JP5388744B2 - Silencer and image forming apparatus - Google Patents

Description

  The present invention, for example, reads an image of a document such as a scanner, a copying machine, a printer, or a facsimile machine, and forms an image on a recording material, and discharges heat in the image reading apparatus or the image forming apparatus to the outside. The present invention relates to a muffler device for preventing noise during operation.

  Conventionally, in an electronic device or OA device, heat is accumulated inside the device by exhausting heat generated from electronic components inside the device to the outside of the device by a fan arranged on the exterior of the device casing. It has been done to prevent. On the other hand, in recent years, with the increase in the density of electronic components, the heat generated in the device has increased and the heat to be exhausted has increased. For this reason, it is necessary to use a large fan or to increase the rotation speed of the fan, which causes problems such as an increase in the size of the device and an increase in noise.

  In view of such a problem, a structure is known in which a fan sound is silenced using a so-called side branch type silencer (see, for example, Patent Document 1). The configuration of such a side branch type silencer will be described with reference to FIG. An axial fan 2 is fixed to an end portion of a duct 1 attached to a housing of an OA device such as a printer. Then, the axial flow fan 2 causes air to flow in the direction of the arrow in the figure, and the heat in the housing is discharged outside the machine. In the middle of the duct 1 is disposed a filter 3 made of a catalyst that removes ozone and the like generated from the components inside the casing. And by flowing air through the filter 3, ozone or the like is prevented from being discharged out of the casing.

  Further, a side branch 4 is provided on the downstream side of the duct 3 in the air flow direction so as to protrude from the side surface of the duct 1. For this reason, the sound generated from the axial fan 2 is divided into one that passes between points A and B and one that passes between points A and C-B. The length L of the side branch 4 is set such that the phase of the sound passing between the paths A and B and the sound passing between the paths A and C-B are shifted by 180 °. By providing such a side branch type silencer, it is possible to reduce noise by reducing the amount of noise generated from the fan to the outside of the housing.

JP-A-8-156367

  However, in recent years, the location where the fan is installed may be limited depending on the downsizing of the device or the component configuration inside the device, and it is difficult to employ the structure in which the side branch type silencer is provided as described above. For example, since the speed of sound in the air is about 331000 mm / s, if the frequency of the fan noise is 2000 Hz, the distance λ that the sound travels in one cycle is 331000/2000 = 165.5 mm. Therefore, in order to cause the side branch 4 to cause the sound to interfere, the phase should be shifted by λ / 2 by the side branch 4, so that even if it is simply calculated, 2L = λ / 2, L = λ / 4 = 41.4 mm It is necessary to. In order to secure this space, it is necessary to consider it from the conception stage of the device, and a dead space is easily generated by the side branch 4. On the other hand, it is conceivable to provide a sound absorbing material in the duct to mute the fan sound. However, since the sound absorbing material is expensive, the cost increases.

  In view of such circumstances, the present invention was invented to realize a structure capable of reducing the size of the apparatus at low cost and preventing the occurrence of dead space.

The silencer of the present invention is installed in a duct through which air flows, and in the direction in which the air flow is shielded, and has a plurality of paths therethrough, and propagates air from upstream in the air flow direction. Sound path changing means for changing the sound propagation path to at least one pair of paths having different path lengths by the plurality of paths, the sound path changing means closing the longitudinal intermediate portion of the duct. In a member having a predetermined thickness, the first hole penetrating in the longitudinal direction of the duct, and the direction toward the wall surface of the duct as it goes downstream in the air flow direction with respect to the longitudinal direction of the duct. The plurality of paths are formed by forming a second hole penetrating so as to incline, and the pair of paths passes through the first hole and the second hole. And the path reflected by the wall surface, Than serial sound path changing means converge in the flow direction downstream of the air, the convergence point of the point of convergence, when starting from the point at which the propagation path of the sound is changed, the pair of these start point to the convergence point Where L _A and L _B are the path lengths, S is the sound velocity of air propagation in the duct, f is the frequency of sound propagating through the duct, and K is an odd number, | L _A −L _B | = It satisfies the relational expression of S × K / (2 × f).

  According to the present invention, the sound path changing means for changing the sound propagation path can be provided in the duct, so that the silencing effect can be obtained. Therefore, the apparatus can be reduced in size and the dead space can be prevented from being generated. it can.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment. FIG. 3 is a perspective view showing a schematic configuration by cutting a part of the front side of the image forming apparatus. The perspective view which shows a schematic structure by cut | disconnecting a part on the back side similarly. The perspective view which shows the schematic structure of a back side similarly. (A) which shows an example of an exhaust fan is a front view, (b) is a side view. (A) which shows the sound path change means which concerns on 1st Embodiment is a front view, (b) is the II sectional drawing of (a). Sectional drawing which shows typically the air blower provided with the silencer which concerns on 1st Embodiment. The figure shown in order to demonstrate that it can mute by interference of sound. Sectional drawing which shows typically the air blower provided with the silencer which concerns on 2nd Embodiment. Sectional drawing which shows typically the air blower provided with the silencer which concerns on 3rd Embodiment. Sectional drawing of the experimental apparatus for confirming the effect of 3rd Embodiment. (A) which shows the filter used in order to contrast by experiment is a front view, (b) is a roll sectional view of (a). The graph which shows an experimental result. Sectional drawing which shows typically the air blower provided with the side branch type silencer.

<First Embodiment>
A first embodiment of the present invention will be described with reference to FIGS. First, an image forming apparatus incorporating the silencer of the present embodiment will be briefly described with reference to FIG. The image forming apparatus 10 includes a document reading unit 11 that reads a document, an image forming unit 12 that forms an image based on image information of the document reading unit 11, and a transport unit 13 that transports a recording material P to the image forming unit 12. And a fixing device 14 for fixing the toner image on the recording material P. The image forming unit 12 includes a photosensitive drum 15 as an image carrier, a charging unit 16 such as a corona charger, an exposure unit such as a laser, a developing unit 17, a transfer unit 18 such as a charger, and a separation unit. 19 and cleaning means 20 and the like.

  In the image forming process, the surface of the photosensitive drum 15 is charged by the charging unit 16, and the exposure unit irradiates laser light based on the image information of the document reading unit 11 to form an electrostatic latent image on the surface of the photosensitive drum. This electrostatic latent image is developed by the developing means 17 and becomes visible as a toner image. Thereafter, a predetermined electrostatic load bias is applied by the transfer means 18 at the transfer portion T, and the toner image is transferred to the recording material P. The recording material P is transported to the transfer unit T from the paper feed cassette 21 constituting the transport unit 12 through the transport path 22 or manually inserted from the outside. The recording material P to which the toner image is transferred by the transfer unit T is separated by the separating unit 19 and conveyed to the fixing device 14 by the conveying belt 23. Then, the toner image transferred to the recording material P by the fixing device 14 is heated and fixed to the recording material P. Thereafter, the recording material P on which the toner image is fixed is discharged onto a discharge tray 25 by a discharge roller 24 serving as a discharge unit.

  As shown in FIG. 2, an air supply fan 26 that sends air into the housing 10 a of the image forming apparatus 10 (in the arrow direction in FIG. 2) is provided on the front (front) side of the image forming apparatus 10. Yes. On the other hand, as shown in FIG. 3, on the rear surface (rear surface) side of the image forming apparatus 10, heat and ozone generated in the housing 10a are discharged to the outside of the housing 10a (in the arrow direction in FIG. 3). A blower (exhaust device) 27 is provided. The blower device 27 includes a duct 28 through which air flows, a sound path changing unit 29 installed at an intermediate portion in the longitudinal direction in the duct 28, and an exhaust fan 30 that is a blower fixed to a base end portion of the duct 28. . Of these, the duct 28 and the sound path changing means 29 constitute a silencer 31. In other words, the blower 27 includes an exhaust fan 30 that exhausts the air in the apparatus to the outside, and a silencer 31 through which the air exhausted by the exhaust fan 30 flows.

  Further, as shown in FIG. 4, a louver 32 is fixed to a portion of the end portion of the duct 28 that opens to the outer cover 10b of the housing 10a. The louver 32 is provided to prevent a user's finger or the like from entering the image forming apparatus 10. Note that the air supply fan 26 and the air blower 27 as described above can be installed on any surface of the housing 10a other than the above-described places. For example, the air supply fan 26 can be installed on the side surface or the back surface of the housing 10a, or the air blower 27 can be installed on the side surface.

  The blower 27 will be described in detail with reference to FIGS. 5 to 7 in addition to FIG. The duct 28 is attached to the opening of the rear plate 10 c of the housing 10 a and communicates with the inside of the housing 10 a through the exhaust fan 30. The air inside the image forming apparatus 1 can be discharged outside the apparatus. The duct 28 is formed in a substantially rectangular parallelepiped cylindrical shape with a resin such as ABS, and the inner side surface is a smooth surface and the plate thickness is such that the vibration of the exhaust fan 30 is not transmitted, for example, 3 mm or more. It is said. Such a duct 28 does not have a step in the air flow direction and has an outer shape that is point-symmetric with respect to the central axis of the duct. That is, the shape does not have a side branch as shown in FIG.

  Further, as shown in FIG. 5, the exhaust fan 30 is a motor 30a such as a DC brushless motor that is a power source, and a plurality of (for example, seven) that are fixed to the rotating shaft of the motor 30a and rotate in synchronization with the rotation of the motor 30a. It is an axial fan provided with the wing | blade 30b. The motor 30 a is supported by a resin fan case 30 c and is driven by applying a DC voltage via a cable connector 30 d connected to a voltage generator (not shown) of the image forming apparatus 10. Further, fan spokes 30e of the same number as each wing 30b are formed in the portion of the fan case 30c facing each wing 30b to secure an opening for flowing air and the motor 30a can be supported by the fan case 30c. It is said. Further, round holes 30f are formed in the four corners of the fan case 52, and are fastened to, for example, protruding pieces provided so as to protrude into the duct 28 through the round holes 30f. In the case of the illustrated example, the exhaust fan 30 rotates in the direction indicated by the arrow in FIG. 5A and causes air to flow in the direction indicated by the arrow in FIG.

  In the present embodiment, wind noise generated mainly when each blade 30 b of the exhaust fan 30 passes through the fan spoke 30 e becomes noise generated from the exhaust fan 30. That is, pressure fluctuations occur when each wing 30b rotates and approaches the fan spoke 30e, and as a result, wind noise is generated. Therefore, the frequency of noise generated from the exhaust fan 30 is determined by the number of rotations of the motor 30a (exhaust fan 30) and the number of blades 30b. Specifically, if the rotational speed per second of the exhaust fan 30 is N (rps) and the number of each blade 30b is Z, the frequency f of the wind noise is N × Z (Hz). Further, since the wind noise of the exhaust fan 30 is generated in the vibration mode of each wing 30b, an order component of N × Z may be radiated. That is, when J is a natural number, the frequency f = N × Z × J. For example, if the rotation speed of the motor 30a is 50 rps and the number of the wings 30b is seven, sounds with frequencies of 50 × 7 = 350 Hz, 700 Hz, 1050 Hz, etc. are generated.

  As shown in FIGS. 6 and 7, the sound path changing means 29 is a plate-like member made of ABS resin, having a predetermined thickness (for example, 10 mm or more) and having a rectangular parallelepiped shape as a whole. It is installed in the direction that shields the air flow. Specifically, it is internally fitted and fixed so as to close the longitudinal intermediate portion of the duct 28. The sound path changing means 29 has a first hole 33 and a second hole 34 that form a plurality of paths that pass therethrough. Of these, the first hole 33 passes through the center of the duct 28 in parallel with the longitudinal direction of the duct 28. Further, the second hole 34 is directed toward the wall surface 28a of the duct 28 toward the downstream in the air flow direction (right to left in FIG. 7) around the first hole 33 with respect to the longitudinal direction of the duct 28. It penetrates so as to be inclined in the direction. In the case of the illustrated example, the second hole 34 is provided in four portions around the first hole 33. The first hole 33 and the second hole 34 are respectively provided with filters 35a and 35b that adsorb (having a catalyst) volatile organic compounds in the air such as ozone. Then, the air in the image forming apparatus 10 is exhausted to the outside through the filters 35a and 35b, and ozone and the like are removed by the catalyst inside the filter. In other words, the sound path changing means 29 also serves as a filter member that removes ozone and the like.

  In the present embodiment, by providing the sound path changing means 29 with the first hole 33 and the second hole 34, the sound generated in the exhaust fan 30 as described above and propagated from the upstream in the air flow direction. Is propagated into two paths when passing through the sound path changing means 29. In other words, as shown in FIG. 6B, air and sound flow through the first hole 33 and the second hole 34 of the sound path changing unit 29 in the directions of arrows α and β, respectively. Has been. Further, the filters 35a and 35b respectively disposed in the first hole 33 and the second hole 34 are provided with a plurality of regular hexagonal cylindrical catalysts so that air flows in the directions of arrows α and β. Each is arranged according to the direction of the hole where it is installed. That is, the filter 35 a disposed in the first hole 33 is disposed in parallel with the longitudinal direction of the duct 28, and the filter 35 b disposed in the second hole 34 is disposed in a direction inclined with respect to the longitudinal direction of the duct 28. To do.

  In addition, one side of the regular hexagon of each catalyst is about 1 mm, for example, and it is preferable that the thickness of the filters 35a and 35b is 10 mm or more so that the sound that has passed through the holes of each catalyst does not interfere. . Further, in order to equalize the energy of the sound divided into the two paths by the sound path changing means 29 (that is, the volume of the two paths is the same), the opening area of the first hole 33 and the second hole The configuration is such that the opening areas of 34 (the total area of all four locations) are equal.

  In addition, as described above, the sound propagation paths separated by the first hole 33 and the second hole 34 have different path lengths. In other words, a pair of paths having different path lengths are formed by the first hole and the second hole. That is, the path length of the sound passing through the first hole 33 is different from the path length of the sound passing through the holes 34 a, 34 b, 34 c, 34 d constituting the second hole 34. Here, the path passing through the first hole 33 and the path passing through the hole portion 34a constitute a pair of paths having different path lengths. Similarly, a path that passes through the first hole 33 and a path that passes through the hole 34b, a path that passes through the first hole 33, and a path that passes through the hole 34c pass through the first hole 33. And a path passing through the hole 34d constitute a pair of paths having different path lengths. In the present embodiment, the difference in length between each pair of path lengths is the same. Therefore, in the following description, a route passing through each hole 34a, 34b, 34c, 34d is simply referred to as a route passing through the second hole 34.

  First, as shown in FIG. 7, a path that passes through the first hole 33 is a path M that passes through the center of the duct 28 in the longitudinal direction. Further, a path passing through the second hole 34 is changed to a direction toward the wall surface 28a of the duct 28 by the sound path changing means 29, and is further set as a path N reflected by the wall surface 28a. The path M and the path N converge on the downstream side of the sound path changing unit 29 in the air flow direction. In the present embodiment, the route M and the route N pass through the louver 32 fixed to the outer cover 10 b of the image forming apparatus 10 shown in FIG. 4 and converge outside the image forming apparatus 10. As described above, the noise generation source of the exhaust fan 30 is the wind noise of the fan, and friction noise is generated when the fan comes into contact with air. Accordingly, the wind noise generation position of the fan is point A, point C, point E, the convergence point where path M and path N converge is point B, and the position where path N is reflected by wall surface 28a is point D, point F. Then, the route M can be represented by AB, and the route N can be represented by CDB and EFB.

Here, it is assumed that the path length of AB is L _A (mm) and the path length of CDB (EFB) is L _B (mm). Further, the sound velocity of air propagation in the duct 28 is S (mm / s), the frequency of sound propagating through the duct 28 (sound of the exhaust fan 30) is f (Hz), and K is an odd number (1, 3, 5, 7 ...). In this case, the pair of paths M and N described above are
| L _A −L _B | = S × K / (2 × f)
The relational expression is satisfied. This relational expression will be described.

As shown in FIG. 8, sound interference is indicated by a broken line whose phase is shifted by 180 ° with respect to the sound indicated by the solid line when time (s) is taken on the horizontal axis and sound pressure (Pa) is taken on the vertical axis. It is known that when a sound is applied, the sound pressure is theoretically reduced to zero. For this reason, if the path length of the path | route M and the path | route N has shifted | deviated by half wavelength, the sound which passes the inside of the duct 28 will be silenced. Therefore, in order to obtain a silencing effect, the relationship of the path length difference | L _A −L _B | may be a half wavelength. That is, assuming that the wavelength of sound is λ, | L _A −L _B | may be λ / 2. Since λ can be expressed by S / f, | L _A −L _B | can be expressed by S / (2 × f). Further, since the path length between the path M and the path N only needs to be shifted by a half wavelength, | L _A −L _B | may be K × λ / 2 if K is an odd number. As a result, the above relational expression is derived. In the case of this embodiment, the inclination angle of the second hole 34 and the filter 35b installed in the second hole 34 is related to the size of the duct 28 so that the paths M and N satisfy the above-described relational expression. It is regulated by.

In addition, there are other paths other than CDB and EFB, but the path passing through the second hole 34 is any point on the path passing through the first hole 33. And the relationship between the path lengths also satisfies the above equation. For example, when the path lengths of C′-D′- _B ′ and E′-F′- _B ′ shown in FIG. 7 are L _B ′, | L _A −L _B ′ | = S × K / (2 Xf) is satisfied.

In the above description, the above relational expression is derived starting from point A, point C (C ′), and point E (E ′). However, the point that the sound propagation path is changed is the first hole. Since this is the opening upstream of 33 and the second hole 34, the above relational expression can be derived based on this point. That is, since the path length between the path M and the path N does not change upstream of the first hole 33 and the second hole 34, the upstream opening of the first hole 33 and the second hole 34 is defined. When the starting point is used, the difference between the path lengths from the starting point to the convergence point B is the same as | L _A −L _B (L _B ′) |.

  Further, as shown in FIG. 7, the louver 32 provided in the opening on the outside of the duct 28 is disposed along the paths AB, CDB, and EFB, and the exhaust fan. The path of air and sound discharged from 30 through the duct 28 is not blocked. Specifically, the angle of each wing of the louver 32 is such that the portion overlapping the first hole 33 in the longitudinal direction is parallel to the longitudinal direction, and this peripheral portion is in the direction in which the path N reflects from the wall surface 28a of the duct 28. Each is adjusted in parallel.

  In the present embodiment, as described above, the first hole 33 and the second hole 34 have the same energy as the sound passing through the first hole 33 and the sound passing through the second hole 34. And regulate the area relationship. For this reason, as described above, the path that passes through the second 34 converges at any point on the path that passes through the first hole 33, and a sound with a different path length interferes, thereby providing a high silencing effect. Can be obtained.

  According to this embodiment, as described above, the sound path changing means 29 for changing the sound propagation path is provided in the duct 28 so that a silencing effect can be obtained. In addition, dead space can be prevented from occurring. That is, in order to obtain a silencing effect, it is not necessary to provide the side branch 4 as shown in FIG. 14 described above, and this can prevent the apparatus from becoming large and generating a dead space. Moreover, since it is not necessary to provide an expensive sound absorbing material, the cost can be reduced.

  Further, according to the present embodiment, the frequency of the fan noise is specified by the number of rotations of the exhaust fan 30 and the number of the blades 30b. Therefore, by designing the sound path changing unit 29 according to this frequency, The sound to be played can be muted reliably. In this case, it is only necessary to appropriately regulate the relationship between the first hole 33 and the second hole 34, such as changing the inclination angle of the second hole 34 of the sound path changing means 29. High degree of freedom. Note that the fan noise is a steady sound, so that the sound-deadening effect is enhanced by reliably muting the target sound as in the present embodiment. And the unpleasant sound which passes through the duct 28 can be reduced.

  In the above description, the structure using the duct 28 having a quadrangular cross-sectional shape has been described. However, it may be a circle or an ellipse, or a polygon such as a hexagon. In this case, the shapes of the exhaust fan 30 and the sound path changing means 29 are matched with the duct 28. The sound path changing means 29 forms a plurality of (even number) types of holes having different inclination angles, and the paths passing through the holes have different path lengths so that a plurality of sets satisfying the relational expression as described above are satisfied. A pair of paths may be configured. For example, four types of holes having different inclination angles may be formed, and any one of the paths may satisfy the above relational expression, and the other set may satisfy the above relational expression. That is, the sound path changing means 29 may be any means that changes the sound propagation path to at least one pair of paths having different path lengths. Further, the positions of the first hole 33 and the second hole 34 in the sound path changing unit 29 are not limited to the above-described embodiment. For example, the second hole 34 may be formed in the central portion of the sound path changing unit 29, and the first hole 33 may be formed around the second hole 34. Further, in the above-described embodiment, the filters 35 a and 35 b are provided in the first hole 33 and the second hole 34 of the sound path changing unit 29, but these filters are provided separately from the sound path changing unit 29. Also good. In other words, the holes 33 and 34 of the sound path changing unit 29 may not be provided with a filter, and a filter may be provided in a portion of the duct 28 that is shifted from the sound path changing unit 29. Further, the sound path changing means may be any means that can change the sound path other than the structure in which a plurality of holes are formed in the plate-like member as described above. For example, a plurality of wings that change the direction of air flow such as louvers may be arranged in the duct, and the angle of the wings at the center and the angle of the surrounding wings may be changed.

<Second Embodiment>
A second embodiment of the present invention will be described with reference to FIG. In the case of this embodiment, the duct 28A, the sound path changing means 29A, and the exhaust fan 30A are circular in cross section. Further, the duct 28A is once increased in diameter downstream in the air flow direction than the sound path changing means 29A, and further inclined in a direction in which the diameter gradually decreases further downstream. Such a change in diameter is less gradual in the reduced diameter portion 37 where the diameter becomes smaller toward the downstream from the enlarged portion than in the enlarged diameter portion 36 where the diameter becomes larger from the downstream than the sound path changing means. ing. Then, all the paths of the sound passing through the second hole 34A of the sound path changing means 29A are reflected by the wall surface 28a of the duct 28A and converge at one point (point H) on the central axis of the duct 28A. ing. That is, the relationship between the inclination angle of the wall surface 28a of the reduced diameter portion 37 and the inclination angle of the second hole 34A (and the filter 35b) is regulated, and the path of the sound passing through the second hole 34A is indicated by a point H. It is converged. The first hole 33A is formed in the center of the sound path changing unit 29A and is a circular hole penetrating in the longitudinal direction of the duct 28A. The second hole 34A is formed in an annular shape around the first hole 33A, and penetrates so as to incline in the direction toward the wall surface 28a as it goes downstream with respect to the longitudinal direction of the duct 28A. Therefore, the second hole 34A is formed in a partial conical shape.

  In the present embodiment, the sound path changed by the sound path changing unit 29A will be described in detail. First, a path that passes through the first hole 33A is a path M that passes through the center of the duct 28A in the longitudinal direction. Further, a path passing through the second hole 34A is changed to a direction toward the wall surface 28a of the duct 28A by the sound path changing means 29A, and is further set as a path N reflected by the wall surface 28a. These paths M and N form a pair of paths having different path lengths. Further, the wind noise generation position of the fan is point G, point I, point O, the convergence point where the path M and the path N converge is the point H, and the position where the path N is reflected by the wall surface 28a is the point K, the point Q. Then, the route M can be represented by GH, and the route N can be represented by IJKH and OPQH.

Here, it is assumed that the path length of GH is L _A (mm), and the path length of IJKH (OPQH) is L _B (L _C ) (mm). Further, the sound velocity of air propagation in the duct 28A is S (mm / s), the frequency of sound propagating through the duct 28A is f (Hz), and K is an odd number (1, 3, 5, 7,...). . In this case, the pair of paths M and N described above are
| L _A −L _B (L _C ) | = S × K / (2 × f)
The relational expression is satisfied. The routes IJKH and OPQH are the same distance from each other. Other structures and operations are the same as those in the first embodiment.

<Third Embodiment>
A third embodiment of the present invention will be described with reference to FIG. The configuration of this embodiment is almost the same as that of the first embodiment described above, but the sound path passing through the second hole 34 of the sound path changing means 29 is connected to the outlet of the duct 28 (downstream in the air flow direction). The first embodiment is different from the first embodiment in that the light is converged near the opening. Specifically, the path N passing through the center portion of the second hole 34 in the thickness direction (vertical direction in FIG. 10) and the path M passing through the center portion of the first hole 33 are on the outlet side of the duct 28. The inclination angle of the second hole 34 (filter 35b) is regulated so as to converge at the end. Thereby, the sound radiated outside from the outlet of the duct 28 is further reduced.

Also in this embodiment, the path length difference between the path M passing through the first hole 33 and the path N passing through the second hole 34 is shifted by half a wavelength. It can also be expressed as: First, the distance between the opposing surfaces of the duct 28 is X, and the distance from the starting point where the sound propagation path is changed by the sound path changing means 29 in the longitudinal direction of the duct 28 to the convergence point B where the paths M and N converge. Is Y. In the illustrated example, Y is the distance from the upstream end face of the sound path changing means 29 to the outlet side end of the duct 28. Also, let W be the distance between the starting points of the sound path M passing through the first hole 33 and the sound path N passing through the second hole 34. In the illustrated example, W is the distance between the most upstream point on the central axis of the first hole 33 and the most upstream point in the center in the thickness direction (vertical direction in FIG. 10) of the second hole 34. It is said. Furthermore, the inclination angle of the second hole 34 with respect to the longitudinal direction of the duct 28 is θ. In this case,
(X−W) / sin θ−Y = S × K / (2 × f)
The inclination angle θ is regulated in relation to the distances D, H, and L so that the following relational expression is satisfied.

This relational expression will be described. As shown in FIG. 9, route M is AB and route N is CDB (EFB). The route N is divided into CD (EF) and DB (FB). Then
CD = (X / 2−W) / sin θ, DB = X / 2 sin θ
It becomes.
Therefore, the path N becomes CD−D−B = (X−W) / sin θ.

Since the route M (AB) is the distance Y, the difference between the route M and the route N is
(X-W) / sin θ-Y
Therefore, this may be set to a half wavelength of the frequency f of the sound. Therefore,
(X−W) / sin θ−Y = S × K / (2 × f)
(S: sound velocity of air propagation in duct 28, f: frequency of sound, K: odd number)

  If this relational expression is satisfied, the sound is muted by interference near the exit. Further, when the wind noise of the exhaust fan 30 is silenced, it may be designed to match f = N × Z (N (rps): the rotational speed of the exhaust fan 30, Z: the number of feathers). As described above, when the sound is silenced near the outlet of the duct 28, if the characteristics of the exhaust fan 30 as a noise source are known, the specifications of the duct 28 and the sound path changing means 29 are appropriately set. By restricting to the above, it is possible to mute the sound near the outlet of the duct 28. That is, the dimensions of the duct 28, the installation position of the sound path changing means 29, the positional relationship between the first and second holes 33 and 34, and the inclination angle of the second hole 34 are appropriately set in relation to the characteristics of the exhaust fan 30. regulate.

  Note that by appropriately changing the above-described X, Y, W, and θ, the point to be silenced can be set to any point inside or outside the duct. Further, the above description has been given for the case where the cross-sectional shape of the duct 28 is rectangular. However, the present embodiment can be applied even when the cross-sectional shape of the duct is other polygonal or circular. Here, when the cross section orthogonal to the central axis of the duct is a polygon, X is the distance between the faces facing each other across the central axis. Similarly, when the cross section is circular, X is the inner diameter of the duct. Other configurations and operations are the same as those in the first embodiment.

  Next, an experiment conducted by the inventor for verifying the effect of the third embodiment described above will be described. FIG. 11 shows the apparatus used in this experiment. In this apparatus, a sound path changing means 29 is arranged at a predetermined position of the duct 28, and a speaker 38 is arranged at the upstream end of the duct 28 instead of the exhaust fan. The speaker 38 outputs a sine wave (sine wave) having a specific frequency from the pulse generator 39. The frequency f of the sine wave was calculated from the relationship (X−W) / sin θ−Y = S × K / (2 × f) described above. In the experiment, a sine wave having a frequency of 414 Hz was output as a result of the calculation. In addition, a microphone 40 is arranged near the outlet of the duct 28 to collect sound coming out of the duct 28, and the sound is subjected to frequency analysis by fast Fourier transform (FFT) by an FFT analyzer 41.

  Further, as a comparative example, the frequency of the sound emitted from the duct 28 was also analyzed when the filter member 42 shown in FIG. This filter member 42 has the same shape except for the portion corresponding to the sound path changing means 29 and the second hole. That is, the second hole 43 of the filter member 42 is not inclined like the second hole 34 of the sound path changing means 29 and is formed in parallel with the longitudinal direction of the duct 28, that is, in parallel with the first hole 33. doing. The first hole 33 and the second hole 43 are provided with filters 35a and 35c that allow air to flow in parallel with the respective holes.

  FIG. 13 shows the results of the experiment conducted under the above conditions. FIG. 13A shows the case where the filter member 42 is used, and FIG. 13B shows the case of the third embodiment. Further, the horizontal axis of the graph represents frequency (Hz), and the vertical axis represents sound pressure level (dB). In this experiment, the sound pressure level of the primary component at 414 Hz is 68.4 dB in FIG. 13A, whereas it is 65.34 dB in FIG. 13B, which is about 3 dB lower. It could be confirmed. It was also confirmed that noise could be reduced from 38.4 dB to 37.4 dB even at the third order component 1242 Hz. As a result, it was confirmed that the sound was reduced by utilizing the sound interference phenomenon by branching the sound propagation path into two systems and converging it in the vicinity of the outlet of the duct 28.

  27 ... Air blower, 28, 28A ... Duct, 28a ... Wall surface, 29, 29A ... Sound path changing means, 30, 30A ... Exhaust fan, 30b ... Wings, 31 ... · Silencer 33, 33A ... 1st hole, 34, 34A ... 2nd hole, 35a, 35b ... Filter

Claims (6)

A duct through which air flows, and a sound propagation path that is installed in a direction that shields the air flow in the duct and that has a plurality of paths that pass therethrough and that propagates air from upstream in the air flow direction, Sound path changing means for changing to at least one pair of paths having different path lengths by a plurality of paths,
The sound path changing means includes a first hole penetrating in the longitudinal direction of the duct, a first hole penetrating in the longitudinal direction of the duct, and a member having a predetermined thickness so as to close the longitudinal middle portion of the duct. On the other hand, forming the plurality of paths by forming a second hole penetrating so as to incline in the direction toward the wall surface of the duct as it goes downstream in the air flow direction,
The pair of paths includes a path that passes through the first hole and a path that passes through the second hole and reflects on the wall surface, and is downstream of the sound path changing unit in the air flow direction. If the convergence point is the convergence point, and the point where the sound propagation path is changed is the starting point,
The path lengths of the pair of paths from the start point to the convergence point are L _A and L _B , the air propagation sound velocity in the duct is S, the frequency of the sound propagating through the duct is f, and K is an odd number. Then
| L _A −L _B | = S × K / (2 × f)
A silencer characterized by satisfying the relational expression: 2. The muffler according to claim 1, wherein the sound path changing means forms the second hole around the first hole . The inclination angle of the second hole with respect to the longitudinal direction of the duct is regulated so that the pair of paths converge at an end of the duct on the downstream side in the air flow direction. The silencer according to 1 or 2. The frequency f of the sound has a plurality of wings, and based on the wind noise of the fan wings that form an air flow in the duct, the number of rotations per second of the fan is N, If the number is Z and J is a natural number,
f = N × Z × J
The silencer according to any one of claims 1 to 3, wherein The silencer according to any one of claims 1 to 4, wherein a filter that adsorbs a volatile organic compound in the air is provided in the first hole and the second hole.   6. The image forming apparatus, comprising: a fan that discharges air inside the apparatus to the outside; and a silencer that flows air discharged by the fan, wherein the silencer is any one of claims 1 to 5. An image forming apparatus characterized by being a muffler.

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