Patent Publication Number: US-8123468-B2

Title: Centrifugal fan

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal fan used as a ventilating fan or the like. 
     BACKGROUND ART 
     There has conventionally been known such a centrifugal fan that is used as a centrifugal fan and has an orifice different from a suction casing with a bell-mouthed suction port at an opening on one side of an outer casing. An example of such a centrifugal fan in the related art is described in Patent Document 1. 
     The centrifugal fan in the related art will be described below with reference to  FIG. 31 . 
     As illustrated in  FIG. 31 , the centrifugal fan in the related art has outer casing  101  having one opened side, motor  104  that has impeller  103  fixed to top surface  102  in outer casing  101 , casing  105  that surrounds a periphery of impeller  103 , and suction casing  107  having suction port  106 . Orifice  109  has suction hole  108  having a diameter equal to or smaller than that of suction port  106 . Orifice  109  is separated from suction casing  107  by predetermined clearance h. Orifice  109  configures resonant space  112  so that lower end  110  of suction casing  107  and end  111  of orifice  109  are separated by clearance distance i. Grill  113  is provided on one side of orifice  109 . Discharge port  114  is provided on one side of outer casing  101 . 
     In the above configuration, when impeller  103  is rotated, sucked air passes from grill  113  through suction hole  108  of orifice  109  and then enters impeller  103  from suction port  106  of suction casing  107 . The sucked air is subjected to pressure rise by impeller  103 . The sucked air passes through an inside of casing  105  and is then discharged from discharge port  114 . Sound waves of rotational noises caused when the sucked air is subjected to pressure rise by impeller  103 , vortex turbulent noises caused when the sucked air passes through casing  105 , and noises caused in casing  105  are emitted from suction port  106 . Some of them are incident from inlet portion  115  having clearance distance i into resonant space  112 . The incident sound waves of the noises at a frequency specified according to a volume and shape of resonant space  112  are resonated and suppressed in such a manner that air column resonance occurs in resonant space  112  and that inlet portion  115  and resonant space  112  function as a Helmholtz resonator. 
     The frequency of the noises that are resonated and suppressed in resonant space  112  is specified according to the volume and shape of resonant space  112 . In such a centrifugal fan in the related art, a range in which the volume and shape of resonant space  112  can be adjusted is small. For this reason, a range of frequencies of noises that can be resonated and suppressed is small. The range of frequencies of noises that can be suppressed is therefore required to be increased. 
     To suppress noises at a low frequency, the volume of resonant space  112  needs to be increased. A size of outer casing  101  thus becomes larger. Noises at a lower frequency are required to be suppressed without changing the size of outer casing  101 . 
     The volume of resonant space  112  cannot be easily changed. If main frequencies of noises are changed according to an installed state of the centrifugal fan or there are noises at a plurality of outstanding frequencies, a noise reduction effect by resonance noise-suppression may be reduced. Frequencies at which noises can be suppressed are required to be adjusted.
     [Patent Document 1] Japanese Patent No. 3279834   

     DISCLOSURE OF THE INVENTION 
     The present invention addresses such problems in the related art and provides a centrifugal fan that can increase a range of frequencies of noises in which noises can be suppressed, can suppress noises at a lower frequency without changing a size of an outer casing, and can adjust frequencies of noises at which noises can be suppressed. 
     A centrifugal fan of the present invention has, in an outer casing having an opening, a motor that couples an impeller so as to rotate the impeller on a rotational axis, a casing that surrounds a periphery of the impeller and has a suction port, and a bell-mouthed orifice that has an opening communicating with the opening of the outer casing. In the centrifugal fan that resonates and suppresses noises released from the suction port by a resonant space formed by the orifice, part of a path where sound waves of noises incident from an inlet portion between an end of the orifice and the casing into the resonant space are reflected is made longer. 
     Noises at a lower frequency can be suppressed without increasing a volume of the resonant space. The centrifugal fan that can reduce noises at a desired frequency without increasing a size of the outer casing can be obtained. 
     A centrifugal fan of the present invention has, in an outer casing having an opening, a motor that couples an impeller so as to rotate the impeller on a rotational axis, a casing that surrounds a periphery of the impeller and has a suction port, and a bell-mouthed orifice that has an opening communicating with the opening of the outer casing. In the centrifugal fan, a member closing a clearance portion between an end of the orifice and the casing is a noise absorbing structural material, and a space formed by the orifice is a rear air layer, thereby forming a resonance type noise absorbing structure. 
     With this structure, the resonance type noise absorbing structure can be formed by the member closing the clearance portion and the space formed by the orifice. Hence, a range of frequencies of noises in which noises can be suppressed can be increased. Frequencies of noises at which noises can be suppressed can be adjusted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side sectional view illustrating a centrifugal fan of Embodiment 1 of the present invention. 
         FIG. 1B  is a surface portion sectional view illustrating the centrifugal fan of Embodiment 1 of the present invention. 
         FIG. 2  is a diagram illustrating a principle of a Helmholtz resonator. 
         FIG. 3A  is a side sectional view illustrating an experimental object of an experiment to observe a noise-suppression effect of the centrifugal fan of Embodiment 1 of the present invention. 
         FIG. 3B  is a lower surface view illustrating the experimental object of the experiment to observe the noise-suppression effect of the centrifugal fan of Embodiment 1 of the present invention. 
         FIG. 4  is a diagram illustrating the noise-suppression effect of the centrifugal fan of Embodiment 1 of the present invention. 
         FIG. 5  is a side sectional view illustrating a centrifugal fan of Embodiment 2 of the present invention. 
         FIG. 6  is a side sectional view illustrating a centrifugal fan of Embodiment 3 of the present invention. 
         FIG. 7  is a side sectional view illustrating a centrifugal fan of Embodiment 4 of the present invention. 
         FIG. 8  is a side sectional view illustrating a centrifugal fan of Embodiment 5 of the present invention. 
         FIG. 9  is a side sectional view illustrating a centrifugal fan of Embodiment 6 of the present invention. 
         FIG. 10  is a side sectional view illustrating a centrifugal fan of Embodiment 7 of the present invention. 
         FIG. 11  is a side sectional view illustrating a centrifugal fan of Embodiment 8 of the present invention. 
         FIG. 12  is a side sectional view illustrating a centrifugal fan of Embodiment 9 of the present invention. 
         FIG. 13A  is a side sectional view illustrating a centrifugal fan of Embodiment 10 of the present invention. 
         FIG. 13B  is a lower surface portion sectional view illustrating the centrifugal fan of Embodiment 10 of the present invention. 
         FIG. 14  is a side sectional view illustrating a centrifugal fan of Embodiment 11 of the present invention. 
         FIG. 15  is a side sectional view illustrating a centrifugal fan of Embodiment 12 of the present invention. 
         FIG. 16  is a side sectional view illustrating a centrifugal fan of Embodiment 13 of the present invention. 
         FIG. 17A  is a side sectional view illustrating a centrifugal fan of Embodiment 14 of the present invention. 
         FIG. 17B  is a lower surface portion sectional view illustrating the centrifugal fan of Embodiment 14 of the present invention. 
         FIG. 18  is a side sectional view illustrating a centrifugal fan of Embodiment 15 of the present invention. 
         FIG. 19  is a side sectional view illustrating a centrifugal fan of Embodiment 16 of the present invention. 
         FIG. 20A  is a side sectional view illustrating a centrifugal fan of Embodiment 17 of the present invention. 
         FIG. 20B  is a lower surface portion sectional view illustrating the centrifugal fan of Embodiment 17 of the present invention. 
         FIG. 21  is a side sectional view illustrating a centrifugal fan of Embodiment 18 of the present invention. 
         FIG. 22  is a side sectional view illustrating a centrifugal fan of Embodiment 19 of the present invention. 
         FIG. 23  is a side sectional view illustrating a centrifugal fan of Embodiment 20 of the present invention. 
         FIG. 24  is a side sectional view illustrating a centrifugal fan of Embodiment 21 of the present invention. 
         FIG. 25  is a side sectional view illustrating a centrifugal fan of Embodiment 22 of the present invention. 
         FIG. 26A  is a side sectional view illustrating a centrifugal fan of Embodiment 23 of the present invention. 
         FIG. 26B  is a lower surface portion sectional view illustrating a centrifugal fan of Embodiment 23 of the present invention. 
         FIG. 27  is a side sectional view illustrating a centrifugal fan of Embodiment 24 of the present invention. 
         FIG. 28  is a side sectional view illustrating a centrifugal fan of Embodiment 25 of the present invention. 
         FIG. 29  is a side sectional view illustrating a centrifugal fan of Embodiment 26 of the present invention. 
         FIG. 30  is a side sectional view illustrating a centrifugal fan of Embodiment 27 of the present invention. 
         FIG. 31  is a side sectional view illustrating a centrifugal fan in the related art. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           1  centrifugal fan 
           2  opening 
           3  outer casing 
           4  rotational axis 
           5  impeller 
           6  motor 
           7  suction port 
           8  side wall 
           9  discharge port 
           10  casing 
           11  discharge opening 
           12  discharge adapter 
           13  electric equipment portion 
           14  flange portion 
           15  ceiling material 
           16  hole 
           17  duct 
           18  opening 
           19  orifice 
           20  resonant space 
           21  end 
           22  wall body 
           23  inlet portion 
           24  inlet portion region 
           25  volume portion 
           26  throat portion 
           27  hollow portion 
           28  experimental object 
           30  circular pipe 
           31  noise receiving side circular pipe 
           32  transmission loss for each frequency in the absence of the wall body 
           33  transmission loss for each frequency in the presence of the wall body 
           34  end face of the wall body 
           35  screwed portion 
           36  screwing portion 
           37  screw 
           38  inside wall 
           39  outside wall 
           40  double pipe 
           41  partition wall 
           42  outer peripheral space 
           43  motor side outer wall 
           44  rear space 
         c sound speed 
         D inside diameter of the wall body 
         Di inside diameter of the suction port 
         F frequency 
         h clearance 
         i clearance distance 
         L throat portion length 
         La axial depth length 
         Lr diametrical depth length 
         S throat portion area 
         TL transmission loss 
         V hollow portion volume 
       
    
     PREFERRED EMBODIMENTS FOR CARRYING OUT OF THE INVENTION 
     The present invention includes, in an outer casing having an opening, a motor that couples an impeller so as to rotate the impeller on a rotational axis, a casing that surrounds a periphery of the impeller and has a suction port, and a bell-mouthed orifice that has an opening communicating with the opening of the outer casing. In the centrifugal fan that resonates and suppresses noises released from the suction port by a resonant space formed by the orifice, part of a path where sound waves of noises incident from an inlet portion between an end of the orifice and the casing into the resonant space are reflected is made longer. Since noises at a lower frequency can be suppressed without increasing a volume of the resonant space, noises at a desired frequency can be reduced without increasing a size of the outer casing. The present invention relates to a centrifugal fan in which the suction port of the casing is bell-mouthed. An inflow of air into the casing can be smoothened, ventilation efficiency can be improved without increasing a size of the outer casing, and noises can be reduced. 
     The present invention relates to a centrifugal fan in which the casing has a scroll shape. A dynamic pressure of air flowed out from the impeller can be efficiently converted to a static pressure. The air can be discharged from a discharge port. Ventilation efficiency can thus be improved without increasing a size of the outer casing, and noises can be reduced. 
     The present invention relates to a centrifugal fan in which the opening of the orifice is concentric with the suction port of the casing. Noises from the suction port of the casing can be smoothly guided to the entire resonant space, and sucked air can be smoothly guided to the casing. Noises at a desired frequency can thus be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the opening of the orifice has a diameter equal to or smaller than that of the suction port. Noises from the suction port of the casing can be efficiently guided to the resonant space. Noises at a desired frequency can thus be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a path where some of sound waves of noises incident from the inlet portion of the resonant space are reflected in the rearmost portion of the resonant space is made longer. Noises at a lower frequency can be suppressed without increasing a volume of the resonant space. Noises at a desired frequency can thus be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a wall body is provided in the resonant space and part of the path where sound waves of noises are reflected is made longer. Sound waves of noises are propagated so as to move around the wall body. The path where some of sound waves of noises are reflected can be made longer. Noises at a lower frequency can also be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the wall body provided in the resonant space has a cylindrical pipe shape that is concentric with the suction port and has one end contacting the casing. A path where sound waves of noises emitted from the suction port move around the wall body has only one U-shaped curve. Sound waves of noises can be smoothly propagated to the rearmost portion of the path where sound waves of noises are reflected, and noises at a lower frequency can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which inside diameter D of the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port establishes a relation of D&gt;Di+2i between inside diameter Di of the suction port and clearance distance i of the inlet portion. The inlet portion and the wall body can be sufficiently separated, and reduction of an amount of sound waves of noises that are incident into the resonant space through reflection of some of sound waves of noises attempting to be incident into the resonant space near the inlet portion by the wall body can be prevented. Hence, noises at a lower frequency can be suppressed without reducing the amount of sound waves that are resonated and suppressed. 
     The present invention relates to a centrifugal fan in which inside diameter D of the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port establishes a relation of D&lt;Di+2i between inside diameter Di of the suction port and clearance distance i of the inlet portion. An area of an inlet portion region that is surrounded by the cylindrical pipe shaped wall body and the orifice and is equivalent to a throat portion area of a Helmholtz resonator can be reduced. A volume of a portion of the resonant space that locates at a rear side of the cylindrical pipe shaped wall body and is equivalent to a hollow portion volume can be increased. Hence, by an operation of the Helmholtz resonator, noises at a lower frequency can be suppressed. 
     The present invention relates to a centrifugal fan in which a thickness of the wall body and the end of the orifice is increased so that the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port and the end of the orifice are overlapped in a diametrical direction. A diametrical depth length of the inlet portion region that is surrounded by an upper end of the cylindrical pipe shaped wall body and the end of the orifice and is equivalent to a throat portion length of the Helmholtz resonator can be made longer. Hence, by the operation of the Helmholtz resonator, noises at a lower frequency can be suppressed. 
     The present invention relates to a centrifugal fan in which inside diameter D of the cylindrical pipe shaped wall body is not uniform in a circumferential direction. A portion in which a length of a path where sound waves of noises emitted from the suction port are reflected is different can be provided. Sound waves of noises at a plurality of frequencies can be resonated and suppressed in the resonant space. Noises at a plurality of low frequencies can be suppressed without increasing a volume of the resonant space. Noises at a desired frequency can also be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a height of the cylindrical pipe shaped wall body is not uniform in a circumferential direction. A portion in which a length of a path where sound waves of noises emitted from the suction port are reflected is different can be provided, and a portion in which an axial depth length of the inlet portion region that is surrounded by the upper end of the cylindrical pipe shaped wall body and the end of the orifice and is equivalent to the throat portion length of the Helmholtz resonator is different can be provided. Hence, sound waves of noises at a plurality of frequencies can be resonated and suppressed in the resonant space, noises at a plurality of low frequencies can be suppressed without increasing a volume of the resonant space, and noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which clearance distance i of the inlet portion into the resonant space is not uniform in a circumferential direction. A portion in which the axial depth length of the inlet portion region that is surrounded by the upper end of the cylindrical pipe shaped wall body and the end of the orifice and is equivalent to the throat portion length of the Helmholtz resonator is different in a circumferential direction. Hence, by the operation of the Helmholtz resonator, sound waves of noises at a plurality of frequencies can be resonated and suppressed in the resonant space. 
     The present invention relates to a centrifugal fan in which inside diameter D of the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port can be adjusted. Even if frequencies of noises emitted from the suction port are changed according to an installed state of the centrifugal fan or the like, the frequencies at which noises are suppressed can be adjusted by changing inside diameter D of the cylindrical pipe shaped wall body. Noises at a lower frequency can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a height of the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port can be adjusted. Even if frequencies of noises emitted from the suction port are changed according to an operation state of the centrifugal fan or the like, the frequencies at which noises are suppressed can be adjusted by changing the height of the cylindrical pipe shaped wall body. Noises at a lower frequency can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which at least one cylindrical pipe shaped wall body that is concentric with the suction port of the casing and has one end contacting the casing and at least one cylindrical pipe shaped wall body that is concentric with the suction port of the casing and has one end contacting the orifice are arranged alternately in a diametrical direction. The centrifugal fan can have a labyrinth structure in which sound waves of noises are propagated so as to alternately move around the wall bodies. A path where some of sound waves of noises are reflected can be made longer. Noises at a lower frequency can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the cylindrical pipe shaped wall body that is provided in the resonant space and is concentric with the suction port is molded integrally with the casing. The cylindrical pipe shaped wall body that is concentric with the suction port and the casing can be made into one component. Hence, the number of components can be reduced, a production cost can be reduced, and noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan provided with a partition wall that divides the resonant space into a plurality of parts. Since sound waves of noises emitted from the suction port are resonated and suppressed in the resonant spaces having a plurality of volumes, noises at a plurality of low frequencies can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the wall body having a different shape is provided in each of a plurality of resonant spaces and part of a path where sound waves of noises are reflected in each of the resonant spaces is made longer. Part of the path where sound waves of noises are propagated in the resonant spaces having a plurality of volumes can be made longer, and noises at a plurality of lower frequencies can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which clearance distances i of the inlet portions of the plurality of resonant spaces are different. A length of the inlet portion region that is surrounded by the cylindrical pipe shaped wall body and the end of the orifice and is equivalent to the throat portion length of the Helmholtz resonator can be set in each of the plurality of resonant spaces. By the operation of the Helmholtz resonator, noises at a plurality of lower frequencies can be suppressed without increasing a volume of the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which an outer peripheral space surrounded by a side wall of the casing and the outer casing is used as the resonant space. A volume of the resonant space can be increased, a path from the inlet portion to the rearmost portion can be made longer, and noises at a lower frequency can be suppressed. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a rear space between a motor side outer wall of the casing and the outer casing is used as the resonant space. A volume of the resonant space can be increased, a path from the inlet portion to the rearmost portion can be made longer, and noises at a lower frequency can be suppressed. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan that includes, in an outer casing having an opening, a motor that couples an impeller so as to rotate the impeller on a rotational axis, a casing that surrounds a periphery of the impeller and has a suction port, and a bell-mouthed orifice that has an opening communicating with the opening of the outer casing, wherein a member closing a clearance portion between an end of the orifice and the casing is a noise absorbing structural material, and a space formed by the orifice is a rear air layer, thereby forming a resonance type noise absorbing structure. The resonance type noise absorbing structure can be formed by the member closing the clearance portion and the orifice. Hence, a range of frequencies in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which the suction port of the casing is bell-mouthed. An inflow of air into the casing can be smoothened, ventilation efficiency can be improved without increasing a size of the outer casing, and noises can be reduced. 
     The present invention relates to a centrifugal fan in which the casing has a scroll shape. A dynamic pressure of air flowed out from the impeller can be efficiently converted to a static pressure. The air can be discharged from a discharge port. Hence, ventilation efficiency can be improved without increasing a size of the outer casing, and noises can be reduced. 
     The present invention relates to a centrifugal fan in which the opening of the orifice is concentric with the suction port of the casing. Noises from the suction port of the casing can be smoothly guided to the entire resonant space, and sucked air can be smoothly guided to the casing. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the opening of the orifice has a diameter equal to or smaller than that of the suction port. Noises from the suction port of the casing can be efficiently guided to the resonant space. Hence, noises at a desired frequency can be reduced without increasing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the member closing the clearance portion is a filmy material. There is formed a vibration system in which the filmy material is a mass and the space as the rear air layer is a spring, so that a filmy noise absorbing structure as the resonance type noise absorbing structure can be formed. Hence, a range of frequencies in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which the member closing the clearance portion is a holed plate. There is formed a vibration system in which air in a hole portion of the holed plate is a mass and the rear air layer is a spring, so that a holed plate noise absorbing structure as the resonance type noise absorbing structure that absorbs noises according to the same principle as the Helmholtz resonator can be formed. Hence, a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which the member closing the clearance portion is a porous material. Noises at a relatively high frequency can be absorbed by the porous material itself. In addition, there is formed a vibration system in which the porous material is a mass and the rear air layer is a spring, so that a porous noise absorbing structure as the resonance type noise absorbing structure can be formed. Hence, noises at a relatively low frequency can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which a wall body is provided in the space and a thickness of the rear air layer is increased. A length of a path where sound waves of noises are propagated so as to move around the wall body in the space can be regarded as a thickness of the rear air layer in the resonance type noise absorbing structure. Hence, the thickness of the rear air layer can be increased, and noises at a lower frequency can be suppressed without changing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which the wall body provided in the space has a cylindrical pipe shape that is concentric with the suction port and has one end contacting the casing. A path where sound waves of noises move around the wall body in the space has only one U-shaped curve. Sound waves of noises can thus be smoothly propagated. Hence, a thickness of the rear air layer can be increased more stably, and noises at a lower frequency can be suppressed without changing a size of the outer casing by the wall body of a simple structure. 
     The present invention relates to a centrifugal fan in which inside diameter D of the cylindrical pipe shaped wall body that is provided in the space and is concentric with the suction port is not uniform in a circumferential direction. As inside diameter D of the cylindrical pipe shaped wall body is partially different, a portion in which a thickness of the rear air layer is different can be provided. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which a height of the cylindrical pipe shaped wall body that is provided in the space and is concentric with the suction port is not uniform in a circumferential direction. As the height of the cylindrical pipe shaped wall body is partially different, a portion in which a thickness of the rear air layer is different can be provided. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which an inside diameter of the cylindrical pipe shaped wall body that is provided in the space and is concentric with the suction port can be adjusted. A thickness of the rear air layer can be adjusted by adjusting the inside diameter of the cylindrical pipe shaped wall body. Hence, frequencies of noises at which noises can be suppressed can be adjusted. 
     The present invention relates to a centrifugal fan in which a height of the cylindrical pipe shaped wall body that is provided in the space and is concentric with the suction port can be adjusted. A thickness of the rear air layer can be adjusted by adjusting the height of the cylindrical pipe shaped wall body. Hence, frequencies of noises at which noises can be suppressed can be adjusted. 
     The present invention relates to a centrifugal fan in which at least one cylindrical pipe shaped wall body that is concentric with the suction port of the casing and has one end contacting the casing and at least one cylindrical pipe shaped wall body that is concentric with the suction port of the casing and has one end contacting the orifice are arranged alternately in a diametrical direction. The centrifugal fan can have a labyrinth structure in which sound waves of noises are propagated in the space so as to alternately move around the wall bodies, a thickness of the rear air layer can be increased, and noises at a lower frequency can be suppressed without changing a size of the outer casing. 
     The present invention relates to a centrifugal fan provided with a partition wall that divides the space formed by the orifice into a plurality of parts. A plurality of resonance type noise absorbing structures in which a shape and volume of the space are different can be formed. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which a member closing the clearance portion is provided in each of a plurality of spaces divided by the partition wall and at least two kinds of members are applied. A plurality of kinds of resonance type noise absorbing structures can be formed. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which the wall body having a different shape is provided in each of a plurality of spaces divided by the partition wall and a thickness of the rear air layer is increased in each of the spaces. The resonance type noise absorbing structure in which the thickness of the rear air layer is different in each of the divided spaces can be formed. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     The present invention relates to a centrifugal fan in which an outer peripheral space surrounded by a side wall of the casing and the outer casing is used as the rear air layer. The outer peripheral space can be used as the rear air layer of the resonance type noise absorbing structure. Hence, a thickness of the rear air layer can be increased, and noises at a lower frequency can be suppressed without changing a size of the outer casing. 
     The present invention relates to a centrifugal fan in which a rear space between a motor side outer wall of the casing and the outer casing is used as the rear air layer. The rear space can be used as the rear air layer of the resonance type noise absorbing structure. Hence, a thickness of the rear air layer can be increased, and noises at a lower frequency can be suppressed without changing a size of the outer casing. 
     Embodiments of the present invention will be described below with reference to the drawings. 
     Embodiment 1 
     As illustrated in  FIGS. 1A and 1B , centrifugal fan  1  used as a ceiling burying type ventilating fan includes, in outer casing  3  having an inner dimension of 265 mm square, a height of 195 mm, and opening  2  in a lower surface, motor  6  that couples multiblade impeller  5  having a diameter of 180 mm so as to rotate impeller  5  on rotational axis  4 , and casing  10  that surrounds a periphery of impeller  5  and has suction port  7  that is bell-mouthed and has inside diameter Di of 150 mm in a lower surface and discharge port  9  on side wall  8 . Side wall  8  of casing  10  has a scroll shape in which an air path is gradually expanded toward discharge port  9 . Discharge port  9  of casing  10  communicates with discharge adapter  12  via discharge opening  11  provided in one side of outer casing  3 . Electric equipment portion  13  that houses an electric component such as a connector or a terminal for electrically connecting motor  6  with an external power supply is arranged in a portion between casing  10  and outer casing  3 . Flange portion  14  is provided on an outer periphery of the lower surface of outer casing  3 . Outer casing  3  is fixed to ceiling material  15  by a screw and the like through hole  16  provided in flange portion  14 . Duct  17  that is disposed on a ceiling and communicates with outdoors is joined to discharge port  9  via discharge adapter  12 . 
     Orifice  19  has bell-mouthed opening  18  that is concentric with suction port  7  of casing  10  and has an inside diameter of 148 mm equal to or smaller than that of suction port  7 . Orifice  19  is separated from suction port  7  of casing  10  by predetermined clearance h (60 mm) and closes opening  2  of outer casing  3 . Orifice  19  forms resonant space  20  between casing  10  and orifice  19 . End  21  of orifice  19  is separated from the lower surface of casing  10  by clearance distance i (20 mm). Cylindrical pipe shaped wall body  22  that is concentric with suction port  7  of casing  10  is provided integrally with the lower surface of casing  10 . Cylindrical pipe shaped wall body  22  has a thickness of 2 mm and a height of 28 mm. Inside diameter D of the wall body is 216 mm and establishes a relation of D&gt;Di+2i between inside diameter Di (150 mm) of the suction port of casing  10  and clearance distance i (20 mm) between end  21  of orifice  19  and the casing. 
     In the above configuration, when impeller  5  is rotated by motor  6 , sucked air passes from opening  2  through opening  18  of orifice  19 . The sucked air smoothly enters impeller  5  from bell-mouthed suction port  7  of casing  10 . The sucked air is subjected to pressure rise by impeller  5 . The sucked air passes through an inside of casing  10  in a scroll shape. A dynamic pressure is thereby efficiently converted to a static pressure. The sucked air is discharged to duct  17  by discharge adapter  12  and is then discharged to outdoors. Opening  18  of orifice  19  has a diameter equal to or smaller than inside diameter Di of the suction port of casing  10 , has a sufficiently large area, and has a bell mouth shape smoothly introducing the sucked air. Thus, lowering of an aerodynamic performance of centrifugal fan  1  due to pressure loss can be prevented. 
     Sound waves of rotational noises caused when the sucked air is subjected to pressure rise by impeller  5 , vortex turbulent noises caused when the sucked air passes through the inside of casing  10 , and noises amplified by resonance in casing  10  are emitted downward from suction port  7 . Some of the sound waves of the noises emitted from suction port  7  of casing  10  are incident from inlet portion  23  configured between end  21  of orifice  19  and casing  10  into resonant space  20 . Suction port  7  of casing  10  and resonant space  20  communicate with each other via inlet portion  23 . Opening  18  of orifice  19  is concentric with suction port  7  of casing  10 . The diameter of opening  18  of orifice  19  is equal to or smaller than inside diameter Di of the suction port of casing  10 . Hence, some of the sound waves of the noises emitted from suction port  7  of casing  10  are easily incident from inlet portion  23  configured between end  21  of orifice  19  and casing  10  into resonant space  20 . 
     Of the sound waves of the noises incident from inlet portion  23  into resonant space  20 , some of the sound waves of the noises at a frequency specified according to a volume and shape of the resonant space  20  are resonated and suppressed in such a manner that air column resonance occurs in resonant space  20  and that inlet portion region  24  surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and volume portion  25  of resonant space  20  that locates at a rear side of cylindrical pipe shaped wall body  22  function as a Helmholtz resonator. 
     Since the frequencies of the noises suppressed by air column resonance depend on a length of a path to a structure (here, casing  10  and outer casing  3 ) where sound waves incident from inlet portion  23  are reflected, the frequencies are lowered as the length of the path is longer. 
     Frequency F at which noises are suppressed by the operation of the Helmholtz resonator is expressed by  FIG. 2  and the following equation (1) where c is the speed of sound, S is a throat portion area, L is a throat portion length, and V is a hollow portion volume. 
     
       
         
           
             
               
                 
                   F 
                   = 
                   
                     
                       c 
                       
                         2 
                         ⁢ 
                         π 
                       
                     
                     ⁢ 
                     
                       
                         S 
                         
                           L 
                           × 
                           V 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     In centrifugal fan  1  of the configuration of Embodiment 1, inlet portion region  24  surrounded by cylindrical pipe shaped wall body  22  and orifice  19  of  FIGS. 1A and 1B  is equivalent to throat portion  26  of  FIG. 2 . Likewise, volume portion  25  of resonant space  20  that locates at the rear side of cylindrical pipe shaped wall body  22  is equivalent to hollow portion  27 . 
     Since cylindrical pipe shaped wall body  22  is located in resonant space  20 , sound waves of noises are propagated so as to move around wall body  22 , and part of a path where sound waves of noises incident into resonant space  20  are reflected can be made longer. Hence, frequencies of noises to be resonated and suppressed can be lowered. In the structure (here, casing  10  and outer casing  3 ) in which sound waves incident from inlet portion  23  are reflected, the path to the rearmost portion in which the path of the sound waves from inlet portion  23  is longest can be made longer. Noises at a lower frequency can thus be resonated and suppressed. 
     Part of the path where sound waves of noises incident into resonant space  20  are reflected is made longer. This means that, for example, wall body  22  is provided as in this example so that some of sound waves move around wall body  22 . As compared with a case of the absence of wall body  22 , part of the path where sound waves are reflected is made longer. For example, some of sound waves may be moved around wall body  22 , as such an example, a structure as means for causing some of sound waves to move around wall body  22  can be provided in resonant space  20 . 
     Wall body  22  has a cylindrical pipe shape concentric with suction port  7  of casing  10  and has one end contacting the lower surface of casing  10 . The path where sound waves of noises emitted from suction port  7  are incident from inlet portion  23  into resonant space  20  and move around wall body  22  has only one U-shaped curve. Hence, sound waves of noises can be smoothly propagated to the rearmost portion of the path where sound waves of noises are reflected. Consequently, the path where some of sound waves of noises incident from inlet portion  23  of resonant space  20  are reflected in the rearmost portion of resonant space  20  is made longer. Noises at a lower frequency can thus be suppressed without increasing a volume of the resonant space. The centrifugal fan in which noises at a desired frequency can be reduced without increasing a size of the outer casing can be obtained. The rearmost portion refers to the rearmost portion of resonant space  20 , that is, the furthest position, seen from inlet portion  23  of resonant space  20 . 
     Between inside diameter Di of the suction port and clearance distance i of inlet portion  23 , a relation of D&gt;Di+2i is established. Inlet portion  23  and wall body  22  can be sufficiently separated. Consequently, reduction of an amount of sound waves of noises incident into resonant space  20  by reflection of some of sound waves of noises attempting to be incident into resonant space  20  near inlet portion  23  by wall body  22  can be prevented, and reduction of the amount of sound waves that are resonated and suppressed can be prevented. 
     As described above, since noises at a lower frequency can be suppressed without increasing the volume of resonant space  20 , noises at a desired frequency can be reduced without increasing the size of outer casing  3 . 
     Wall body  22  and casing  10  can be made into one component. Thus, the number of components can be reduced, and a production cost can be reduced. 
       FIGS. 3A ,  3 B, and  4  illustrate descriptions and graphs of experimental object  28  when noise suppression in resonant space  20  is experimentally observed. In the experiment, for improving efficiency, a size of experimental object  28  equivalent to centrifugal fan  1  is a quarter of centrifugal fan  1 , and for simplification, casing  10  has a cylindrical pipe shape. Distance h between casing  10  and orifice  19  is equivalent to 60 mm in centrifugal fan  1 . Clearance distance i of inlet portion  23  is equivalent to 20 mm. Inside diameter D of cylindrical pipe shaped wall body  22  is equivalent to 216 mm, and a height thereof is equivalent to 28 mm. 
     An amount in which sound waves emitted from circular pipe  30  equivalent to suction port  7  of casing  10  are transmitted to noise receiving side circular pipe  31  is measured to measure an amount of resonant noise-suppression in resonant space  20 , that is, transmission loss TL. 
     In the graph of  FIG. 4 , a horizontal axis indicates frequency F of a sound wave and a vertical axis indicates transmission loss TL as a noise reduction effect. The dotted line in the graph of  FIG. 4  indicates transmission loss  32  for each frequency in the absence of the wall body, and the solid line indicates transmission loss  33  for each frequency in the presence of the wall body. In the presence of wall body  22 , frequency F at which transmission loss TL is maximal is found to be reduced. 
     Cylindrical pipe shaped wall body  22  may have a shape that can increase part of the path of sound waves of noises incident into resonant space  20  and can bend the path in a U-shape on a cross section passing through rotational axis  4 . Accordingly, cylindrical pipe shaped wall body  22  may have a polygonal pipe shape or an oval pipe shape. Even if cylindrical pipe shaped wall body  22  is not concentric with suction port  7  of casing  10 , a similar effect can be obtained. 
     In this configuration, resonant space  20  formed by orifice  19  is formed so as to be surrounded by orifice  19 , casing  10 , and outer casing  3 , but resonant space  20  may be surrounded by other structures of products equipped with orifice  19  and centrifugal fan  1 , e.g., a filter, a heater, an electric component, and a case of an electric component. A similar effect can be obtained by a configuration in which air column resonance occurs in resonant space  20 , or in which inlet portion region  24  surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and volume portion  25  of resonant space  20  that locates at the rear side of cylindrical pipe shaped wall body  22  function as the Helmholtz resonator for resonance noise suppression. 
     Centrifugal fan  1  may be horizontally installed on a wall, not on a ceiling. In this case also, a similar effect can be obtained. 
     Embodiment 2 
     Embodiment 2 of the present invention will be described next. Description of similar parts to those in Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 2 will be described. 
     In Embodiment 2, as illustrated in  FIG. 5 , inside diameter D of the cylindrical pipe shaped wall body that is provided in resonant space  20  and is concentric with suction port  7  is 160 mm, and between inside diameter Di (150 mm) of suction port  7  and clearance distance i (20 mm) of inlet portion  23 , a relation of D&lt;Di+2i is established. 
     By this configuration, an area of inlet portion region  24  that is surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and is equivalent to throat portion area S of a Helmholtz resonator can be reduced. A volume of resonant space  20  that locates at a rear side of cylindrical pipe shaped wall body  22  and is equivalent to hollow portion volume V can be increased. Hence, by an operation of the Helmholtz resonator, noises at a lower frequency can be suppressed. 
     Embodiment 3 
     Embodiment 3 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given and only the parts that are unique to Embodiment 3 will be described. 
     In Embodiment 3, as illustrated in  FIG. 6 , end  21  of orifice  19  is arranged such that cylindrical pipe shaped wall body  22  that is provided in resonant space  20  and is concentric with suction port  7  has a thickness of 20 mm and faces end face  34  of the wall body with a clearance of 5 mm. A region sandwiched between end face  34  of the wall body and end  21  of orifice  19  configures inlet portion region  24 . 
     By this configuration, diametrical depth length Lr of inlet portion region  24  that is surrounded by end face  34  of the cylindrical pipe shaped wall body and end  21  of orifice  19  and is equivalent to throat portion length L of a Helmholtz resonator can be increased. Hence, by an operation of the Helmholtz resonator, noises at a lower frequency can be suppressed. 
     End  21  of orifice  19  and end face  34  of the wall body have only to be longer in a diametrical direction and only the end face may be thickened. A similar effect can also be obtained from the configuration. 
     Embodiment 4 
     Embodiment 4 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 4 will be described. 
     In Embodiment 4, as illustrated in  FIG. 7 , inside diameter D of the cylindrical pipe shaped wall body is not uniform in a circumferential direction such that one portion thereof is 216 mm and the other portion thereof is 160 mm. 
     By this configuration, inside diameter D of the cylindrical pipe shaped wall body is partially different. A portion in which a length of a path where sound waves of noises emitted from suction port  7  are reflected is different can thus be provided. Hence, sound waves of noises at a plurality of frequencies can be resonated and suppressed in resonant space  20 . 
     Embodiment 5 
     Embodiment 5 of the present invention will be described. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 5 will be described. 
     In Embodiment 5, as illustrated in  FIG. 8 , a height of cylindrical pipe shaped wall body  22  is not uniform in a circumferential direction such that one portion thereof is 28 mm and the other portion thereof is 40 mm. 
     By this configuration, the height of cylindrical pipe shaped wall body  22  is partially different. A portion in which a length of a path where sound waves of noises emitted from suction port  7  are reflected is different can be provided. A portion in which axial depth length La of inlet portion region  24  that is surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and is equivalent to throat portion length L of a Helmholtz resonator can be provided. Hence, sound waves of noises at a plurality of frequencies can be resonated and suppressed in resonant space  20 . 
     Embodiment 6 
     Embodiment 6 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 6 will be described. 
     In Embodiment 6, as illustrated in  FIG. 9 , clearance distance i of inlet portion  23  into resonant space  20  is not uniform in a circumferential direction such that one portion thereof is 20 mm and the other portion thereof is 14 mm. 
     By this configuration, axial depth length La of inlet portion region  24  that is surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and is equivalent to throat portion length L of a Helmholtz resonator is different in a circumferential direction. Hence, by an operation of the Helmholtz resonator, noises at a plurality of frequencies can be resonated and suppressed in resonant space  20 . 
     Embodiment 7 
     Embodiment 7 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 7 will be described. 
     In Embodiment 7, as illustrated in  FIG. 10 , screwed portion  35  is provided on the lower surface of casing  10 , and cylindrical pipe shaped wall body  22  having screwing portion  36  is detachably fixed by screw  37 . Cylindrical pipe shaped wall body  22  having different inside diameter D can be attached thereto. 
     By this configuration, even if frequencies of noises emitted from suction port  7  are changed according to an installed state of centrifugal fan  1 , the frequencies at which noises are suppressed can be adjusted by changing inside diameter D of cylindrical pipe shaped wall body  22 . 
     Embodiment 8 
     Embodiment 8 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 8 will be described. 
     In Embodiment 8, as illustrated in  FIG. 11 , cylindrical pipe shaped wall body  22  that is provided in resonant space  20  and is concentric with suction port  7  is a double pipe  40  having inside wall  38  and outside wall  39 . A height of wall body  22  can be adjusted by sliding outside wall  39 . 
     By this configuration, even if frequencies of noises emitted from suction port  7  are changed according to an operation state of centrifugal fan  1 , the frequencies at which noises are suppressed can be adjusted by changing the height of cylindrical pipe shaped wall body  22 . 
     Embodiment 9 
     Embodiment 9 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 9 will be described. 
     In Embodiment 9, as illustrated in  FIG. 12 , in cylindrical pipe shaped wall body  22  that is concentric with suction port  7  of casing  10  and has one end contacting casing  10 , two wall bodies  22  having wall body inside diameters D of 180 mm and 240 mm and cylindrical pipe shaped wall body  22  that is concentric with suction port  7  of casing  10  and has one end contacting orifice  19  and inside diameter D of 216 mm are arranged in resonant space  20 . 
     By this configuration, centrifugal fan  1  can have a labyrinth structure in which sound waves of noises are propagated so as to alternately move around wall bodies  22  in resonant space  20 , and a path where some of sound waves of noises are reflected can be made longer. Hence, noises at a lower frequency can be suppressed without increasing a volume of resonant space  20 . 
     Embodiment 10 
     Embodiment 10 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 10 will be described. 
     In Embodiment 10, as illustrated in  FIGS. 13A and 13B , partition wall  41  that divides resonant space  20  into two by different volumes is provided on a plane passing through rotational axis  4 , and cylindrical pipe shaped wall body  22  that is concentric with suction port  7  and has one end having a different height and contacting casing  10  is provided in each of resonant spaces  20 . 
     By this configuration, sound waves of noises emitted from suction port  7  are resonated and suppressed in resonant space  20  having a different volume, and part of a path where sound waves of noises are propagated in each of divided resonant spaces  20  can be made longer. Hence, noises at a plurality of lower frequencies can be suppressed without increasing a volume of resonant space  20 . 
     Embodiment 11 
     Embodiment 11 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 11 will be described. 
     In Embodiment 11, as illustrated in  FIG. 14 , clearance distances i of inlet portions  23  of resonant spaces  20  divided into two by partition wall  41  are 20 mm and 14 mm, and clearance distances i of inlet portions  23  are different. 
     By this configuration, a length of inlet portion region  24  that is surrounded by cylindrical pipe shaped wall body  22  and orifice  19  and is equivalent to throat portion length L of a Helmholtz resonator can be set in each of two resonant spaces  20 . Hence, by an operation of the Helmholtz resonator, noises at a plurality of lower frequencies can be suppressed without increasing a volume of resonant space  20 . 
     Embodiment 12 
     Embodiment 12 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 12 will be described. 
     In Embodiment 12, as illustrated in  FIG. 15 , outer peripheral space  42  that is surrounded by side wall  8  of casing  10  and outer casing  3  communicates with resonant space  20  surrounded by orifice  19 , casing  10 , and outer casing  3 . 
     By this configuration, outer peripheral space  42  can be used as resonant space  20 . Hence, a volume of resonant space  20  can be increased, and a path from inlet portion  23  to the rearmost portion can be made longer. Noises at a lower frequency can thus be suppressed. 
     Embodiment 13 
     Embodiment 13 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 13 will be described. 
     In Embodiment 13, as illustrated in  FIG. 16 , in centrifugal fan  1  in which part of motor  6  is protruded from casing  10  in outer casing  3 , rear space  44  between motor side outer wall  43  of casing  10  and outer casing  3  communicates with outer peripheral space  42  surrounded by side wall  8  of casing  10  and outer casing  3 , and outer peripheral space  42  communicates with resonant space  20  surrounded by orifice  19 , casing  10 , and outer casing  3 . 
     By this configuration, rear space  44  and outer peripheral space  42  can be used as resonant space  20 . Hence, a volume of resonant space  20  can be increased, a path from inlet portion  23  to the rearmost portion can be made longer, and noises at a lower frequency can be suppressed. 
     Embodiment 14 
     Embodiment 14 of the present invention will be described next. Description of similar parts to those of Embodiment 1 of the present invention is not given, and only the parts that are unique to Embodiment 14 will be described. 
     In Embodiment 14, as illustrated in  FIGS. 17A and 17B , centrifugal fan  1  used as a ceiling burying type ventilating fan includes, in outer casing  3  having an inner dimension of 265 mm square, a height of 195 mm, and opening  2  in a lower surface, motor  6  that couples multiblade impeller  5  having a diameter of 180 mm so as to rotate impeller  5  on rotational axis  4 , and casing  10  that surrounds a periphery of impeller  5  and has suction port  7  that is bell-mouthed and has suction port inside diameter Di of 150 mm in a lower surface and discharge port  9  on side wall  8 . Side wall  8  of casing  10  has a scroll shape in which an air path is gradually expanded toward discharge port  9 . Discharge port  9  of casing  10  communicates with discharge adapter  12  via discharge opening  11  provided in one side of outer casing  3 . Electric equipment portion  13  that houses an electric component such as a connector or a terminal for electrically connecting motor  6  with an external power supply is arranged in a portion between casing  10  and outer casing  3 . Flange portion  14  is provided on an outer periphery of the lower surface of outer casing  3 . Outer casing  3  is fixed to ceiling material  15  by a screw or the like through hole  16  provided in flange portion  14 . Duct  17  that is disposed on a ceiling and communicates with outdoors is joined to discharge port  9  via discharge adapter  12 . 
     Orifice  19  has bell-mouthed opening  18  that is concentric with suction port  7  of casing  10  and has an inside diameter of 148 mm equal to or smaller than that of suction port  7 . Orifice  19  is separated from suction port  7  of casing  10  by 60 mm and closes opening  2  of outer casing  3 . Orifice  19  forms space  70  surrounded by casing  10 , orifice  19 , and outer casing  3 . End  21  of orifice  19  is separated from the lower surface of casing  10  by 20 mm to form clearance portion  72 . Clearance portion  72  has a vinyl sheet that is filmy material  73  as a member closing clearance portion  72  so as to close clearance portion  72 . There is formed a vibration system in which the vinyl sheet that is filmy material  73  as a noise absorbing structural material that is a member closing clearance portion  72  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring. A filmy noise absorbing structure as a resonance type noise absorbing structure is formed. 
     Clearance portion  72  is provided between suction port  7  of casing  10  and space  70  such that suction port  7  of casing  10  communicates with space  70 . The vinyl sheet as filmy material  73  is provided in communicating clearance portion  72  so as to close clearance portion  72 . Suction port  7  and space  70  are blocked. Space  70  is rear air layer  74  of filmy material  73 . 
     In the above configuration, when impeller  5  is rotated by motor  6 , sucked air passes from opening  2  through opening  18  of orifice  19  of the main body. The sucked air smoothly enters impeller  5  from bell-mouthed suction port  7  of casing  10 . The sucked air is subjected to pressure rise by impeller  5 . The sucked air passes through an inside of casing  10  in a scroll shape. A dynamic pressure is thereby efficiently converted to a static pressure. The sucked air is discharged to duct  17  by discharge adapter  12  and is then discharged to outdoors. Opening  18  of orifice  19  has a diameter equal to or smaller than inside diameter Di of the suction port of casing  10 , has a sufficiently large area, and has a bell mouth shape smoothly introducing the sucked air. Lowering of an aerodynamic performance of centrifugal fan  1  due to pressure loss can thus be prevented. 
     Sound waves of rotational noises caused when the sucked air is subjected to pressure rise by impeller  5 , vortex turbulent noises caused when the sucked air passes through the inside of casing  10 , and noises amplified by resonance in casing  10  are emitted downward from suction port  7 ; however, some of the sound waves of the noises emitted from suction port  7  of casing  10  are incident into the vinyl sheet as filmy material  73  provided in clearance portion  72  configured between end  21  of orifice  19  and casing  10 . Opening  18  of orifice  19  is concentric with suction port  7  of casing  10 . The diameter of opening  18  of orifice  19  is equal to or smaller than inside diameter Di of the suction port of casing  10 . Some of the sound waves of the noises emitted from suction port  7  of casing  10  are easily incident into filmy material  73  provided in clearance portion  72  configured between end  21  of orifice  19  and casing  10 . A filmy noise absorbing structure as a resonance type noise absorbing structure is formed by space  70  surrounded by orifice  19 , casing  10 , and outer casing  3  and the vinyl sheet as filmy material  73  closing clearance portion  72  between end  21  of orifice  19  and the lower surface of casing  10 . When frequencies of the sound waves of the noises incident into the vinyl sheet as filmy material  73  coincides with a resonance frequency of the vibration system of the filmy noise absorbing structure, the vinyl sheet as filmy material  73  is vibrated to absorb the sound waves by internal friction, thereby suppressing some of the noises. Frequencies at which noises can be suppressed by an operation of the filmy noise absorbing structure can be changed according to a thickness, surface density, and mass of filmy material  73 , a tension to provide filmy material  73 , a thickness of rear air layer  74 , and the like. A range of frequencies of noises in which noises can be suppressed can thus be increased. 
     Cellophane, an aluminum film, and a polyethylene film, or the like, in addition to the vinyl sheet, may be used as filmy material  73 , which can provide a similar effect. 
     Space  70  to be rear air layer  74  has only to be surrounded by the structure so as to form the resonance type noise absorbing structure. Even if there is a clearance communicating with outside to some extent, a similar effect can be obtained, although a degree of the effect is inferior. 
     In this configuration, space  70  formed by orifice  19  is formed so as to be surrounded by orifice  19 , casing  10 , and outer casing  3 . Space  70  may be surrounded by other structures of products equipped with orifice  19  and centrifugal fan  1 , e.g., a filter, a heater, an electric component, and a case of an electric component. As long as there is formed a vibration system in which space  70  is rear air layer  74  of filmy material  73  so as to be a spring and a filmy noise absorbing structure as a resonance type noise absorbing structure is thus formed, a similar effect can be obtained. 
     Centrifugal fan  1  may be horizontally installed on a wall, not on a ceiling. In this case also, a similar effect can be obtained. 
     Embodiment 15 
     Embodiment 15 of the present invention will be described. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 15 will be described. 
     In Embodiment 15, as illustrated in  FIG. 18 , a hard fiberboard that has a hole area rate of 10% and a large number of small holes  75  having a diameter of 5 mm is provided so as to close clearance portion  72  as holed plate  76 . 
     By this configuration, there is formed a vibration system in which air in small holes  75  of holed plate  76  is a mass and space  70  as rear air layer  74  surrounded by orifice  19 , casing  10 , and outer casing  3  is a spring. A holed plate noise absorbing structure that absorbs noises according to the same principle as a Helmholtz resonator and is a resonance type noise absorbing structure is formed. When some of sound waves of noises emitted from suction port  7  of casing  10  are incident into holed plate  76 , an energy of the incident sound waves of the noises at a frequency that coincides with a resonance frequency of the vibration system of the holed plate noise absorbing structure is absorbed by friction loss due to severe vibration of air in small holes  75 . Hence, some of the noises can be suppressed. Frequencies at which noises can be suppressed by an operation of the holed plate noise absorbing structure can be changed according to a plate thickness and a hole area rate of holed plate  76 , a diameter and a pitch of small holes  75 , and a thickness of rear air layer  74 . A range of frequencies of noises in which noises can be suppressed can thus be increased. 
     A plasterboard or an aluminum plate, in addition to the hard fiberboard, may be used as holed plate  76 , which can provide a similar effect. 
     A plate having a large number of slit-like slots, not holed plate  76  having a large number of circular small holes  75 , can provide a similar effect. 
     Embodiment 16 
     Embodiment 16 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 16 will be described. 
     In Embodiment 16, as illustrated in  FIG. 19 , a soft urethane foam is provided as porous material  77  so as to close clearance portion  72 . 
     By this configuration, some of sound waves of noises emitted from suction port  7  of casing  10  incident into porous material  77 , at a frequency depending on noise absorption properties of porous material  77  itself can be absorbed. The sound waves of the noises that pass through porous material  77  and are reflected by orifice  19 , casing  10 , and outer casing  3  are incident into porous material  77  again so as to be absorbed. There is formed a vibration system in which porous material  77  is a mass and space  70  as rear air layer  74  surrounded by orifice  19 , casing  10 , and outer casing  3  is a spring. A porous noise absorbing structure as a resonance type noise absorbing structure is thus formed. Energy of the incident sound waves of the noises at a frequency that coincides with a resonance frequency of the vibration system of the porous noise absorbing structure is absorbed by friction loss due to vibration of porous material  77  itself and air in porous material  77 . Hence, some of the noises can be suppressed. Frequencies at which noises can be suppressed by an operation of the porous noise absorbing structure can be changed according to, e.g., a material, a thickness, and noise absorption properties of porous material  77 , a thickness of rear air layer  74 , and by overlapping of porous materials  77  having different noise absorption properties. A range of frequencies of noises in which noises can be suppressed can thus be increased. 
     Typically, frequencies of noises at which noises can be absorbed by porous material  77  are relatively high, and frequencies of noises at which noises can be suppressed by the porous noise absorbing structure are relatively low. A range of frequencies of noises in which noises can be suppressed at the same time can thus be increased. 
     Glass wool or rock wool, in addition to the soft urethane foam, may be used as porous material  77 , which can provide a similar effect. 
     Porous material  77  may be covered with a material, such as a cloth, that transmits sound waves in order to prevent scattering of porous material  77 . In this case also, a similar effect can be obtained. 
     A material having noise absorption properties, in addition to porous material  77 , e.g., a porous molded plate material such as a rock wool plate or a flexible material such as sponge may also be applied. In this case also, a similar effect can be obtained. 
     Embodiment 17 
     Embodiment 17 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 17 will be described. 
     In Embodiment 17, as illustrated in  FIGS. 20A and 20B , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring. In centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, cylindrical pipe shaped wall body  78  concentric with suction port  7  of casing  10  is integrally provided in the lower surface of casing  10 . Cylindrical pipe shaped wall body  78  has a thickness of 2 mm and a height of 28 mm, and inside diameter D of the wall body is 216 mm. 
     There has typically been known that frequencies at which noises can be suppressed by the filmy noise absorbing structure are inversely proportional to a square root of a thickness of rear air layer  74 . By this configuration, a length of a path where sound waves of noises are propagated so as to move around wall body  78  in space  70  can be regarded as the thickness of rear air layer  74 . Hence, the thickness of rear air layer  74  can be increased, and the frequencies of noises at which noises can be suppressed can be reduced. Noises at a lower frequency can thus be suppressed without changing a size of outer casing  3 . 
     Cylindrical pipe shaped wall body  78  has a cylindrical pipe shape that is concentric with suction port  7  and has one end integrated with casing  10 . Accordingly, a path where sound waves of noises move around wall body  78  in space  70  has only one U-shaped curve. Hence, sound waves of noises can be smoothly propagated, and the thickness of rear air layer  74  can be increased more stably. Noises at a lower frequency can be suppressed without changing the size of outer casing  3  by wall body  78  of a simple configuration. 
     In this configuration, clearance portion  72  is closed by filmy material  73 . When clearance portion  72  is closed by holed plate  76 , frequencies of noises suppressed by the holed plate noise absorbing structure are inversely proportional to the square root of the thickness of rear air layer  74 . Frequencies that can be suppressed can thus be reduced. When clearance portion  72  is closed by porous material  77 , the frequencies of noises suppressed by the porous noise absorbing structure is inversely proportional to the thickness of rear air layer  74 . The frequencies at which noises can be suppressed can thus be reduced likewise. 
     In addition, cylindrical pipe shaped wall body  78  may have a shape that can increase the path of sound waves in space  70  and bend the path in a U-shape on a cross section passing through rotational axis  4 . Cylindrical pipe shaped wall body  78  may have a polygonal pipe shape or an oval pipe shape. Even if cylindrical pipe shaped wall body  78  is not concentric with suction port  7  of casing  10 , a similar effect can be obtained. 
     Embodiment 18 
     Embodiment 18 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 18 will be described. 
     In Embodiment 18, as illustrated in  FIG. 21 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring. In centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, cylindrical pipe shaped wall body  78  concentric with suction port  7  of casing  10  is integrally provided in the lower surface of casing  10 . Cylindrical pipe shaped wall body  78  has a thickness of 2 mm and a height of 28 mm. Inside diameter D of the wall body is not uniform in a circumferential direction such that one portion thereof is 216 mm and the other portion thereof is 160 mm. 
     By this configuration, inside diameter D of the cylindrical pipe shaped wall body is partially different. A portion where a thickness of rear air layer  74  is different can thus be provided. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     Embodiment 19 
     Embodiment 19 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 19 will be described. 
     In Embodiment 19, as illustrated in  FIG. 22 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, cylindrical pipe shaped wall body  78  concentric with suction port  7  of casing  10  is integrally provided in the lower surface of casing  10 . Cylindrical pipe shaped wall body  78  has a thickness of 2 mm and inside diameter D of the wall body of 216 mm. A height is not uniform in a circumferential direction such that one portion thereof is 28 mm and the other portion thereof is 40 mm. 
     By this configuration, the height of cylindrical pipe shaped wall body  78  is partially different. A portion where a thickness of rear air layer  74  is different can thus be provided. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     Embodiment 20 
     Embodiment 20 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 20 will be described. 
     In Embodiment 20, as illustrated in  FIG. 23 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, screwed portion  79  is provided in the lower surface of casing  10 , and cylindrical pipe shaped wall body  78  having screwing portion  80  is detachably fixed to the lower surface of casing  10  by screw  81 . Cylindrical pipe shaped wall body  78  having different inside diameter D of the wall body can thus be attached. 
     By this configuration, even if frequencies of noises emitted from suction port  7  are changed according to an installed state of centrifugal fan  1  or the like, a thickness of rear air layer  74  can be adjusted by changing inside diameter D of the cylindrical pipe shaped wall body. Frequencies of noises at which noises can be suppressed can thus be adjusted. 
     Embodiment 21 
     Embodiment 21 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 21 will be described. 
     In Embodiment 21, as illustrated in  FIG. 24 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, cylindrical pipe shaped wall body  78  that is provided in space  70  and is concentric with suction port  7  is double pipe  84  having inside wall  82  and outside wall  83 . A height of wall body  78  can be adjusted by sliding outside wall  83 . 
     By this configuration, a thickness of rear air layer  74  can be adjusted by adjusting the height of cylindrical pipe shaped wall body  78 . Frequencies of noises at which noises can be suppressed can thus be adjusted. 
     Embodiment 22 
     Embodiment 22 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 22 will be described. 
     In Embodiment 22, as illustrated in  FIG. 25 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, in cylindrical pipe shaped wall body  78  that is concentric with suction port  7  of casing  10  and has one end contacting casing  10 , there are arranged two wall bodies  78  having wall body inside diameters D of 180 mm and 240 mm and cylindrical pipe shaped wall body  78  that is concentric with suction port  7  of casing  10  and has one end contacting orifice  19  and wall body inside diameter D of 216 mm. 
     By this configuration, centrifugal fan  1  can have a labyrinth structure in which sound waves of noises are propagated so as to alternately move around wall bodies  78  in space  70 . Thus, a thickness of rear air layer  74  can be increased, and noises at a lower frequency can be suppressed without changing a size of outer casing  3 . 
     Embodiment 23 
     Embodiment 23 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 23 will be described. 
     In Embodiment 23, as illustrated in  FIGS. 26A and 26B , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, partition wall  85  that divides space  70  into two by different volumes is provided on a plane passing through rotational axis  4 . 
     By this configuration, a plurality of resonance type noise absorbing structures in which a shape and volume of space  70  as rear air layer  74  are different can be formed. Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     Embodiment 24 
     Embodiment 24 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 24 will be described. 
     In Embodiment 24, as illustrated in  FIG. 27 , clearance portion  72  of one of spaces  70  divided into two by partition wall  85  is closed by a vinyl sheet as filmy material  73 . Clearance portion  72  of the other space  70  is closed by a hard fiberboard that is holed plate  76  and has a hole area rate of 10% with a large number of small holes  75  having a diameter of 5 mm. 
     By this configuration, two kinds of resonance type noise absorbing structures can be formed. Thus, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     Clearance portion  72  of one of divided spaces  70  may be unclosed and resonant noise-suppression may be performed by air column resonance in space  70 . 
     Embodiment 25 
     Embodiment 25 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 25 will be described. 
     In Embodiment 25, as illustrated in  FIG. 28 , cylindrical pipe shaped wall body  78  that is concentric with suction port  7  and has one end having a different height contacting casing  10  is provided in each of spaces  70  divided into two by partition wall  85  and a thickness of rear air layer  74  is increased in each of spaces  70 . 
     By this configuration, wall body  78  having a different shape is provided in each of spaces  70  divided into two by partition wall  85  and the resonance type noise absorbing structure in which a thickness of rear air layer  74  is different can be formed in each of divided spaces  70 . Hence, noises at a plurality of frequencies can be suppressed, and a range of frequencies of noises in which noises can be suppressed can be increased. 
     In cylindrical pipe shaped wall body  78  that is provided in each of spaces  70  divided into two by partition wall  85 , has one end contacting casing  10 , and is concentric with suction port  7 , an inside diameter, a thickness, and a combination of these as well as a height, may be different. In this case also, a similar effect can be obtained. 
     Embodiment 26 
     Embodiment 26 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 26 will be described. 
     In Embodiment 26, as illustrated in  FIG. 29 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, outer peripheral space  86  surrounded by side wall  8  of casing  10  and outer casing  3  communicates with space  70  surrounded by orifice  19 , casing  10 , and outer casing  3 . 
     By this configuration, outer peripheral space  86  can be used as rear air layer  74  of the resonance type noise absorbing structure. Hence, a thickness of rear air layer  74  can be increased, and noises at a lower frequency can be suppressed without changing a size of outer casing  3 . 
     Embodiment 27 
     Embodiment 27 of the present invention will be described next. Description of similar parts to those of Embodiment 14 of the present invention is not given, and only the parts that are unique to Embodiment 27 will be described. 
     In Embodiment 27, as illustrated in  FIG. 30 , a vinyl sheet as filmy material  73  so as to close clearance portion  72  is provided, and there is formed a vibration system in which the vinyl sheet as filmy material  73  is a mass and space  70  is rear air layer  74  of filmy material  73  so as to be a spring; therefore, in centrifugal fan  1  formed with a filmy noise absorbing structure as a resonance type noise absorbing structure, part of motor  6  is protruded from casing  10  in outer casing  3 . Rear space  88  between motor side outer wall  87  of casing  10  and outer casing  3  communicates with outer peripheral space  86  surrounded by side wall  8  of casing  10  and outer casing  3 . Outer peripheral space  86  communicates with space  70  surrounded by orifice  19 , casing  10 , and outer casing  3 . 
     By this configuration, rear space  88  can be used as rear air layer  74  of the resonance type noise absorbing structure together with outer peripheral space  86 . Hence, a thickness of rear air layer  74  can be increased, and noises at a lower frequency can be suppressed without changing a size of the outer casing. 
     INDUSTRIAL APPLICABILITY 
     The present invention provides a centrifugal fan that can increase a range of frequencies of noises in which noises can be suppressed, can suppress noises at a lower frequency without changing a size of the outer casing, and can adjust frequencies of noises at which noises can be suppressed. The industrial applicability is therefore extremely high.