Abstract:
A piezoelectric electroacoustic transducer is miniaturized and has a reduced frequency, and even when a diaphragm has a warp or winding, the resonance frequency is be stabilized. The piezoelectric electroacoustic transducer includes a substantially rectangular piezoelectric diaphragm flexurally vibrating in a thickness direction in response to the application of an alternating signal between electrodes and a casing for accommodating the piezoelectric diaphragm, wherein the casing is provided with supporting members for supporting four corners of the piezoelectric diaphragm.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric buzzer and a piezoelectric receiver.  
           [0003]    2. Description of the Related Art  
           [0004]    In electronic devices, electronic home appliances, portable telephones, and other such electronic devices, piezoelectric electroacoustic transducers have been widely used as a piezoelectric buzzer or piezoelectric receiver for producing an alarm sound or an operating sound. A piezoelectric electroacoustic transducer of this type generally includes a circular piezoelectric element bonded to one surface of a circular metallic plate to form a unimorph-type diaphragm, and the periphery of the metallic plate is mounted on a circular case using silicone rubber while an opening of the case is closed with a cover.  
           [0005]    However, when using a circular diaphragm, there has been a problem of low production efficiency, low acoustic conversion efficiency, and moreover, difficulty in miniaturizing the piezoelectric electroacoustic transducer.  
           [0006]    A piezoelectric electroacoustic transducer is described in Japanese Unexamined Patent Application Publication No. 2000-310990 as improving production efficiency and acoustic conversion efficiency, and can be miniaturized by using a rectangular diaphragm. The piezoelectric electroacoustic transducer includes a rectangular piezoelectric diaphragm, an insulating case having a bottom wall, four sidewalls, and a supporting member, which is disposed inside between two of the sidewalls opposing each other for supporting the diaphragm, the supporting member being provided with first and second conductive members formed therein for external connection, and a lid plate having holes formed thereon for releasing sound. The diaphragm is accommodated within the case, where two opposite sides of the diaphragm are fixed to the supporting member with an adhesive or an elastic sealant, while gaps between the remaining two sides of the diaphragm and the case are sealed with the elastic sealant, so that the diaphragm and the first and second conductive parts are electrically connected together with a conductive adhesive, and the lid plate is bonded on the open end of the sidewalls of the case.  
           [0007]    As another method of fixing the piezoelectric diaphragm, there is also a method of fixing the four sides of the diaphragm to the supporting member with an adhesive or an elastic sealant.  
           [0008]    In the diaphragm, the bending vibration may occur in a length-bending mode and the bending vibration in an area-bending mode, corresponding to a manner in which the diaphragm is supported. The former is a mode of bending-vibration in the plate-thickness direction with both ends in the longitudinal direction functioning as the supports where two sides of the diaphragm are fixed to the case, and the latter is a mode in which by fixing four sides of the diaphragm to the case, the entire area of the diaphragm flexurally vibrates in the plate-thickness direction with the four sides functioning as the supports such that the intersection of the diagonal lines has the maximum displacement.  
           [0009]    However, the bending vibration in any of a conventional length-bending mode and area-bending mode has a defect in that the resonance frequency of the diaphragm is high and the sound pressure in the low frequency range cannot be increased. When increasing the size of the case and the diaphragm, the resonance frequency can be reduced in frequency. However, the piezoelectric electroacoustic transducer will be thereby increased in size.  
           [0010]    Also, the conventional piezoelectric electroacoustic transducer has had a problem that when the diaphragm has a warp or winding, the resonance frequency is not stabilized, because two sides or four sides of the diaphragm are strongly restrained.  
         SUMMARY OF THE INVENTION  
         [0011]    In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer that is miniaturized and has a reduced frequency, and even when a diaphragm has a warp or winding, the resonance frequency is stabilized.  
           [0012]    According to a preferred embodiment of the present invention, a piezoelectric electroacoustic transducer includes a substantially rectangular piezoelectric diaphragm having electrodes and flexurally vibrating in a thickness direction in response to application of an alternating signal between the electrodes, and a casing for accommodating the piezoelectric diaphragm, wherein the casing is provided with supporting members that support four corners of the piezoelectric diaphragm.  
           [0013]    When four sides of the diaphragm are supported on the casing as usual, and also when four corners of the diaphragm are supported thereon according to preferred embodiments of the present invention, the diaphragm vibrates in an area-bending mode in both cases. However, as shown in FIGS. 1A and 1B, the node of vibration is different. That is, when supporting the four sides of the diaphragm, as shown in FIG. 1A, the diaphragm vibrates using a circle inscribed in the diaphragm as a node, whereas, when supporting the four corners of the diaphragm, as shown in FIG. 1 B, the diaphragm vibrates using a circle that is substantially circumscribed with the diaphragm as a node. Therefore, the maximum displacement, i.e., the sound pressure, at the center of the circle is larger in the latter in comparison with that of the former. Also, the displacing area of the diaphragm is larger in the latter in comparison with that of the former, so that the vibration frequency is lower in the latter, enabling the low frequency to be achieved even in the diaphragm with the same external size.  
           [0014]    [0014]FIGS. 1A and 1B show the comparison between supporting the four sides of the diaphragm in the different manners described above. Even when compared to the case of supporting two sides of the diaphragm, in the supporting structure according to preferred embodiments of the present invention, the restricting force for the diaphragm is greatly reduced, so that low frequency can be achieved and moreover, the sound pressure in a low-frequency range can be increased.  
           [0015]    As described above, by supporting the four corners of the diaphragm, miniaturization and reduction in frequency can be achieved because the diaphragm is freely deflectable, and even when the diaphragm has a warp or winding, variations in the resonance frequency are prevented.  
           [0016]    A piezoelectric electroacoustic transducer according to preferred embodiments of the present invention may further include terminal electrodes having internal connections exposed in vicinities of the supporting members and external connections exposed on the external surface of the casing and electrically connecting to the internal connections, and conductive adhesives, wherein the electrodes of the piezoelectric diaphragm and the internal connections of the terminal electrodes may be electrically connected together with the conductive adhesives.  
           [0017]    That is, in order to cause the diaphragm to vibrate flexurally in an area bending mode, it is necessary to apply an alternating signal between the electrodes of the diaphragm. By connecting the electrodes of the diaphragm to the internal connections exposed in the vicinities of the supporting members of the casing with the conductive adhesives, the alternating signal can be applied by reducing the restraining force acting against the vibration of the diaphragm as much as possible.  
           [0018]    In addition, when a urethane conductive paste is used as the conductive adhesive, for example, the restraining force against the diaphragm need not be large due to its elasticity after curing.  
           [0019]    Preferably, a gap between the external periphery of the piezoelectric diaphragm and the internal periphery of the casing is sealed with an elastic sealant.  
           [0020]    Even when the diaphragm flexurally vibrates in the area bending mode, if there is a gap between the diaphragm and the casing, air leaks therethrough so that sound pressure cannot be obtained. By sealing the gap between the external periphery of the diaphragm and the internal periphery of the casing with the elastic sealant, the air leakage can be eliminated without impeding the vibration of the diaphragm.  
           [0021]    For the elastic sealant, a silicone adhesive may be used, for example, and other sealants may also be used.  
           [0022]    Preferably, the piezoelectric diaphragm includes a film attached at least to the external periphery of the diaphragm, and a gap between the piezoelectric diaphragm and the casing is sealed by melting or bonding the film in the internal periphery of the casing.  
           [0023]    Although the gap between the diaphragm and the casing can be sealed by applying the elastic sealant, when the elastic sealant having an excessive thickness adheres to the clearance between the diaphragm and the casing, there is a possibility of damping the vibration. Whereas, when the diaphragm and the casing are connected with the thin film, the diaphragm is liable to vibrate so as to able to obtain high sound pressure.  
           [0024]    The film may be attached only to the periphery of the diaphragm or may be attached to the entire surface of the diaphragm.  
           [0025]    Preferably, the supporting members provided in the casing are projections for point-supporting vicinities of four corners of the piezoelectric diaphragm.  
           [0026]    That is, although the supporting members may be flat so as to support the four corners of the diaphragm on its surfaces, wherein the contact area is increased so that the vibration may be damped. Whereas, when the supporting members are projections, the diaphragm is scarcely restrained, thereby improving sound pressure characteristics.  
           [0027]    In this case, when opposing projections are arranged in the upper and lower portion of the casing so as to clamp the corners of the diaphragm between the projections, fixing with the adhesive can be eliminated to simplify the manufacturing process and damping of the vibration can be eliminated due to the reduced supporting area.  
           [0028]    Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIGS. 1A and 1B are drawings for comparing vibration nodes when supporting a diaphragm on four sides and when supporting the diaphragm at the corners thereof;  
         [0030]    [0030]FIG. 2 is an assembly view of a piezoelectric electroacoustic transducer according to a first preferred embodiment of the present invention;  
         [0031]    [0031]FIG. 3 is a plan view of the piezoelectric electroacoustic transducer shown in FIG. 2 in which a lid plate and an elastic sealant are removed;  
         [0032]    [0032]FIG. 4 is a sectional view along the line A-A of FIG. 3;  
         [0033]    [0033]FIG. 5 is a perspective view of a piezoelectric diaphragm used in the piezoelectric electroacoustic transducer shown in FIG. 2;  
         [0034]    [0034]FIG. 6 is a step sectional view along the line B-B of FIG. 5;  
         [0035]    [0035]FIG. 7 is a chart for comparing sound pressures when supporting the piezoelectric diaphragm on two sides and when supporting the piezoelectric diaphragm at the corners thereof;  
         [0036]    [0036]FIG. 8 is a plan view of a piezoelectric electroacoustic transducer according to a second preferred embodiment of the present invention in which a lid plate and an elastic sealant are removed;  
         [0037]    [0037]FIG. 9 is a sectional view along the line C-C of FIG. 8;  
         [0038]    [0038]FIG. 10 is a perspective view of a case included in the piezoelectric electroacoustic transducer shown in FIG. 8;  
         [0039]    [0039]FIG. 11 is a perspective view of a piezoelectric electroacoustic transducer according to a third preferred embodiment of the present invention in which a lid plate is removed; and  
         [0040]    [0040]FIG. 12 is an assembly view of a case and a diaphragm shown in FIG. 11. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0041]    FIGS.  2  to  6  show a surface-mount-type piezoelectric electroacoustic transducer according to a first preferred embodiment of the present invention.  
         [0042]    The electroacoustic transducer according to the present preferred embodiment is for use in wide-range frequencies, such as a piezoelectric receiver, and includes a layered-structure piezoelectric diaphragm  1 , a case  10 , and a lid plate  20 . The case  10  and the lid plate  20  constitute a casing.  
         [0043]    The diaphragm  1 , as shown in FIGS. 5 and 6, is preferably made by depositing two piezoelectric ceramic layers  1   a  and  1   b  and includes principal-surface electrodes  2  and  3  disposed on both principal surfaces and an internal electrode  4  disposed between the ceramic layers  1   a  and  1   b . The two ceramic layers  1   a  and  1   b,  as shown by the thick-line arrow in the drawings, are polarized in one thickness direction. The top principal-surface electrode  2  and the bottom principal-surface electrode  3  are preferably slightly smaller than the side length of the diaphragm  1 , and one end thereof is connected to an end surface electrode  5  disposed on one end surface of the diaphragm  1 . Therefore, the two principal surface electrodes  2  and  3  are connected to each other. The internal electrode  4  is substantially symmetrical with the principal-surface electrodes  2  and  3 , and one end of the internal electrode  4  is separated from the end surface electrode  5  while the other end is connected to an end surface electrode  6  disposed on the other end surface of the diaphragm  1 . In addition, on both surfaces of the diaphragm  1  at the other end, auxiliary electrodes  7  with small widths are arranged for being electrically connected to the end surface electrode  6 .  
         [0044]    On the top and bottom surfaces of the diaphragm  1 , resin layers  8  and  9  are provided for respectively covering the principal-surface electrodes  2  and  3 . The resin layers  8  and  9  are provided for increasing the shatter strength. On the top and bottom resin layers  8  and  9 , cut-outs  8   a  and  9   a  exposing the principal-surface electrodes  2  and  3  and cut-outs  8   b  and  9   b  exposing the auxiliary electrodes  7  are disposed in the vicinities of corners of the diaphragm  1  diagonally opposing each other.  
         [0045]    In addition, the cut-outs  8   a,    8   b,    9   a,  and  9   b  may be disposed on only one of the top and bottom surfaces. However, for eliminating the top and bottom directionality, the cut-outs  8   a,    8   b,    9   a  and  9   b  are disposed on both surfaces according to the present preferred embodiment of the present invention.  
         [0046]    The auxiliary electrode  7  is not necessarily a band electrode with a predetermined width, and it may be disposed only at a position corresponding to the cut-outs  8   b  and  9   b.    
         [0047]    According to the present preferred embodiment, PZT ceramics having approximate dimensions of 10 mm×10 mm×20 μm is preferably used as the ceramic layers  1   a  and  1   b,  and a polyamidoimide resin with a thickness of about 5 μm to about 10 μm is preferably used as the resin layers  8  and  9 .  
         [0048]    The case  10 , preferably made of an insulating material such as ceramics and a resin, has a substantially rectangular box-shape having a bottom wall  10   a  and four sidewalls  10   b  to  10   e.  When forming the case  10  of a resin, a heat-resistant resin may be preferable, such as an LCP (liquid crystal polymer), SPS (syndiotactic polystyrene), PPS (polyphenylene sulfide), and an epoxy resin. In internal peripheries of the four sidewalls  10   b  to  10   e,  steps  10   f  are configured in an annular arrangement. Over the steps  10   f  inside the two opposing sidewalls  10   b  and  10   d , internal connections  11   a  and  12   a  of a pair of terminals  11  and  12  are exposed. The terminals  11  and  12  are disposed in the case  10  by insert molding, in which external connections  11   b  and  12   b  protruding outside the case  10  are bent along external surfaces of the sidewalls  10   b  and  10   d  toward the bottom wall  10   a  of the case  10 . According to the present preferred embodiment, the internal connections  11   a  and  12   a  of the terminals  11  and  12  are preferably two-forked, respectively. These two-forked internal connections  11   a  and  12   a  are located in vicinities of the corners of the case  10 .  
         [0049]    Four corners inside the steps  10   f,  as shown in FIGS. 3 and 4, are provided with supporting members  10   g  disposed at positions lower than the steps  10   f  by one step for supporting the four corners of the diaphragm  1 . Therefore, when placing the diaphragm  1  onto the supporting members  10   g , the top surface of the diaphragm  1  substantially corresponds in height with the top surfaces of the internal connections  11   a  and  12   a  of the terminals  11  and  12 . Wherein the supporting member  10   g  is substantially triangular-shaped in plan view, and the four supporting members  10   g  are arranged on the same circumference.  
         [0050]    In addition, the bottom wall  10   a  is provided with a first sound-releasing hole  10   h  disposed therein.  
         [0051]    The diaphragm  1  is accommodated within the case  10  so as to be fixed to the supporting members  10   g  or to the vicinities thereof at four positions with elastic supporting materials  13 . That is, the elastic supporting materials  13  are applied between the principal electrode  2  exposed in the cut-out  8   a  and the internal connection  11   a  of the terminal  11 , and between the auxiliary electrodes  7  exposed in the cut-out  8   b,  which diagonally opposes the cut-out  8   a,  and the internal connection  12   a  of the terminal  12 . The remaining two positions, which are diagonally opposed to each other, are also coated with the elastic supporting materials  13 . In addition, the elastic supporting materials  13  are applied in longitudinally aligned, substantially elliptical shapes. However, the coating shape is not limited to the substantially elliptical shape. As the elastic supporting material  13 , the urethane adhesive having a Young&#39;s modulus after curing of about 3.7×10 6  Pa may be used, for example. Since the viscosity of the elastic supporting material  13  before curing is high (50 dpa·s to about 120 dpa·s, for example) so as to make it very difficult for the elastic supporting material  13  to spread, and it is also difficult for the elastic supporting material  13  to flow down to the bottom wall  10   a  through the gap between the diaphragm  1  and the case  10 , when it is applied. After applying the elastic supporting material  13 , it is heated and cured.  
         [0052]    In addition, as a method for fixing the diaphragm  1 , after accommodating the diaphragm  1  within the case  10 , the elastic supporting material  13  may be applied by a dispenser. Alternatively, the diaphragm  1  may be accommodated within the case  10  in a state in which the diaphragm  1  is coated with the elastic supporting materials  13  in advance.  
         [0053]    The coating position of the elastic supporting material  13  may preferably be as close to the supporting member  10   g  as possible. In FIG. 3, the elastic supporting material  13  is applied at a position that is slightly spaced away from the supporting member  10   g . This is because a conductive adhesive  14  is located along side the elastic supporting material  13 . Whereas, when the electrodes of the diaphragm  1  and the internal connections  11   a  and  12   a  can be arranged at corners of the case  10 , the coating position of the elastic supporting material  13  may also be the supporting member  10   g.    
         [0054]    After curing the elastic supporting materials  13 , the conductive adhesives  14  are applied preferably in substantially elliptic shapes on the elastic supporting materials  13  which are applied in substantially elliptic shapes so as to intersect with the elastic supporting materials  13 , so that the principal-surface electrode  2  and the internal connection  11   a  of the terminal  11  are connected together, and the auxiliary electrode  7  and the internal connection  12   a  of the terminal  12  are also connected together. As the conductive adhesive  14 , the urethane conductive paste having a Young&#39;s modulus after curing of about 0.3×10 9  Pa may be used, for example. After the conductive adhesive  14  is applied, it is heated and cured. The coating shape of the conductive adhesives  14  is not limited to a substantially elliptical shape. The coating shape can be any shape that is sufficient to be able to connect between the principal-surface electrode  2  and the internal connection  11   a,  and between the auxiliary electrode  7  and the internal connection  12   a  by being located along side the elastic supporting materials  13 .  
         [0055]    After applying and curing the conductive adhesives  14 , the gap between the entire periphery of the diaphragm  1  and the internal periphery of the case  10  is coated with an elastic sealant  15 , so that air leakage between the top and bottom surfaces of the diaphragm  1  is prevented. After applying the elastic sealant  15  in an annular arrangement, it is heated and cured. As the elastic sealant  15 , the silicone adhesive having a Young&#39;s modulus after curing of about 3.0×10 5  Pa is used, for example.  
         [0056]    After fixing the diaphragm  1  to the case  10  as described above, the lid plate  20  is bonded to the upper opening of the case  10  with an adhesive  21 . The lid plate  20  is preferably made of the same material as that of the case  10 . By bonding the lid plate  20 , an acoustic space is formed between the lid plate  20  and the diaphragm  1 . The lid plate  20  is provided with a second sound-releasing hole  22  formed therein.  
         [0057]    In the manner described above, the surface-mount piezoelectric electroacoustic transducer is completed.  
         [0058]    In the electroacoustic transducer according to the present preferred embodiment, when a predetermined alternating voltage is applied to between the terminals  11  and  12 , the diaphragm  1  can flexurally vibrate in an area-bending mode. The piezoelectric ceramic layer, which is polarized in the same direction as that of an electric field, contracts in a planar direction while the piezoelectric ceramic layer, polarized in the opposite direction to that of the electric field, expands in the planar direction, so that the entire structure bends in the thickness direction.  
         [0059]    According to the preferred embodiment, since the diaphragm  1  has a deposited ceramic structure, and two vibrating regions (ceramic layers) sequentially arranged in the thickness direction vibrate in directions opposite to each other, so that a larger displacement, i.e., larger sound pressure can be obtained in comparison with a unimorph-type diaphragm.  
         [0060]    [0060]FIG. 7 shows sound characteristics when two opposing sides of the diaphragm  1  are supported on the case and when four corners thereof are supported on the case. As is apparent from the drawing, it is understood that the resonance frequency is approximately 1200 Hz in the former while the resonance frequency is approximately 800 Hz in the latter, so that the resonance frequency is reduced by supporting the diaphragm with the four corners and moreover, the sound pressure at the resonance frequency is increased.  
         [0061]    FIGS.  8  to  10  show a piezoelectric electroacoustic transducer according to a second preferred embodiment of the present invention. According to the second preferred embodiment, while seats  10   i  are arranged at four corners of the case, there are projections  10   j  disposed on the top surfaces of the seats  10   i  protruding therefrom, so that bottom surfaces of the corners of the diaphragm  1  are substantially point-supported by the projections  10   j.    
         [0062]    In this case, the contact area between the diaphragm  1  and the projections  10   j  is extremely reduced so as not to damp the vibration, thereby improving sound characteristics.  
         [0063]    In addition, the steps  10   f  inside the sidewalls  10   c  and  10   e  are omitted in FIG. 10.  
         [0064]    According to the second preferred embodiment, the lid plate  20  may be provided with projections disposed on the bottom surface and opposing the projections  10   j  of the case  10 , so as to clamp the diaphragm between these projections from the upper and lower directions.  
         [0065]    [0065]FIGS. 11 and 12 show a piezoelectric electroacoustic transducer according to a third preferred embodiment of the present invention.  
         [0066]    According to the preferred embodiment, while a step  10   k  is disposed along the entire periphery inside the case  10 , supporting members  10   l  extended inside are disposed at four corners of the step  10   k.  The step  10   k  and the supporting members  10   l  are the same in height. The diaphragm  1  is bonded on a film  30  larger than the diaphragm  1  in size. The four corners of the diaphragm  1  are placed on the supporting members  10   l  while the four sides thereof are arranged not to mount on the step  10   k.  For the film  30 , an elastic thin-thickness film which does not impede the bending vibration of the diaphragm  1 , such as polyimide may be used. The entire periphery of the film  30  is bonded or melted on the step  10   k  and the supporting members  10   l  of the case  10 .  
         [0067]    In this case, because the film  30  performs the functions of fixing the diaphragm  1  to the case  10  and of sealing the clearance to the case  10  as well, the elastic supporting material  13  or the conductive adhesive  14  can be omitted. Also, there is no possibility of damping the vibration of the diaphragm  1  due to excessive coating with the elastic sealant  15 .  
         [0068]    In addition, the film  30  is not limited to a substantially rectangular bonded on the entire surface of the diaphragm  1 , and it may be a frame-shape bonded only to the periphery of the diaphragm  1 . The film  30  is also not limited to being bonded only to the bottom surface of the diaphragm  1 , and it may be bonded to the top surface or to both surfaces.  
         [0069]    According to the present preferred embodiment, in order to stably and reliably support the four corners of the diaphragm  1 , convex members may be formed on the bottom surface of the lid plate  20  so as to urge the corners of the diaphragm  1  toward the supporting members  10   l  with the convex members.  
         [0070]    Although the terminals provided in the case  10  are omitted from FIGS. 11 and 12, they are substantially the same as those shown in FIG. 2 or  10 . In this case, it may be also required only to connect between the internal connections of the terminals and the electrodes of the diaphragm  1  with the conductive adhesives.  
         [0071]    The present invention is not limited to the preferred embodiments described above and it can be modified within the spirit and scope of the present invention.  
         [0072]    The piezoelectric diaphragm according to the preferred embodiments is preferably made by depositing two piezoelectric ceramic layers. Alternatively, the piezoelectric diaphragm may be made by depositing three or more piezoelectric ceramic layers.  
         [0073]    Also, the piezoelectric diaphragm is not limited to the deposited structure of the piezoelectric ceramic layers. A diaphragm made by bonding a piezoelectric plate on one surface or on both surfaces of a metallic plate may be used.  
         [0074]    The terminal electrode according to the present invention is not limited to the inserted terminal according to the preferred embodiments. The terminal electrode may be a thin-film or thick-film electrode extending from the top surface of the supporting member of the case toward outside.  
         [0075]    The casing according to the present invention may be any structure as long as it accommodates the piezoelectric diaphragm and has a function to support four corners thereof, and it is not limited to the casing formed of a case having a concave section and a lid plate to be bonded on the top surface of the case, as in the preferred embodiments described above.  
         [0076]    While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.