Patent Publication Number: US-11046098-B2

Title: Noise reducing structure and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2017-187528 filed Sep. 28, 2017. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a noise reducing structure and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided a noise reducing structure including a first resonance tube that extends in a first direction, that takes in from a sound absorbing opening portion a sound wave that is generated from a noise source, and that causes the sound wave to resonate to reduce leakage to outside; and a second resonance tube that extends in a second direction differing from the first direction, and that, along with the first resonance tube, causes the sound wave that is generated from the noise source to resonate to reduce the leakage to the outside. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view of a structure of an image forming apparatus to which a noise reducing structure according to a first exemplary embodiment of the present invention is applied; 
         FIGS. 2A and 2B  each are a perspective view of a structure of an apparatus body of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 3  illustrates a structure of a driving device; 
         FIG. 4  is a perspective view of the structure of the driving device; 
         FIG. 5  is a graph showing a frequency distribution of noises that are generated by the image forming apparatus; 
         FIG. 6  illustrates the principles of a resonance tube; 
         FIG. 7  is a schematic view illustrating a sound pressure distribution of a two-dimensional resonance tube; 
         FIGS. 8A and 8B  illustrate a structure of the two-dimensional resonance tube; 
         FIG. 9  illustrates a structure of a three-dimensional resonance tube; 
         FIG. 10  is a front view of a structure of a right side frame; 
         FIG. 11  is a front view of a structure of a portion of the right side frame; 
         FIG. 12  is a perspective view of the structure of the portion of the right side frame; 
         FIG. 13  is an exploded perspective view of the structure of the portion of the right side frame; 
         FIG. 14  is an exploded perspective view of the structure of the portion of the right side frame; 
         FIG. 15  is a schematic view of a resonance tube; 
         FIG. 16  is a partly cutaway perspective view of a resonance tube; 
         FIG. 17  is a partly cutaway perspective view of the resonance tube; 
         FIG. 18  is a schematic view of a structure of an image forming apparatus to which a noise reducing structure according to a second exemplary embodiment of the present invention is applied; and 
         FIG. 19  provides explanatory views each showing a relationship between the length of a resonance tube and the wavelength of a sound wave. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention are described below with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a schematic view of a structure of an entire image forming apparatus  1  to which a noise reducing structure according to a first exemplary embodiment is applied. 
     Structure of Entire Image Forming Apparatus 
     The image forming apparatus  1  according to the first exemplary embodiment is, for example, a monochrome printer. The image forming apparatus  1  includes, for example, an image forming unit  2  that forms a toner image (image) formed by performing development with toner of developer; a sheet-feeding unit  4  that supplies recording paper  3 , serving as an exemplary recording medium, to the image forming unit  2 ; a transporting unit  5  that transports to, for example, the image forming unit  2  pieces of recording paper  3  that are supplied one at a time from the sheet-feeding unit  4 ; and a fixing unit  6  that performs fixing on the recording paper  3  on which the toner image has been formed by the image forming unit  2 . 
     The image forming unit  2  forms an image on a surface of recording paper  3  by performing an electrophotographic process that uses developer. The image forming unit  2  includes, for example, a photoconductor drum  21 , serving as an exemplary image carrier; a charging device  22  that charges a peripheral surface of the photoconductor drum  21 ; an exposure device  23  that exposes the photoconductor drum  21  to light and forms an electrostatic latent image; a developing device  24  that supplies developer to the electrostatic latent image on the photoconductor drum  21  and develops the electrostatic latent image; a transfer device  25  that transfers the toner image formed on the photoconductor drum  21  to the recording paper  3 ; and a cleaning device  26  that cleans the peripheral surface of the photoconductor drum  21 . The transfer device  25  may be one that does not directly transfer the toner image to the recording paper  3  from the photoconductor drum  21 . That is, the transfer device  25  may be one that transfers the toner image to the recording paper  3  via an intermediate transfer body, such as an intermediate transfer belt. The developer may contain, for example, black toner. The developer may contain, in addition to black toner, color toners, such as yellow toner, magenta toner, and cyan toner. 
     The sheet-feeding unit  4  includes, for example, a holding container  41  that holds recording paper  3  and a sheet-feeding roller  42  that feeds pieces of the recording paper  3  one at a time from the holding container  41 . By setting the holding container  41  at an apparatus body  1   a  of the image forming apparatus  1 , the sheet-feeding unit  4  is capable of supplying the pieces of recording paper  3  held in the holding container  41 . The holding container  41  is mounted such that, for example, the holding container  41  is capable of being drawn out towards the front of the apparatus body  1   a  (towards a side surface that a user faces when the user operates the image forming apparatus  1 ), that is, towards a side of a left side surface in the illustrated example. 
     The transporting unit  5  transports recording paper  3  that is fed from the sheet-feeding unit  4  to the image forming unit  2  and the fixing unit  6  to discharge the recording paper  3  on which the image has been formed to a discharging section  7  that is disposed at a top portion of the apparatus body  1   a . When images are to be formed on both surfaces of the recording paper  3 , the transporting unit  5  re-transports the recording paper  3  on which the image has been formed on one surface thereof to the image forming unit  2  with the front and back surfaces of this recording paper  3  being reversed without discharging this recording paper  3  to the discharging section  7 . 
     The fixing unit  6  fuses the toner image, formed on the surface of the recording paper  3  by the image forming unit  2 , by using heat and pressure, and fixes the toner image to the recording paper  3 . The recording paper  3  to which the image has been fixed by the fixing unit  6  is discharged to and is held by the discharging section  7  with the recording paper  3  placed thereon. 
     In  FIG. 1 , reference numeral  100  denotes a controlling device that performs overall control on the operation of the image forming apparatus  1 . 
     Structure of Apparatus Body of Image Forming Apparatus 
     As illustrated in  FIG. 2A , the apparatus body  1   a  of the image forming apparatus  1  is formed as a box body whose external shape is a substantially rectangular-parallelepiped shape. The apparatus body  1   a  includes a front cover  11 , a rear cover  12 , left and right side covers  13  and  14 , and an upper cover  15 . The front cover  11  is an example of an exterior body that covers a front surface (a left side surface in  FIG. 2A ) of the apparatus body  1   a . The rear cover  12  is an example of an exterior body that covers a rear surface of the apparatus body  1   a . The left and right side covers  13  and  14  are examples of exterior bodies that cover left and right side surfaces of the apparatus body  1   a , corresponding thereto. The upper cover  15  is an example of an exterior body that covers an upper portion of the apparatus body  1   a . Of these covers, for example, the rear cover  12  and the right side cover  14  are provided so as to be openable and closable as appropriate. 
     As illustrated in  FIG. 2B  in which the right side cover  14  is removed, the apparatus body  1   a  includes a frame structural member serving as an exemplary internal structural body that is covered by the exterior bodies. The frame structural member includes, for example, left and right side frames  16  (the left side frame is not illustrated) and a connecting frame (not illustrated). The left and right side frames  16  are disposed on the left and right side surfaces of the apparatus body  1   a  corresponding thereto. The connecting frame connects the left and right side frames  16  on a forward surface side and on a rear surface side of the apparatus body  1   a  corresponding thereto. 
     Various members that constitute, for example, the image forming unit  2 , the sheet-feeding unit  4 , the transporting unit  5 , and the fixing unit  6  are mounted on the left and right side frames  16 . A driving device  80  that drives, for example, the image forming unit  2 , the sheet-feeding unit  4 , and the transporting unit  5  is mounted on the right side frame  16 . Furthermore, as illustrated in  FIG. 11 , an exhaust fan  165  and an intake fan (not illustrated) are attached to the right side frame  16 . The exhaust fan  165  serves as an exemplary air sending unit that discharges the air in the apparatus body  1   a  to the outside. The intake fan (not illustrated) serves as an exemplary air sending unit that introduces the outside air into the apparatus body  1   a . In  FIG. 2A , reference sign  142  denotes a louver corresponding to the intake fan (not illustrated), and reference sign  143  denotes a louver corresponding to the exhaust fan  165 . 
     As illustrated in  FIG. 3 , the driving device  80  includes, for example, a driving motor  81  and multiple driving force transmission gears  821  to  830 . The driving motor  81  serves as a driving source. The multiple driving force transmission gears  821  to  830  transmit driving force of the driving motor  81  to rotary bodies, such as the photoconductor drum  21  and the developing device  24  of the image forming unit  2 , the sheet-feeding unit  4 , the transporting unit  5 , and the fixing unit  6 . 
     As illustrated in  FIG. 1 , as rotary bodies that are rotationally driven by the driving device  80 , there exist rotary bodies having, for example, various outside diameters, made of various materials, and having various weights, such as the photoconductor drum  21 , a developing roller and stirring-and-transporting member of the developing device  24 , the sheet-feeding roller  42  of the sheet-feeding unit  4 , transporting rollers of the transporting unit  5 , and a heating roller of the fixing unit  6 . Of these rotary bodies, the rotary body having the largest outside diameter and weight is the photoconductor drum  21 . When the speed (the peripheral speed) of each rotary body that is determined on the basis of a process speed of the image forming apparatus  1  is fixed, the rotation speed of the photoconductor drum  21  having the largest outside diameter is the lowest. Therefore, of the driving force transmission gears that transmit rotational driving force of the driving motor  81 , as illustrated in  FIG. 4 , the outside diameter of a driving force transmission gear  831  that transmits the rotational driving force to the photoconductor drum  21  is the largest. As a result, the frequency of a driving sound that is generated from, for example, the driving force transmission gear  831  that transmits the rotational driving force to the photoconductor drum  21  becomes the lowest, so that the driving sound becomes a sound having a relatively low frequency of 1000 Hz (1 KHz) or less. 
     When performing an image forming operation, the image forming apparatus  1  generates a driving sound due to the driving device  80  rotationally driving, for example, the image forming unit  2 , the sheet-feeding unit  4 , the transporting unit  5 , and the fixing unit  6 . In addition, as illustrated in  FIG. 5 , the image forming apparatus  1  generates, for example, an electrostatic discharge sound or a mechanical sliding friction sound that is generated when each step, such as a charging step on the surface of the photoconductor drum  21 , a developing step, a transfer step, a sheet-feeding step, and a transporting step, is performed; and rotation sounds of the exhaust fan  165  and the intake fan are generated. For example, various driving sounds, discharge sounds, sliding friction sounds, and rotation sounds that are generated by the image forming apparatus  1  leak to the outside of the apparatus body  1   a  and become noises. Among the various noises that are generated by the image forming apparatus  1 , the principal noise is a mechanical driving sound that is generated by the driving device  80  and a rotation sound of the exhaust fan  165 . Of mechanical driving sounds that are generated by the driving device  80 , in particular, a sound having a relatively low frequency of 1000 Hz (1 KHz) or less is difficult to attenuate sufficiently at, for example, the front cover  11 , the rear cover  12 , the side covers  13  and  14 , and the upper cover  15 , which have required thicknesses and are made of synthetic resin or the like (refer to paragraph [0012] of Japanese Unexamined Patent Application Publication No. 2000-235396). 
     In Japanese Unexamined Patent Application Publication No. 2000-235396, a resonance space corresponding to the frequency that is generated during operation is formed between an exterior member and an interior member. The resonance space in Japanese Unexamined Patent Application Publication No. 2000-235396 constitutes a Helmholtz resonator as described in the detailed description of the invention. As is publicly known, a Helmholtz resonator is a device in which the air existing in a container having an open portion acts as a spring and resonates, and has a silencing effect of attenuating sound due to resonating air vibration passing through the open portion. 
     However, a Helmholtz resonator has technical problems in that since the air existing in the container acts as a spring, the device tends to be large; and in that since the attenuating effect is produced by using the open portion, the silencing effect is not easily sufficiently produced. In particular, when a Helmholtz resonator is used to absorb a sound having a low frequency, the size of the device is increased. 
     Regarding such technical problems, paragraph [0007] in Japanese Unexamined Patent Application Publication No. 2015-169701 that provides an electrical device including a Helmholtz arrester states that “However, in the case described in PTL 2, the noise reducing effect that is actually obtained is less than the expected noise reducing effect.” Incidentally, PTL 2 that is discussed in paragraph [0007] in Japanese Unexamined Patent Application Publication No. 2015-169701 refers to Japanese Unexamined Patent Application Publication No. 2003-43861 in which a Helmholtz resonator is similarly used. 
     In the exemplary embodiment, attention is paid to a function as a resonance tube that generates a standing wave of a sound of a particular frequency in a space formed with a tubular shape or the like, instead of to a Helmholtz resonator in which the air existing in a container having an open portion acts as a spring. Moreover, this is based on a new technical idea that, instead of forming a resonance tube as a structural body extending simply straight, forms a resonance tube that is disposed two-dimensionally or three-dimensionally. 
       FIG. 6  schematically illustrates the basic principles of a resonance tube. 
     When sound is incident upon a tube  200  (hereunder referred to as “resonance tube”) having one end  201  open and the other end  202  closed from a sound absorbing opening portion  203  open at the other end  202 , resonance occurs at a frequency dependent upon a length L of the resonance tube  200 . Therefore, by setting the length L of the resonance tube  200  as appropriate, it is possible to cause a sound having a target frequency to resonate to reduce leakage to outside. In addition, when a sound absorbing material or a sound absorbing mechanism is provided in the resonance tube  200  (an antinode of particle speed or an antinode of sound pressure), it is possible to increase a noise reducing effect of reducing the incident sound. The one end  201  may be closed, in which case the sound pressure distribution of the one end  201  becomes a node. In general, when the one end  201  is closed, the length L of the resonance tube  200  may be L=λ/4, which is shorter than the length L=λ/2 of the resonance tube  200  when the one end  201  is open. 
     In the resonance tube  200  that causes noise to resonate, the wavelength λ of sound is increased when the sound is a low-frequency sound whose frequency is relatively low, and hence it is required to set the length L of the resonance tube  200  at a large value. 
     However, in the image forming apparatus  1 , it may be difficult to ensure the length L of the resonance tube  200  corresponding to a target low-frequency sound at a relatively low frequency only in one direction, due to reduction in size of the apparatus body  1   a  and the layout of various members. 
     Owing to this, in the exemplary embodiment, to form a resonance tube corresponding to a low-frequency sound at a relatively low frequency even if it is difficult to form the resonance tube  200  only in one direction due to limitation on size, there are provided a first resonance tube that extends in a first direction, that takes in from a sound absorbing opening portion a sound wave that is generated from a noise source, and that causes the sound wave to resonate to reduce leakage to outside, and a second resonance tube that extends in a second direction differing from the first direction, and that, along with the first resonance tube, causes the sound wave that is generated from the noise source to resonate to reduce the leakage to the outside. Also, in the exemplary embodiment, there is provided a third resonance tube that extends in a third direction differing from the first and second directions, and that, along with the first and second resonance tubes, causes the sound wave that is generated from the noise source to resonate to reduce the leakage to the outside. 
       FIG. 7  schematically illustrates a distribution of sound pressures, with gradation, in a resonance tube  210  that is formed two-dimensionally.  FIGS. 8A and 8B  schematically illustrate an internal structure of the resonance tube  210  that is formed two-dimensionally.  FIG. 9  schematically illustrates a resonance tube  210  that is formed three-dimensionally. 
     A resonance tube  210  is formed with a tube shape having a rectangular cross-section and bent in an L shape or a substantial L shape. The cross-sectional shape of the resonance tube  210  is not limited to the rectangular shape, and may be a circular shape. The resonance tube  210  has a sound absorbing opening portion  211  in a surface of one end portion closed in a longitudinal direction of the resonance tube  210 . Also, the resonance tube  210  has an opening  212  at an end portion opposite to the air absorbing opening portion  211  in the longitudinal direction. Also, a sound absorbing material  213  is disposed at a position corresponding to an antinode of the particle speed if required. The end portion opposite to the sound absorbing opening portion  211  may be closed. 
     In the exemplary embodiment illustrated in  FIG. 8A , the resonance tube  210  includes a first resonance tube  214  having a length L 1  and a second resonance tube  215  having a length L 2 . When the resonance tube  200  illustrated in  FIG. 7  functions as a resonance tube that causes a sound of a frequency of 500 Hz to resonate, since sound wavelength=sound speed/frequency, if the length L is set at λ/4, the length L of the resonance tube  200  is about 17 cm. In the case of an open tube in which one end of the resonance tube  200  is open, the length L is set at λ/2. In contrast, in the case of the resonance tube  210  illustrated in  FIG. 8A , the lengths of the first resonance tube  214  and the second resonance tube  215  may be, for example, 10 cm and 7 cm, and the total length L 1 +L 2  may be about 17 cm. In the case of an open end in which one end of the resonance tube  210  is open, regarding an antinode present at an end portion of the resonance tube  210 , the end portion in which sound resonates more than resonance of sound in a tube is actually located at a slightly outer side with respect to the tube, and it is required to perform fine adjustment by an amount corresponding to an open-end-portion correction value+ΔL (in the case of open tube, +2ΔL). ΔL is at the outer side by 0.6 in a case of a cylindrical tube with a radius a. The total length of the resonance tube  210  (=L 1 +L 2 ) is not limited to λ/4 of the wavelength λ of the sound, and of course may be set at λ/2, 1λ, 2λ, . . . . Also, the open tube and the closed tube have different intervals. 
     When the relationship between the resonance wavelength, at which the first to third resonance tubes  721  to  723  make resonance, and the length of the tube is formulated, the formula is as follows as illustrated in  FIG. 19 .
 
Open tube λ n =2 L/n  ( n= 1, 2, . . . )
 
Closed tube λ n =4 L /(2 n− 1) (λ: wavelength (=sound speed/frequency))
 
     These are rewritten according to the lengths of the first to third resonance tubes  721  to  723  as follows.
 
Open tube  L =(λ/2) n  
 
Closed tube  L =(λ/4) (2 n− 1)
 
     The exemplary embodiment is further specifically described. As illustrated in  FIGS. 10 and 11 , the exhaust fan  165  is attached to an outer side surface of the right side frame  16  by screwing or the like, at a lower end portion of the right side frame  16  on a rear surface side. The right side frame  16  has an exhaust opening  166  having a substantially rectangular shape at a position corresponding to the exhaust fan  165 , and plural exhaust holes  167  being open above the opening  166 . The right side frame  16  also has a datum hole  168  being thin and long and serving as a reference when the right side frame  16  is handled, for example, when the right side frame  16  is assembled, at a position below the opening  166  on the rear surface side. 
     As illustrated in  FIG. 10 , the right side frame  16  is formed with rectangular side surfaces by, for example, press working or welding a metal sheet. The right side frame  16  is formed with a high rigidity by forming it with the shape of a frame body as a result of outwardly bending outer peripheral edges  161  to  164  thereof. A housing (bracket)  840  of the driving device  80  that is made from, for example, a metal sheet or synthetic resin is mounted on an outer side surface of the right side frame  16  in a fixed state. The driving force transmission gears  821  to  830  and  831  of the driving device  80  and multiple rotatory shafts (not illustrated) that support the driving force transmission gears  821  to  830  and  831  are disposed in the housing  840  of the driving device  80  perpendicularly to a surface of the right side frame  16 . 
     At a central portion of the housing  840  of the driving device  80 , a drum supporting cover (bracket)  841  is mounted on the right side frame  16  by, for example, screwing. The drum supporting cover  841  is formed with a substantially rhombic shape by using, for example, a metal sheet; and rotatably supports an end portion of the photoconductor drum  21  in an axial direction via a bearing member (not illustrated). An open portion  842  corresponding to the shape of the drum supporting cover  841  is provided in a region of the right side frame  16  corresponding to the drum supporting cover  841 . As illustrated in  FIG. 4 , a flange portion  843  is formed on an outer peripheral end edge of the drum supporting cover  841  by, for example, burring. The driving force transmission gear  831  for rotationally driving the photoconductor drum  21  is rotatably disposed at a lower portion of the drum supporting cover  841 . An opening  844  is disposed at a lower end portion of the drum supporting cover  841 , for avoiding interference between the driving force transmission gear  831  and the flange portion  843 . A surface of the housing  840  and a surface of the drum supporting cover  841  of the driving device  80  form substantially the same plane. 
     As illustrated in  FIGS. 12 to 14 , a first duct member  70  made of synthetic resin is attached to the right side frame  16 . The first duct member  70  constitutes a portion of a guide portion that guides the holding container  41  of the sheet-feeding unit  4  when the holding container  41  is inserted to or removed from an inner side surface of the right side frame  16  at a position corresponding to the exhaust fan  165 . The first duct member  70  also constitutes an exhaust duct. As illustrated in  FIG. 13 , the first duct member  70  is formed with a box body whose side surfaces have a substantially rectangular shape by subjecting, for example, synthetic resin to injection molding, and which has a relatively small depth. The first duct member  70  has a side surface  701  and an upper end portion  702  on the right side frame  16  side. The side surface  701  and the upper end portion  702  are open. An end surface of the first duct member  70  on the right side frame  16  side is provided with three engagement protrusions  703  to  705  having substantially L-shaped cross-sectional shapes, and a snap-fit portion  706 . The engagement protrusions  703  to  705  cause the first duct member  70  to be hermetically attached to the right side frame  16 , and form a space between the first duct member  70  and the right side frame  16  so that only an upper end portion of the space is partially open. The snap-fit portion  706  positions and fixes the first duct member  70  to the right side frame  16 . The snap-fit portion  706  has a base end portion that is connected to a side surface of the first duct member  70  in an elastically deformable manner. Also, a protrusion  707  protruding toward the right side frame  16  is formed at a tip end of the snap-fit portion  706 . The first duct member  70  is positioned and fixed by engaging the three engagement protrusions  703  to  705  with engagement hole portions  708  to  710  of the right side frame  16  (see  FIGS. 10 and 11 ), and engaging the protrusion  707  of the snap-fit portion  706  with an engagement hole portion  711  of the right side frame  16 . 
     As illustrated in  FIG. 15 , the first duct member  70  includes a first resonance tube  721  and a second resonance tube  722 . The first resonance tube  721  extends in a vertical direction serving as an exemplary first direction, takes in from a sound absorbing opening portion a sound wave that is generated from a noise source, and causes the sound wave to resonate to reduce leakage to outside. The second resonance tube  722  extends in a horizontal direction serving as an exemplary second direction differing from the first direction, and, along with the first resonance tube  721 , causes the sound wave that is generated from the noise source to resonate to reduce the leakage to the outside. 
     As illustrated in  FIG. 13 , the first resonance tube  721  is formed by a first partition portion  731  disposed along the vertical direction of partition walls  730  provided in a substantial L shape in the first duct member  70 . An upper end portion of the first resonance tube  721  is open to the upper side, and constitutes a sound absorbing opening portion  724 . Also, the second resonance tube  722  is formed of a second partition portion  732  disposed along the horizontal direction of the partition walls  730  provided in the substantial L shape in the first duct member  70 . The above-described datum hole  168  of the right side frame  16  is located at a tip end portion along the longitudinal direction of the second resonance tube  722 . The datum hole  168  constitutes a communication hole through which the second resonance tube  722  is connected with a third resonance tube  723  (described later). 
     In addition, a second duct member  90  made of synthetic resin and constitutes an exhaust duct is attached to an outer side surface of the right side frame  16  at a position corresponding to the exhaust fan  165 . The second duct member  90  is integrally formed with the exterior body of the exhaust fan  165  at a lower end portion of the exhaust fan  165 . The second duct member  90  is formed with a laterally elongated substantially rectangular-parallelepiped shape whose side surface at the right side frame  16  side being open. The second duct member  90  constitutes the third resonance tube  723  that extends in the third direction differing from the first and second directions, and that, along with the first and second resonance tubes  721  and  722 , causes the sound wave that is generated from the noise source to resonate to reduce the leakage to the outside. As illustrated in  FIG. 15 , the third resonance tube  723  is disposed to be adjacent to the second resonance tube  722  with the right side frame  16  interposed therebetween in a substantially horizontal plane. 
     Consequently, the first resonance tube  721 , the second resonance tube  722 , and the third resonance tube  723  constitute a single continuous resonance tube. The length of the single resonance tube is the sum of the lengths L 1 , L 2 , and L 3  of the first to third resonance tubes  721  to  723 . 
     Action of Image Forming Apparatus 
     In the image forming apparatus  1  according to the exemplary embodiment, even if it is difficult to form a resonance tube only in one direction due to limitation on size, it is possible to form a resonance tube as follows. 
     In the image forming apparatus  1 , when the controlling device  100  receives command information regarding a request for an image forming operation (print), the driving device  80  drives, for example, the image forming unit  2 , the sheet-feeding unit  4 , the transporting unit  5 , and the fixing unit  6 . In the image forming apparatus  1 , the intake fan (not illustrated) and the exhaust fan  165  are driven in synchronization with an image forming operation. 
     As illustrated in  FIG. 3 , in the driving device  80 , the driving motor  81  is rotationally driven, and rotational driving force of the driving motor  81  is transmitted to the rotary bodies, such as the photoconductor drum  21  of the image forming unit  2 , via, for example, the driving force transmission gears  821  to  830  and  831 . 
     At this time, the driving device  80  generates driving noises resulting from, for example, meshing of the driving force transmission gears  821  to  830  and  831 . Of the driving noises resulting from the meshing of the driving force transmission gears  821  to  830  and  831 , in particular, the driving noise resulting from the meshing of the driving force transmission gear  831  having a large outside diameter tends to have a low frequency of 1000 Hz or less because the rotation speed of the driving force transmission gear  831  having the large outside diameter is less than the rotation speeds of driving force transmission gears having small outside diameters. 
     Also, the intake fan (not illustrated) and the exhaust fan  165  generate rotation sounds resulting from driving of the intake fan and the exhaust fan  165 . The rotation sounds of the intake fan and the exhaust fan  165  tend to have low frequencies of 1000 Hz or less. 
     As illustrated in  FIGS. 15 to 17 , the noises that are generated from, for example, the driving force transmission gears  821  to  830  and  831  of the driving device  80  are introduced to the inside of the first resonance tube  721  via the opening  724  that functions as the sound absorbing opening portion of the first duct member  70 , and a sound at a wavelength λ resonates, the wavelength λ corresponding to the sum of the lengths L 1  to L 3  of the second and third resonance tubes  722  and  723  continued from the first resonance tube  721 . Hence the noises having frequencies of 1000 Hz or less that are generated from the driving device  80  and the air sending sound resonate in the first to third resonance tubes  721  to  723  that function as the single resonance tube although the individual lengths L 1 , L 2 , and L 3  of the first to third resonance tubes  721  to  723  are small. Output of the noises to the outside of the image forming apparatus  1  is prevented or reduced. Accordingly, even if it is difficult to ensure the length L of a single resonance tube only in one direction for a noise having a relatively low frequency, the resonance tube having the sum of the lengths L 1 , L 2 , and L 3  in total of the first to third resonance tubes  721  to  723  may be constituted, and a noise having a relatively low frequency is reduced. 
     Second Exemplary Embodiment 
       FIG. 18  schematically illustrates an entire image forming apparatus  1  to which a noise reducing structure according to a second exemplary embodiment is applied. 
     As illustrated in  FIG. 18 , the image forming apparatus  1  according to the second exemplary embodiment includes a side cover  14  as an exemplary exterior body. The side cover  14  is openably and closably mounted on an apparatus body  1   a . The side cover  14  is disposed so as to cover an outer side surface of a driving device  80  of the apparatus body  1   a . Multiple reinforcing ribs  171  to  176  that are tilted so as to be parallel to each other are integrally formed with an inner side surface of the side cover  14 . Spaces that are formed by one end portion of each of the multiple reinforcing ribs  171  to  176  are closed by a reinforcing rib  177 . In addition, lower end portions  171   a  to  176   a  of the multiple reinforcing ribs  171  to  176  are bent downward. The multiple reinforcing ribs  171  to  176  including the lower end portions  171   a  to  176   a  constitute a resonance tube. The resonance tube constituted by the multiple reinforcing ribs  171  to  176  have lengths differing from each other by the lengths of the lower end portions  171   a  to  176   a , and causes multiple sounds with different frequencies to resonate. 
     By closing the spaces formed by the multiple reinforcing ribs  171  to  177  that are adjacent to each other, the open sides are closed to constitute multiple resonance tubes formed by closed spaces. In this way, by closing the side cover  14 , the open sides of the multiple reinforcing ribs  171  to  177  are closed by a housing  840  and a drum supporting cover  841  of the driving device  80 . When the lengths of the multiple resonance tubes formed by the multiple reinforcing ribs  171  to  177  are made to differ from each other, it is possible to cause sounds having different wavelengths to resonate. The opening of the driving device  80  constitutes the sound absorbing opening portion of each resonance tube. 
     Although, in the exemplary embodiments, a monochrome image forming apparatus that forms a black toner image is described as the image forming apparatus, the type of image forming apparatus is not limited thereto. Obviously, as the image forming apparatus, a full-color image forming apparatus that forms toner images of four colors, yellow (Y), magenta (M), cyan (C), and black (K) may also be similarly used. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.