Jet generating device and electronic apparatus

A jet generating device includes a vibrating member which vibrates gas, a driving unit which drives the vibrating member, and a housing which has a first opening and a first chamber connected to the first opening and containing the gas. The housing supports the vibrating member, and is such that, of sounds generated as a result of vibrating the vibrating member, the sound having a maximum noise level has a predetermined frequency. In addition, the housing discharges the gas as pulsating gas through the first opening as a result of driving the vibrating member.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2005-123432 filed in the Japanese Patent Office on Apr. 21, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a jet generating device which generates a jet of gas and an electronic apparatus including the jet generating device.

2. Description of the Related Art

Hitherto, an increase in the performance of a personal computer (PC) has caused an increase in the amount of heat generated from a heating element such as an integrated circuit (IC). Therefore, various heat-dissipation technologies or products have been proposed. Examples of heat-dissipation methods include the following. In one method, heat is dissipated by bringing a radiating fin, formed of a metal such as aluminum, into contact with an IC and conducting heat from the IC to the radiating fin. In another method, a fan is used to dissipate heat. The fan, for example, forcefully removes air warmed in a housing of a personal computer and introduces surrounding low-temperature air to the vicinity of a heating element. In still another method, a radiating fin and a fan are both used to dissipate heat. With the radiating fin increasing the area of contact between a heating element and air, the fan forcefully removes warmed air existing around the radiating fin.

However, in forceful convection of air with such a fan, a temperature boundary layer at a surface of the fin is produced at a downstream side of the fin, thereby giving rise to the problem that the heat from the radiating fin is not efficiently removed. This problem may be solved by, for example, reducing the thickness of the temperature boundary layer by increasing fan air velocity. However, increasing the rotational speed of the fan for the purpose of increasing the fan air velocity causes noise to be generated, such as noise from a fan bearing or noise of wind produced by the fan.

Methods using a vibrating plate that reciprocates periodically (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2000-223871 (FIG. 2), 2000-114760 (FIG. 1), 2-213200 (FIG. 1), and 3-116961 (FIG. 3)) are available as methods which efficiently allow heat from a radiating fin to escape to outside air by destroying the temperature boundary layer without using a fan as an air blower. Of devices in these four documents, in particular, the devices in Japanese Unexamined Patent Application Publication Nos. 2-213200 and 3-116961 include a vibrating plate which roughly divides space in a chamber in two, a resilient member disposed in the chamber and supporting the vibrating plate, and a unit which vibrates the vibrating plate. In these devices, for example, when the vibrating plate is displaced upwards, the volume of an upper space of the chamber is reduced. Therefore, the pressure in the upper space is increased. Since the upper space is connected to outside air through a suction-exhaust opening, a portion of the air in the upper space is discharged to the outside air by the pressure increase in the upper space. At this time, the volume of a lower space that is opposite to the upper space (the vibrating plate is disposed between the lower space and the upper space) is increased, causing the pressure in the lower space to decrease. Since the lower space is connected to the outside air through a suction-exhaust opening, the pressure reduction in the lower space causes a portion of the outside air existing near the suction-exhaust opening to be sucked into the lower space. In contrast, when the vibrating plate is displaced downwards, the volume of the upper space of the chamber is increased. Therefore, the pressure in the upper space is decreased. Since the upper space is connected to the outside air through the suction-exhaust opening, the pressure reduction in the upper space causes a portion of the outside air existing near the suction-exhaust opening to be sucked into the upper space. At this time, the volume of the lower space that is opposite to the upper space (the vibrating plate is disposed between the lower space and the upper space as mentioned above) is decreased, causing the pressure in the lower space to increase. The pressure increase in the lower space causes a portion of the air in the lower space to be discharged to the outside air. The vibrating plate is driven by, for example, an electromagnetic driving method. Accordingly, by reciprocating the vibrating plate, the discharging of the air in the chamber to the outside air and the sucking of the outside air into the chamber are periodically repeated. Pulsating air induced by a periodic reciprocating movement of the vibrating plate is blown against a heating element such as the radiating fin (heat sink), so that the temperature boundary layer at the surface of the radiating fin is efficiently broken, as a result of which the radiating fin is cooled with high efficiency.

SUMMARY OF THE INVENTION

In recent years, the amount of heat that is generated by increased IC clocking is increasing. Therefore, for example, in order to destroy a temperature boundary layer formed near a radiating fin due to the generation of the heat thereof, a larger amount of air is typically sent towards the IC or the radiating fin than it has been up until now. In a method for discharging air with a vibrating plate which reciprocates periodically, such as those described in the aforementioned four documents, the amount of air that is discharged can be increased by increasing the vibration amplitude of the vibrating plate. However, the larger the vibration amplitude of the vibrating plate, the larger the noise. Therefore, for practical purposes, the vibrating plate is typically operated with a small vibration amplitude at which the noise level does not bother anyone.

One cause of this noise is sound waves that are generated by a periodic variation in air pressure in a chamber caused by a reciprocating movement of a vibrating plate. The sound waves vibrate a wall surface of the chamber or propagate through the outside air via a suction-exhaust opening, as a result of which sound waves having the same frequency as the vibration frequency of the vibrating plate are discharged into the outside air. Therefore, the higher the vibration amplitude, the more serious is the problem of noise produced by these sound waves.

Another cause of the noise is airflow sound caused by a disturbance in airflow resulting from a reciprocating movement of the vibrating plate. The larger the vibration amplitude, the higher a maximum flow velocity of the air undergoing reciprocating movement. Therefore, in the chamber, a disturbance occurs in the airflow due to a structural member (such as a driving unit or the suction-exhaust opening) which inhibits the smooth flow of the air, or a disturbance occurs in the flow of air passing through the suction-exhaust opening at a high speed or in the airflow outside the suction-exhaust opening. Consequently, noise resulting from the airflow sound produced by such disturbances becomes a problem.

Although, as in, for example, Japanese Unexamined Patent Application Publication Nos. 2-213200 and 3-116961 (lower left column on page 2 in each of these documents), the noise generated by sound waves that are produced by air vibration resulting from a reciprocating movement of the vibrating plate can be reduced by moving the vibration frequency away from an audible area, the lower the frequency, the smaller an air discharge amount per unit time (the air discharge amount is proportional to the product of the vibration amplitude, effective sectional area, and the frequency of the vibrating plate). In contrast, when the vibrating plate is vibrated with a high frequency outside the audible area, the vibration amplitude of the vibrating plate is considerably reduced by amplitude-frequency characteristics of a mechanical vibrating system including a driving unit. Therefore, the air discharge amount per unit time is reduced as expected. The airflow sound generated by a disturbance in the airflow in the chamber depends upon the maximum velocity of the vibrating plate rather than the vibration frequency of the vibrating plate. Therefore, the airflow sound is not typically restricted by a method which merely moves the vibration frequency away from the audible area.

In view of the aforementioned problems, it is desirable to provide a jet generating device which can restrict generation of noise without reducing a gas discharge amount and cooling capacity, and an electronic apparatus including the jet generating device.

According to an embodiment of the present invention, there is provided a jet generating device including a vibrating member which vibrates gas, a driving unit which drives the vibrating member, and a housing which has at least one first opening and a first chamber containing the gas, supports the vibrating member, and is such that, of sounds generated as a result of vibrating the vibrating member, the sound having a maximum noise level has a predetermined frequency. The housing discharges the gas as pulsating gas through the at least one first opening as a result of driving the vibrating member. The first chamber is connected to the at least one first opening.

According to the embodiment of the present invention, the housing is formed so that a sound having a maximum noise level of the sounds generated by the vibration of the vibrating member has a predetermined frequency. Therefore, for example, if this frequency is properly set, the generation of noise can be restricted without reducing a gas discharge amount.

The noise generated by the vibration of the vibrating member refers to a sound wave generated by periodically varying the air pressure in the first chamber by a reciprocating movement of the vibrating member (hereafter referred to as “first sound”) or to an airflow sound generated by a disturbance in airflow produced by a reciprocating movement of the vibrating member (hereafter referred to as “second sound”).

The predetermined frequency is, for example, equal to or greater than 1 Hz and less than 1 kHz, or equal to or greater than 60 kHz and less than 40 kHz. If the predetermined frequency is less than 1 Hz, an expected lowest air discharge amount per unit time is not typically obtained. If the predetermined frequency is equal to or greater than 40 kHz, the vibration amplitude becomes too small as a practical value.

The type of gas which may be used is not only air, but also nitrogen, helium gas, argon gas, or other types of gas.

A driving method of the driving unit may make use of, for example, an electromagnetic action, a piezoelectric action, or an electrostatic action.

In one form of the present invention, in order for the sound having the maximum noise level to have the predetermined frequency, at least one of a length of the at least one first opening, an opening area of the at least one first opening, and a volume of the first chamber is set, the length being in a direction in which the gas which is vibrated as a result of driving the vibrating member is discharged from the at least one first opening. In this form of the present invention, the at least one first opening has, in the aforementioned discharge direction, a tubular form in which the area of the at least one first opening is substantially constant, that is, a tubular form having a substantially constant thickness. In another form of the present invention, when a sound velocity is C[m/s], the volume of the first chamber is V[m3], an equivalent circular radius of the opening area is r[m], the number of the at least one first opening is n, and the length of the at least one first opening is L[m], it is desirable that, for example, {C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2<1000 be satisfied. In other words, the housing is formed so that a Helmholtz resonance frequency is less than a predetermined frequency of the predetermined frequencies mentioned above or equal to or greater than a predetermined frequency of the predetermined frequencies mentioned above.

If an area having an area corresponding to the opening area is circular, the equivalent circular radius refers to the radius of this circle. If this area is other than circular, such as rectangular, the equivalent circular radius refers to the radius of a circle having an area corresponding to the area of the rectangle.

In still another form, the housing further has at least one second opening and a second chamber connected to the at least one second opening, disposed opposite to the first chamber, and containing the gas, the gas being alternately discharged as the pulsating gas through the at least one first opening and the at least one second opening by driving the vibrating member. Accordingly, when the gas is alternately discharged through the at least one first and second openings, in particular, the phases of the second sound generated from the respective openings are opposite to each other, so that the sound waves weaken each other. Therefore, the noise can be further reduced.

In still another form, similarly to the above, in order for the sound having the maximum noise level to have the predetermined frequency, at least one of a length of the at least one second opening, an opening area of the at least one second opening, and a volume of the second chamber is set, the length being in a direction in which the gas which is vibrated as a result of driving the vibrating member is discharged from the at least one second opening.

In still another form, the housing further has a partition plate disposed at a surface of the housing between the at least one first opening and the at least one second opening. By this, for example, when the at least one first and second openings are too close to each other, it is possible to, for example, prevent entrance and leaving of the gas between the openings. Therefore, it is possible to, for example, prevent the reduction of an amount of gas blown against a heating element.

In still another form, when an equivalent circular radius of an opening area of the at least one first opening and an equivalent circular radius of an opening area of the at least one second opening, which are substantially the same, are r[m], a distance between the at least one first opening and the at least one second opening is d[m], and a wavelength of the sound having the maximum noise level is λ [m], 3r≦d<λ/2 is satisfied. The distance d is a distance between substantially the centers of the openings. Therefore, when the distance from an edge defining the at least one first opening to an edge defining the at least one second opening is d2, the aforementioned formula becomes r≦d2<λ/2. By making 3r≦d, it is possible to, for example, prevent the entrance and the leaving of the gas between the openings. By making d<λ/2, for example, there is no location where substantially maximum vibration amplitudes of sound waves generated from the openings strengthen each other, so that it is possible to prevent the generation of noise.

In still another form, the at least one first opening has a first end which is disposed adjacent to the first chamber so that an opening area of the at least one first opening increases towards the first chamber. This smoothens airflow when the gas flows into and out of the at least one first opening, so that the noise level of the second sound is reduced. The same effect is achieved even if the at least one first opening has a first end which is disposed at an outer side of the housing and which is formed so that the opening area of the first opening increases towards the outer side. Alternatively, if the at least one first opening has a first end and a second end, the first end being disposed adjacent to the first chamber so that an opening area of the at least one first opening increases towards the first chamber, the second end being disposed at an outer side of the housing so that the opening area of the at least one first opening increases towards the outer side of the housing, the airflow is expected to be further smoothened. This also applies to the case in which the housing has a second chamber and at least one second opening connected to the second chamber.

In still another form, the housing further has a first nozzle for forming the at least one first opening therein, the first nozzle having a first inclined surface which is such as to reduce a width of the first nozzle towards an outer side of the housing. This makes it easier to suck in gas around the nozzle when the gas is discharged from the at least one first opening, that is, a combined jet gas amount can be increased. This also applies to the case in which the housing has a second chamber and at least one second opening connected to the second chamber.

According to another embodiment of the present invention, there is provided an electronic apparatus including a heating element, a vibrating member which vibrates gas, a driving unit which drives the vibrating member, and a housing which has at least one first opening and a first chamber containing the gas, supports the vibrating member, and is such that, of sounds generated as a result of vibrating the vibrating member, the sound having a maximum noise level has a predetermined frequency. The housing discharges the gas towards the heating element as pulsating gas through the at least one first opening as a result of driving the vibrating member. The first chamber is connected to the at least one first opening.

Examples of the electronic apparatus are a computer (which may be a laptop computer or a desktop computer when the computer is a personal computer), a Personal Digital Assistance (PDA), an electronic dictionary, a camera, a display device, an audio/visual device, a cellular phone, a game device, and other electronic products. The heating element may be an electronic part, such as an IC or a resistor, a radiating fin (heat sink), or any other element as long as it generates heat.

As mentioned above, according to the embodiments and forms of the present invention, it is possible restrict the generation of noise without reducing the gas discharge amount and cooling capacity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will hereunder be described with reference to the drawings.

FIG. 1is a perspective view of a jet generating device and a heat sink according to an embodiment of the present invention.FIG. 2is a sectional view of a jet generating device10shown inFIG. 1.

The jet generating device10includes a housing1having a rectangular hole1bin an upper portion of the housing1. A rectangular resilient supporting member6is mounted to a periphery of the hole1bof the housing1, and supports a vibrating plate3serving as a vibrating member. A chamber11is formed by the vibrating plate3, the resilient supporting member6, and the housing1. A plurality of nozzles2for discharging air in the chamber11towards a heat sink20disposed outside the housing1are mounted to a side surface1aof the housing1. The nozzles2may be integrated to the housing1.

An actuator5for driving the vibrating plate3is disposed at a top portion of the housing1. For example, a magnet14which is magnetized in a vibration direction R of the vibrating plate3is disposed within a cylindrical yoke8, and, for example, a disc-shaped yoke18is mounted to the magnet14. A magnetic circuit is formed by the magnet14and the yokes8and18. A coil bobbin9upon which a coil17is wound moves into and out of a space between the magnet14and the yoke8. In other words, the actuator5is a voice coil motor. A power feed wire16is connected to the actuator5. The power feed wire16is electrically connected to a control circuit13(such as a driving IC) through a terminal24mounted to a cover4. An electrical signal is supplied to the actuator5from the control circuit13.

The yoke8is integrally formed with the cover4covering the top portion of the housing1. However, from the viewpoint of preventing spreading of magnetic flux generated by the magnet14from the yoke8to the cover4, the yoke8and the cover4may be formed of different materials. The coil bobbin9is secured to a surface of the vibrating plate3. The vibrating plate3can be vibrated in the directions of a double-headed arrow R by such an actuator5.

The housing1is formed of, for example, resin, rubber, metal, or ceramic. Resin and rubber facilitate the formation of the housing1and are suited for mass-production. In addition, resin and rubber can increase a sound attenuation factor and can thus restrict noise, and can be used to reduce weight and costs. Considering heat dissipation of the housing1, it is desirable that the metal be copper or aluminum having high thermal conductivity. The cover4is also formed of, for example, resin, rubber, metal, or ceramic. The housing1and the cover4may be formed of the same material or different materials. The resilient supporting member6is formed of, for example, resin or rubber.

The vibrating plate3is formed of, for example, resin, paper, rubber, or metal. Although the vibrating plate3is illustrated as having the shape of a flat plate, it may be cone-shaped like a vibrating plate having a speaker mounted thereto or may have a three-dimensional shape.

An operation of the jet generating device10having the above-described structure will be described.

When, for example, a sinusoidal alternating voltage is applied to the actuator5, the vibrating plate3undergoes sinusoidal vibration, causing the volume of the chamber11to increase or decrease. The change in the volume of the chamber11changes the pressure in the chamber11, causing airflow from the nozzles2to be produced as pulsating airflow. For example, when the vibrating plate3is displaced in the direction in which the volume of the chamber11is increased, the pressure in the chamber11is reduced. This causes air outside the housing1to flow into the chamber11through the nozzles2. In contrast, when the vibrating plate3is displaced in the direction in which the volume of the chamber11is reduced, the pressure in the chamber11is increased. This causes air in the chamber11to be discharged outside the chamber11through the nozzles2and to be blown against the heat sink20. A reduction in air pressure around the nozzles2when the air is discharged from the nozzles2causes the air around the nozzles2to be sucked into the air that is discharged from the nozzles2. That is, jets are combined. Such combined jets make it possible to cool the heat sink20by blowing the combined jets against the heat sink20.

Here, the housing1according to the embodiment is formed so that a sound having a maximum noise level of sounds generated by vibration of the vibrating plate3has a predetermined frequency. Therefore, for example, if this frequency is set to a suitable value, it is possible to restrict the generation of noise without reducing an air discharge amount.

One type of sound generated by the vibration of the vibrating plate3refers to a sound wave produced by periodically varying the air pressure in the chamber11by a reciprocating movement of the vibrating plate3((hereafter referred to as “first sound”). The first sound is primarily a sound which is generated by transmitting its vibration to, for example, the housing1or the cover4as a result of, for example, periodically varying the air pressure in the chamber11. The other type of sound generated by the vibration of the vibrating plate3refers to an airflow sound generated by a disturbance in airflow produced by a reciprocating movement of the vibrating member3(hereafter referred to as “second sound”). The second sound is primarily an airflow sound produced when air is discharged from the nozzles.

As described above, in the embodiment, the housing1is formed so that, of the first and second sounds, the sound having a maximum noise level has a predetermined frequency. Here, the noise level refers to a noise level that is corrected so as to match auditory sensation characteristics of human beings.

FIG. 3is a graph of auditory sensation characteristics of human beings. This graph is an equal loudness curve (A characteristic curve) based on Japanese Industrial Standards (JIS), and shows how loud the sound in a frequency range of from 20 Hz to 20 kHz is heard when a person is exposed to a same sound pressure level. In other words, the graph shows how loud the sound at each frequency is heard with a 1-kHz sound wave being a standard. From the graph, when the sound pressure level is the same, a 50-Hz sound can be heard more softly than the 1-kHz sound by 30 dB. That is, the noise level is lower. A sound pressure level Lp[dB] is expressed by the following Formula (1):
Lp=20 log(p/p0)  (1)
(where p is the sound pressure [Pa], and p0is the standard sound pressure [20 μPa])

Considering this fact, for example, the frequency of the sound having the maximum noise level is equal to or greater than 1 Hz and less than 1 [kHz]. This is because, when the frequency is in the range of from 1 [kHz] to 6 [kHz], the noise level is perceived by a human being as being relatively high. Therefore, if the frequency is less than 1 [kHz], the noise is not easily perceived, and, if the frequency is less than 1 [Hz], an expected lowest air discharge amount per unit time is not typically obtained.

The frequency may be equal to or greater than 1 [Hz] and less than 500 [Hz], or equal to or greater than 1 [Hz] and less than 20 [Hz]. This is because, if the frequency is less than 500 [Hz], from the graph shown inFIG. 3, the sound pressure level is reduced by at least 3 [dB] compared to when the frequency is 1 [kHz], so that noise is considerably reduced. The frequency is less than 20 [Hz] because this frequency falls outside an audible area of human beings.

As mentioned above, when the frequency is in the range of from 1 [kHz] to 6 [kHz], the noise level is perceived by a human being as being relatively high. Therefore, the frequency is equal to or greater than 6 [kHz] and less than 40 [kHz]. When the frequency is equal to or greater than 40 kHz, the vibration amplitude becomes too small as a practical value.

The frequency may be equal to or greater than 10 [kHz] and less than 40 [kHz], or equal to or greater than 20 [kHz] and less than 40 [kHz]. This is because, if the frequency is equal to or greater than 10 [kHz], the sound pressure level is reduced by at least 3 [dB] compared to when the frequency is 1 [kHz], so that noise is considerably reduced. The frequency is equal to or greater than 20 [kHz] because this frequency falls outside the audible area of human beings.

More specifically, in order for the sound having a maximum noise level to have a frequency equal to any of the frequencies mentioned above, as inFIG. 2, at least one of a length L[m] of the nozzles2(opening length), an opening area S[m2] (a flow-path sectional area of the nozzles2and, more specifically, an area of the nozzles2in a plane substantially perpendicular to the direction of the length), and a volume V[m3] of the chamber11is set. In this case, when the sound velocity is C[m/s], the radius of an opening plane of the openings is r[m], and the number of openings (number of nozzles2) is n, any one of the following Formulas (2) to (7) is satisfied:
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2<1000  (2)
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2<500  (3)
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2<20  (4)
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2≧6000  (5)
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2≧10000  (6)
{C/(2π)}·[πr2/{V/(1.2r·n1/2+L)}]1/2≧20000  (7)

In other words, the housing is formed so that a Helmholtz resonance frequency is less than a predetermined frequency of the predetermined frequencies mentioned above or equal to or greater than a predetermined frequency of the predetermined frequencies mentioned above. The numbers on the right side in these Formulas (2) to (7) have these values due the same reasons mentioned above.

FIG. 4is a sectional view of a jet generating device according to another embodiment of the present invention. Description of parts, functions, etc. of the jet generating device of this embodiment that correspond to those of the jet generating device10of the previous embodiment will be simplified or omitted, that is, the description will be given focusing on the differences.

The inside of a housing21of a jet generating device30is partitioned by a vibrating plate3and a resilient supporting member6, so that a first chamber11aand a second chamber11bare formed in the housing21. The housing21has nozzles2aand2bdefining openings connected to the first chamber11aand the second chamber11b, respectively. In such a jet generating device30, applying, for example, a sinusoidal alternating voltage to an actuator5causes the vibrating plate3to vibrate. The vibration of the vibrating plate3alternately increases and decreases the pressure in the chambers11aand11b, so that air alternately flows into and out of the chambers11aand11bthrough the respective nozzles2aand2b. In other words, when air is discharged from the first chamber11ato the outside of the housing21through the nozzle2a, air flows into the second chamber11bfrom the outside through the nozzle2b. In contrast, when air is discharged from the second chamber11bto the outside of the housing21through the nozzle2b, air flows into the first chamber11afrom the outside through the nozzle2a.

As with the jet generating device10, the jet generating device30according to the embodiment is formed so that the frequency of a sound having a maximum noise level is a predetermined frequency. This makes it possible to reduce noise. In this case, it is desirable that the frequency of either a first sound or a second sound that is generated at least one of the first chamber11aand the second chamber11bfall within any of the aforementioned frequency ranges, such as equal to or greater than 1 Hz and less than 1 [kHz]. Alternatively, it is desirable that at least one of the first chamber11aand the second chamber lib satisfy any one of the Formulas (2) to (7).

When air is discharged from the nozzles2aand2b, for example, sound is generated independently from each of the nozzles2aand2b. This sound is identified as the first sound. However, since sound waves that are generated at the nozzles2aand2bhave opposite phases, the sound waves weaken each other. This makes it possible to further reduce noise.

It is desirable that the volumes of the first and second chambers11aand11bbe the same. This causes the amount of air that is discharged to be the same, so that noise is further reduced.

FIGS. 5A and 5Bshow housings of a jet generating device according to still another embodiment of the present invention. In each ofFIGS. 5A and 5B, the left side is a front view and the right side is a side view of the housing, respectively. A plurality of nozzles22in a housing1shown inFIG. 5Ahave rectangular openings. A plurality of nozzles32in a housing1shown inFIG. 5Bhave elliptical openings. For the nozzles having such forms, when the opening area is considered, for example, a radius r of a circle having an area that is equal to the area of a rectangle if the nozzles are rectangular is considered.

FIGS. 6A to 6Dshow housings of a jet generating device according to still another embodiment of the present invention. A plurality of tubular openings31aare formed in a housing31shown inFIG. 6A. These openings31afunction as nozzles. In other words, tubular holes may only be formed in the housing31instead of nozzles.

FIG. 6Bshows a housing41, such as that shown inFIG. 4, having two chambers. Openings41aand42bare connected to the respective chambers of the housing41.

A housing51shown inFIG. 6Chas an inclined surface51bformed at a nozzle having openings51a. In other words, the housing51has the inclined surface51bwhich is such as to reduce a vertical width (in the figure) of the nozzle towards the outside of a chamber. Such a structure makes it easier for gas around the nozzle to be sucked in when air is discharged from the openings51a. In other words, it is possible to increase a combined jet air amount. Instead of or in addition to the inclined surface51bwhich is such as to reduce the vertical width, a different inclined surface which is such as to reduce a horizontal width (perpendicular to the plane ofFIG. 6C) of the nozzle may be used.

FIG. 7is a sectional view of a portion of a housing of a jet generating device of still another embodiment of the present invention. A housing71has, for example, a plurality of openings71aand openings71b. For example, as shown inFIG. 6B, the housing71has two chambers therein and the plurality of openings71aand the plurality of openings71bare connected to the chambers.

Opening areas of the openings71aand71bare substantially the same and their equivalent circular diameters are r. In this case, as shown inFIG. 7, a distance d1between the openings71aand the respective openings71b(distance between the centers of the openings71aand the respective openings71b) is set equal to or greater than three times r, that is, a distance d2is set equal to or greater than 1 times r. For example, when air from the openings71ais discharged to the outside, air flows into the housing71from the openings71b. Therefore, when the openings71aand71bare too close to each other, the air discharged from the openings71aflows into the housing71from the openings71b. This may reduce the amount of air blown against a heating element (not shown). However, when 3r≦d1as in the embodiment, such a problem can be overcome.

When, for example, the wavelength of a second sound generated from each of the openings71aand71bis λ[m], it is desirable that d1<λ/2. In the case where the centers in the opening planes of the openings71aand71bare sound sources, respectively, when d1<λ/2, a location where substantially maximum vibration amplitudes of sound waves generated from the openings71aand71bstrengthen each other no longer exists, so that it is possible to prevent noise from being generated. When the medium is air and the sound velocity is approximately 340 [m/s], the wavelength of sound in an audible range of human beings is on the order of from 1.7 [mm] to 17 [m].

FIG. 8is a perspective view of a modification of the housing71shown inFIG. 7. A partition plate81cis mounted between openings81aand81bformed in a housing81. The partition plate81cmay be formed of the same material as that of the housing81and integrated thereto. Similarly to the above, such a structure makes it possible to prevent the entering and leaving of air between the openings81aand81b. In this case, the distance between the openings81aand the respective openings81bdoes not necessarily have to be equal to or greater than 1 times r as it is inFIG. 7, that is, it may be less than 1 times r.

FIGS. 9A to 9Care side views showing portions of housings of a jet generating device according to still another embodiment of the present invention. As shown inFIG. 9A, an opening91afor discharging air from a chamber92in a housing91is formed so that its opening area is increased at an end91badjacent the chamber92in the direction of the chamber92. This smoothens airflow when the air flows in and out of the chamber92, so that a noise level of a second sound is reduced.

At an opening101aof a housing101shown inFIG. 9B, an end which is situated at a side opposite to a chamber102, that is, an outer end101bis formed so that its opening area is increased towards an outer side of the housing101. Such a structure also smoothens airflow, thereby reducing the noise level of the second sound.

An opening111aof a housing111shown inFIG. 9Cis formed so that both ends111band111care widened. This further smoothens airflow.

The present invention is not limited to the above-described embodiments, so that various modifications may be made.

For example, although the housing1shown inFIG. 1has a rectangular parallelepiped shape, it may have a cylindrical shape or a triangular prismatic shape. When the housing1has a cylindrical shape, that is, when its planar shape is, for example, circular, it is desirable that the vibrating plate3be disc-shaped. In other words, it is desirable that the housing1and the vibrating plate3have similar shapes.

The planar shape of the flat-plate yoke18shown inFIG. 1is, for example, circular. However, it may be elliptical or rectangular. Although the vibrating plate3is rectangular in the relevant figures, the vibrating plate3may have a circular shape that is similar to the planar shape of the actuator5.

Although, inFIGS. 9A to 9C, the ends91b,101b,111b, and111care curved, they may have tapering shapes having flat inclined surfaces.

At least two of the above-described embodiments may be combined. For example, the ends defining the openings41aand41bof the respective chambers shown inFIG. 6Bmay having any one of the forms shown inFIGS. 9A to 9C. Alternatively, the openings51a, etc., of the housing51shown inFIG. 6Cor the openings61a, etc., of the housing61shown inFIG. 6Dmay have any one of the forms shown inFIGS. 9A to 9C. Still alternatively, the distance between the nozzles2aand2bof the housing21shown inFIG. 4may be d1as shown inFIG. 7.