cover member  32  to deform in a sagging manner from the vent holes  28 . In the event that the cover member  32  is deformed due to the pressure, especially if the opening portion  36  is larger than the vent hole  28 , the cover member  32  may by worn by the outer circumference of the vent hole  28 , or noises may leak to the outside from a clearance formed between the cover member  32  and duct body  12 . 
     The structure for holding the cover member  32  is not limited to the protrusions  71 ,  72  described above. For example, the holding member  52  may be formed to have a thin area where the opening portions  36  are formed and a thick area at respective end portions in the circumferential direction, so that the cover member  32  is held by the step portions formed at the boundary between these areas. 
     Further, the present embodiment is not limited to the example in which the protrusions  71 ,  72  are provided on the holding member  52 .  FIG. 12  is a sectional view of an air intake duct  70 A according to a first modification of the fourth embodiment of the present invention. As shown in the sectional view of  FIG. 12 , groove portions (holding receiving portions)  73 ,  74  into which the protrusions  71 ,  72  are inserted may be formed in the duct body  12  at locations corresponding to the protrusions  71 ,  72 . 
     Further, the protrusions  71 ,  72 , may be provided continuously along the longitudinal direction of the duct body  12  as shown in  FIG. 10 , or may be provided intermittently. Preferably, the height of the protrusion  71  is larger than the height of the protrusion  72  so that, when the holding member  52  rotates while pulling the cover member  32  by the protrusion  72 , the cover member  32  firmly held by the protrusion  71 . 
     By modifying the shape of the outer circumferential surface of the duct body  12  and the shape of the inner circumferential surface of the holding member  52 , or by modifying the attaching position of the hinge portion  56  of the holding member  52  with respect to the duct body  12 , it can be configured such that the protrusion  71  near the rotation center of the holding member  52  holds the cover member  32  earlier than the protrusion  72  that is far from the rotation center. 
     The present embodiment is not limited to the example in which the protrusions  71 ,  72  are formed along the longitudinal direction as described above.  FIG. 13  is a perspective view of an air intake duct  70 B according to a second modification of the fourth embodiment, and  FIG. 14  is a sectional view taken along the longitudinal direction. As shown in  FIGS. 13 and 14 , a pair of protrusions  81 ,  82  for applying a tension to the cover member  32  may be provided on the holding member  52  such that they extend in the circumferential direction and interpose the opening portions  36  therebetween, and circumferential grooves  83 ,  84  for receiving the protrusions  81 ,  82  may be formed in the duct body  12 . Also with this structure, tension can be applied to the cover member  32  as shown by an arrow is in  FIG. 14 . Here, in  FIG. 13 , for easy explanation, the illustration of the cover member  32  is omitted. In the examples shown in  FIGS. 12 to 14 , the protrusions  71 ,  72  are provided on the holding member  52  and the groove portions  73 ,  74  are formed in the duct body  12 . However, the protrusions  71 ,  72  may be provided on the duct body  12  and the groove portions  73 ,  74  may be formed in the holding member  52 . 
     &lt;Fifth Embodiment 
     In the fourth embodiment, the tension is applied to the entire cover member  32 . However, as long as the tension is applied to at least areas on the cover member  32  that correspond to the vent holes  28 , the above noise reduction effect can be expected.  FIG. 15  is a perspective view of an air intake duct  90  according to a fifth embodiment of the present invention, and  FIG. 16  is an enlarged sectional view taken along the plane perpendicular to the longitudinal direction. 
     As shown in  FIGS. 15 and 16 , according to the air intake duct  90  of the fifth embodiment, a ring-shaped protrusion (a holding receiving portion)  91  is formed along the outer circumferential edge of the vent hole  28  of the duct body  12 , and a corresponding ring-shaped protrusion (a holding portion)  92  is formed along the outer circumferential edge of the opening portion  36 . Also with this structure, the cover member  32  can be held on the duct body  12  by the holding member  52  with tension being applied to the cover member  32 . 
     Here, when forming the protrusions  91 ,  92  on the outer circumferential edges of the vent hole  28  and opening portion  36  in this manner, it is preferable to employ the structure shown in  FIG. 16 , so that tension can reliably be applied to the cover member  32 . In  FIG. 16 , when the holding member  52  is secured to the duct body  12 , the radially outer side wall of the protrusion  91  presses the radially inner side wall of the protrusion  92  via the cover member  32 . 
     On the other hand, as shown in a reference example of  FIG. 17 , in a case in which the radially inner side wall of a protrusion  91 A presses the radially outer side wall of a protrusion  92 A, a stress cs is applied to the cover member  32  such that it compresses the cover member  32  toward the vent hole  28 . That is, when securing the holding member  52  to the duct body  12 , the inner side wall of the protrusion  91 A and the outer side wall of the protrusion  92 A act to move the cover member  32  from outside to inside with respect to the vent holes  28 . Therefore, a compression stress is applied to the areas on the cover member  32  that correspond to the vent holes  28 , so that tension is less likely to be applied to the areas on the cover member  32  that correspond to the vent holes  28 . 
     In contrast, according to the structure shown in  FIG. 16 , as the holding member  52  presses the cover member  32  against the duct body  12 , the cover member  32  that has been in contact with the outer side wall of the protrusion  91  is pressed toward the outside of the vent holes  28  by the inner side wall of the protrusion  92 . Therefore, the tension is can be applied to the areas on the cover member  32  that correspond to the vent holes  28 . 
     &lt;Sixth Embodiment 
     In the first to fifth embodiments, description has been given of the examples where the holding member  34 ,  52  is provided on the outer circumferential surface of the duct body  12 . However, as shown in the sectional view of an air intake duct of  FIG. 18 , a holding member  52 A may be provided on the inner circumferential surface of the duct body  12 . 
       FIG. 18  is a sectional view of the duct body  12  of an air intake duct according to a sixth embodiment of the present invention. The holding member  52 A is rotatably attached to the first half body  16  of the duct body  12  via a hinge portion  56 A. The pawl portion  54  is provided on a side opposite to the hinge portion  56 A in the circumferential direction. When the pawl portion  54  is engaged with the pawl hole  22  of the flange  20 , the holding member  52 A is held by the first half body  16 . The first half body  16  and the second half body  18  are joined together at a position shifted along the longitudinal direction of the duct body  12  from the section shown in  FIG. 18  with the flange portions  16   a ,  18   a  butted against each other, by means of a vibration welding, bonding using an adhesive, or a mechanical fastening using such as a rivet or a bolt. 
     In this embodiment, the holding member  52 A extends along the inner circumferential surface of the duct body  12 , and the protrusions  71 ,  72  provided on the outer circumferential surface of the holding member  52 A press the cover member  32  against the first half body  16 , whereby the cover member  32  is secured to the duct body  12 . The holding member  52 A holds a portion of the cover member  32  at a location different from the vent holes  28  of the duct body  12 . 
     As described above, also in this embodiment, pressure variations inside the air flow passage  14  can be released outward from the vent holes  28  through the opening portions  36  and the cover member  32 , thereby suppressing the pipe resonance inside the air flow passage  14 . Also, similarly to the first embodiment, a pair of ribs opposed to each other across the vent holes  28  may be provided on the inner circumferential surface of the duct body  12  such that they extend in the circumferential direction of the duct body  12 . 
     While the present invention has been described with reference to embodiments thereof, the technical scope of the present invention is not limited to the scope of the description of the above embodiments, and it is apparent for those skilled in the art that various changes and modifications can be made in the above embodiments. 
     For example, while the flow section of the air flow passage  14  is formed in a circular form in the above embodiments, it may have a polygonal shape, such as a quadrangular shape, an elliptic shape or other shapes. 
     Further, although description has been given with regard to the vent hole  28  having a circular shape, the vent hole  28  may have a polygonal shape, such as a quadrangular shape, an elliptic shape or other shapes. The vent holes  28  of the set of openings  26  may have the same dimension as described above, may have similar figures to each other, may be different in shapes, or may be a combination of thereof. 
     The sets of openings  26 A to  26 C disposed at the respective positions spaced from each other in the longitudinal direction of the duct body  12 , the total opening length of the vent holes  28  of each of the sets in circumferential direction may be designed to become smaller as it approach the inlet port. That is, the total opening length of the second set of openings  26 B located nearer to the inlet port than the first set of openings  26 A may be smaller than the total opening length of the first set of openings  26 A, and the total opening length of the third set of openings  26 C located nearer to the inlet port than the second set of openings  26 B may be smaller than the total opening length of the second set of openings  26 B. According to this configuration, the pipe resonance can be suitably suppressed while minimizing the number and size of the vent holes  28  formed in the duct body  12 . 
     The above description has been given with regard to examples having the two flanges  20  that are spaced from each other in the circumferential direction. However, the number of flanges  20  may be one, or three or more. Further, while the attaching structure for the holding member is configured such that the pawl portion engages with a pawl hole, it may be configured such that the pawl portion is formed in the duct body and the pawl hole is opened in the holding member, or may have other structures. 
     Further, in the above description, the pair of protrusions (the holding portion and the holding receiving portion)  71 ,  72 ,  91 ,  92  formed on the duct body  12  and the holding member  34 ,  52  respectively are ring-shaped protrusions. However, the present invention is not limited to these examples. For example, they may be in a form of a frame having a polygonal shape, such as a triangular or quadrangular shape, surrounding the vent hole  28  or the opening portion  36 , or may be a plurality of intermittent protrusions formed along the outer peripheries of the vent holes  28  and the opening portions  36 . 
     The present application is based on Japanese Patent Application No. 2010-131358 filed on Jun. 8, 2010, and the content of which is incorporated herein by reference. 
     According to the present invention, there is provided an air intake duct in which a cover member can easily be attached to a duct body while ensuring ventilation, by using a holding member that holds a portion at a location different from a vent hole of the duct body.