Abstract:
A jet generator includes a casing containing a gas and having an opening, vibrators attached to the casing, and actuators for actuating the vibrators. The vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other, thereby vibrating the gas to eject a pulsating jet thereof through the opening.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention contains subject matter related to Japanese Patent Application JP 2005-134302 filed in the Japanese Patent Office on May 2, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to jet generators for generating gas jets and electronic devices including the jet generators.  
         [0004]     2. Description of the Related Art  
         [0005]     Increased performance of personal computers (PCs) has posed the problem of increased amounts of heat generated from heat sources such as integrated circuits (ICs). Accordingly, a wide variety of heat dissipation techniques have been proposed or commercialized. For example, radiation fins formed of a metal such as aluminum are brought into contact with an IC to transmit heat from the IC to the fins and dissipate it. In addition, a fan is used to forcibly eject warm air in a PC casing and introduce ambient cool air to the vicinity of a heat source. Furthermore, a fan and radiation fins are used in combination to forcibly eject warm air around the radiation fins with increased contact area between the air and a heat source.  
         [0006]     The forced convection of air using a fan, however, causes a thermal boundary layer at the surfaces of radiation fins on the downstream side thereof. The thermal boundary layer undesirably makes it difficult to draw heat away from the radiation fins effectively. One of the possible solutions to this problem is to increase the air velocity of the fan to reduce the thickness of the thermal boundary layer. However, increasing the number of revolutions of the fan for increased air velocity undesirably causes noise, such as noise from a fan bearing and wind noise due to wind from the fan.  
         [0007]     Japanese Unexamined Patent Application Publication Nos. 2000-223871, 2000-114760, 2-213200, and 3-116961, for example, disclose methods for efficiently dissipating heat from radiation fins to the outside air by breaking the thermal boundary layer without using a fan as an air blower. These methods involve the use of a diaphragm that reciprocates periodically. In particular, Japanese Unexamined Patent Application Publication Nos. 2-213200 and 3-116961 disclose devices including a diaphragm that separates the space in a chamber substantially in half, an elastic member disposed in the chamber so as to support the diaphragm, and means for vibrating the diaphragm. The diaphragm, when displaced upward, decreases the volume of the upper space of the chamber to increase the pressure therein. The increased pressure in the upper space forces part of the air contained therein into the outside air. The upper space communicates with the outside air through inlet/outlet openings. At the same time, the diaphragm increases the volume of the lower space, opposite the upper space across the diaphragm, to decrease the pressure therein. The decreased pressure in the lower space forces part of the outside air into the lower space. The lower space communicates with the outside air through inlet/outlet openings. When displaced downward, on the other hand, the diaphragm increases the volume of the upper space of the chamber to decrease the pressure therein. The decreased pressure in the upper space forces part of the outside air into the upper space through the inlet/outlet openings. At the same time, the diaphragm decreases the volume of the lower space to increase the pressure therein. The increased pressure in the lower space forces part of the air contained therein into the outside air through the inlet/outlet openings. The diaphragm is, for example, electromagnetically actuated. The diaphragm thus reciprocates and periodically repeats the ejection of the air contained in the chamber to the outside air and the suction of the outside air into the chamber. The periodic reciprocating motion induces a pulsating air jet which impinges on a heat source such as radiation fins (heatsink). The pulsating air jet efficiently breaks a thermal boundary layer on the surface of the heat source, thus efficiently cooling the heat source.  
       SUMMARY OF THE INVENTION  
       [0008]     In recent years, the amounts of heat generated from ICs have been rising with increasing clock speed. Accordingly, for example, a larger amount of air supply is demanded for ICs and radiation fins to break a thermal boundary layer caused near the fins after heat generation. In air ejection techniques using a diaphragm that reciprocates periodically as disclosed in the above publications, the amount of air ejected can be increased by increasing the amplitude of vibration of the diaphragm. If the amplitude of vibration is increased, however, the vibration of the diaphragm is undesirably transmitted through, for example, a casing of a jet generator and a casing of an electronic device including the jet generator.  
         [0009]     This problem arises from a vibrational force produced by the reciprocating motion of the diaphragm, which has weight, and an actuator that actuates the diaphragm. The transmission of vibration can be reduced by, for example, decreasing the weight or amplitude of vibration of the diaphragm or the frequency used. However, there are trade-offs between the reduction in the weight of the diaphragm and the maintenance of the strength thereof and between the reduction in amplitude of vibration and frequency and the increase in the amount of air ejected for increased cooling efficiency (the amount of air ejected is proportional to the product of the amplitude of vibration, the effective cross-sectional area, and the frequency).  
         [0010]     Accordingly, it is desirable to provide a jet generator that can inhibit the transmission of vibration to the outside thereof without decreasing the amount of gas ejected or cooling capability and also provide an electronic device including the jet generator.  
         [0011]     A jet generator according to an embodiment of the present invention includes a casing containing a gas and having an opening, vibrators attached to the casing, and actuators for actuating the vibrators. The vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other, thereby vibrating the gas to eject a pulsating jet thereof through the opening.  
         [0012]     This jet generator can inhibit the transmission of vibration to the outside of the casing or the jet generator because the vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other. In addition, the jet generator can avoid a decrease in the amount of gas ejected, or rather can increase it, because the vibrational forces attenuate each other even for increased amplitudes of vibration.  
         [0013]     For example, at least one of the mass, structure, amplitude of vibration, and phase of the vibrators may be adjusted so that the vibrational forces attenuate each other. Alternatively, the vibrators may be arranged in such a manner that the vibrational forces attenuate each other, as described later.  
         [0014]     The vibrators may be arranged in any manner that allows the vibrational forces thereof to attenuate each other after synthesis. For example, the vibrators may be arranged in the vibration direction or perpendicularly thereto. In addition, the vibrators may be arranged in three dimensions. For example, three vibrators may be arranged with the vibration directions thereof tilted 120° from each other (such that they define, for example, a triangular prism), or four vibrators may be arranged with the vibration directions thereof tilted 90° from each other (such that they define, for example, a rectangular parallelepiped). The term “vibration direction” herein is unrelated to phase; this term represents the direction of reciprocating motion, namely vibration, and is hereinafter used with this meaning.  
         [0015]     Although the gas used is typically air, other gases may also be used, including nitrogen gas, helium gas, and argon gas.  
         [0016]     The actuators may actuate the vibrators with, for example, an electromagnetic effect, a piezoelectric effect, or an electrostatic effect.  
         [0017]     The vibrators may have a three-dimensional structure, rather than a flat structure. Such vibrators are exemplified by those having side plates or ribs for increasing rigidity, although any three-dimensional structure may be used for any purpose. Examples of the shape of the vibrators in a plane perpendicular to the vibration direction include a circle, an ellipse, and a rectangle.  
         [0018]     In this embodiment, two of the vibrators may face each other and be actuated by the actuators so as to move toward and away from each other. This allows the vibrational forces to attenuate each other. In this case, the vibrators may, for example, have different sizes, have different shapes, or be formed of different materials.  
         [0019]     In this embodiment, preferably, the vibrators have the same size and shape, are formed of the same material, and vibrate with the same frequency, and the actuators actuate the vibrators with a phase difference of substantially 360/n° from each other where n is the number of the vibrators. This allows the vibrational forces to attenuate each other. The same size, shape, and material described above mean sizes, shapes, and materials, respectively, that are sufficiently similar to achieve the embodiment of the present invention, that is, that can be construed as being substantially identical in terms of mass production, rather than as being physically completely identical.  
         [0020]     This embodiment preferably meets the following conditions: the number of the vibrators is at least three; the vibrators have the same size and shape, are formed of the same material, and vibrate with the same frequency; a first vibrator group including at least two of the vibrators is actuated to vibrate at a first phase; the sum of the amplitudes of vibration of the first vibrator group is a first amplitude of vibration; at least one of the vibrators other than the first vibrator group is actuated to vibrate at a second phase opposite the first phase; and the sum of the amplitude of vibration of the at least one vibrator is a second amplitude of vibration equal to the first amplitude of vibration. The vibration of the vibrators may thus be controlled so that the vibrational forces thereof attenuate each other after synthesis.  
         [0021]     In this embodiment, at least two of the vibrators may differ in at least one of size, shape, and material. Even if the jet generator includes two or more different types of vibrators, the amplitudes of vibration or phases thereof, for example, may be controlled so that the vibrational forces thereof attenuate each other after synthesis.  
         [0022]     A jet generator according to another embodiment of the present invention includes casings that contain a gas and each have an opening, vibrators attached to the individual casings, and actuators disposed in the individual casings to actuate the vibrators. The vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other, thereby vibrating the gas to eject a pulsating jet thereof through the openings.  
         [0023]     This jet generator can inhibit the transmission of vibration to the outside of the casings or the jet generator because the vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other. Each of the casings may have a single opening or a plurality of openings.  
         [0024]     In this embodiment, preferably, the number of the vibrators is at least three, a first vibrator group including at least two of the vibrators is actuated to vibrate at a first phase in a first direction, and at least one of the vibrators other than the first vibrator group is actuated to vibrate at a second phase opposite the first phase in the first direction. The vibrators do not necessarily have to have the same size and shape or be formed of the same material, and may be arranged and actuated by the actuators  5  so that the vibrational forces thereof attenuate each other.  
         [0025]     In this embodiment, preferably, the vibrators vibrate in the same direction, and the casings are arranged in the vibration direction. In this case, at least two of the vibrators vibrate at different phases in the same direction. This allows effective ejection of the gas toward objects, such as heat sources, arranged in one or two dimensions in a plane including the vibration direction. Alternatively, preferably, the vibrators vibrate in the same direction, and the casings are arranged in a plane substantially perpendicular to the vibration direction. This allows the ejection of the gas toward objects, such as heat sources, arranged in one or two dimensions in the plane substantially perpendicular to the vibration direction.  
         [0026]     In this embodiment, the casings may have engaging portions that engage with each other. These engaging portions allow the casings to be stacked on top of each other or to be arranged in a plane according to the shapes and positions of objects of interest, such as heat sources, to achieve, for example, effective heat dissipation.  
         [0027]     An electronic device according to another embodiment of the present invention includes a heat source, a jet generator casing containing a gas and having an opening, vibrators attached to the casing, and actuators for actuating the vibrators. The vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other, thereby vibrating the gas to eject a pulsating jet thereof through the opening toward the heat source.  
         [0028]     An electronic device according to another embodiment of the present invention includes a heat source, jet generator casings that contain a gas and each have an opening, vibrators attached to the individual casings, and actuators disposed in the individual jet generator casings to actuate the vibrators. The vibrators vibrate with the vibrational forces thereof being synthesized so as to attenuate each other, thereby vibrating the gas to eject a pulsating jet thereof through the openings toward the heat source.  
         [0029]     Examples of the electronic devices include computers (such as laptop PCs and desktop PCs), personal digital assistants (PDAs), electronic dictionaries, cameras, displays, audio/video equipment, cellular phones, game machines, and other electrical appliances. The heat source may be any object that releases heat. Examples of the heat source include, though not limited to, electronic components such as ICs and resistors and radiation fins (heatsinks).  
         [0030]     The jet generators and the electronic devices according to the embodiments described above can inhibit the transmission of vibration to the outside of the jet generators without decreasing the amount of gas ejected or cooling capability. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a perspective view of a jet generator according to an embodiment of the present invention;  
         [0032]      FIG. 2  is a sectional view of the jet generator shown in  FIG. 1 ;  
         [0033]      FIG. 3  is a graph showing how diaphragms vibrate with the vibrational forces thereof attenuating each other;  
         [0034]      FIG. 4  is a sectional view of a jet generator according to another embodiment of the present invention;  
         [0035]      FIG. 5  is a sectional view of a jet generator according to another embodiment of the present invention;  
         [0036]      FIG. 6  is a sectional view of a jet generator according to another embodiment of the present invention;  
         [0037]      FIG. 7  is a sectional view of a jet generator according to another embodiment of the present invention;  
         [0038]      FIG. 8  is a graph showing variations in the amplitudes of vibration of diaphragms included in jet-generating units;  
         [0039]      FIG. 9  is another graph showing variations in the amplitudes of vibration of the diaphragms included in the jet-generating units;  
         [0040]      FIG. 10  is a sectional view of a jet generator according to another embodiment of the present invention;  
         [0041]      FIGS. 11A and 11B  are sectional views of jet generators that inhibit the occurrence of a moment according to other embodiments of the present invention;  
         [0042]      FIGS. 12A  to  12 F are schematic diagrams of jet generators including jet-generating units according to other embodiments of the present invention;  
         [0043]      FIGS. 13A  to  13 F are schematic diagrams of jet generators including diaphragms in a single casing according to other embodiments of the present invention;  
         [0044]      FIGS. 14A  to  14 F are schematic diagrams of electronic devices including jet generators according to other embodiments of the present invention;  
         [0045]      FIGS. 15A  to  15 F are schematic diagrams illustrating the relative positions of heat sources and jet-generating units in other embodiments of the present invention;  
         [0046]      FIGS. 16A and 16B  are sectional views of an electronic device including a casing integrated with casings of jet-generating units according to another embodiment of the present invention;  
         [0047]      FIGS. 17A and 17B  are sectional view of casings of jet-generating units stacked on top of each other according to another embodiment of the present invention;  
         [0048]      FIG. 18  is a bottom view of the casing of each jet-generating unit shown in  FIG. 17A ;  
         [0049]      FIG. 19  is a sectional view of casings according to a modification of the embodiment shown in  FIG. 17B ;  
         [0050]      FIG. 20  is a sectional view of an electronic device including the jet generator shown in  FIG. 10  according to another embodiment of the present invention;  
         [0051]      FIG. 21  is a sectional view of an electronic device including the jet generator shown in  FIG. 10  according to another embodiment of the present invention;  
         [0052]      FIG. 22  is a sectional view of an electronic device including the jet generator shown in  FIG. 10  according to another embodiment of the present invention;  
         [0053]      FIG. 23  is a plan view of the jet generator shown in  FIG. 22 ;  
         [0054]      FIGS. 24A and 24B  are partial side views of the electronic device shown in  FIG. 22 ; and  
         [0055]      FIG. 25  is a side view of an example of a movable member. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0056]     Embodiments of the present invention will now be described with reference to the drawings.  
         [0057]      FIG. 1  is a perspective view of a jet generator according to an embodiment of the present invention.  FIG. 2  is a sectional view of the jet generator.  
         [0058]     A jet generator  10  includes a casing  1  containing air. This casing  1  has, for example, a rectangular parallelepiped shape. The casing  1  includes, for example, two opposing diaphragms  3   a  and  3   b  and actuators  5   a  and  5   b  for actuating the diaphragms  3   a  and  3   b , respectively. For example, the actuator  5   a  is disposed on the top side of the casing  1 , and the actuator  5   b  is disposed on the bottom side of the casing  1 . Elastic supports  6   a  and  6   b  are attached to the peripheries of the diaphragms  3   a  and  3   b , respectively. The elastic supports  6   a  and  6   b  are also attached to ribs  7  protruding from the inner walls of the casing  1 . That is, the diaphragms  3   a  and  3   b  are attached to the elastic supports  6   a  and  6   b  so as to be vibratable with respect to the casing  1 . The diaphragms  3   a  and  3   b  and the elastic supports  6   a  and  6   b  separate the space in the casing  1  into three chambers  11   a ,  11   b , and  11   c.    
         [0059]     The chamber  11   b  has a larger volume than the chambers  11   a  and  11   c . This structure, however, does not necessarily have to be employed, and the chambers  11   a ,  11   b , and  11   c  may all have identical or different volumes.  
         [0060]     Arrays of openings  1   a  to id are provided in a side surface  12  of the casing  1 . The openings  1   a  communicate with the chamber  11   a . The openings  1   b  and  1   c  communicate with the chamber  11   b . The openings id communicate with the chamber  11   c . The air contained in the chambers  11   a ,  11   b , and  11   c  is ejected through the openings  1   a  to id toward a heat source (not shown) such as a heatsink.  
         [0061]     The two actuators  5   a  and  5   b , which have the same structure, each include, for example, a cylindrical yoke  8 , a magnet  14  accommodated in the yoke  8  and magnetized in the vibration direction R of the diaphragms  3   a  and  3   b , and a disc-shaped yoke  18  attached to the magnet  14 . The magnet  14  and the yokes  8  and  18  constitute a magnetic circuit. A coil bobbin  9  having a coil  17  wound therearound moves into and out of the space between the magnet  14  and the yoke  8 . That is, the actuators  5   a  and  5   b  are composed of voice coil motors. The actuators  5   a  and  5   b  are connected to drive ICs (not shown) through feed lines (not shown) connected to the coils  17 . The drive ICs supply electrical signals to the actuators  5   a  and  5   b  through the feed lines to vibrate the diaphragms  3   a  and  3   b  in the vibration direction R.  
         [0062]     The casing  1  is formed of, for example, resin, rubber, metal, or ceramic. In particular, resin and rubber are suitable for mass production because of their formability. In addition, resin and rubber can inhibit, for example, noise from the actuators  5   a  and  5   b  and jet noise due to the vibration of the diaphragms  3   a  and  3   b . That is, if the casing  1  is formed of resin or rubber, it can inhibit the noise with high attenuation. Furthermore, these materials allow for reductions in weight and cost. Among metals, copper and aluminum are preferred for their high thermal conductivity in view of heat dissipation from the casing  1 . The elastic supports  6   a  and  6   b  are formed of, for example, resin or rubber.  
         [0063]     The diaphragms  3   a  and  3   b  are formed of, for example, resin, paper, rubber, or metal. The diaphragms  3   a  and  3   b  do not necessarily have to have a flat shape as shown in the drawings and may also have a three-dimensional shape such as a conical shape like diaphragms for loudspeakers. The planar shape (the shape in a plane substantially perpendicular to the vibration direction R) of the diaphragms  3   a  and  3   b  is not limited to the rectangular shape shown in  FIG. 1 ; the diaphragms  3   a  and  3   b  may also have, for example, a circular shape, an elliptical shape, or a combination of a circle and a rectangle, that is, a rectangular shape with rounded corners.  
         [0064]     The operation of the jet generator  10  is then described below.  
         [0065]     The actuators  5   a  and  5   b  are supplied with, for example, a sinusoidal AC voltage to induce the sinusoidal vibration of the diaphragms  3   a  and  3   b . Specifically, the actuators  5   a  and  5   b  actuate the diaphragms  3   a  and  3   b , respectively, so that they move toward and away from each other to increase or decrease the volumes of the chambers  11   a ,  11   b , and  11   c . The changes in the volumes thereof vary the pressures therein to produce a pulsating air jet through the openings  1   a  to  1   d . If, for example, the diaphragms  3   a  and  3   b  are displaced in such directions as to increase the volumes of the chambers  11   a  and  11   c , respectively, the pressures in the chambers  11   a  and  11   c  decrease and the pressure in the chamber  11   b  increases. As a result, the air outside the casing  1  flows into the chambers  11   a  and  11   c  through the openings  1   a  and  1   d , respectively, while the air contained in the chamber  11   b  is ejected to the outside of the casing  1  through the openings  1   b  and  1   c . If, on the other hand, the diaphragms  3   a  and  3   b  are displaced in such directions as to decrease the volumes of the chambers  11   a  and  11   c , respectively, the pressures in the chambers  11   a  and  11   c  increase so that the air contained in the chambers  11   a  and  11   c  is ejected to the outside through the openings  1   a  and  1   d.    
         [0066]     When the air is ejected through the openings  1   a  to id, the atmospheric pressure outside the casing  1  decreases around the openings  1   a  to  1   d . As a result, the ambient air is drawn to the air ejected through the openings  1   a  to id to produce a synthetic jet. The synthetic jet is allowed to impinge on a heat source, such as a heatsink, and cool it.  
         [0067]      FIG. 3  is a graph showing the attenuation by synthesis of vibrational forces produced by the vibration of the diaphragms  3   a  and  3   b . In  FIG. 3 , the thin line represents variations in the amplitude of vibration of the diaphragm  3   a , and the dashed line represents variations in the amplitude of vibration of the diaphragm  3   b . This graph shows the variations in the amplitudes of vibration of the diaphragms  3   a  and  3   b  for the configuration shown in  FIG. 2 . The thick line represents the amplitude of the two superposed waves, which is ideally zero. The variations in amplitude of vibration are in phase with the variations in vibrational force because an equation describing the amplitude of vibration (Y=A sin ωt where A is the amplitude, ω is angular velocity, and t is time) is differentiated two times with respect to time to yield an equation describing acceleration. Accordingly, the variations in amplitude of vibration are proportional to the variations in vibrational force. If, therefore, the vibration of one diaphragm is out of phase with that of the other diaphragm, the vibrational forces thereof are synthesized so as to attenuate each other.  
         [0068]     Sound waves occur in the vicinities of the openings  1   a  to id when the air is ejected to the outside through the openings  1   a  to id. These sound waves attenuate each other and result in reduced noise because the vibration of the diaphragm  3   a  is out of phase with that of the diaphragm  3   b  and thus the timing when the air is ejected through the openings  1   b  and  1   c  is out of phase with the timing when the air is ejected through the openings  1   a  and  1   d.    
         [0069]     The jet generator  10 , as described above, can inhibit the transmission of the vibration of the diaphragms  3   a  and  3   b  to the outside of the casing  1  or the jet generator  10  because the diaphragms  3   a  and  3   b  vibrate so that the vibrational forces thereof attenuate each other. In addition, the jet generator  10  can avoid a decrease in the amount of air ejected, or rather can increase it, because the vibrational forces of the diaphragms  3   a  and  3   b  attenuate each other even for increased amplitudes of vibration.  
         [0070]      FIG. 4  is a sectional view of a jet generator according to another embodiment of the present invention. The description below will focus on differences from the jet generator  10  according to the embodiment described above, and the same members and functions, for example, as in the above embodiment are not or only briefly described.  
         [0071]     A jet generator  20  includes a first jet-generating unit  120  and a second jet-generating unit  220  that are stacked on top of each other. The first jet-generating unit  120  includes a casing  121  accommodating a diaphragm  3  and an elastic support  6  which separate the space in the casing  121  into a first chamber  131   a  and a second chamber  131   b . The second jet-generating unit  220  includes a casing  221  having the same structure as the casing  121  of the first jet-generating unit  120 . The second jet-generating unit  220  is disposed upside down with respect to the position of the first jet-generating unit  120  with the diaphragms  3  thereof facing each other.  
         [0072]     Actuators  5  actuate the diaphragms  3  so as to decrease the volumes of the chambers  131   b  and  231   a  while increasing the volumes of the chambers  131   a  and  231   b . On the other hand, the actuators  5  actuate the diaphragms  3  so as to increase the volumes of the chambers  131   b  and  231   a  while decreasing the volumes of the chambers  131   a  and  231   b . These operations eject a pulsating air jet through openings  121   a ,  121   b ,  221   a , and  221   b.    
         [0073]     The two jet-generating units  120  and  220  can allow the vibrational forces of the diaphragms  3  to attenuate each other. The jet generator  20  thus has the same advantages as the jet generator  10  shown in  FIGS. 1 and 2 .  
         [0074]      FIG. 5  is a sectional view of a jet generator according to another embodiment of the present invention. A jet generator  30  includes two jet-generating units  130  and  230  having the same structure and arranged with the diaphragms  3  thereof facing away from each other in the vibration direction R. The jet generators  130  and  230  include casings  131  and  231 , respectively, accommodating actuators  5 . For example, the jet generator  30  allows the diaphragms  3  to move toward and away from each other so that the vibrational forces thereof attenuate each other.  
         [0075]      FIG. 6  is a sectional view of a jet generator according to another embodiment of the present invention. A jet generator  40  includes two jet-generating units  140  and  240  that are stacked on top of each other. This jet generator  40  differs from the jet generator  20  shown in  FIG. 4  in the shape of diaphragm. In  FIG. 6 , a diaphragm  33   b  of the jet-generating unit  240 , for example, is thicker than a diaphragm  33   a  of the jet-generating unit  140 .  
         [0076]     Even if the diaphragms  33   a  and  33   b  have different sizes, have different shapes, or are formed of different materials, for example, the diaphragms  33   a  and  33   b  may be allowed to move toward or away from each other so that the vibrational forces thereof attenuate each other after synthesis. A residual force may be left after the attenuation of the vibrational forces by synthesis. The vibrational forces may also be substantially eliminated by, for example, increasing the amplitude of vibration of the diaphragm  33   a  to larger than that of the diaphragm  33   b , which has a larger mass than the diaphragm  33   a.    
         [0077]      FIG. 7  is a sectional view of a jet generator according to another embodiment of the present invention. A jet generator  50  includes three jet-generating units  150 ,  250 , and  350  stacked on top of each other and having the same structure as the jet-generating units  120  and  220  shown in  FIG. 4 . The jet-generating units  150  and  250  face the same direction while the jet-generating unit  350  faces the opposite direction.  FIG. 8  is a graph showing variations in the amplitudes of vibration of diaphragms  3   a ,  3   b , and  3   c  included in the jet-generating units  150 ,  250 , and  350 , respectively.  FIG. 8  shows that the diaphragms  3   a ,  3   b , and  3   c  vibrate with a phase difference of 120° from each other. As in  FIG. 8 , waves representing the amplitudes of vibration of n diaphragms are superposed to leave no vibrational force if the diaphragms vibrate with a phase difference of 360/n° from each other.  
         [0078]     The three diaphragms  3   a ,  3   b , and  3   c  may also vibrate as shown in  FIG. 9 . If one diaphragm has an amplitude of vibration of 1.0 in the graph of  FIG. 9 , for example, the other two diaphragms each have an amplitude of vibration of 0.5 in opposite phase.  
         [0079]     The diaphragms  3   a ,  3   b , and  3   c  preferably have the same size and shape and be formed of the same material, for example, to achieve waveforms as shown in  FIGS. 7 and 8 .  
         [0080]      FIG. 10  is a sectional view of a jet generator according to another embodiment of the present invention. A jet generator  110  includes jet-generating units  120  (which are the same as the jet-generating unit  120  or  220  shown in  FIG. 4 ) arranged in a plane perpendicular to the vibration direction R of diaphragms  3   a  and  3   b . In the drawing, openings  121   a  and  121   b  are positioned so that air is ejected perpendicularly to the page. In the vibration of the jet generator  110 , the diaphragm  3   a  moves downward when the diaphragm  3   b  moves upward, and vice versa. The vibrational forces of the diaphragms  3   a  and  3   b  are then synthesized and converted into a moment acting on the overall jet generator  110  in a direction indicated by arrow T. This arrangement can therefore inhibit an adverse effect on an electronic device including the jet generator  110  and can also reduce noise. It should be noted that the synthesized vibrational force is also said to be “attenuated” when the force is converted into a moment, as in this embodiment, because the conversion results in a reduction in the vibrational force acting on the overall device.  
         [0081]     The occurrence of the moment may be inhibited by arranging at least three jet-generating units  120  longitudinally, as shown in  FIGS. 11A and 11B . In  FIG. 11A , for example, diaphragms  3   a  and  3   c  move upward when a diaphragm  3   b  moves downward. If the diaphragms  3   a  to  3   c  have the same size and shape and are formed of the same material, for example, the resultant vibrational forces may be minimized by substantially balancing the synthesized vibrational force (amplitude of vibration) of the diaphragms  3   a  and  3   c  with the vibrational force (amplitude of vibration) of the diaphragm  3   b . In  FIG. 11B , for example, the synthesized vibrational force can be attenuated by allowing the diaphragms  3   a  and  3   d  to move upward when the diaphragms  3   b  and  3   c  move downward.  
         [0082]      FIGS. 12A  to  12 F are schematic diagrams of jet generators including jet-generating units.  FIG. 12A  shows a jet generator including jet-generating units  120  stacked on top of each other as shown in  FIG. 4 .  FIG. 12B  shows a jet generator as shown in  FIG. 10 .  FIG. 12C  shows a jet generator including jet-generating units  120  arranged in two columns and two rows.  FIG. 12D  shows a jet generator including n jet-generating units  120  stacked on top of each other.  FIG. 12E  shows a jet generator including m jet-generating units  120  arranged longitudinally.  FIG. 12F  shows a jet generator including jet-generating units  120  arranged in n columns and m rows. In these embodiments, the vibrational forces of diaphragms can be allowed to attenuate each other after synthesis by adjusting, for example, the amplitudes of vibration, phases, or arrangements of the diaphragms. In addition, these embodiments provide greater versatility because the jet-generating units  120 , which have the same structure, can be arranged and combined according to the size and shape of a heat source of interest.  
         [0083]     Jet generators shown in  FIGS. 13A  to  13 F according to other embodiments of the present invention are similar to those shown in  FIGS. 12A  to  12 F. The jet generators shown in  FIGS. 13A  to  13 F include a single casing accommodating diaphragms.  FIG. 13A , for example, shows a jet generator as shown in  FIG. 2 . That is, the number of regions separated in a single casing is equal to the number of diaphragms. These embodiments can allow the resultant vibrational forces to attenuate each other after synthesis. If, particularly, a jet generator is designed for cooling a heat source of a given size, these embodiments have advantages such as reductions in the amount of material used and the size of the overall jet generator.  
         [0084]      FIGS. 14A  to  14 F are schematic diagrams of electronic devices including jet generators according to other embodiments of the present invention.  FIG. 14A  shows a casing  100  of an electronic device, such as a PC, and jet-generating units  60  and  70  included in the casing  100 . Although the jet-generating units  60  and  70  differ in, for example, the size of casing in the drawing, they have the same basic structure and principle as those described above. The jet-generating unit  60  has the same structure as, for example, the jet-generating unit  120  shown in  FIG. 4 . Various arrangements of jet-generating units are permitted as exemplified in  FIGS. 14A  to  14 F.  
         [0085]     The jet-generating units  60  and  70  (and other jet-generating units  80  and  90 ) are in contact with each other in  FIGS. 14A  to  14 C while they are separated from each other in  FIGS. 14D  to  14 F, in which the vibrational forces of the jet-generating units  60  and  70 , for example, attenuate each other through the casing  100 .  
         [0086]      FIGS. 15A  to  15 F are schematic diagrams illustrating the relative positions of heat sources and jet-generating units in other embodiments of the present invention. In  FIGS. 15A  to  15 C, a single heat source  95 , such as a heatsink, is disposed in a casing  100  of an electronic device such as a PC. In  FIGS. 15D  to  15 F, heat sources  95   a  and  95   b , for example, are disposed in the casing  100 . Jet-generating units may be assigned to individual heat sources. Any of the arrangements shown in  FIGS. 15A  to  15 F can allow the vibrational forces to attenuate each other. The optimum arrangement may be determined with consideration given to the size of electronic devices, the capacities and arrangement of heat sources, and the sizes and capacities of jet-generating units.  
         [0087]      FIGS. 16A and 16B  are sectional views of an electronic device according to another embodiment of the present invention. This electronic device includes a casing integrated with casings of jet-generating units. In  FIG. 16A , the electronic device includes a casing  200  having walls  200   a ,  200   b , and  200   c  protruding from the inner bottom surface thereof. The casing  200  can be integrally formed with the walls  200   a ,  200   b , and  200   c . In  FIG. 16B , jet-generating units  130  and  135  are fixed to the walls  200   a ,  200   b , and  200   c . The jet-generating units  130  and  135  have the same structure as those shown in  FIG. 5 . As compared to, for example, the case where the jet generator  110  shown in  FIG. 10  is directly attached to the casing  200 , this embodiment allows for a reduction in the thickness of the electronic device by the thickness of the casings of the jet-generating units  130  and  135 . In this embodiment, a synthesized vibrational force is converted into a moment by allowing the diaphragm  3   a  to move downward while the diaphragm  3   b  moves upward.  
         [0088]      FIGS. 17A and 17B  illustrate the casing structure of a jet generator according to another embodiment of the present invention. This jet generator includes jet-generating units  120 , as shown in  FIG. 4 , including casings  121  stacked on top of each other.  FIG. 17B  is an enlarged view of parts X, Y, and Z circled by the dotted lines in  FIG. 17A . The jet-generating units  120  have bumps  121   c  on the top surfaces of the casings  121  and recesses  121   d  on the bottom surfaces of the casings  121 . The bumps  121   c  and the recesses  121   d  are disposed in, for example, the vicinities of the four corners, as shown in  FIG. 18 . This structure allows the bumps  121   c  to engage with the recesses  121   d  so that the jet-generating units  120  can readily be stacked and aligned.  
         [0089]     Although the four bumps  121   c  and the four recesses  121   d  are disposed on each casing  121  in  FIG. 18 , more or less than four bumps  121   c  and more or less than four recesses  121   d  may also be provided. If the bumps  121   c  and the recesses  121   d  are provided on, for example, all six surfaces of each casing  121 , including the top and bottom surfaces thereof, the casings  121  can be arranged in every direction. This allows the casings  121  to be stacked on top of each other or to be readily arranged in a plane according to the shapes and positions of objects of interest, such as heat sources, to achieve, for example, effective heat dissipation.  
         [0090]     The sizes and shapes of the bumps  121   c  and the recesses  121   d  are not limited to those in  FIGS. 17B and 18 . Although the bumps  121   c  and the recesses  121   d  have a circular shape in  FIG. 18 , they may also have other shapes including a rectangular shape and an elongated rail shape.  
         [0091]      FIG. 19  is a sectional view of the casings  121  shown in  FIG. 17B  according to a modification of the embodiment described above. In this modification, the bumps  121   c  each have a depression  121   e  which may be filled with, for example, a bonding material  123  such as an adhesive. These depressions  121   e  may also be disposed on other portions of the surfaces of the casings  121 .  
         [0092]      FIG. 20  is a sectional view of an electronic device including the jet generator  110  shown in  FIG. 10  according to another embodiment of the present invention. In this embodiment, the jet generator  110  is attached to the inner bottom surface of a casing  200  of the electronic device, such as a PC, with a damping member  15  disposed therebetween to inhibit the transmission of vibration from the jet generator  110  to the casing  200 . The damping member  15  may be formed of a material that can readily absorb vibration and impact, such as resin, rubber, and a low-repulsion material.  
         [0093]     Alternatively, in  FIG. 21 , the casing  200  may have a suspension structure for elastically supporting the jet generator  110  with elastic members  13  formed of, for example, springs or rubber.  
         [0094]      FIG. 22  illustrates a suspension structure of an electronic device according to another embodiment of the present invention.  FIG. 23  is a plan view of a jet generator  160  shown in  FIG. 22 .  FIG. 22  is a sectional view taken along line XXII-XXII in  FIG. 23 . This jet generator  160  includes two jet-generating units  120  including casings  121  coupled by a coupling member  165 . Two pillars  19 , for example, protrude from the inner bottom surface of a casing  200  of the electronic device. These pillars  19  support the jet-generating units  120  with a movable member  16  movably in the vertical direction and tiltably with respect to the horizontal direction (see  FIG. 24B ). The movable member  16  has elastic force in the vertical direction and the tilt direction (the rotation direction) indicated by the arrows shown in  FIG. 24B . The coupling member  165  is fixed to the movable member  16  to prevent the jet generator  160  from coming into contact with the casing  200 , that is, to suspend the jet generator  160  in the casing  200 . The coupling member  165  may be integrally formed with the casings  121 .  
         [0095]     In  FIG. 24A , for example, the two casings  121  (see  FIGS. 22 and 23 ) are in a horizontal position. When diaphragms  3  of the jet-generating units  120  are actuated, a moment acts on the overall the jet generator  160 , as described in the embodiment shown in  FIG. 10 , to tilt the jet generator  160  in the rotation direction, as shown in  FIG. 24B . The resulting vibration is then negligibly transmitted to the electronic device because the jet generator  160  is suspended.  
         [0096]      FIG. 25  illustrates an example of the structure of the movable member  16 . The movable member  16  includes, for example, two plates  16   a  and  16   b  stacked with springs  16   c  disposed therebetween. The coupling member  165  is fixed to the upper plate  16   a  so that the jet generator  160  can move in the vertical direction and the rotation direction.  
         [0097]     Which structure has the best effect of attenuating the vibration of an electronic device among the structures shown in  FIGS. 20, 21 , and  22  depends on various factors, including the size, shape, and weight of the electronic device; the size, shape, and weight of the jet generator used; and the direction of reciprocating motion and drive frequency of the diaphragms used.  
         [0098]     The present invention is not limited to the embodiments described above, and various modifications are permitted.  
         [0099]     Although the simple openings  1   a  to id are provided on the casing  1  in  FIGS. 1 and 2 , nozzles may be attached to the openings  1   a  to id. The nozzles may then be integrally formed with the casing  1 .  
         [0100]     At least two of the features of the embodiments shown in the drawings may be used in any combination.  
         [0101]     The jet generators described above may also be used to supply fuel to fuel cells. Specifically, the nozzles (or openings) of the jet generators according to the embodiments described above may be disposed opposite oxygen (air) inlets of fuel cell bodies. The jet generators can thus inject a jet into the inlets as an oxygen fuel.  
         [0102]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.