Patent Publication Number: US-11653802-B2

Title: Cleaner and method for setting cleaner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-193471, filed on Nov. 20, 2020, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a cleaner and a method for setting the cleaner. 
     2. Description of the Background 
     A known cleaner includes a suction unit as described in Japanese Patent No. 6686131. 
     BRIEF SUMMARY 
     The cleaner produces noise that may cause discomfort to the user and other persons nearby. 
     One or more aspects of the present disclosure are directed to a cleaner that reduces noise. 
     A first aspect of the present disclosure provides a cleaner, including: 
     a motor; 
     a fan including a number Z of blades, the fan being rotatable at a rotational speed N about a rotation axis by the motor, the rotational speed N indicating revolutions per minute; 
     a cover having an inlet located frontward from the fan; and 
     a number V of ribs located in the inlet, the ribs extending in a radial direction from the rotation axis and arranged in a circumferential direction about the rotation axis, 
     wherein the fan is rotatable to produce noise having a frequency f NZ  of 20,000 Hz or higher, and f NZ =(m−k×V)×N/60 and m=n×z+k×V, where n is an order, m is an integer, and k is an integer. 
     A second aspect of the present disclosure provides a method for setting a cleaner, the method including: 
     determining a number Z of blades included in a fan rotatable about a rotation axis by a motor); 
     determining a number V of ribs located in an inlet located frontward from the fan, the ribs extending in a radial direction from the rotation axis; and 
     determining the number Z of blades, the number V of ribs, and a rotational speed N of the fan indicating revolutions per minute to cause noise produced by rotation of the fan to have a frequency f NZ  of 20,000 Hz or higher, 
     wherein f NZ =(m−k×V)×N/60 and m=n×z+k×V, where n is an order, m is an integer, and k is an integer. 
     The cleaner according to the above aspects of the present disclosure reduces noise. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of a cleaner according to an embodiment. 
         FIG.  2    is a side view of the cleaner according to the embodiment. 
         FIG.  3    is a cross-sectional view of the cleaner according to the embodiment. 
         FIG.  4    is a view of a connection pipe in the embodiment. 
         FIG.  5    is a perspective view of a left housing, a sound absorber, and a controller in the embodiment. 
         FIG.  6    is a perspective view of a right housing, a sound absorber, and a controller in the embodiment. 
         FIG.  7    is a side view of the left housing, the sound absorber, and the controller in the embodiment. 
         FIG.  8    is a perspective view of the left housing and the sound absorber in the embodiment that are shown separately. 
         FIG.  9    is a side view of the left housing and the sound absorber in the embodiment that are shown separately. 
         FIG.  10    is a front perspective view of a suction unit in the embodiment. 
         FIG.  11    is a rear perspective view of the suction unit in the embodiment. 
         FIG.  12    is a front view of the suction unit in the embodiment. 
         FIG.  13    is an exploded perspective view of the suction unit in the embodiment as viewed from the front. 
         FIG.  14    is a perspective view of a motor and a fan in the embodiment. 
         FIG.  15    is a schematic diagram of the suction unit in the embodiment describing airflow. 
         FIG.  16    is a schematic diagram of a rib and a blade in the embodiment. 
         FIG.  17    is a schematic diagram of a rib and blades in an embodiment. 
         FIG.  18    is a schematic diagram of ribs and blades in an embodiment. 
         FIG.  19    is an example table showing the characteristic Mach number in the embodiment. 
         FIG.  20    is a table describing a method for calculating the frequency of noise in the embodiment. 
         FIG.  21    is a block diagram of a computer system in the embodiment. 
         FIG.  22    is a flowchart showing a method for setting the suction unit in the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Although one or more embodiments of the present disclosure will now be described with reference to the drawings, the present disclosure is not limited to the present embodiments. The components in the embodiments described below may be combined as appropriate. One or more components may be eliminated. 
     In the embodiments, the positional relationships between the components will be described using the directional terms such as front and rear (or forward and backward), right and left (or lateral), and up and down (or vertical). The terms indicate relative positions or directions with respect to the center of a cleaner  1 . 
     The cleaner  1  includes a motor  14 . The motor  14  includes a rotor rotatable about a rotation axis AX. A radial direction from the rotation axis AX is referred to as a radial direction or radially for convenience in the embodiments. A direction about the rotation axis AX is referred to as a circumferential direction (or circumferentially) or a rotation direction for convenience. A direction parallel to the rotation axis AX is referred to as an axial direction or axially for convenience. 
     A position nearer the rotation axis AX in the radial direction, or a radial direction toward the rotation axis AX, is referred to as radially inward for convenience. A position farther from the rotation axis AX in the radial direction, or a radial direction away from the rotation axis AX, is referred to as radially outward for convenience. A position in one circumferential direction (first direction), or one circumferential direction (first direction), is referred to as a first circumferential direction for convenience. A position in the other circumferential direction (second direction), or the other circumferential direction (second direction), is referred to as a second circumferential direction for convenience. A position in one axial direction (first direction), or one axial direction (first direction), is referred to as a first axial direction for convenience. A position in the other axial direction (second direction), or the other axial direction (second direction), is referred to as a second axial direction for convenience. 
     In the embodiments, the rotation axis AX extends in a front-rear direction. The first axial direction is the front direction. The second axial direction is the rear direction. 
     Overview of Cleaner 
       FIG.  1    is a perspective view of the cleaner  1  according to an embodiment.  FIG.  2    is a side view of the cleaner  1  according to the embodiment.  FIG.  3    is a cross-sectional view of the cleaner  1  according to the embodiment. 
     The cleaner  1  includes a housing  2 , a suction unit  3 , a filter holder  4 , sound absorbers  5 , a battery mount  6 , a controller  7 , and an interface unit  8 . 
     The housing  2  includes a handle  9  gripped by a user of the cleaner  1 . The cleaner  1  is a handheld cleaner including the handle  9  that is gripped by the user for cleaning. 
     The housing  2  accommodates the suction unit  3 , the filter holder  4 , the sound absorbers  5 , and the controller  7 . The housing  2  has a suction port  10  on its front end, and exhaust ports  11  on both its rear right and its rear left. The suction port  10  connects the outside and the inside of the housing  2 . The exhaust ports  11  connect the inside and the outside of the housing  2 . 
     The suction unit  3  generates a suction force through the suction port  10 . The suction unit  3  includes a motor assembly  12  and a cover  13 . The motor assembly  12  includes the motor  14 , a fan  15 , a motor case  16 , and a control board  17 . The motor  14  generates a rotational force for rotating the fan  15 . The fan  15  rotates with the rotational force generated by the motor  14 . The motor case  16  accommodates the motor  14  and the fan  15 . The control board  17  outputs control signals for controlling the motor  14 . The control board  17  includes, for example, field-effect transistors (FETs). The cover  13  surrounds and accommodates the motor assembly  12 . 
     The motor  14  runs and rotates the fan  15 , which then generates a suction force through the suction port  10 . Air outside the housing  2  flows through the suction port  10  into the housing  2 . Air inside the housing  2  flows through the exhaust ports  11  out of the housing  2 . 
     The filter holder  4  includes multiple linear members to hold a filter  18 . The filter  18  surrounds the filter holder  4  to collect dust from air flowing into the housing  2  through the suction port  10 . The filter holder  4  and the filter  18  are between the suction port  10  and the suction unit  3  inside the housing  2 . 
     The sound absorbers  5  are located inside the housing  2  and face the exhaust ports  11 . The sound absorbers  5  are formed from a porous material with open-cell foam. The sound absorbers  5  absorb sound traveling through air to reduce noise. The cleaner  1  produces, for example, noise from airflow and noise (NZ) resulting from rotation of the fan  15 . 
     The battery mount  6  is located below the housing  2  at the rear. The battery mount  6  receives a battery pack  19  in a detachable manner. 
     The battery pack  19  serves as a power supply for the cleaner  1 . The battery pack  19  is attached to the battery mount  6  and supplies power to the cleaner  1 . The motor  14  runs on power supplied from the battery pack  19 . The controller  7  operates on power supplied from the battery pack  19 . The battery pack  19  is a general-purpose battery for powering various electrical instruments. The battery pack  19  is usable for powering power tools or other electrical instruments. The battery pack  19  is usable for powering cleaners other than the cleaner  1  according to the embodiment. The battery pack  19  is a rechargeable battery such as a lithium-ion battery. The battery mount  6  has a structure similar to the structure of a battery mount in a power tool. 
     The user of the cleaner  1  attaches and detaches the battery pack  19  to and from the battery mount  6 . The battery mount  6  includes a guide and a mount terminal. The battery pack  19  includes a battery terminal. The guide on the battery mount  6  guides the battery pack  19 . The mount terminal on the battery mount  6  is connectable to the battery terminal on the battery pack  19 . The user places the battery pack  19  from the rear and moves the battery pack  19  along the guide to attach the battery pack  19  to the battery mount  6 . This electrically connects the battery terminal on the battery pack  19  and the mount terminal on the battery mount  6 . The battery pack  19  includes a release button. The user of the cleaner  1  operates the release button on the battery pack  19  to move the battery pack  19  backward to remove the battery pack  19  from the battery mount  6 . 
     The controller  7  controls electronic devices in the cleaner  1 . The controller  7  controls the motor  14  with the control board  17 . The controller  7  controls the drive current to be supplied from the battery pack  19  to the motor  14 . The controller  7  and the control board  17  are connected to each other with a cable (not shown). The cable is used to, for example, supply power from the battery pack  19  to the motor  14  as a power line and provide control signals to the control board  17  as a signal line. The controller  7  includes a board incorporating multiple electronic components. Examples of the electronic components on the board include a processor such as a central processing unit (CPU), a nonvolatile memory such as a read-only memory (ROM) or a storage, a volatile memory such as a random-access memory (RAM), and a resistor. 
     The interface unit  8  is located on the handle  9 . The interface unit  8  includes a drive button  81 , a mode switch button  82 , and an indicator  83 . The user gripping the handle  9  can push the drive button  81  and the mode switch button  82 . 
     The motor  14  that is stopped starts running in response to the drive button  81  being pushed. This causes the fan  15  to rotate to generate a suction force through the suction port  10 . The suction force causes air outside the housing  2  to be sucked through the suction port  10  together with dust. The air sucked through the suction port  10  flows into the housing  2 . 
     After flowing into the housing  2 , the air flows through the filter  18 , which collects dust in the air. The air flows through the filter  18  and the suction unit  3  and is then discharged out of the housing  2  through the exhaust ports  11 . 
     The rotational speed of the motor  14  is adjusted in four steps in response to the mode switch button  82  being pushed while the motor  14  is running. In response to a push on the mode switch button  82 , the running motor  14  switches from a first rotational speed to a second rotational speed. In response to another push on the mode switch button  82 , the motor  14  switches from the second rotational speed to a third rotational speed. In response to still another push on the mode switch button  82 , the motor  14  switches from the third rotational speed to a fourth rotational speed. In response to still another push on the mode switch button  82 , the motor  14  switches back to the first rotational speed. As the motor  14  changes its rotational speed, the suction force through the suction port  10  changes accordingly. The running motor  14  stops in response to the drive button  81  being pushed. 
     The indicator  83  includes four light emitters. The light emitters are, for example, light-emitting diodes (LEDs). With the motor  14  running at the first rotational speed, one of the light emitters is on. With the motor  14  running at the second rotational speed, two of the light emitters are on. With the motor  14  running at the third rotational speed, three of the light emitters are on. With the motor  14  running at the fourth rotational speed, the four light emitters are on. With the motor  14  stopped, the four light emitters are off. 
     Housing 
     The housing  2  includes a front housing  21  and a rear housing  22 . The front housing  21  has an opening  211  at the rear to receive the front of the rear housing  22 . This allows the front housing  21  and the rear housing  22  to be connected together in a detachable manner. 
     The front housing  21  includes a connection pipe  212  at the front. The connection pipe  212  has the suction port  10  at the front end. The front housing  21  accommodates the filter holder  4  and the filter  18  in the internal space. 
     The rear housing  22  includes the handle  9 . The battery mount  6  is located below the rear housing  22 . The rear housing  22  includes a left housing  22 L and a right housing  22 R. The left housing  22 L is located on the left of the right housing  22 R. The left housing  22 L and the right housing  22 R are fastened together with multiple screws  22 S. The left housing  22 L and the right housing  22 R each have the exhaust ports  11 . The rear housing  22  accommodates the suction unit  3 , the sound absorbers  5 , and the controller  7  in the internal space. 
       FIG.  4    is a view of the connection pipe  212  in the embodiment.  FIG.  4    shows the housing  2  as viewed in direction A in  FIG.  1   . 
     As shown in  FIGS.  3  and  4   , the connection pipe  212  has an inner surface  213  defining a flow path. The inner surface  213  surrounds the rotation axis AX. The inner surface  213  extends substantially parallel to the rotation axis AX. 
     The connection pipe  212  has a recess  214 . The recess  214  is recessed rearward from the upper front end of the connection pipe  212 . The recess  214  has an inner surface including a first face  214 A, a second face  214 B, a third face  214 C, and a fourth face  214 D. 
     The first face  214 A, the second face  214 B, and the third face  214 C extend substantially parallel to the rotation axis AX. The fourth face  214 D is substantially orthogonal to an axis parallel to the rotation axis AX. The first face  214 A faces downward. The second face  214 B faces leftward. The third face  214 C faces rightward. The fourth face  214 D faces frontward. The second face  214 B has an upper end connected to the right end of the first face  214 A. The third face  214 C has an upper end connected to the left end of the first face  214 A. The fourth face  214 D has an upper end connected to the rear end of the first face  214 A. The second face  214 B, the third face  214 C, and the fourth face  214 D each have a lower end connected to the inner surface  213 . The lower end of the second face  214 B and the inner surface  213  define a corner. The lower end of the third face  214 C and the inner surface  213  define a corner. The lower end of the fourth face  214 D and the inner surface  213  define a corner. 
     The suction port  10  receives a basal end of a suction pipe (not shown). The suction pipe is detachable from the connection pipe  212 . The connection pipe  212  includes a lock  215 . The lock  215  allows the suction pipe to be fastened to the connection pipe  212 . 
     The lock  215  is at least partly located in an opening  216  in the first face  214 A. The lock  215  includes a hook  217 . The hook  217  faces the flow path in the connection pipe  212 . The hook  217  protrudes toward the flow path in the connection pipe  212 . The lock  215  is pivotably supported on the connection pipe  212  to allow the hook  217  to be switchable between the positions where the hook  217  protrudes and does not protrude from the first face  214 A. A spring (not shown) is located between the connection pipe  212  and the lock  215 . The spring applies an elastic force to the lock  215  to cause the hook  217  to protrude from the first face  214 A. The suction pipe has a recess. The hook  217  is hooked in the recess on the suction pipe to fasten the suction pipe to the connection pipe  212 . The lock  215  is unlocked to release the suction pipe from the connection pipe  212 . 
     The hook  217  is located frontward from the fourth face  214 D. The hook  217  protruding from the first face  214 A faces the fourth face  214 D. 
     The suction port  10  being blocked may cause surging depending on the structure of the fan  15 . Surging refers to a phenomenon in which the suction port  10  is at least partly blocked and decreases airflow through the suction port  10 , producing abnormal noise. As described above, the lock  215  is at least partly located in the opening  216  in the connection pipe  212 . The connection pipe  212  and the rear of the lock  215  define a first gap. The connection pipe  212  and the rear of the hook  217  also define a second gap. When the suction port  10  is blocked, air flows between the connection pipe  212  and the lock  215  through the first gap and into the flow path in the connection pipe  212  through the second gap. This structure reduces surging. The suction pipe is received in the connection pipe  212  and fastened with the hook  217  to block the second gap. This avoids leakage of air through the second gap to generate an adequate suction force. 
     Sound Absorber 
       FIG.  5    is a perspective view of the left housing  22 L, the sound absorber  5 , and the controller  7  in the embodiment.  FIG.  6    is a perspective view of the right housing  22 R, the sound absorber  5 , and the controller  7  in the embodiment.  FIG.  7    is a side view of the left housing  22 L, the sound absorber  5 , and the controller  7  in the embodiment.  FIG.  8    is a perspective view of the left housing  22 L and the sound absorber  5  in the embodiment that are shown separately.  FIG.  9    is a side view of the left housing  22 L and the sound absorber  5  in the embodiment that are shown separately. 
     Two sound absorbers  5  are located inside the rear housing  22 . One sound absorber  5  inside the rear housing  22  faces the exhaust ports  11  on the left housing  22 L. The other sound absorber  5  inside the rear housing  22  faces the exhaust ports  11  on the right housing  22 R. 
     Each sound absorber  5  has a first surface  51 , a second surface  52 , and a peripheral surface  53 . The second surface  52  faces in the direction opposite to the first surface  51 . The peripheral surface  53  connects the peripheries of the first surface  51  and the second surface  52  to each other. Each sound absorber  5  is a block. 
     The sound absorber  5  is a porous material with open-cell foam and including many micropores. Open-cell foam includes interconnecting multiple pores. The porous material with open-cell foam may be at least one selected from flexible urethane sponge, glass wool, mineral wool, and felt. 
     Each sound absorber  5  has air passages  54  and a support slit  55 . The air passages  54  extend through the first surface  51  and the second surface  52 . The support slit  55  extends through the first surface  51  and the second surface  52 . Each sound absorber  5  has multiple air passages  54 . The air passages  54  extend substantially parallel to each other. Each sound absorber  5  has a single support slit  55 . 
     Each air passage  54  has substantially perfectly circular openings at both ends (first and second openings). Each air passage  54  has an inner diameter larger than the size of each pore. 
     The support slit  55  has substantially elliptical openings at both ends. The support slit  55  has the openings each with a short-side dimension smaller than the dimension (diameter) of each air passage  54 . 
     The exhaust ports  11  are elongated slits. The exhaust ports  11  are located at regular intervals in the short-side direction of the exhaust ports  11 . 
     Each sound absorber  5  is located inside the rear housing  22  to have the air passages  54  with first openings at least partly overlapping the exhaust ports  11  and with second openings facing the internal space of the rear housing  22 . 
     The rear housing  22  includes supports  221 , supports  222 , and supports  223 . The supports  221  face the first surfaces  51  of the respective sound absorbers  5 . The supports  222  face the peripheral surfaces  53  of the respective sound absorbers  5 . The supports  223  are received in the respective support slits  55 . The left housing  22 L and the right housing  22 R each include the support  221 , the support  222 , and the support  223 . 
     The support  221 , the support  222 , and the support  223  in the left housing  22 L protrude rightward from the inner surface of the left housing  22 L. The support  221 , the support  222 , and the support  223  in the right housing  22 R protrude leftward from the inner surface of the right housing  22 R. 
     Each support  221  includes a peripheral wall  221 A and ribs  221 B. The peripheral wall  221 A surrounds the exhaust ports  11 . Each rib  221 B is between adjacent exhaust ports  11 . The peripheral wall  221 A has substantially the same profile as the first surface  51 . The peripheral wall  221 A is in contact with the periphery of the first surface  51 . The support  221  includes multiple ribs  221 B. The ribs  221 B are in contact with the first surface  51 . 
     Each support  222  includes front supports  222 A, an upper support  222 B, and a lower support  222 C. The front supports  222 A are located frontward from the exhaust ports  11 . The upper support  222 B is located above the exhaust ports  11 . The lower support  222 C is located below the exhaust ports  11 . 
     Each support  222  includes two front supports  222 A. The front supports  222 A are in contact with the front of the peripheral surface  53 . The upper support  222 B is in contact with the top of the peripheral surface  53 . The lower support  222 C is in contact with the bottom of the peripheral surface  53 . The lower support  222 C serves as a screw boss to receive at least a part of a screw  22 S. 
     The support  223  protrudes from one rib  221 B. The support  223  is received in the support slit  55 . 
     The rear housing  22  includes a controller support  224  supporting the controller  7 . The controller  7  is supported on the controller support  224  and located at the rear of the sound absorbers  5 . The controller  7  is supported on the controller support  224  and faces the rear of the peripheral surfaces  53 . 
     The sound absorbers  5  inside the housing  2  face the exhaust ports  11  to reduce noise. The sound absorbers  5  have the air passages  54 . Air inside the housing  2  flows through the air passages  54  and then flows out of the housing  2 . This structure allows air to smoothly flow from the inside to the outside of the housing  2 . The sound absorbers  5  having the air passages  54  reduce noise without increasing the resistance to the air discharge flow. The cleaner  1  has smooth airflow and is less likely to decrease its suction power. 
     Each sound absorber  5  has the air passages  54  that allow air to flow smoothly. The sound absorbers  5  having the air passages  54  have larger surface areas, thus more effectively absorbing sound. 
     The air passages  54  extend substantially parallel to each other to allow air to flow smoothly. 
     Each sound absorber  5  is located to have the air passages  54  with their first openings at least partly facing the exhaust ports  11  and their second openings facing the center of the internal space of the housing  2 . This structure allows air flowing through the second openings into the air passages  54  to flow out through the first openings and then to be smoothly discharged out of the housing  2  through the exhaust ports  11 . 
     The exhaust ports  11  are elongated slits. This structure reduces the likelihood that external foreign objects enter the housing  2  through the exhaust ports  11 . Each air passage  54  has an inner diameter at its first opening larger than the short-side dimension of each exhaust port  11 . This structure allows air in the air passages  54  flowing out through the first openings to be smoothly discharged out of the housing  2  through the exhaust ports  11 . 
     Multiple exhaust ports  11  are arranged in the short-side direction of the exhaust ports  11  to allow air to be smoothly discharged outside through the exhaust ports  11 . Each air passage  54  has an inner diameter at its first opening larger than the interval between the exhaust ports  11  in the short-side direction. The first openings of the air passages  54  at least partly overlap the exhaust ports  11 . In other words, the first openings of the air passages  54  are less likely to be blocked by the inner surface of the housing  2  between the exhaust ports  11 . This structure allows air in the air passages  54  flowing out through the first openings to be smoothly discharged out of the housing  2  through the exhaust ports  11 . 
     The air passages  54  are arranged in the short-side and long-side directions of the exhaust ports  11 . This structure allows air inside the housing  2  to flow through the air passages  54  and then to be smoothly discharged out of the housing  2  through the exhaust ports  11 . 
     The sound absorbers  5  are supported on the supports  223  protruding from the inner surface of the housing  2 . The supports  223  are received in the respective support slits  55 . The sound absorbers  5  are readily attached to the housing  2  by simply placing the supports  223  into the support slits  55 . This structure improves the workability for attaching and detaching the sound absorbers  5  to and from the housing  2 . 
     Each support  223  includes a hook at its distal end. The supports  223  may be received in the respective support slits  55  with the hooks hooked onto the second surfaces  52  of the sound absorbers  5 . This structure allows the sound absorbers  5  to be stably attached to the housing  2  with the supports  223 . 
     Suction Unit 
       FIG.  10    is a front perspective view of the suction unit  3  in the embodiment.  FIG.  11    is a rear perspective view of the suction unit  3  in the embodiment.  FIG.  12    is a front view of the suction unit  3  in the embodiment.  FIG.  13    is an exploded perspective view of the suction unit  3  in the embodiment as viewed from the front.  FIG.  14    is a perspective view of the motor  14  and the fan  15  in the embodiment. The suction unit  3  includes the motor assembly  12 , the cover  13 , and an elastic member  60 . 
     Motor Assembly 
     The motor assembly  12  includes the motor  14 , the fan  15 , the motor case  16 , and the control board  17 . 
     The motor  14  generates a rotational force for rotating the fan  15 . The motor  14  is an inner-rotor motor. The motor  14  includes a rotor shaft  141  rotatable about the rotation axis AX. The fan  15  is fixed to the front of the rotor shaft  141 . 
     The fan  15  is located frontward from the motor  14 . The fan  15  is rotatable about the rotation axis AX with the rotational force generated by the motor  14 . The fan  15  is a centrifugal fan. As shown in  FIG.  14   , the fan  15  includes a front plate  152 , a rear plate  153 , and blades  154 . The front plate  152  includes an inlet  151 . The rear plate  153  is located rearward from the front plate  152 . The blades  154  are between the front plate  152  and the rear plate  153 . 
     Multiple blades  154  are arranged to surround the rotation axis AX. Each blade  154  has the same shape, and has the same dimensions in the circumferential, radial, and axial directions. The blades  154  are located at equal intervals in the circumferential direction. 
     Adjacent blades  154  define an outlet  155 . The rotating fan  15  draws air frontward from the fan  15  into the inlet  151 . The air drawn into the inlet  151  flows between the blades  154  and is then discharged radially outward through the outlets  155 . 
     The motor case  16  accommodates the motor  14  and the fan  15 . The motor case  16  includes a cylinder  23 , a fan cover  24 , a support  25 , and legs  26 . 
     The cylinder  23  has the rotation axis AX at the center. The fan cover  24  is located frontward from the fan  15 . The fan cover  24  is at the front end of the cylinder  23 . The support  25  supports the motor  14  and the control board  17 . The legs  26  are fixed to the support  25 . The motor case  16  includes two legs  26 . The legs  26  are located radially outward from the outer surface of the cylinder  23 . 
     The motor case  16  has an inflow port  27  and an outflow port  28 . The inflow port  27  is at the front end of the motor case  16 . The outflow port  28  is located rearward from the inflow port  27 . The inflow port  27  in the embodiment is at the center of the fan cover  24 . The outflow port  28  is defined by the rear end of the cylinder  23  and the outer surface of the support  25 . The air from the fan  15  is discharged backward from the motor case  16  through the outflow port  28 . 
     The control board  17  outputs control signals for controlling the motor  14 . The control board  17  is located rearward from the support  25 . The control board  17  faces the rear of the support  25 . The control board  17  is supported on the support  25 . The control board  17  is between the two legs  26 . 
     Cover 
     The cover  13  surrounds and accommodates the motor assembly  12 . The cover  13  is fixed to the housing  2 . 
     The cover  13  includes a first cover  31  and a second cover  32 . The second cover  32  is at least partly located rearward from the first cover  31 . The second cover  32  is connected to the first cover  31  in a detachable manner. The first cover  31  and the second cover  32  define the internal space of the cover  13  for accommodating the motor assembly  12 . 
     The first cover  31  includes a cylinder  33 , a front plate  34 , an inlet  35 , a flow straightener  36 , protrusions  37 , and a protrusion  38 . 
     The cylinder  33  is substantially cylindrical. The cylinder  33  has the rotation axis AX at the center. The cylinder  33  has an outer surface facing radially outward and an inner surface facing radially inward. 
     The front plate  34  is connected to the front end of the cylinder  33 . The front plate  34  has a substantially circular profile. The front plate  34  has a front surface facing frontward and a rear surface facing rearward. 
     The inlet  35  is at the center of the front plate  34 . The inlet  35  has a through-hole connecting the front surface and the rear surface of the front plate  34 . 
     The front plate  34  includes a ring  341 , a ring  342 , multiple ribs  343 , and multiple ribs  344  on the front surface. The ring  341 , the ring  342 , the ribs  343 , and the ribs  344  protrude frontward from the front surface of the front plate  34 . 
     The ring  341  surrounds the inlet  35 . The ring  342  surrounds the ring  341 . The ribs  343  extend in the radial direction. The ribs  343  are between the ring  341  and the ring  342  in the radial direction. The ribs  343  are connected to each of the ring  341  and the ring  342 . The ribs  343  are located at intervals in the circumferential direction. The ribs  344  extend in the radial direction. The ribs  344  are located radially outward from the ring  342 . The ribs  344  are connected to the ring  342 . The ribs  344  are located at intervals in the circumferential direction. 
     The ring  341  has the front end located frontward from the front ends of the ribs  343  and the front ends of the ribs  344 . The ring  342  has the front end located frontward from the front ends of the ribs  343  and the front ends of the ribs  344 . 
     The flow straightener  36  is located at the inlet  35 . The flow straightener  36  guides air being sucked into the inlet  35 . The flow straightener  36  includes an inner ring  361 , an outer ring  362 , and multiple ribs  363 . 
     The inner ring  361 , the outer ring  362 , and the ribs  363  have the front ends located frontward from the front surface of the front plate  34 . The inner ring  361  is at the center of the inlet  35 . The outer ring  362  surrounds the inner ring  361 . The outer ring  362  defines the profile of the inlet  35 . The inlet  35  in the embodiment has a circular profile. 
     The ribs  363  extend in the radial direction. Each rib  363  has the same dimension in the circumferential direction. Each rib  363  has the same dimension in the radial direction. The ribs  363  are located at equal intervals in the circumferential direction. The ribs  363  are between the inner ring  361  and the outer ring  362  in the radial direction. The ribs  363  are connected to each of the inner ring  361  and the outer ring  362 . The ribs  363  have radially inner ends connected to the inner ring  361 . The ribs  363  have radially outer ends connected to the outer ring  362 . 
     The outer ring  362  is fixed to the front plate  34 . The inner ring  361  is fixed to the outer ring  362  with the ribs  363 . 
     The inlet  35  in the embodiment includes a first inlet  351  and second inlets  352 . The first inlet  351  is defined inside the inner ring  361 . Each second inlet  352  is defined between adjacent ribs  363 . The first inlet  351  has a circular profile. The rotation axis AX extends through the first inlet  351 . Multiple second inlets  352  are arranged in the circumferential direction. Each second inlet  352  has a substantially triangular profile. The first inlet  351  and the second inlets  352  allow air to flow. 
     The outer ring  362  includes first portions  362 A and second portions  362 B. Each first portion  362 A has a first dimension in the axial direction. Each second portion  362 B has a second dimension larger than the first dimension in the axial direction. The first portions  362 A have the front ends located rearward from the front ends of the second portions  362 B. The outer ring  362  includes multiple (three in the present embodiment) first portions  362 A located at intervals in the circumferential direction. Each second portion  362 B is between adjacent first portions  362 A in the circumferential direction. The outer ring  362  in the present embodiment includes three second portions  362 B arranged in the circumferential direction. 
     The second portions  362 B have the front ends located frontward from the front ends of the first portions  362 A. The second portions  362 B have the front ends at substantially the same position as the front end of the inner ring  361  in the axial direction. The front end of each first portion  362 A and the front end of the corresponding second portion  362 B form a step between them. 
     The ribs  363  include first ribs  363 A and second ribs  363 B. The first ribs  363 A are connected to the first portions  362 A. The second ribs  363 B are connected to the second portions  362 B. The first portions  362 A have the front ends at substantially the same position as the front, radially outer ends of the first ribs  363 A in the axial direction. The second portions  362 B have the front ends at substantially the same position as the front, radially outer ends of the second ribs  363 B in the axial direction. In other words, the first ribs  363 A have the radially outer ends not protruding frontward from the first portions  362 A. The second ribs  363 B have the radially outer ends not protruding frontward from the second portions  362 B. The first ribs  363 A have the front ends at least partly located rearward from the front ends of the second ribs  363 B. 
     The protrusions  37  are located on the outer surface of the cylinder  33 . The protrusions  37  protrude radially outward from the outer surface of the cylinder  33 . Four protrusions  37  are located at intervals in the circumferential direction. Two protrusions  37  are arranged in the axial direction. In other words, eight protrusions  37  are arranged on the outer surface of the cylinder  33 . 
     The protrusion  38  is located on the outer surface of the cylinder  33 . The protrusion  38  protrudes radially outward from the outer surface of the cylinder  33 . The single protrusion  38  is located on the outer surface of the cylinder  33  at the top. 
     The front plate  34  has multiple (four in the present embodiment) screw openings  39 . 
     The cylinder  33 , the front plate  34 , the flow straightener  36 , the protrusions  37 , and the protrusion  38  are integral with one another. The first cover  31  is formed by insert molding. The first cover  31  includes a base formed from a synthetic resin. The synthetic resin is, for example, polypropylene. The base is covered with an elastomer. The elastomer is, for example, synthetic rubber. 
     The outer surface of the cylinder  33 , the ring  341 , the ring  342 , the ribs  343 , the ribs  344 , the protrusions  37 , and the protrusion  38  in the embodiment are formed from an elastomer. The inner surface of the cylinder  33  and the flow straightener  36  are formed from a synthetic resin. 
     As shown in  FIG.  3   , the rear housing  22  includes a ring  225  located frontward from the cover  13 . The ring  225  is fixed to the inner surface of the rear housing  22 . The ring  225  has the rear surface in contact with the front surfaces of the rings  341  and  342 . Thus, the cover  13  and the housing  2  are positioned relative to each other. 
     The protrusions  37  are in contact with the inner surface of the rear housing  22 . Thus, the cover  13  and the housing  2  are positioned relative to each other. 
     The protrusion  38  is at least partly in contact with the inner surface of the rear housing  22 . Thus, the cover  13  and the housing  2  are positioned relative to each other. 
     The ring  341 , the ring  342 , the protrusions  37 , and the protrusion  38  in contact with the rear housing  22  are elastically deformable. This reduces transmission of vibrations from the suction unit  3  to the housing  2 . 
     The second cover  32  includes a cylinder  41 , a rear plate  42 , an opening  43 , screw bosses  44 , supports  47 , protrusions  48 , protrusions  49 , and a guide  70 . Each screw boss  44  has a screw hole  45 . The supports  47  support pins  46 . 
     The cylinder  41  is substantially cylindrical. The cylinder  41  has the rotation axis AX at the center. The cylinder  41  has an outer surface facing radially outward and an inner surface facing radially inward. The cylinder  41  in the embodiment has a rear portion with its inner diameter decreasing toward the rear end. 
     The rear plate  42  is connected to the rear end of the cylinder  41 . The rear plate  42  has a substantially circular profile. The rear plate  42  has a front surface facing frontward and a rear surface facing rearward. 
     The opening  43  is in the rear plate  42 . The opening  43  includes a through-hole connecting the front surface and the rear surface of the rear plate  42 . 
     The second cover  32  includes multiple (four in the present embodiment) screw bosses  44  at the front. The screw bosses  44  each have the screw hole  45 . 
     The supports  47  support the pins  46 . The pins  46  are formed from rubber. The supports  47  define recesses on the inner surface of the cylinder  41 . Four supports  47  are located at intervals in the circumferential direction. The four supports  47  support the respective pins  46 . The four pins  46  are in contact with the outer surface of the cylinder  23  in the motor case  16 . The four pins  46  allow the second cover  32  to be positioned relative to the motor case  16 . 
     The protrusions  48  protrude rearward from upper positions on the rear surface of the rear plate  42 . The second cover  32  includes two protrusions  48  aligning in the lateral direction. As shown in  FIG.  3   , the protrusions  48  are in contact with the sound absorbers  5 . The left protrusion  48  supports the sound absorber  5  at the exhaust ports  11  in the left housing  22 L. The right protrusion  48  supports the sound absorber  5  at the exhaust ports  11  in the right housing  22 R. 
     The protrusions  49  protrude rearward from lower positions on the rear surface of the rear plate  42 . The second cover  32  includes two protrusions  49  aligning in the lateral direction. As shown in  FIG.  3   , the protrusions  49  are in contact with the sound absorbers  5 . The left protrusion  49  supports the sound absorber  5  at the exhaust ports  11  in the left housing  22 L. The right protrusion  49  supports the sound absorber  5  at the exhaust ports  11  in the right housing  22 R. 
     The guide  70  is fixed to the outer surface of the cylinder  41 . The guide  70  protrudes radially outward from the outer surface of the cylinder  41 . The guide  70  in the embodiment includes first guides  71 , second guides  72 , and third guides  73 . The guide  70  in the embodiment includes two first guides  71 , two second guides  72 , and two third guides  73 . 
     The two first guides  71  are located opposite to each other in the radial direction. The two second guides  72  are located opposite to each other in the radial direction. The two third guides  73  are located opposite to each other in the radial direction. 
     One first guide  71  includes the first and second screw bosses  44 . The other first guide  71  includes the third and fourth screw bosses  44 . The screw bosses  44  protrude frontward from the first guides  71 . 
     The cylinder  41 , the rear plate  42 , the screw bosses  44 , the supports  47 , the protrusions  48 , the protrusions  49 , and the guide  70  in the embodiment are integral with one another. The second cover  32  is formed from a synthetic resin. The synthetic resin used for the second cover  32  is, for example, acrylonitrile butadiene styrene (ABS). 
     The elastic member  60  is between the cover  13  and at least a part of the motor case  16 . The elastic member  60  reduces transmission of vibrations from the motor  14  or the fan  15  to the cover  13 . 
     The elastic member  60  in the embodiment includes a first elastic member  61  and a second elastic member  62 . The first elastic member  61  is at least partly located frontward from the motor case  16 . The second elastic member  62  is at least partly located rearward from the motor case  16 . 
     The first elastic member  61  reduces transmission of vibrations from the motor  14  or the fan  15  to the first cover  31 . The first elastic member  61  is between the surface of the fan cover  24  in the motor case  16  and the rear surface of the front plate  34  in the first cover  31 . The rear surface of the front plate  34  in the first cover  31  faces at least a part of the surface of the fan cover  24  in the motor case  16 . The front plate  34  in the first cover  31  has the inlet  35  located frontward from the fan cover  24 . The first elastic member  61  is annular and surrounds the inlet  35 . The first elastic member  61  is connected to the rear surface of the front plate  34 . 
     The first elastic member  61  is molded integrally with the first cover  31 . The first elastic member  61  may be a part of the first cover  31 . The first cover  31  may be formed by insert molding as described above. In this case, the first elastic member  61  may be formed from an elastomer that covers the base of the first cover  31 . 
     The second elastic member  62  reduces transmission of vibrations from the motor  14  or the fan  15  to the second cover  32 . The second elastic member  62  is between the motor case  16  and the rear plate  42  in the second cover  32 . The second cover  32  has the opening  43  located rearward from the motor case  16 . The second elastic member  62  is connected to at least a part of the motor case  16  with the second elastic member  62  blocking the opening  43 . The second elastic member  62  in the embodiment is connected to the legs  26  on the motor case  16 . 
     The second elastic member  62  includes a first connector  63 , a second connector  64 , a blocker  65 , and a pipe  66 . The first connector  63  is fixed to one leg  26 . The second connector  64  is fixed to the other leg  26 . The blocker  65  is between the first connector  63  and the second connector  64 . The blocker  65  is inside the opening  43 . The pipe  66  protrudes rearward from the rear surface of the blocker  65 . The pipe  66  has a support hole  67 . 
     Assembling Suction Unit 
     Assembling the suction unit  3  involves connecting the second elastic member  62  to the legs  26  on the motor case  16 . A cable (not shown) connected to the control board  17  is placed in the support hole  67  in the second elastic member  62 . The second elastic member  62  is connected to the legs  26  on the motor case  16  with the cable received in the support hole  67 . The first connector  63  in the second elastic member  62  is hooked on one leg  26 , and the second connector  64  is hooked on the other leg  26 . The second elastic member  62  is thus connected to the motor case  16 . 
     The two legs  26  protrude radially outward from the outer surface of the cylinder  23 . The second elastic member  62 , which has a larger radial dimension than the cylinder  23 , can be appropriately connected to the motor case  16  with the legs  26 . 
     The second elastic member  62  is connected to the motor case  16  with the blocker  65  facing the control board  17 . The first connector  63  and the second connector  64  are located radially outward from the control board  17 . 
     The motor case  16  and the second elastic member  62  connected together are then connected to the second cover  32 . The motor case  16  and the second elastic member  62  are placed inside the second cover  32  through the front opening in the second cover  32 . The motor case  16  and the second elastic member  62  are placed inside the second cover  32  with the blocker  65  blocking the opening  43  in the second cover  32 . The pins  46  come in contact with the outer surface of the cylinder  23  in the motor case  16  placed inside the second cover  32 . The motor case  16  is positioned with the pins  46 . The cable connected to the control board  17  at least partly extends rearward from the rear end of the support hole  67 . 
     The second cover  32  is connected to the motor assembly  12  and the second elastic member  62 , and is then connected to the first cover  31 . The cylinder  33  in the first cover  31  has an inner diameter larger than the outer diameter of the cylinder  41  in the second cover  32 . The first cover  31  and the second cover  32  are connected together with the inner surface of the cylinder  33  and at least a part of the outer surface of the cylinder  41  facing each other. The cylinder  41  includes the guide  70  on the outer surface. The first cover  31  and the second cover  32  are connected together with the guide  70  located inside the cylinder  33 . The guide  70  is between the inner surface of the cylinder  33  and the outer surface of the cylinder  41 . 
     The first cover  31  has the screw openings  39 . The second cover  32  has the screw holes  45 . The first cover  31  and the second cover  32  are fastened together with four screws  90 . The screws  90  are placed into the screw openings  39  from the front of the first cover  31 . The screws  90  are then placed into and received in the screw holes  45 . The screws  90  thus fasten the first cover  31  and the second cover  32  together. 
     Operation 
     The operation of the cleaner  1  according to the embodiment will now be described. The motor  14  that is stopped starts running in response to the drive button  81  being pushed by the user. The motor  14  runs on power supplied from the battery pack  19 . The motor  14  runs and rotates the fan  15 , which then generates a suction force through the suction port  10 . This causes air outside the housing  2  to be sucked through the suction port  10  into the front housing  21 . 
     Dust in the air entering the front housing  21  is then collected on the filter  18 . The air through the filter  18  is sucked into the inlet  35  in the suction unit  3 . 
       FIG.  15    is a schematic diagram of the suction unit  3  in the embodiment describing airflow. Air is sucked into the inlet  35  as the fan  15  rotates, and flows into the motor case  16  through the inflow port  27 . The air from the fan  15  is discharged backward from the motor case  16  through the outflow port  28 . 
     The air discharged backward through the outflow port  28  hits the front surface of the rear plate  42  in the second cover  32 , and then flows forward between the outer surface of the cylinder  23  in the motor case  16  and the inner surface of the cylinder  41  in the second cover  32 . The air hits the rear surface of the front plate  34  in the first cover  31  and then flows onto and along the outer surface of the cylinder  41  from its front end. 
     The air flowing onto and along the outer surface of the cylinder  41  is guided by the guide  70  in the circumferential direction. The air flows in the circumferential direction through a flow path  74  defined by the guide  70 . The air through the flow path  74  then flows backward from the second cover  32  through a discharge port  75  defined by the guide  70 . The air flowing through the discharge port  75  is then discharged out of the housing  2  through the exhaust ports  11 . 
     The guide  70  guides air in the circumferential direction along the outer surface of the second cover  32 . This structure allows air to travel a longer distance inside the housing  2 , thus reducing noise. 
     Relationship Between Ribs and Fan 
     As described above, the suction unit  3  includes the motor  14 , the fan  15 , the cover  13  having the inlet  35 , and the ribs  363 . The fan  15  is rotatable about the rotation axis AX by the motor  14 . The inlet  35  in the cover  13  is located frontward from the fan  15 . The ribs  363  are located in the inlet  35  and extend radially from the rotation axis AX. The fan  15  includes the blades  154 . The ribs  363  are arranged in the circumferential direction. 
     Each blade  154  has the same shape, and has the same dimensions in the circumferential, radial, and axial directions. The blades  154  are located at equal intervals in the circumferential direction. 
     Each rib  363  has the same dimension in the circumferential direction. Each rib  363  has the same dimension in the radial direction. The ribs  363  are located at equal intervals in the circumferential direction. 
     The fan  15  rotating behind the ribs  363  may produce noise (NZ). The rotating fan  15  causes the blades  154  to move behind the ribs  363 . This causes air to be compressed to increase the pressure between the ribs  363  and the blades  154 . The increasing pressure between the ribs  363  and the blades  154  may cause noise. 
       FIGS.  16  to  18    are schematic diagrams of the ribs  363  and the blades  154  in embodiments. 
       FIG.  16    shows the structure including one blade  154  (Z=1, where Z is the number of blades  154 ) and one rib  363  (V=1, where V is the number of ribs  363 ). In this case, the pressure between the rib  363  and the blade  154  increases once per rotation of the fan  15 . For the fan  15  rotating at a rotational speed N of 60 rpm, the pressure increases 60 times per minute. This produces noise with the frequency f NZ  being 1 Hz. 
       FIG.  17    shows the structure including three blades  154  (Z=3) and one rib  363  (V=1). In this case, the pressure between the rib  363  and the blades  154  increases three times per rotation of the fan  15 . For the fan  15  rotating at a rotational speed N of 60 rpm, the pressure increases 180 times per minute. This produces noise with the frequency f NZ  being 3 Hz. 
       FIG.  18    shows the structure including three blades  154  (Z=3) and two ribs  363  (V=2). In this case, the pressure between the ribs  363  and the blades  154  increases six times per rotation of the fan  15 . For the fan  15  rotating at a rotational speed N of 60 rpm, the pressure increases 360 times per minute. This produces noise with the frequency f NZ  being 6 Hz. 
     In the present embodiment, the number Z of blades  154 , the number V of ribs  363 , and the rotational speed N of the fan  15  indicating the revolutions per minute are determined to cause the rotating fan  15  to produce noise having the frequency f NZ  of 20,000 Hz or higher. 
     The range of human hearing is specified to be higher than or equal to 15 Hz and lower than 20,000 Hz. Humans cannot hear noise from the rotating fan  15  having the frequency f NZ  of 20,000 Hz or higher. In other words, setting the frequency f NZ  of noise to 20,000 Hz or higher can reduce noise audible to humans. 
     The frequency f NZ  of noise is expressed by formula (1) below.
 
 f   NZ =( m−k×V )× N/ 60  (1)
 
     In the formula, m is an integer, k is an integer, V is the number of ribs  363 , and N is the rotational speed of the fan  15  indicating the revolutions per minute (rpm). 
     The integer m in formula (1) is expressed by formula (2) below.
 
 m=n×Z+k×V   (2)
 
     In the formula, n is the order (natural number), and Z is the number of blades  154 . 
     The integer m is a value associated with the attenuation rate of noise with respect to the distance from the cleaner  1  (the fan  15  or the ribs  363 ). The integer m having a smaller absolute value |m| indicates a smaller attenuation rate of noise from the cleaner  1 , indicating that the noise reaches farther. 
     Formula (3) below is satisfied, where Δx is the distance from the cleaner  1  (the fan  15  or the ribs  363 ) as a noise source, ΔdB is the value of noise from the cleaner  1 , Mc is the characteristic Mach number, Mm is the Mach number at the distal end of the blade  154 , and R is the duct radius.
 
Δ dB/Δx=− 8.69×| m |×( Mc   2   −Mm   2 ) 1/2   /R   (3)
 
     In formula (3), the absolute value |m| being smaller indicates the noise value ΔdB being less likely to attenuate at a longer distance Δx from the cleaner  1 . In other words, the absolute value |m| being smaller indicates a smaller attenuation rate of noise from the cleaner  1 , indicating that the noise reaches a longer distance from the cleaner  1 . 
     In the embodiment, the number Z of blades  154 , the number V of ribs  363 , and the rotational speed N of the fan  15  are determined to cause the frequency f NZ  to be 20,000 Hz or higher for the integer m being −1 to +1 inclusive (|m|≤1). The frequency f NZ  of noise may be set to 20,000 Hz or higher to reduce noise audible to humans under the conditions in which the cleaner  1  produces noise reaching far with the noise value ΔdB that is less likely to attenuate. 
     The characteristic Mach number Mc is determined using the integer m and a hub ratio σ. The hub ratio σ is the ratio of the hub diameter to the duct diameter, or in other words, (hub diameter)/(duct diameter). For an open space with no duct used, the hub ratio σ is zero. 
       FIG.  19    is an example table showing the characteristic Mach number Mc in the embodiment. As shown in  FIG.  19   , the characteristic Mach number Mc is determined using the integer m and the hub ratio σ. 
     The Mach number Mm at the distal end of the blade  154  is expressed by formula (4) below.
 
 Mm =(π× D×N )/(60× a   0 )  (4)
 
where D is the diameter (m) of the blade  154  and a 0  is the sound speed.
 
       FIG.  20    is a table describing a method for calculating the frequency f NZ  of noise in the embodiment. 
     In the example shown in  FIG.  20   , the number Z of blades  154 , the number V of ribs  363 , the rotational speed N of the fan  15 , the integer k, and the order n are first set to selected values. The integer m is then calculated using formula (2). The frequency f NZ  is then calculated using formula (1). 
       FIG.  20    shows the relationships between the integer k, the integer m, and the frequency f NZ  for the settings with the number Z being 3, the number V being 5, the rotational speed N being 600 rpm, the integer k being each value shown in  FIG.  20   , and the order n being 1. 
     Setting the integer k to a value shown in  FIG.  20    yields the integer m using formula (2). For example, setting the integer k to −5 yields the integer m=1×3+(−5)×5, which is −22. Setting the integer k to −4 yields the integer m=1×3+(−4)×5, which is −17. 
     The integer k and the integer m being determined yield the frequency f NZ  using formula (1). For example, the integer k being −5 and the integer m being −22 yield the frequency f NZ =(−22−(−5)×5)×600/60, which is 30 Hz. The integer k being −4 and the integer m being −17 yield the frequency f NZ =(−17−(−4)×5)×600/60, which is 30 Hz. 
     As shown in  FIG.  20   , the number Z, the number V, the rotational speed N, and the order n may be set to selected values to yield the constant frequency f NZ  (30 Hz) independently of the varying integers k and m. 
     In the example shown in  FIG.  20   , the absolute value |m| is minimum for the integer m being −2. This indicates that the noise value ΔdB is less likely to attenuate at a longer distance Δx from the cleaner  1  for the integer m being −2 in the example shown in  FIG.  20   . 
     In the embodiment, the number Z, the number V, the rotational speed N, the integer k, and the order n are determined to cause the frequency f NZ  to be 20,000 Hz or higher for the integer m being −1 to +1 inclusive (|m|≤1). Thus, the frequency f NZ  of noise may be set to 20,000 Hz or higher to reduce noise audible to humans under the conditions with the noise value ΔdB that is less likely to attenuate. 
     In the embodiment, the number Z is selected from between 2 and 20 inclusive. The number V is selected from between 1 and 50 inclusive. The rotational speed N is selected from between 1,000 and 40,000 inclusive. The integer k is selected from between −10 and +10 inclusive. The order n is selected from between 1 and 10 inclusive. The number Z, the number V, the rotational speed N, the integer k, and the order n are selected from their respective limited numerical ranges. This reduces the load for calculating the frequency f NZ . 
     Method for Setting Suction Unit 
     The method for setting the suction unit  3  will now be described. Setting the suction unit  3  includes determining the number Z of blades  154 , the number V of ribs  363 , and the rotational speed N of the fan  15  indicating the revolutions per minute to cause the frequency f NZ  to be 20,000 Hz or higher. These settings of the suction unit  3  are performed with a computer system. 
       FIG.  21    is a block diagram of a computer system  1000  in the embodiment. The computer system  1000  includes a processor  1001 , a main memory  1002 , a storage  1003 , and an interface  1004  including an input-output circuit. The processor  1001  may be a CPU. The main memory  1002  may be a nonvolatile memory such as a ROM and a volatile memory such as a RAM. The processor  1001  reads a computer program from the storage  1003 , loads the computer program in the main memory  1002 , and sets the suction unit  3  in accordance with the loaded computer program. The computer program may be distributed to the computer system  1000  through a network. 
     For setting the suction unit  3  to produce noise with the frequency f NZ  of 20,000 Hz or higher, the computer system  1000  first determines, as selected values, the number Z, the number V, the integer k, and the order n. The computer system  1000  then calculates the absolute value |m| of the integer m using formula (2). The computer system  1000  determines the number Z, the number V, the integer k, and the order n to cause the integer m to be −1 to +1 inclusive, or in other words, |m|≤1. 
     After determining the integer m satisfying |m|≤1, the number Z, the number V, the integer k, and the order n, the computer system  1000  determines the rotational speed N to cause the frequency f NZ  to be 20,000 Hz or higher using the integer m satisfying |m|≤1, the number V, and the integer k using formula (1). 
       FIG.  22    is a flowchart showing the method for setting the suction unit  3  in the embodiment. 
     The computer system  1000  determines the number Z of blades  154 . The number Z is a natural number. The computer system  1000  selects the number Z from between 2 and 20 inclusive (step S 1 ). 
     The computer system  1000  determines the number V of ribs  363 . The number V is a natural number. The computer system  1000  selects the number V from between 1 and 50 inclusive (step S 2 ). 
     The computer system  1000  determines the integer k. The computer system  1000  selects the integer k from between −10 and 10 inclusive (step S 3 ). 
     The computer system  1000  determines the order n. The order n is a natural number. The computer system  1000  selects the order n from between 1 and 10 inclusive (step S 4 ). 
     Steps S 1  to S 4  may be performed in any order. At least two of steps S 1  to S 4  may be performed in parallel. 
     The computer system  1000  calculates the integer m using the number Z determined in step S 1 , the number V determined in step S 2 , the integer k determined in step S 3 , and the order n determined in step S 4  using formula (2). More specifically, the computer system  1000  calculates the integer m by substituting the number Z determined in step S 1 , the number V determined in step S 2 , the integer k determined in step S 3 , and the order n determined in step S 4  into formula (2) (step S 5 ). 
     The computer system  1000  determines whether the absolute value |m| of the integer m calculated in step S 5  is 1 or less (step S 6 ). 
     In response to the absolute value |m| not being 1 or less in step S 6  (No in step S 6 ), the computer system  1000  varies at least one of the number Z, the number V, the integer k, or the order n. The computer system  1000  varies the combination of the number Z, the number V, the integer k, and the order n until the absolute value |m| of the integer m calculated using formula (2) is 1 or less. 
     In response to the absolute value |m| being 1 or less in step S 6  (Yes in step S 6 ), the computer system  1000  determines the rotational speed N of the fan  15 . The rotational speed N is a natural number. The computer system  1000  selects the rotational speed N from between 1,000 and 40,000 inclusive (step S 7 ). 
     The computer system  1000  calculates the frequency f NZ  using the integer m determined in step S 5 , the integer k determined in step S 3 , the number V determined in step S 2 , and the rotational speed N determined in step S 7  using formula (1). More specifically, the computer system  1000  calculates the frequency f NZ  by substituting the integer m determined in step S 5 , the integer k determined in step S 3 , the number V determined in step S 2 , and the rotational speed N determined in step S 7  into formula (1) (step S 8 ). 
     The computer system  1000  determines whether the frequency f NZ  calculated in step S 8  is 20,000 Hz or higher (step S 9 ). 
     In response to the frequency f NZ  not being 20,000 Hz or higher in step S 9  (No in step S 9 ), the computer system  1000  varies the rotational speed N. The computer system  1000  varies the rotational speed N until the frequency f NZ  calculated using formula (1) is 20,000 Hz or higher while using the integer m determined in step S 5 , the integer k determined in step S 3 , and the number V determined in step S 2 . 
     In response to the frequency f NZ  being 20,000 Hz or higher in step S 9  (Yes in step S 9 ), the computer system  1000  ends the settings of the suction unit  3 . 
     In the embodiment described above, the number Z, the number V, the integer k, and the order n are determined using formula (2) to cause the integer m to be −1 to +1 inclusive (steps S 1  to S 6 ). The rotational speed N is then determined using formula (1) to cause the frequency f NZ  to be 20,000 Hz or higher (step S 7  to step S 9 ). 
     For example, the number Z is determined to be 11, the number V is determined to be 14, the integer k is determined to be −4, and the order n is determined to be 5. These numbers are substituted into formula (2) to cause the integer m to be −1. The rotational speed N is determined to be 22,000 rpm. These numbers are substituted into formula (1) to cause the frequency f NZ  to be about 20,167 Hz. The rotational speed N may be determined to be 40,000 rpm to cause the frequency f NZ  to be about 36,666 Hz. Thus, the rotational speed N may be set to 22,000 to 40,000 inclusive to cause the frequency f NZ  to be 20,000 Hz or higher when the number Z is 11, the number V is 14, the integer k is −4, and the order n is 5. 
     In the embodiment described above, the fan  15  rotates behind the ribs  363 . In this structure, the number Z of blades  154 , the number V of ribs  363 , and the rotational speed N of the fan  15  indicating the revolutions per minute are determined to cause noise from the rotating fan  15  to have the frequency f NZ  of 20,000 Hz or higher. The range of human hearing is specified to be higher than or equal to 15 Hz and lower than 20,000 Hz. Humans cannot hear noise from the rotating fan  15  having the frequency f NZ  of 20,000 Hz or higher. In other words, setting the frequency f NZ  of noise to 20,000 Hz or higher can reduce noise audible to humans. This reduces noise from the cleaner  1 . 
     The frequency f NZ  is calculated using formula (1). The integer m is calculated using formula (2). The integer m (=n×Z+k×V) is included in formula (3). The integer m is a value associated with the attenuation rate of the noise value ΔdB with respect to the distance Δx from the fan  15  or the ribs  363 . The integer m having a smaller absolute value |m| indicates a smaller attenuation rate of noise from the cleaner  1 , indicating that the noise reaches farther. In other words, the absolute value |m| being smaller indicates the cleaner  1  having the noise value ΔdB that is less likely to attenuate. For the absolute value |m| being 1 or less, the cleaner  1  has the noise value ΔdB that is less likely to attenuate. The number Z of blades  154 , the number V of ribs  363 , and the rotational speed N of the fan  15  are determined to cause the frequency f NZ  to be 20,000 Hz or higher for the integer m being −1 to +1 inclusive (|m|≤1). The frequency f NZ  of noise may be set to 20,000 Hz or higher to reduce noise from the cleaner  1  audible to humans under the conditions in which the cleaner  1  produces noise with the noise value ΔdB that is less likely to attenuate. 
     As described with reference to steps S 1  to S 6  in  FIG.  22   , the number Z, the number V, the integer k, and the order n are determined using formula (2) to cause the integer m to be −1 to +1 inclusive. 
     As described with reference to steps S 7  to S 9  in  FIG.  22   , the rotational speed N is determined using formula (1) to cause the frequency f NZ  to be 20,000 Hz or higher after the number Z, the number V, the integer k, and the order n are determined to cause the integer m to be −1 to +1 inclusive. 
     In step S 1 , the number Z is selected from natural numbers between 2 and 20 inclusive. In step S 2 , the number V is selected from natural numbers between 1 and 50 inclusive. In step S 3 , the integer k is selected from between −10 and 10 inclusive. In step S 4 , the order n is selected from natural numbers between 1 and 10 inclusive. In step S 7 , the rotational speed N is selected from natural numbers between 1,000 and 40,000 inclusive. The number Z, the number V, the integer k, the order n, and the rotational speed N are selected from their respective limited numerical ranges. This reduces the load for calculating the frequency f NZ  with the computer system  1000 . 
     In the embodiment, the number Z is 11, the number V is 14, the integer k is −4, and the order n is 5. The rotational speed N is set to 22,000 to 40,000 inclusive. 
     The inner ring  361  is at the center of the inlet  35 . The first inlet  351  is thus formed inside the inner ring  361 . The ribs  363  have radially inner ends connected to the inner ring  361 . The ribs  363  are thus connected together with the inner ring  361 . Each second inlet  352  is formed between adjacent ribs  363 . 
     The outer ring  362  surrounds the inner ring  361 . The outer ring  362  defines the profile of the inlet  35  (second inlets  352 ). The ribs  363  have radially outer ends connected to the outer ring  362 . The ribs  363  are thus supported by each of the inner ring  361  and the outer ring  362 . 
     The outer ring  362  includes the first portions  362 A each having the first dimension in the axial direction, and the second portions  362 B each having the second dimension larger than the first dimension. The first portions  362 A are located at intervals in the circumferential direction. Each second portion  362 B is between adjacent first portions  362 A in the circumferential direction. The outer ring  362  has steps at the front end. This structure allows smooth flow of air sucked into the inlet  35 , thus reducing noise. 
     The ribs  363  include the first ribs  363 A connected to the first portions  362 A and the second ribs  363 B connected to the second portions  362 B. The first ribs  363 A have the front ends at least partly located rearward from the front ends of the second portions  362 B. This structure allows smooth flow of air sucked into the inlet  35 , thus reducing noise. 
     Other Embodiments 
     The second cover  32  and the second elastic member  62  may be integrally molded in the above embodiments. 
     The suction unit  3  is included in a handheld cleaner in the above embodiments. In some embodiments, the suction unit  3  may be included in a wheeled cleaner. 
     REFERENCE SIGNS LIST 
     
         
           1  cleaner 
           2  housing 
           3  suction unit 
           4  filter holder 
           5  sound absorber 
           6  battery mount 
           7  controller 
           8  interface unit 
           9  handle 
           10  suction port 
           11  exhaust port 
           12  motor assembly 
           13  cover 
           14  motor 
           15  fan 
           16  motor case 
           17  control board 
           18  filter 
           19  battery pack 
           21  front housing 
           22  rear housing 
           22 L left housing 
           22 R right housing 
           22 S screw 
           23  cylinder 
           24  fan cover 
           25  support 
           26  leg 
           27  inflow port 
           28  outflow port 
           31  first cover 
           32  second cover 
           33  cylinder 
           34  front plate 
           35  inlet 
           36  flow straightener 
           37  protrusion 
           38  protrusion 
           39  screw opening 
           41  cylinder 
           42  rear plate 
           43  opening 
           44  screw boss 
           45  screw hole 
           46  pin 
           47  support 
           48  protrusion 
           49  protrusion 
           51  first surface 
           52  second surface 
           53  peripheral surface 
           54  air passage 
           55  support slit 
           60  elastic member 
           61  first elastic member 
           62  second elastic member 
           63  first connector 
           64  second connector 
           65  blocker 
           66  pipe 
           67  support hole 
           70  guide 
           71  first guide 
           72  second guide 
           73  third guide 
           74  flow path 
           75  discharge port 
           81  drive button 
           82  mode switch button 
           83  indicator 
           90  screw 
           141  rotor shaft 
           151  inlet 
           152  front plate 
           153  rear plate 
           154  blade 
           155  outlet 
           211  opening 
           212  connection pipe 
           213  inner surface 
           214  recess 
           214 A first face 
           214 B second face 
           214 C third face 
           214 D fourth face 
           215  lock 
           216  opening 
           217  hook 
           221  support 
           221 A peripheral wall 
           221 B rib 
           222  support 
           222 A front support 
           222 B upper support 
           222 C lower support 
           223  support 
           224  controller support 
           225  ring 
           341  ring 
           342  ring 
           343  rib 
           344  rib 
           351  first inlet 
           352  second inlet 
           361  inner ring 
           362  outer ring 
           362 A first portion 
           362 B second portion 
           363  rib 
           363 A first rib 
           363 B second rib 
           1000  computer system 
           1001  processor 
           1002  main memory 
           1003  storage 
           1004  interface 
         AX rotation axis