Patent Publication Number: US-9834398-B2

Title: Sheet conveying device, sheet feeder, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2015-206041, filed on Oct. 20, 2015, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Example embodiments generally relate to a sheet conveying device, a sheet feeder, and an image forming apparatus, and more particularly, to a sheet conveying device for conveying a sheet, a sheet feeder incorporating the sheet conveying device, and an image forming apparatus incorporating the sheet conveying device. 
     Background Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction peripherals having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. 
     Such image forming apparatus includes a body, an image scanner disposed atop the body, and an auto document feeder (ADF) disposed atop the image scanner. In order to downsize the ADF, the ADF includes an original tray and an ejection tray situated below the original tray. A user places an original sheet bearing an image to be read by the image scanner on the original tray. The ejection tray receives the original sheet bearing the image that has been read by the image scanner. A sheet conveying device conveys the original sheet from the original tray to the ejection tray. 
     The image forming apparatus may be a multifunction peripheral including a sheet conveying device that conveys a recording sheet onto which an image is formed with toner, ink, or the like according to image data sent from the image scanner or a client computer connected to the multifunctional peripheral. 
     While the original sheet or the recording sheet is conveyed through the sheet conveying device, the original sheet or the recording sheet slides over a component disposed inside the sheet conveying device, generating slide noise. The slide noise leaks out of the image forming apparatus as undesired noise, degrading an environment of the image forming apparatus. 
     SUMMARY 
     At least one embodiment provides a novel sheet conveying device that includes a conveyer to convey a sheet and a primary sheet guide including a bending portion to bend the sheet while the sheet slides over the bending portion to change a sheet conveyance direction. A secondary sheet guide is disposed opposite the primary sheet guide with an interval between the primary sheet guide and the secondary sheet guide. A noise attenuator intakes and attenuates slide noise generated by the bending portion while the sheet slides over the bending portion. The noise attenuator includes at least one resonant cavity disposed opposite an outer face of one of the primary sheet guide and the secondary sheet guide, at least one sound inlet, disposed in proximity to the bending portion, to intake the slide noise generated by the bending portion, and at least one sound guide communicating with the sound inlet and the resonant cavity to guide the slide noise from the sound inlet to the resonant cavity. 
     At least one embodiment further provides a novel sheet feeder that includes a roller pair to feed a sheet and a sheet conveying device to convey the sheet fed by the roller pair. The sheet conveying device includes a conveyer to convey the sheet and a primary sheet guide including a bending portion to bend the sheet while the sheet slides over the bending portion to change a sheet conveyance direction. A secondary sheet guide is disposed opposite the primary sheet guide with an interval between the primary sheet guide and the secondary sheet guide. A noise attenuator intakes and attenuates slide noise generated by the bending portion while the sheet slides over the bending portion. The noise attenuator includes at least one resonant cavity disposed opposite an outer face of one of the primary sheet guide and the secondary sheet guide, at least one sound inlet, disposed in proximity to the bending portion, to intake the slide noise generated by the bending portion, and at least one sound guide communicating with the sound inlet and the resonant cavity to guide the slide noise from the sound inlet to the resonant cavity. 
     At least one embodiment further provides a novel image forming apparatus that includes an image scanner to read an image on a sheet and a sheet conveying device to convey the sheet to the image scanner. The sheet conveying device includes a conveyer to convey the sheet and a primary sheet guide including a bending portion to bend the sheet while the sheet slides over the bending portion to change a sheet conveyance direction. A secondary sheet guide is disposed opposite the primary sheet guide with an interval between the primary sheet guide and the secondary sheet guide. A noise attenuator intakes and attenuates slide noise generated by the bending portion while the sheet slides over the bending portion. The noise attenuator includes at least one resonant cavity disposed opposite an outer face of one of the primary sheet guide and the secondary sheet guide, at least one sound inlet, disposed in proximity to the bending portion, to intake the slide noise generated by the bending portion, and at least one sound guide communicating with the sound inlet and the resonant cavity to guide the slide noise from the sound inlet to the resonant cavity. 
     Additional features and advantages of example embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of example embodiments and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic vertical cross-sectional view of an image forming apparatus according to an example embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view of an auto document feeder incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a block diagram of the image forming apparatus depicted in  FIG. 1 , illustrating control of the auto document feeder depicted in  FIG. 2 ; 
         FIG. 4  is a block diagram of the image forming apparatus depicted in  FIG. 1 , illustrating transmission of signals between the auto document feeder depicted in  FIG. 2  and a body of the image forming apparatus; 
         FIG. 5  is a perspective view of a Helmholtz resonator; 
         FIG. 6  is a cross-sectional view of a sheet conveying device incorporated in the auto document feeder depicted in  FIG. 2 , illustrating a noise attenuator; 
         FIG. 7  is a perspective view of the noise attenuator depicted in  FIG. 6 ; 
         FIG. 8  is an exploded perspective view of the noise attenuator depicted in  FIG. 6 ; 
         FIG. 9  is a graph illustrating a relation between a frequency of friction noise and a sound pressure level; 
         FIG. 10  is a cross-sectional view of a noise attenuator as a first variation of the noise attenuator depicted in  FIG. 6 ; 
         FIG. 11  is a cross-sectional view of a noise attenuator as a second variation of the noise attenuator depicted in  FIG. 6 ; 
         FIG. 12A  is a perspective view of a sound guide as a rectangular tube incorporated in the noise attenuator depicted in  FIG. 6 ; 
         FIG. 12B  is a perspective view of a sound guide as a circular tube incorporated in the noise attenuator depicted in  FIG. 6 ; 
         FIG. 13  is a cross-sectional view of a sound guide as a first variation of the sound guide depicted in  FIG. 12A ; 
         FIG. 14  is a graph illustrating noise reduction by Helmholtz resonance with the sound guide depicted in  FIG. 13 ; and 
         FIG. 15  is a perspective view of a noise attenuator incorporating a sound guide as a second variation of the sound guide depicted in  FIG. 12A . 
     
    
    
     The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     It will be understood that if an element or layer is referred to as being “on”, “against”, “connected to”, or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on”, “directly connected to”, or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, a term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly. 
     Although the terms first, second, and the like may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present disclosure. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing example embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus  1  according to an example embodiment is explained. 
       FIG. 1  is a schematic vertical cross-sectional view of the image forming apparatus  1 . The image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this example embodiment, the image forming apparatus  1  is a color MFP that forms a color toner image on a recording medium by electrophotography. Alternatively, the image forming apparatus  1  may be a monochrome MFP that forms a monochrome toner image on a recording medium. Yet alternatively, the image forming apparatus  1  may form an image on a recording medium by inkjet printing. 
     Referring to  FIG. 1 , a description is provided of a construction of the image forming apparatus  1 . 
     As illustrated in  FIG. 1 , the image forming apparatus  1  is a digital multifunction peripheral including a body  1 M and an auto document feeder (ADF)  5  disposed atop the body  1 M. The body  1 M includes a sheet feeding device  7 , an image forming device  3 , and an image scanner  4 . The image scanner  4  and the ADF  5  construct an image reading device  6 . The ADF  5  serves as a sheet feeder. The ADF  5  includes a noise attenuator  2  described below. 
     A detailed description is now given of a construction of the sheet feeding device  7 . 
     The sheet feeding device  7  includes three paper trays  21 A,  21 B, and  21 C being layered vertically and loading a plurality of sheets P serving as recording media of different sizes, respectively. Each of the paper trays  21 A,  21 B, and  21 C loads the sheets P (e.g., plain paper) having a size selected from the different sizes in portrait orientation or landscape orientation. 
     The sheet feeding device  7  includes a plurality of sheet feeders  22 A,  22 B, and  22 C that picks up and separates an uppermost sheet P from other sheets P placed on the paper trays  21 A,  21 B, and  21 C and feeds the uppermost sheet P to conveyance rollers, respectively. The sheet feeding device  7  further includes a sheet feeding path  24  provided with the conveyance rollers that convey the sheet P conveyed from one of the sheet feeders  22 A,  22 B, and  22 C to a given image forming position inside the image forming device  3 . 
     A detailed description is now given of a construction of the image forming device  3 . 
     The image forming device  3  includes an exposure device  31 , a plurality of photoconductive drums  32 K,  32 Y,  32 M, and  32 C, a plurality of developing devices  33 K,  33 Y,  33 M, and  33 C replenished with toners in different colors, that is, black, yellow, magenta, and cyan toners, respectively, a transfer belt  34 , a secondary transfer device  35 , and a fixing device  36 . 
     The exposure device  31  generates laser beams L according to black, yellow, magenta, and cyan image data created by the image reading device  6 , which expose the photoconductive drums  32 K,  32 Y,  32 M, and  32 C, respectively. The exposure device  31  exposes the photoconductive drums  32 K,  32 Y,  32 M, and  32 C with the laser beams L, forming electrostatic latent images corresponding to the black, yellow, magenta, and cyan image data on an outer circumferential surface of the photoconductive drums  32 K,  32 Y,  32 M, and  32 C, respectively. 
     The developing devices  33 K,  33 Y,  33 M, and  33 C disposed in proximity to the photoconductive drums  32 K,  32 Y,  32 M, and  32 C supply the black, yellow, magenta, and cyan toners to the electrostatic latent images formed on the photoconductive drums  32 K,  32 Y,  32 M, and  32 C so that the black, yellow, magenta, and cyan toners construct thin layers, thus developing the electrostatic latent images into visible black, yellow, magenta, and cyan toner images, respectively. 
     The black, yellow, magenta, and cyan toner images formed on the photoconductive drums  32 K,  32 Y,  32 M, and  32 C are primarily transferred onto the transfer belt  34 . The secondary transfer device  35  disposed in proximity to the transfer belt  34  secondarily transfers the black, yellow, magenta, and cyan toner images from the transfer belt  34  onto the sheet P conveyed from the sheet feeding device  7 , thus forming a color toner image on the sheet P. The fixing device  36  melts and fixes the color toner image on the sheet P under heat and pressure. 
     The image forming device  3  further includes a conveyance path  39 A through which the sheet P conveyed from the sheet feeding path  24  of the sheet feeding device  7  is further conveyed to the secondary transfer device  35 . The conveyance path  39 A is provided with a registration roller pair  37  that adjusts a conveyance time and a conveyance speed of the sheet P. The sheet P is conveyed through a secondary transfer nip formed between the transfer belt  34  and the secondary transfer device  35  at a conveyance speed equivalent to a rotation speed of the transfer belt  34 . After the sheet P passes through the secondary transfer nip and the fixing device  36 , the sheet P is ejected onto an output tray  38  by an output roller pair  90 . 
     The image forming device  3  further includes a bypass tray  25  that loads a plurality of sheets P and a bypass conveyance path  39 B that delivers a sheet P from the bypass tray  25  to the conveyance path  39 A at a position upstream from the registration roller pair  37  in a sheet conveyance direction. 
     Below the secondary transfer device  35  and the fixing device  36  are a switchback conveyance path  39 C and a reverse conveyance path  39 D, each of which includes a plurality of conveyance rollers and conveyance guides. 
     If the image forming apparatus  1  receives a duplex print job to form a toner image on both sides of the sheet P, the switchback conveyance path  39 C performs switchback conveying to feed back and convey the sheet P bearing a toner image on a front side thereof to the reverse conveyance path  39 D. 
     The reverse conveyance path  39 D reverses the sheet P conveyed from the switchback conveyance path  39 C and conveys the sheet P to the registration roller pair  37 . 
     Thus, the switchback conveyance path  39 C feeds back the sheet P bearing the toner image on the front side thereof and the reverse conveyance path  39 D reverses and conveys the sheet P to the registration roller pair  37  which conveys the sheet P to the secondary transfer nip. As the sheet P is conveyed through the secondary transfer nip, the secondary transfer device  35  secondarily transfers another toner image formed on the transfer belt  34  onto a back side of the sheet P. After the fixing device  36  fixes the toner image on the sheet P, the sheet P is ejected onto the output tray  38  by the output roller pair  90 . 
     A detailed description is now given of a construction of the image scanner  4 . 
     The image scanner  4  includes a first carriage  41  mounting a light source (e.g., a lighting unit) and a mirror, a second carriage  42  mounting a mirror, an image forming lens  43 , an imaging device  44 , and a first exposure glass  45 . The above-described components of the image scanner  4  are situated in the body  1 M and construct a first side reader  40  that reads an image on a first side (e.g., a front side) of a sheet S (e.g., an original sheet) conveyed over the first exposure glass  45 . The first side of the sheet S is one side of the sheet S, for example, the front side of the sheet S, conveyed automatically by the ADF  5 . 
     The image scanner  4  further includes a second exposure glass  46  on which a sheet S (e.g., an original sheet) bearing an image is placed and an abutment  47   a  to abut on one edge of the sheet S to position the sheet S on the second exposure glass  46 . 
     The first carriage  41  is disposed below the first exposure glass  45  and the second exposure glass  46  such that the first carriage  41  is movable horizontally and positioned adjustably. Light emitted by the light source is reflected by the mirror and irradiates the sheet S through the first exposure glass  45  or the second exposure glass  46 . The light reflected by the sheet S is deflected by the mirrors mounted on the first carriage  41  and the second carriage  42 , respectively, and enters the image forming lens  43  to form an image in the imaging device  44  that produces image data. 
     For example, while the light source is energized, the first carriage  41  moves at a speed that is twice as great as a speed of the second carriage  42  to allow the light to irradiate and scan the sheet S placed on the second exposure glass  46 . While the light irradiates the sheet S, the imaging device  44  reads the image on the sheet S. Thus, the image scanner  4  performs stationary original reading, that is, flat bed scanning. 
     The first carriage  41  halts at a home position immediately below the first exposure glass  45 . While an optical system including the light source and the mirrors halts, the first carriage  41  reads the image on the first side of the sheet S conveyed by the ADF  5 . Thus, the image scanner  4  performs moving original reading, that is, document feeding (DF) scanning. 
     In addition to the first side reader  40  situated inside the image scanner  4 , the image forming apparatus  1  includes a second side reader  48  situated inside the ADF  5 . The second side reader  48  reads an image on a second side (e.g., a back side) of the sheet S after the sheet S passes over the first exposure glass  45 . 
     A detailed description is now given of a construction of the ADF  5 . 
     The ADF  5  is coupled to a top face of the body  1 M such that the ADF  5  is pivotable about a hinge. As the ADF  5  is lifted, the ADF  5  moves to an open position where the ADF  5  exposes the first exposure glass  45  and the second exposure glass  46  of the image scanner  4 . Conversely, as the ADF  5  is lowered, the ADF  5  moves to a close position where the ADF  5  covers the first exposure glass  45  and the second exposure glass  46 . 
     Referring to  FIGS. 2 to 4 , a description is provided of the construction of the ADF  5  in more detail. 
       FIG. 2  is a cross-sectional view of the ADF  5 .  FIG. 3  is a block diagram of the image forming apparatus  1 , illustrating control of the ADF  5 .  FIG. 4  is a block diagram of the image forming apparatus  1 , illustrating transmission of signals between the ADF  5  and the body M 1  of the image forming apparatus  1 . 
     As illustrated in  FIG. 2 , the ADF  5  further includes an original set portion A, a separate-feed portion B, a registration portion C, a turn portion D, a first read-convey portion E, a second read-convey portion F, an ejection portion G, and a stack portion H. As illustrated in  FIG. 3 , the image forming apparatus  1  further includes a plurality of drivers that drives the original set portion A, the separate-feed portion B, the registration portion C, the turn portion D, the first read-convey portion E, the second read-convey portion F, and the ejection portion G to convey the sheet S, that is, a pickup motor  101 , a feed motor  102 , a reading motor  103 , an ejection motor  104 , and a bottom plate lift motor  105 . The image forming apparatus  1  further includes a controller  100  that controls the pickup motor  101 , the feed motor  102 , the reading motor  103 , the ejection motor  104 , and the bottom plate lift motor  105 . 
     As illustrated in  FIG. 2 , the ADF  5  employs a sheet-through feeding method. The original set portion A loads a plurality of sheets S facing up, each of which bears an image to be read at least on the first side of the sheet S. For duplex printing, an image on the second side of the sheet S faces down. The separate-feed portion B separates a single sheet S from other sheets S placed on the original set portion A and feeds the single sheet S to the registration portion C. The registration portion C contacts and temporarily halts the sheet S conveyed from the separate-feed portion B to correct skew of the sheet S and feeds the sheet S to the turn portion D. The turn portion D turns the sheet S conveyed from the registration portion C to direct the image on the sheet S to face down and conveys the sheet S to the first read-convey portion E. The first read-convey portion E allows the image on the sheet S conveyed from the turn portion D to be read by the image scanner  4  through the first exposure glass  45  and conveys the sheet S to the second read-convey portion F. The second read-convey portion F reads the image on the second side of the sheet S and conveys the sheet S to the ejection portion G. The ejection portion G ejects the sheet S to an outside of the ADF  5 . The stack portion H receives and stacks the sheet S. 
     A detailed description is now given of a construction of the original set portion A. As illustrated in  FIG. 2 , a user places the plurality of sheets S on an original table  51  incorporating a movable original table  51 A such that the image on the first side of each sheet S faces up. The user moves a side guide in a width direction of the sheets S that is perpendicular to a sheet conveyance direction DS to restrict and position the sheets S in the width direction thereof. A set feeler  57 A and an original set sensor  57 B detect the position of the sheet S and send a signal to a body controller  111  through an interface (I/F) circuit  207  depicted in  FIG. 3 . 
     A plurality of original length sensors  91 A and  91 B mounted on the original table  51  detects a schematic length of the sheet S in the sheet conveyance direction DS. Each of the original length sensors  91 A and  91 B is a reflection sensor or an actuator type sensor that detects the sheet S even when the single sheet S is placed on the original table  51 . 
     The bottom plate lift motor  105  depicted in  FIG. 3  lifts and lowers the movable original table  51 A in directions a and b depicted in  FIG. 2 . As the set feeler  57 A and the original set sensor  57 B detect the sheet S placed on the original table  51 , the controller  100  rotates the bottom plate lift motor  105  forward to lift the movable original table  51 A until an uppermost sheet S of the plurality of sheets S placed on the original table  51  contacts a pickup roller  58 . 
     A proper position sensor  92  detects the uppermost sheet S lifted by the movable original table  51 A to a proper height. When the proper position sensor  92  is turned on, the controller  100  controls the bottom plate lift motor  105  to stop the movable original table  51 A. When the sheets S are fed repeatedly and the height of the uppermost sheet S is lowered gradually, the proper position sensor  92  is turned off. The controller  100  controls the bottom plate lift motor  105  repeatedly to lift the movable original table  51 A until the proper position sensor  92  is turned on again. Thus, the uppermost sheet S is retained at the proper height constantly. 
     When the sheets S have been fed from the original table  51  and therefore the original table  51  is clear, the controller  100  rotates the bottom plate lift motor  105  backward to lower the movable original table  51 A to a home position where the user sets a next sheaf of sheets S on the original table  51 . 
     The pickup motor  101  and a cam rotate the pickup roller  58  in directions c and d depicted in  FIG. 2 . As the movable original table  51 A is lifted, the uppermost original S pushes up the pickup roller  58  placed on the movable original table  51 A in the direction c so that the proper position sensor  92  detects the uppermost original S. The user presses a key on a control panel  150  depicted in  FIG. 3  to select a one-sided print mode to form a toner image on one side of a sheet P or a two-sided print mode to form a toner image on both sides of a sheet P. Thereafter, the user presses a print key on the control panel  150  to start printing. As an original feeding signal is transmitted from the body controller  111  to the controller  100  through the interface circuit  207 , the controller  100  rotates the feed motor  102  forward to drive and rotate the pickup roller  58 . Thus, the pickup roller  58  picks up several sheets S, preferably a single sheet S, from the plurality of sheets S placed on the original table  51 . The pickup roller  58  rotates in a rotation direction that directs the uppermost sheet S to an original inlet of the separate-feed portion B. 
     The user may select the one-sided print mode or the two-sided print mode for a whole sheaf of sheets S placed on the original table  51 . Alternatively, the user may select different modes for a part and another part of the sheaf of sheets S. For example, when ten sheets S are placed on the original table  51 , the user may select the two-sided print mode for a first sheet S and a tenth sheet S and the one-sided print mode for second to ninth sheets S. 
     A detailed description is now given of a construction of the separate-feed portion B. 
     The controller  100  rotates the feed motor  102  forward to drive and rotate a feed belt  59  in the sheet conveyance direction DS. The controller  100  rotates the feed motor  102  forward to drive and rotate a reverse roller  60  in a direction opposite the sheet conveyance direction DS. Accordingly, the reverse roller  60  separates the uppermost sheet S from underneath sheets S to feed the uppermost sheet S to the registration portion C. For example, while the reverse roller  60  is in direct contact with and pressed against the feed belt  59  with given pressure or the reverse roller  60  is pressed against the feed belt  59  via the single sheet S, the reverse roller  60  rotates counterclockwise in  FIG. 2  in accordance with rotation of the feed belt  59 . If two or more sheets S enter a nip formed between the feed belt  59  and the reverse roller  60  accidentally, a rotation force of the feed belt  59  that rotates the reverse roller  60  is set to be smaller than a torque of a torque limiter. Accordingly, the reverse roller  60  rotates clockwise in  FIG. 2  in a default rotation direction to feed back the underneath sheets S to the original table  51 , preventing multiple feeding of the sheets S. 
     The feed belt  59  conveys the uppermost sheet S separated from the underneath sheets S by the feed belt  59  and the reverse roller  60  to an abutting sensor  93 . The abutting sensor  93  detects a leading edge of the sheet S. 
     A detailed description is now given of a construction of the registration portion C. 
     The sheet S is conveyed to a pullout roller pair  61  and the leading edge of the sheet S comes into contact with the pullout roller pair  61  that is halted. The sheet S is further conveyed for a given amount after the abutting sensor  93  detects the sheet S. When the sheet S is pressed against the pullout roller pair  61  and bent for a given amount, the controller  100  halts the feed motor  102  to halt the feed belt  59 . The controller  100  rotates the pickup motor  101  to retract the pickup roller  58  from an upper face of the sheet S to cause the feed belt  59  to convey the sheet S. As the leading edge of the sheet S enters a nip formed between an upper roller and a lower roller constructing the pullout roller pair  61 , the pullout roller pair  61  contacts the leading edge of the sheet S to correct skew of the sheet S. 
     The pullout roller pair  61  corrects skew of the sheet S and conveys the sheet S to an intermediate roller pair  62 . The controller  100  rotates the feed motor  102  backward to drive and rotate the pullout roller pair  61 . While the feed motor  102  rotates backward, the pullout roller pair  61  and the intermediate roller pair  62  are driven and the pickup roller  58  and the feed belt  59  are not driven. The pullout roller pair  61 , the intermediate roller pair  62 , the pickup roller  58 , and the feed belt  59  serve as a conveyer that conveys the sheet S. 
     A detailed description is now given of a construction of the turn portion D. 
     A plurality of original width sensors  94  is aligned in a depth direction of the ADF  5  that is parallel to the width direction of the sheet S and perpendicular to the sheet conveyance direction DS. The original width sensors  94  detect a width of the sheet S in the width direction thereof that is conveyed by the pullout roller pair  61 . The controller  100  calculates a length of the sheet S in the sheet conveyance direction DS based on a motor pulse defined when the abutting sensor  93  detects the leading edge and a trailing edge of the sheet S. 
     While the pullout roller pair  61  and the intermediate roller pair  62  are driven and rotated to convey the sheet S from the registration portion C to the turn portion D, a conveyance speed at which the sheet S is conveyed through the registration portion C is higher than a conveyance speed at which the sheet S is conveyed through the first read-convey portion E to shorten a conveyance time to convey the sheet S to the first read-convey portion E. 
     A detailed description is now given of a construction of the first read-convey portion E. 
     When an entry sensor  95  detects the leading edge of the sheet S, before the leading edge of the sheet S enters a nip formed between an upper roller and a lower roller constructing an entry roller pair  63 , the controller  100  starts decreasing the conveyance speed of the sheet S to cause a conveyance speed at which the entry roller pair  63  conveys the sheet S through the first read-convey portion E to be equivalent to a conveyance speed at which the first read-convey portion E conveys the sheet S while reading the image on the sheet S. 
     Simultaneously, the controller  100  rotates the reading motor  103  forward to drive and rotate the entry roller pair  63 , an exit roller pair  64 , and a contact image sensor (CIS) exit roller pair  65 . When a registration sensor  96  detects the leading edge of the sheet S, the conveyance speed of the sheet S is decreased while the sheet S is conveyed for a given distance. When the sheet S halts temporarily before a reading position  20 , the controller  100  transmits a registration position stop signal to the body controller  111  through the interface circuit  207 . When the controller  100  receives a reading start signal from the body controller  111 , the sheet S halted at a registration position is conveyed at an accelerated speed so that the sheet S is conveyed at a given conveyance speed before the leading edge of the sheet S reaches the reading position  20 . At a time when the leading edge of the sheet S detected by a pulse count of the reading motor  103  reaches the reading position  20 , the controller  100  transmits a gate signal indicating a valid imaged region in a sub-scanning direction on the first side of the sheet S to the body controller  111  until the trailing edge of the sheet S passes through the reading position  20 . 
     A detailed description is now given of a construction of the ejection portion G and the stack portion H. 
     In the one-sided print mode, the sheet S having passed through the first read-convey portion E is conveyed to the ejection portion G through the second side reader  48 . When an ejection sensor  97  detects the leading edge of the sheet S, the controller  100  rotates the ejection motor  104  forward to rotate an ejection roller pair  67  counterclockwise in  FIG. 2 . Based on a pulse count of the ejection motor  104  counted after the ejection sensor  97  detects the leading edge of the sheet S, the controller  100  deceases a rotation speed of the ejection motor  104  immediately before the trailing edge of the sheet S is ejected from a nip formed between an upper roller and a lower roller constructing the ejection roller pair  67 , thus preventing the sheet S ejected by the ejection roller pair  67  onto an ejection tray  53  from protruding beyond the ejection tray  53 . The entry roller pair  63 , the exit roller pair  64 , the CIS exit roller pair  65 , and the ejection roller pair  67  serve as a conveyer that conveys the sheet S. 
     A detailed description is now given of a construction of the second read-convey portion F. 
     In the two-sided print mode, at a time when the leading edge of the sheet S reaches the second side reader  48 , which is determined based on a pulse count of the reading motor  103  counted after the ejection sensor  97  detects the leading edge of the sheet S, the controller  100  transmits a gate signal indicating a valid imaged region in the sub-scanning direction on the second side of the sheet S to the second side reader  48  until the trailing edge of the sheet S passes through the second side reader  48 . A second reading roller  70  prevents the sheet S from being lifted while the sheet S is conveyed through the second side reader  48 . The second reading roller  70  also serves as a reference white portion to obtain shading data in the second side reader  48 . 
     Referring to  FIG. 3 , a description is provided of a configuration that controls an operation of the ADF  5 . 
     As illustrated in  FIG. 3 , the image forming apparatus  1  includes the controller  100  that controls the ADF  5 , the body controller  111  that controls the components disposed inside the body  1 M depicted in  FIG. 1 , and the control panel  150  coupled to the body controller  111 . 
     The controller  100  receives a detection signal sent from each of the original set sensor  57 B, the proper position sensor  92 , a table lift sensor  98 , the abutting sensor  93 , the original width sensors  94 , the entry sensor  95 , the registration sensor  96 , and the ejection sensor  97 . 
     The controller  100  drives the pickup motor  101  that drives and rotates the pickup roller  58 , the feed motor  102  that drives and rotates the feed belt  59 , the pullout roller pair  61 , and the intermediate roller pair  62 , and the reading motor  103  that drives and rotates the entry roller pair  63 , the exit roller pair  64 , and the CIS exit roller pair  65 . The controller  100  drives the ejection motor  104  that drives and rotates the ejection roller pair  67  and the bottom plate lift motor  105  that lifts the movable original table  51 A. 
     The controller  100  sends a timing signal and the like to the second side reader  48 . The timing signal notifies a time when the leading edge of the sheet S reaches a reading position where a second side scanning unit  69  reads an image on the second side of the sheet S. Image data created after the timing signal is recognized as valid data. 
     The controller  100  is connected to the body controller  111  through the interface circuit  207 . When the user presses the print key on the control panel  150 , the body controller  111  sends an original feed signal and a reading start signal to the controller  100  through the interface circuit  207 . 
     Referring to  FIG. 4 , a description is provided of a signal path between the ADF  5  and the body M 1  of the image forming apparatus  1 . 
     As illustrated in  FIG. 4 , the second side reader  48  includes a light source  200  including a light-emitting diode (LED) array, a fluorescent lamp, or a cold cathode tube. The light source  200  emits light onto the sheet S according to a lighting signal sent from the controller  100 . The second side reader  48  receives from the controller  100  the timing signal that notifies the time when the leading edge of the sheet S reaches the reading position where the second side scanning unit  69  reads the image on the second side of the sheet S. The second side reader  48  also receives power to be supplied to the light source  200 . 
     The second side reader  48  further includes a plurality of sensor chips  201 , a plurality of operational (OP) amplifier circuits  202 , and a plurality of analog digital (A/D) converters  203 . The plurality of sensor chips  201  is aligned in a main scanning direction. The plurality of OP amplifier circuits  202  is coupled to the plurality of sensor chips  201 , respectively. The plurality of A/D converters  203  is coupled to the plurality of OP amplifier circuits  202 , respectively. The second side reader  48  further includes an image processor  204 , a frame memory  205 , an output control circuit  206 , and the interface circuit  207 . 
     The sensor chip  201  includes a photoelectric transducer called an equal magnification contact image sensor and a condenser lens. The condenser lens of each of the plurality of the sensor chips  201  condenses reflection light reflected by the second side of the sheet S into the photoelectric transducer which reads the reflection light into image data. 
     The OP amplifier circuits  202  amplify the image data created by the sensor chips  201 , respectively. Thereafter, the A/D converters  203  convert the amplified image data into digital image data. 
     The digital image data enters the image processor  204  which performs shading correction and the like on the digital image data. Thereafter, the frame memory  205  stores the digital image data temporarily. The output control circuit  206  converts the digital image data into image data having a data format acceptable by the body controller  111 . Thereafter, the digital image data enters the body controller  111  through the interface circuit  207 . 
     The turn portion D depicted in  FIG. 2  includes a bending portion that changes the sheet conveyance direction DS substantially. While the sheet S slides over the bending portion frictionally, slide noise may occur from the bending portion. To address this circumstance, the turn portion D includes the noise attenuator  2  depicted in  FIG. 1  that attenuates the slide noise. 
     Referring to  FIG. 5 , a description is provided of Helmholtz resonance relating to the noise attenuator  2  of the turn portion D. 
       FIG. 5  is a perspective view of a Helmholtz resonator  900 . As illustrated in  FIG. 5 , the Helmholtz resonator  900  includes a body  901  including a cavity  901   a  having a volume V and a neck  902  including a through-hole  902   a  (e.g., an inlet) having a diameter d and a length l. As a sonic wave enters the through-hole  902   a  from an outside of the Helmholtz resonator  900 , the sonic wave involves air in the through-hole  902   a  into the cavity  901   a  as the sonic wave presses the air into the cavity  901   a . Pressure sealed inside the body  901  increases and presses the air back to the through-hole  902   a . Although the air is pressed back to the outside of the through-hole  902   a , the air returns to the through-hole  902   a  by inertia. Such repeated motion of the air defines a spring  903  with simple harmonic oscillation, which has a mass m and a spring constant k. Hence, a resonance frequency f is calculated by a following formula (1). Even if the neck  902  includes a plurality of through-holes  902   a  that corresponds to the single cavity  901   a , cross-sectional areas of the through-holes  902   a  are combined into a cross-sectional area S in the formula (1) to calculate the resonance frequency f. 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       C 
                       
                         2 
                         ⁢ 
                         π 
                       
                     
                     ⁢ 
                     
                       
                         S 
                         
                           
                             ( 
                             
                               1 
                               + 
                               δ 
                             
                             ) 
                           
                           ⁢ 
                           V 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the formula (1), f represents the resonance frequency in hertz (Hz). C represents a sound velocity in meter per second (m/s). S represents a cross-sectional area of the through-hole  902   a  in square meter (m 2 ). l represents a length of the through-hole  902   a  in meter (m). δ represents a correction factor by an opening edge in meter (m). V represents a volume of the cavity  901   a  in cubic meter (m 3 ). 
     The air inside the through-hole  902   a  vibrates aggressively at a frequency near the resonance frequency. However, in a boundary layer in proximity to a wall of the through-hole  902   a , the air serving as a fluid receives a viscous resistance. Accordingly, vibration energy is converted into thermal energy by the viscous resistance. Consequently, sound energy generated by the sonic wave that enters the Helmholtz resonator  900  from the outside thereof is converted into thermal energy, decreasing the sound energy and attaining sound absorption. 
     A description is provided of a construction to reduce noise that leaks from a comparative sheet conveying device. 
     The comparative sheet conveying device includes a sheet guide incorporating a resonant cavity disposed in a sheet ejection path. A duct is disposed in proximity to a sheet outlet. The resonant cavity adjoins an aperture disposed opposite a recording sheet. The duct attenuates noise that generates from a printing device and moves through the sheet ejection path adjoining the sheet outlet. The resonant cavity reduces the noise by using Helmholtz resonance. 
     However, due to the construction of the comparative sheet conveying device, the sheet guide incorporating the resonant cavity may not be situated appropriately relative to a sound source that generates the noise. Accordingly, the resonant cavity may reduce the noise locally and absorb the noise partially, reducing the noise ineffectively. The resonant cavity of the comparative sheet conveying device attenuates the noise generated by the printing device. Accordingly, the sound source that generates the noise (e.g., the printing device) is separated apart from a noise attenuator (e.g., the duct and the resonant cavity) that attenuates the noise in a recording medium conveyance direction. Accordingly, a part of the noise may diffuse inside the comparative sheet conveying device before the noise reaches the noise attenuator. Consequently, the noise attenuator may not attenuate the noise sufficiently. 
     To address the circumstance of the comparative sheet conveying device, the image forming apparatus  1  depicted in  FIG. 1  includes a sheet conveying device  800  depicted in  FIG. 2  and described below, which incorporates a noise attenuator (e.g., the noise attenuator  2 ) that attenuates slide noise that generates while a sheet (e.g., an original sheet, that is, the sheet S, and a recording sheet, that is, the sheet P) is conveyed through the sheet conveying device  800 , for example, while the sheet S conveyed through the sheet conveying device  800  slides over a component disposed inside the sheet conveying device  800 . The sheet conveying device  800  is installed in an image reading device (e.g., the image reading device  6 ) incorporating an auto document feeder (e.g., the ADF  5 ) or an image forming apparatus (e.g., the image forming apparatus  1 ) that forms an image on a recording medium according to image data created by the image reading device. Alternatively, the sheet conveying device  800  may be installed in an image forming apparatus that forms an image by inkjet printing or other machines that convey a sheet. 
     Referring to  FIGS. 6 to 8 , a description is provided of a construction of the noise attenuator  2  situated in the turn portion D depicted in  FIG. 2 . 
       FIG. 6  is a cross-sectional view of the sheet conveying device  800  incorporating the noise attenuator  2 .  FIG. 7  is a perspective view of the noise attenuator  2 .  FIG. 8  is an exploded perspective view of the noise attenuator  2 . As illustrated in  FIG. 6 , the sheet conveying device  800  situated in the turn portion D of the ADF  5  depicted in  FIG. 2  includes the pullout roller pair  61  depicted in  FIG. 2  and the intermediate roller pair  62  that serve as a conveyer or an original conveyer that conveys the sheet S, a primary sheet guide  54  (e.g., a primary original guide), and a secondary sheet guide  55  (e.g., a secondary original guide) disposed opposite the primary sheet guide  54 . The primary sheet guide  54  and the secondary sheet guide  55  define a sheet conveyance path  56 , that is, an interval between the primary sheet guide  54  and the secondary sheet guide  55 . The sheet conveyance path  56  directs the sheet S conveyed by the pullout roller pair  61  and the intermediate roller pair  62  in the given sheet conveyance direction DS. 
     The primary sheet guide  54  includes a bending portion  54   a  that bends the sheet S and changes the sheet conveyance direction DS. While the sheet S slides over the bending portion  54   a  with substantial friction, the bending portion  54   a  generates slide noise. Since the slide noise is propagated through an interval between the sheet S and the secondary sheet guide  55 , the sheet conveying device  800  incorporates the noise attenuator  2 . However, the turn portion D does not spare a space great enough to accommodate the noise attenuator  2  at a position disposed opposite an outer face of each of the primary sheet guide  54  and the secondary sheet guide  55  and disposed in proximity to each of the primary sheet guide  54  and the secondary sheet guide  55 . In order to attenuate noise by Helmholtz resonance in a resonant cavity effectively, the resonant cavity is requested to have a given capacity or a given volume. 
     To address this request, the noise attenuator  2  depicted in  FIGS. 6 to 8  may include a resonant cavity  305  disposed opposite the outer face of the primary sheet guide  54  or the secondary sheet guide  55  such that the resonant cavity  305  is disposed opposite the sheet conveyance path  56  via the primary sheet guide  54  or the secondary sheet guide  55 . According to this example embodiment, the resonant cavity  305  serving as a Helmholtz resonator is disposed in a space above an outer face  54   b  of the primary sheet guide  54 . The noise attenuator  2  further includes a sound inlet  304  and a sound guide  306 . The sound inlet  304  is disposed in proximity to the bending portion  54   a . The slide noise generated by the bending portion  54   a  moves to the resonant cavity  305  through the sound inlet  304 . The sound guide  306  communicates with the sound inlet  304  and the resonant cavity  305  separated apart from the sound inlet  304  with a substantial distance therebetween. 
     The noise attenuator  2  may include at least one sound inlet  304 , at least one sound guide  306 , and at least one resonant cavity  305 . According to this example embodiment, the noise attenuator  2  includes two resonant cavities  305 , that is, a resonant cavity  305   a  and a resonant cavity  305   b  aligned with the resonant cavity  305   a  in the width direction of the sheet S perpendicular to the sheet conveyance direction DS as illustrated in  FIG. 7 . Each of the resonant cavity  305   a  and the resonant cavity  305   b  communicates with one end of each of six sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e , and  306   f  aligned in the width direction of the sheet S. Another end of each of the six sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e , and  306   f  communicates with six sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e , and  304   f  aligned in the width direction of the sheet S. The six sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e , and  304   f  are disposed opposite or abut on the bending portion  54   a  of the primary sheet guide  54  and aligned in the width direction of the sheet S. 
     Since the slide noise generates by friction between the sheet S and the bending portion  54   a , the slide noise is called friction noise. The friction noise does not have a particular frequency.  FIG. 9  is a graph illustrating a relation between the frequency of the friction noise and the sound pressure level. As illustrated in  FIG. 9 , the friction noise has a broad frequency distribution not smaller than about 3.5 kHz. A human auditory sense is sensitive to a sound having a frequency near 4 kHz. For example, female scream and cry of a baby have the frequency near 4 kHz. If the sound having the frequency near 4 kHz is reduced, the noise attenuator  2  may achieve a substantial advantage against an A-weighting noise corrected for the human auditory sense. Accordingly, the shape of each of the sound inlet  304 , the sound guide  306 , and the resonant cavity  305  depicted in  FIG. 6  is adjusted to attain the resonance frequency of about 4 kHz according to the formula (1) above. 
     A description is provided of an operation of the noise attenuator  2 . 
     As illustrated in  FIG. 6 , the resonant cavity  305  is separated apart from the bending portion  54   a . Since the resonant cavity  305  is separated apart from the sound inlet  304  with the substantial distance therebetween, the sound guide  306  communicates with the sound inlet  304  and the resonant cavity  305 . Since the sound inlet  304  is in proximity to the bending portion  54   a , the slide noise generated by the bending portion  54   a  enters the sound inlet  304  effectively before the slide noise diffuses. Thus, the sound inlet  304  intakes the slide noise effectively. While the slide noise having entered through the sound inlet  304  moves through the sound guide  306 , the slide noise attenuates and enters the resonant cavity  305 . The resonant cavity  305  attenuates the slide noise by Helmholtz resonance. Thus, even if there is not a space for the resonant cavity  305  at a position abutting on the bending portion  54   a  and therefore the resonant cavity  305  is separated apart from the bending portion  54   a , the resonant cavity  305  attenuates the slide noise effectively. 
     A description is provided of a plurality of variations of the noise attenuator  2 . 
       FIG. 10  is a cross-sectional view of a noise attenuator  2 A as a first variation of the noise attenuator  2 . As illustrated in  FIG. 10 , the noise attenuator  2 A includes the single resonant cavity  305  disposed above the primary sheet guide  54  and disposed opposite the outer face  54   b  of the primary sheet guide  54 . The resonant cavity  305  communicates with one end of each of eight sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e ,  306   f ,  306   g , and  306   h . Another end of each of the eight sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e ,  306   f ,  306   g , and  306   h  communicates with eight sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e ,  304   f ,  304   g , and  304   h , respectively, that are disposed opposite or abut on the bending portion  54   a  of the primary sheet guide  54 . 
     Like the noise attenuator  2  depicted in  FIG. 6 , the noise attenuator  2 A includes at least one sound inlet  304 , at least one sound guide  306 , and at least one resonant cavity  305 . That is, the number of each of the sound inlet  304 , the sound guide  306 , and the resonant cavity  305  is not limited. As illustrated in  FIG. 7 , the noise attenuator  2  includes the two resonant cavities  305 , that is, the resonant cavity  305   a  and the resonant cavity  305   b  abutting on or being aligned with the resonant cavity  305   a  in the width direction of the sheet S. The resonant cavities  305   a  and  305   b  may have an identical capacity or an identical volume to enhance the attenuation factor with respect to a target frequency, reducing the slide noise effectively. Alternatively, the resonant cavities  305   a  and  305   b  may have different capacities, respectively, to reduce the slide noise further for a broad frequency range. 
       FIG. 11  is a cross-sectional view of a noise attenuator  2 B as a second variation of the noise attenuator  2 . As illustrated in  FIG. 11 , the noise attenuator  2 B includes a plurality of resonant cavities  305   c  and  305   d . If there is a space above and below the primary sheet guide  54 , the resonant cavities  305   c  and  305   d  are disposed above and below the primary sheet guide  54 , respectively, and disposed opposite the outer face  54   b  of the primary sheet guide  54 . The resonant cavity  305   d  is disposed downstream from the resonant cavity  305   c  in the sheet conveyance direction DS. The upper resonant cavity  305   c  communicates with one end of each of the eight sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e ,  306   f ,  306   g , and  306   h . The lower resonant cavity  305   d  communicates with one end of each of seven sound guides  306   i ,  306   j ,  306   k ,  306   l ,  306   m ,  306   n , and  306   o . Another end of each of the eight sound guides  306   a ,  306   b ,  306   c ,  306   d ,  306   e ,  306   f ,  306   g , and  306   h  communicates with the eight sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e ,  304   f ,  304   g , and  304   h , respectively, that are disposed opposite or abut on the bending portion  54   a  of the primary sheet guide  54 . Another end of each of the seven sound guides  306   i ,  306   j ,  306   k ,  306   l ,  306   m ,  306   n , and  306   o  communicates with seven sound inlets  304   i ,  304   j ,  304   k ,  304   l ,  304   m ,  304   n , and  304   o , respectively, that are disposed opposite the bending portion  54   a  of the primary sheet guide  54 . Thus, the eight sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e ,  304   f ,  304   g , and  304   h  are arranged alternately with the seven sound inlets  304   i ,  304   j ,  304   k ,  304   l ,  304   m ,  304   n , and  304   o , respectively, in the width direction of the sheet S perpendicular to the sheet conveyance direction DS. 
     Accordingly, the fifteen sound inlets  304   a ,  304   b ,  304   c ,  304   d ,  304   e ,  304   f ,  304   g ,  304   h ,  304   i ,  304   j ,  304   k ,  304   l ,  304   m ,  304   n , and  304   o  (hereinafter referred to as the sound inlets  304 ) are arranged closely to each other in the width direction of the sheet S and disposed opposite the bending portion  54   a  serving as a sound source that generates the slide noise. The noise attenuator  2 B does not have a non-inlet that does not intake the slide noise and is interposed between the adjacent sound inlets  304 . The sound inlets  304  aligned closely to each other in the width direction of the sheet S are disposed opposite the sound source (e.g., the bending portion  54   a ) to intake the slide noise before the slide noise diffuses. Accordingly, the noise attenuator  2 B reduces the slide noise substantially with the resonant cavities  305   c  and  305   d  that have the identical capacity without increasing the number and the capacity of the resonant cavities  305   c  and  305   d.    
     A description is provided of the shape of the sound guide  306 . 
       FIG. 12A  is a perspective view of the sound guide  306  as a rectangular tube.  FIG. 12B  is a perspective view of the sound guide  306  as a circular tube. As illustrated in  FIG. 12A , the sound guide  306  is made of resin and is a straight tube having a rectangular cross-section. As illustrated in  FIG. 12B , the sound guide  306  is made of resin and is a cylinder or a straight tube having a circular cross-section. Alternatively, the sound guide  306  may not be the straight tube. The sound guide  306  may have any shape that does not prohibit air from flowing into the resonant cavity  305 . For example, the sound guide  306  may be a curved tube curved according to a space between the sound inlet  304  and the resonant cavity  305 . The sound guide  306  may have different cross-sections that are interposed between the sound inlet  304  and the resonant cavity  305 . For example, the sound guide  306  may be a tube having a cross-section that changes from a rectangle to a circle or from a circle to a rectangle. The sound guide  306  has flexibility in the arrangement in space, the size in cross-section, and the shape in cross-section. 
     A description is provided of a plurality of variations of the sound guide  306 . 
       FIG. 13  is a cross-sectional view of a sound guide  306 S as a first variation of the sound guide  306 . As illustrated in  FIG. 13 , the sound guide  306 S includes a porous plastic portion  306   p  and a crust  306   q  surrounding the porous plastic portion  306   p . The crust  306   q  is a tube that is rectangular in cross-section and made of resin. The porous plastic portion  306   p  is a plate attached to a whole interior face of the crust  306   q  that is rectangular in cross-section. The porous plastic portion  306   p  has an open cell structure made of polyurethane foam or the like. 
       FIG. 14  is a graph illustrating a relation between the frequency of the friction noise and the sound pressure level.  FIG. 14  illustrates noise reduction by Helmholtz resonance with the sound guide  306 S depicted in  FIG. 13 . In  FIG. 14 , a dark line illustrates noise reduction by Helmholtz resonance with the sound guide  306 S incorporating the porous plastic portion  306   p . A light line illustrates noise reduction by Helmholtz resonance with a sound guide not incorporating the porous plastic portion  306   p . The porous plastic portion  306   p  of the sound guide  306 S converts the slide noise into thermal energy by the viscous resistance against the slide noise, improving attenuation of vibration of the slide noise that moves from the sound inlet  304  to the resonant cavity  305  and reducing the slide noise effectively. 
       FIG. 15  is a perspective view of a noise attenuator  2 C incorporating a sound guide  306 T as a second variation of the sound guide  306 . As illustrated in  FIG. 15 , the sound guide  306 T includes two first end portions  306 T 1  disposed at one end of the sound guide  306 T, a second end portion  306 T 2  disposed at another end of the sound guide  306 T, and an intermediate portion  306 T 3  interposed between the first end portions  306 T 1  and the second end portion  306 T 2 . The first end portions  306 T 1  adjoin the two sound inlets  304   a  and  304   b , respectively. The intermediate portion  306 T 3  bridges the two first end portions  306 T 1  and serves as a joint that combines the two first end portions  306 T 1 . The second end portion  306 T 2  adjoins the intermediate portion  306 T 3  and the single resonant cavity  305 . Thus, the sound guide  306 T is bifurcate. 
     If there is a space spared for the sound guide  306  at a position disposed opposite the outer face of the secondary sheet guide  55 , the sound inlet  304  is not disposed opposite the bending portion  54   a  of the primary sheet guide  54  directly but the sound inlet  304  is disposed opposite the bending portion  54   a  of the primary sheet guide  54  via the secondary sheet guide  55 . For example, the sound inlet  304  abuts on a bending portion of the secondary sheet guide  55 . 
     As illustrated in  FIG. 6 , the bending portion  54   a  is disposed at a portion of the primary sheet guide  54  where the primary sheet guide  54  has a decreased radius of curvature. Alternatively, the bending portion  54   a  may be a curved portion that contacts the sheet S linearly. The noise attenuators  2 ,  2 A,  2 B, and  2 C are disposed opposite the bending portion  54   a  situated in the turn portion D of the ADF  5  depicted in  FIG. 2 . Alternatively, the noise attenuators  2 ,  2 A,  2 B, and  2 C may be disposed opposite a bending portion situated at a position other than the turn portion D. 
     A description is provided of advantages of a sheet conveying device (e.g., the sheet conveying device  800 ). 
     As illustrated in  FIG. 6 , the sheet conveying device includes a conveyer (e.g., the pullout roller pair  61 , the intermediate roller pair  62 , the pickup roller  58 , or the feed belt  59 ), a primary sheet guide (e.g., the primary sheet guide  54 ), a secondary sheet guide (e.g., the secondary sheet guide  55 ), a sheet conveyance path (e.g., the sheet conveyance path  56 ), and a noise attenuator (e.g., the noise attenuators  2 ,  2 A,  2 B, and  2 C). The conveyer conveys a sheet (e.g., the sheet S serving as an original sheet or the sheet P serving as a recording sheet). The secondary sheet guide is disposed opposite the primary sheet guide with an interval therebetween to define the sheet conveyance path. The sheet conveyance path conveys the sheet conveyed by the conveyer in a sheet conveyance direction (e.g., the sheet conveyance direction DS). The primary sheet guide or the secondary sheet guide includes a bending portion (e.g., the bending portion  54   a ) that bends the sheet and changes the sheet conveyance direction. The noise attenuator intakes and attenuates slide noise generated by the bending portion while the sheet slides over the bending portion. The noise attenuator includes at least one resonant cavity, at least one sound inlet, and at least one sound guide. The resonant cavity is disposed opposite an outer face (e.g., the outer face  54   b ) of the primary sheet guide or the secondary sheet guide. The sound inlet is disposed in proximity to the bending portion of the primary sheet guide or the secondary sheet guide to intake the slide noise generated by the bending portion. The sound guide communicates with the sound inlet and the resonant cavity to guide the slide noise from the sound inlet to the resonant cavity. 
     Accordingly, even if there is not a space great enough to accommodate the resonant cavity of the noise attenuator such that the resonant cavity abuts on the bending portion serving as a sound source of the slide noise that generates while the sheet is conveyed over the bending portion, the resonant cavity great enough to attenuate the slide noise is connected to the bending portion through the sound guide although the resonant cavity is separated apart from the bending portion. Consequently, the sheet conveying device incorporating the noise attenuator reduces the slide noise effectively. The sheet conveying device is installed in the ADF  5  or the image forming apparatus  1 . 
     As described above, the bending portion of the primary sheet guide is a sound source that generates the slide noise while the sheet slides over the bending portion. The primary sheet guide and the secondary sheet guide define the sheet conveyance path. Even if there is not the space great enough to accommodate the resonant cavity that abuts on the sound source, the noise attenuator is disposed opposite the sound source. The noise attenuator includes a plurality of sound inlets (e.g., the sound inlets  304 ), the resonant cavity separated apart from the sound source, and the sound guide that couples the sound inlets to the resonant cavity. Accordingly, before the slide noise generated by the sound source diffuses, the sound inlets intake the slide noise. The sound guide attenuates the slide noise while the slide noise moves through the sound guide to the resonant cavity. The resonant cavity attenuates the slide noise by Helmholtz resonance. Thus, even if there is not the space great enough to accommodate the resonant cavity that abuts on the sound source, the noise attenuator reduces the slide noise substantially. The noise attenuator is installed in the sheet conveying device. 
     The present disclosure has been described above with reference to specific example embodiments. Note that the present disclosure is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative example embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.