Patent Publication Number: US-8989412-B2

Title: Piezoelectric acoustic transducer

Description:
TECHNICAL FIELD 
     The present invention relates to a piezoelectric acoustic transducer, and, more particularly, to a piezoelectric loudspeaker that achieves both space-saving and high quality sound. 
     BACKGROUND ART 
     Recently, there is an accelerating tendency that mobile devices, such as mobile phones, personal digital assistances (PDA), and portable navigation devices, are thinned and downsized. The need for thinner and smaller components to be mounted in audiovisual equipment and the like has also been increasing. 
     In general, electrodynamic loudspeakers are used as a loudspeaker for reproducing audio signals or music signals in the mobile devices. The electrodynamic loudspeakers, however, employ a driving type that requires a magnet and a voice coil, and thus it is difficult to reduce the thickness of the loudspeakers. Further, the electrodynamic loudspeakers use a magnetic circuit, and thus a problem arises that countermeasures must be taken against magnetic leakage, or the like. Therefore, piezoelectric loudspeakers, which have been widely used for reproducing sounds in the audiovisual equipment or the like, are attracting attention as a loudspeaker of a driving type that is suitable for reducing the thickness. Thus, there is an increasing tendency to mount piezoelectric loudspeakers in mobile devices. 
     Conventionally, the piezoelectric loudspeaker has been known as an acoustic transducer, in which a piezoelectric member is used for an electro acoustic transducer element, and which is used as a sound output means of small-sized devices (e.g., see Patent Literature 1). A structure of the piezoelectric loudspeaker is such that the piezoelectric element is bonded on a metal plate, or the like. Therefore, the piezoelectric loudspeaker is readily reduced in its thickness, as compared to the electrodynamic loudspeaker that requires a magnet and a voice coil. The piezoelectric loudspeakers also have an advantage that no countermeasure is required against the magnetic leakage. When viewed as an electric element, the piezoelectric loudspeakers operate as a capacitor, while the electrodynamic loudspeakers operate mainly as a resistance component. Therefore, the lower the frequency is, the higher the electric impedance becomes, and thereby the piezoelectric loudspeakers have an advantage that the power consumption in a low-frequency band is significantly low, as compared to the electrodynamic loudspeakers. For example, when used in mobile devices, the piezoelectric loudspeakers can reduce the power consumption over the electrodynamic loudspeakers in a normal voice-band, particularly in a frequency band ranging from 1 kHz to 2 kHz. 
     On the other hand, the piezoelectric loudspeakers have a disadvantage that an amount of displacement of a piezoelectric diaphragm is small, as compared to the electrodynamic loudspeakers, when the same voltage is applied. Because of this, in a low-frequency band where a large displacement is required, a sound pressure becomes small (i.e., voltage sensitivity becomes low), and thereby a problem arises that audio signals cannot be reproduced with a sufficient sound pressure. Therefore, in order to overcome the above problems, one of the following methods needs to be chosen. 
     A first method is a method of enlarging the area of the piezoelectric diaphragm to obtain the sound pressure. If the amount of displacement of the piezoelectric diaphragm is constant, the sound pressure of the piezoelectric loudspeaker is proportional to an effective vibration area of the piezoelectric diaphragm, and therefore the effective vibration area is to be enlarged. For example, if the effective vibration area of the piezoelectric diaphragm is doubled, the sound pressure is also doubled, that is, a sound pressure level increases by 6 dB. 
     A second method is a method of increasing a driving voltage to obtain the sound pressure. If the effective vibration area is constant, the amount of displacement of the piezoelectric diaphragm of the piezoelectric loudspeaker is proportional to the driving voltage, and therefore the driving voltage is to be increased. For example, if the driving voltage is doubled, the sound pressure is also doubled. 
     A third method is a method of multi-layering the piezoelectric element to obtain the sound pressure. The number of laminations of the piezoelectric elements is to be increased because driving force of the piezoelectric loudspeaker is proportional to the number of laminations of the piezoelectric elements, if the total thickness of the piezoelectric elements and the driving voltage are constant in a state where directions of deformations of the piezoelectric members align with each other. Therefore, if the number of laminations of the piezoelectric elements is increased, the sound pressure of the loudspeaker increases without the need for changing the effective vibration area of the piezoelectric diaphragm and the driving voltage. 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2003-230193 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the following problems, concerned with mounting the piezoelectric loudspeaker in the mobile devices, persists in the first through third methods described above, in aspects of disposition space and tone quality performance. 
     In the first method, the effective vibration area needs to be enlarged. However, how much the size can be enlarged is limited in the mobile devices or the audiovisual equipment, which require the reduction in thickness and size. Particularly, in a cabinet having a limited volume, the deterioration in a bass range reproduction performance due to an effect caused by insufficient volume at the back of the piezoelectric diaphragm is large. 
     In the second method, the driving voltage needs to be increased. However, a booster amplifier for driving the loudspeaker is separately required to increase the driving voltage, and thereby undesirably inviting an increase in space and cost, because of an increase in number of components. 
     In the third method, the number of laminations of the piezoelectric elements needs to be increased. However, the cost of the piezoelectric element increases according to the number of laminations of the piezoelectric elements. Further, the thickness of a piezoelectric member or an electrode per layer is constrained by a material used or a production method, and therefore the number of laminations of the piezoelectric elements is limited. 
     Accordingly, an objective of the present invention is to provide a piezoelectric acoustic transducer that allows effective reproduction of a high sound pressure in a limited space and with a limited cost. 
     Solution to the Problems 
     The present invention is directed to a piezoelectric acoustic transducer that vibrates in accordance with a voltage applied thereto. In order to achieve the above objective, the piezoelectric acoustic transducer of the present invention a plurality of piezoelectric diaphragms each having a piezoelectric element mounted on at least one main surface of a board; and at least one coupling member for aligning a vibration axis of the piezoelectric element of each of the plurality of piezoelectric diaphragms with each other, and for coupling adjacent piezoelectric diaphragms of the plurality of piezoelectric diaphragms to each other, and the polarity of the piezoelectric element of each of the plurality of piezoelectric diaphragms is set so that the adjacent piezoelectric diaphragms are displaced in directions opposite to each other, in accordance with a voltage applied thereto. 
     One of the piezoelectric diaphragms that is disposed on one side is coupled, in the center of the board, to a non-vibrating fixed frame of the piezoelectric acoustic transducer via the at least one coupling member, and the piezoelectric diaphragm is coupled to the piezoelectric diaphragm adjacent thereto on end portions, of the board, perpendicular to directions in which the piezoelectric element expands or contracts. Alternatively, in one of the piezoelectric diaphragms that is disposed on one side, end portions, of the board, perpendicular to directions in which the piezoelectric element expands or contracts, are coupled to a non-vibrating fixed frame of the piezoelectric acoustic transducer, and the one piezoelectric diaphragm is coupled, in a center of the board, to the piezoelectric diaphragm adjacent thereto via the at least one coupling member. 
     Preferably, the piezoelectric acoustic transducer further includes a surround, which is capable of expansion and contraction, for supporting the board of one of the piezoelectric diaphragms that is disposed on an other side, by means of the non-vibrating fixed frame of the piezoelectric acoustic transducer. Typically, the plurality of piezoelectric diaphragms are formed in rectangular shapes. Typically, the piezoelectric element has a structure in which a printed wiring formed on a surface of the board and a flat plate electrode interpose therebetween a piezoelectric member. It is conceivable that the piezoelectric member is any of piezoelectric single crystal, piezoelectric ceramic, and a piezoelectric polymer. 
     The adjacent piezoelectric diaphragms may be electrically connected with each other via a conductive part provided inside or outside of the at least one coupling member. In this case, the conductive part, provided outside of the at least one coupling member, can be integrally formed with the board which is included in the piezoelectric diaphragm, and which has formed on the surface thereof the printed wiring. 
     Advantageous Effects of the Invention 
     According to the present invention, the piezoelectric diaphragms of the plurality of speaker circuits are alternately displaced in antiphase. Accordingly, greater displacement can be obtained using the voltage used for one speaker circuit, and therefore, the voltage sensitivity in the low-frequency band is increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view showing a structure of a piezoelectric acoustic transducer  1  according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the piezoelectric acoustic transducer  1  taken along a line A-A. 
         FIG. 3A  is a diagram illustrating a vibration operation of the piezoelectric acoustic transducer  1 . 
         FIG. 3B  is a diagram illustrating a vibration operation of the piezoelectric acoustic transducer  1 . 
         FIG. 4  is a cross-sectional view showing another structure of the piezoelectric acoustic transducer  1  according to the first embodiment of the present invention. 
         FIG. 5  is an exploded view showing a structure of a piezoelectric acoustic transducer  2  according to a second embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of the piezoelectric acoustic transducer  2  taken along a line B-B. 
         FIG. 7  is an exploded view showing a structure of a piezoelectric acoustic transducer  3  according to a third embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of the piezoelectric acoustic transducer  3  taken along a line C-C. 
         FIG. 9A  is a diagram illustrating a vibration operation of the piezoelectric acoustic transducer  3 . 
         FIG. 9B  is a diagram illustrating another vibration operation of the piezoelectric acoustic transducer  3 . 
         FIG. 10A  is a cross-sectional view of structuring for a piezoelectric acoustic transducer according to another embodiment of the present invention. 
         FIG. 10B  is a cross-sectional view of a structuring for a piezoelectric acoustic transducer according to still another embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of a structuring for a piezoelectric acoustic transducer according to still another embodiment of the present invention. 
         FIG. 12  is an external view of the piezoelectric acoustic transducers of the present invention in a mounting example 1. 
         FIG. 13  is an external view of the piezoelectric acoustic transducers of the present invention in a mounting example 2. 
         FIG. 14  is a top view of the piezoelectric acoustic transducer of the present invention in a mounting example 3. 
         FIG. 15  is a cross-sectional view of a housing  111 , taken along a line D-D, in which the piezoelectric acoustic transducer  1  is mounted. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, description is given specifically of a piezoelectric acoustic transducer of the present invention with reference to the accompanying drawings. 
     In embodiments provided below, description is given of examples in which the piezoelectric acoustic transducer of the present invention is applied to a loudspeaker. The piezoelectric acoustic transducer of the present invention, however, may be applied to a vibrator, a sensor, a microphone, and the like. 
     First Embodiment 
       FIG. 1  is an exploded view showing a structure of a piezoelectric acoustic transducer  1  according to a first embodiment of the present invention.  FIG. 2  is a cross-sectional view of the piezoelectric acoustic transducer  1 , shown in  FIG. 1 , taken along a line A-A.  FIG. 3A  and  FIG. 3B  are diagrams each illustrating a vibration operation of the piezoelectric acoustic transducer  1  shown in  FIG. 1 . 
     The piezoelectric acoustic transducer  1  according to the first embodiment of the present invention includes an upper speaker circuit  10 , a lower speaker circuit  20 , coupling members  74  and  75 , a surround  76 , an upper frame  77 , and a lower frame  78 . The upper speaker circuit  10 , the lower speaker circuit  20 , the surround  76 , the upper frame  77 , and the lower frame  78  are each formed in a polygon with four right angles that has the same size.  FIG. 1  illustrates an example in a case where this shape is rectangle having a perimeter R. 
     First, description is given of a structure of the piezoelectric acoustic transducer  1 . 
     The upper speaker circuit  10  includes an outer frame portion  11 , first conductive portions  12 , second conductive portions  13 , and a piezoelectric diaphragm  14 . The outer frame portion  11  is a board shaped in a rectangular frame having the perimeter R and a predetermined width w. On this outer frame portion  11 , a first electric wiring  11   a  and a second electric wiring  11   b  are formed, being insulated from each other. The piezoelectric diaphragm  14  includes the following components: a board  15 , formed in a rectangular shape, which has a perimeter r smaller than an inner perimeter of the outer frame portion  11 ; a piezoelectric element  16  mounted on a portion of a top surface of the board  15 ; and a piezoelectric element  17  mounted on a portion of a bottom surface of the board  15 . This piezoelectric diaphragm  14  is connected with the outer frame portion  11  via the first conductive portions  12  and the second conductive portions  13  such that the piezoelectric diaphragm  14  can be curved. Typically, the outer frame portion  11 , the board  15 , the first conductive portions  12 , and the second conductive portions  13  are not configured by using separate components, but are integrally formed by punching a board material. 
     The piezoelectric elements  16  and  17  are thin flat elements each having a structure (not shown) that flat plate electrodes are disposed on top and bottom of a piezoelectric member. The piezoelectric member is formed of a piezoelectric material that expands or contracts in accordance with a voltage applied thereto. Each of the electrodes is formed of an electrically conductive material, such as a metal. The electrode formed on the surface of the board is also called a printed wiring. Electrodes of each of the piezoelectric elements  16  and  17  are electrically connected with a first electric wiring  11   a  and a second electric wiring  11   b , which are formed on the outer frame portion  11 , via the board  15 , the first conductive portions  12 , and the second conductive portions  13  so that a voltage having polarity, which causes the piezoelectric elements  16  and  17  to expand or contract in directions opposite to each other, is concurrently applied between the respective electrodes of each of the piezoelectric elements  16  and  17 . Because of this connection, the upper speaker circuit  10  curves in up-down directions in accordance with the voltage applied between the first electric wiring  11   a  and the second electric wiring  11   b.    
     The lower speaker circuit  20  also has a structure similar to that of the upper speaker circuit  10 , and includes an outer frame portion  21 , first conductive portions  22 , second conductive portions  23 , and a piezoelectric diaphragm  24 . The outer frame portion  21  is a board shaped in a rectangular frame having the perimeter R and the width w. On this outer frame portion  21 , a first electric wiring  21   a  and a second electric wiring  21   b  are formed, being insulated from each other. The piezoelectric diaphragm  24  includes the following components: a board  25  having the perimeter r; a piezoelectric element  26  mounted on a portion of a top surface of the board  25 ; and a piezoelectric element  27  mounted on a portion of a bottom surface of the board  25 . This piezoelectric diaphragm  24  is connected with the outer frame portion  21  via the first conductive portions  22  and the second conductive portions  23  such that the piezoelectric diaphragm  24  can be curved. 
     The piezoelectric elements  26  and  27  are thin flat elements each having a structure (not shown) that flat plate electrodes are disposed on top and bottom of a piezoelectric member. Electrodes of each of the piezoelectric elements  26  and  27  are electrically connected with a first electric wiring  21   a  and a second electric wiring  21   b , which are formed on the outer frame portion  21 , via the board  25 , the first conductive portions  22 , and the second conductive portions  23  so that a voltage having polarity, which causes the piezoelectric elements  26  and  27  to expand or contract in directions opposite to each other, is concurrently applied between the respective electrodes of each of the piezoelectric elements  26  and  27 . Because of this connection, the lower speaker circuit  20  operates for curving in up-down directions in accordance with the voltage applied between the first electric wiring  21   a  and the second electric wiring  21   b.    
     The first electric wiring  11   a  and the second electric wiring  11   b  of the upper speaker circuit  10  are each electrically connected with either of the first electric wiring  21   a  and the second electric wiring  21   b  of the lower speaker circuit  20  such that a voltage having polarity, which causes the upper speaker circuit  10  and the lower speaker circuit  20  to curve in directions opposite to each other, is concurrently applied between the electrodes disposed on the respective speaker circuits. 
     The upper frame  77  is formed of a substance in a rectangular frame shape, having the perimeter R and the width w. The lower frame  78  is formed of a substance in a rectangular frame shape, having the perimeter R and the width w, and a beam part  79  is disposed in the center of the lower frame  78 . In the lower speaker circuit  20 , a bottom surface of the outer frame portion  21  and a portion of the electrode disposed on a bottom surface of the piezoelectric element  27  are bonded with a top surface of the lower frame  78 , and a top surface of the outer frame portion  21  is bonded with a bottom surface of the upper frame  77 . In the upper speaker circuit  10 , a bottom surface of the outer frame portion  11  is bonded with a top surface of the upper frame  77 , and the surround  76  formed of a laminate material, which is capable of expansion and contraction, is mounted across a top surface of the upper speaker circuit  10  (see  FIG. 2 ). Portions of the board  15 , on which neither the piezoelectric elements  16  nor  17  of the upper speaker circuit  10  are mounted, and portions of the board  25 , on which neither the piezoelectric element  26  nor  27  of the lower speaker circuit  20  are mounted, are coupled (structurally connected) to each other via the coupling members  74  and  75  such that a vibration axis of each of the piezoelectric elements  16  and  17  aligns with a vibration axis of each of the piezoelectric elements  26  and  27 . Preferably, the coupling members  74  and  75  are each formed of a material having rigidity lower than those of the boards  15  and  25 . 
     Next, description is given of vibration operations of the piezoelectric acoustic transducer  1  having the structure described above. 
     When a voltage having a first polarity is applied between the first electric wiring  11   a  and the second electric wiring  11   b  of the upper speaker circuit  10 , the piezoelectric element  16  and the piezoelectric element  17  expand or contract in directions opposite to each other. As a result, the board  15  curves in accordance with a difference in expansion and contraction between these two piezoelectric elements, and the piezoelectric diaphragm  14  is displaced by X in a thickness direction thereof. On the other hand, this voltage having the first polarity is also applied between the first electric wiring  21   a  and the second electric wiring  21   b  of the lower speaker circuit  20 , causing the piezoelectric element  26  and the piezoelectric element  27  to expand or contract in directions opposite to each other. As a result, the board  25  curves in accordance with a difference in expansion and contraction between these two piezoelectric elements, and the piezoelectric diaphragm  24  is displaced by −x in a thickness direction thereof. See  FIG. 3A . 
     Further, when a voltage having a second polarity, which is an opposite polarity to the first polarity, is applied between the first electric wiring  11   a  and the second electric wiring  11   b  of the upper speaker circuit  10 , the respective directions, in which the piezoelectric element  16  and the piezoelectric element  17  expand or contract, are changed. As a result, the board  15  curves in a direction opposite to that in the case where the voltage having the first polarity is applied, and thus the piezoelectric diaphragm  14  is displaced by −X in the thickness direction thereof. Meanwhile, the respective directions, in which the piezoelectric element  26  and the piezoelectric element  27  expand or contract, are also changed. As a result, the board  25  curves in a direction opposite to that in the case where the voltage having the first polarity is applied. Thus, the piezoelectric diaphragm  24  is displaced by x in the thickness direction thereof. See  FIG. 3B . 
     The piezoelectric diaphragm  24  is coupled to a non-vibrating fixed frame of the piezoelectric acoustic transducer  1  via the beam part  79  which functions as a coupling member. The piezoelectric diaphragm  14  and the piezoelectric diaphragm  24  are coupled to the coupling members  74  and  75 . Accordingly, the entire displacement of the piezoelectric acoustic transducer  1 , when the voltage having the first polarity is applied, is represented by “X+x” which is a difference between the displacement X of the piezoelectric diaphragm  14  and the displacement −x of the piezoelectric diaphragm  24 . See  FIG. 3A . Further, the entire displacement of the piezoelectric acoustic transducer  1 , when the voltage having the second polarity is applied, is represented by “−X−x” which is a difference between the displacement −X of the piezoelectric diaphragm  14  and the displacement x of the piezoelectric diaphragm  24 . See  FIG. 3B . Accordingly, the piezoelectric acoustic transducer  1  that has two piezoelectric diaphragms can obtain greater displacement by using the same voltage, as compared to a piezoelectric acoustic transducer that has one piezoelectric diaphragm. Namely, a higher sound pressure can be generated. 
     As described above, according to the piezoelectric acoustic transducer  1  of the first embodiment of the present invention, the piezoelectric diaphragm  14  of the speaker circuit  10  and the piezoelectric diaphragm  24  of the speaker circuit  20  are displaced in respective directions opposite to each other, and thereby displacement greater than that in the case where one speaker circuit is used can be obtained by using the same voltage. Therefore, the voltage sensitivity in the low-frequency band is increased. Further, as compared to the first and the third methods described in background art, the piezoelectric acoustic transducer  1  having high quality sound, which is space-saving and low cost, and which has excellent voltage sensitivity in the low-frequency band, can be achieved. 
     Further, according to the piezoelectric acoustic transducer  1  of the first embodiment of the present invention, the piezoelectric diaphragms  14  and  24 , each formed in a rectangular shape, are supported by the conductive portions  12 ,  13 , and the conductive portions  22 , and  23 , respectively, such that the piezoelectric diaphragms  14  and  24  each can be curved. According to this, resonance of each of the piezoelectric diaphragms  14  and  24  in the long side directions is efficiently excited, and thereby, the piezoelectric diaphragms  14  and  24  are subjected to vibrate in the low-frequency. Therefore, it is possible to reproduce the bass with high quality sound (ameliorate the reproduction limit of a bass range). 
     The surround  76  is mounted on the upper speaker circuit  10  to insulate an antiphase sound wave, which is generated from the bottom of the piezoelectric acoustic transducer  1 , and which interferes with a sound wave emitted to the front of the piezoelectric acoustic transducer  1 , and thereby, preventing the reduction of the sound pressure. Therefore, the surround  76  may support the piezoelectric diaphragm  14  flexibly without obstructing the displacement of the piezoelectric diaphragm  14  in the thickness direction. Thus, the surround  76  need not be mounted across the top surface of the upper speaker circuit  10 , as shown in the first embodiment of the present invention, and may be configured so as to fill gaps formed between the piezoelectric diaphragm  14  and the outer frame portion  11 . See  FIG. 4 . 
     Moreover, the structures of the coupling members  74  and  75  are not limited to those of the embodiment shown in  FIG. 1 , in which the coupling members  74  and  75 , formed in rectangular shapes, couple the piezoelectric diaphragm  14  and the piezoelectric diaphragm  24  to each other in end portions of the boards  15  and  25 . For example, the structures of the coupling members  74  and  75  may be formed in cubical shapes or cylindrical shapes such that the piezoelectric diaphragm  14  and the piezoelectric diaphragm  24  are coupled to each other at four corners of each of the boards  15  and  25 . According to the such structures, the resonance of each of the piezoelectric diaphragms  14  and  24  in diagonal directions is efficiently excited. Therefore, it is possible to ameliorate the reproduction limit of the bass range. Further, the resonance of each of the piezoelectric diaphragms  14  and  24  in the short side directions (which have a higher resonance frequency than that in the diagonal directions) is efficiently excited. Therefore, it is possible to obtain greater displacement in the frequency band between the resonance frequency in the diagonal directions and the resonance frequency in the short side directions. 
     Second Embodiment 
       FIG. 5  is an exploded view showing a structure of a piezoelectric acoustic transducer  2  according to a second embodiment of the present invention.  FIG. 6  is a cross-sectional view of the piezoelectric acoustic transducer  2 , shown in  FIG. 5 , taken along a line B-B. 
     The piezoelectric acoustic transducer  2  according to the second embodiment of the present invention includes an upper speaker circuit  30 , a lower speaker circuit  20 , coupling members  74  and  75 , a surround  76 , an upper frame  77 , and a lower frame  78 . This piezoelectric acoustic transducer  2  is different from the piezoelectric acoustic transducer  1  described above in a configuration of the upper speaker circuit  30 . Hereinafter, the same reference characters are given to the components that are the same as those of the piezoelectric acoustic transducer  1 , and description thereof is omitted. Hereinafter, different configurations are mainly described. 
     The upper speaker circuit  30  includes a piezoelectric diaphragm  34  and third conductive portions  38 . Similar to the piezoelectric diaphragm  14  described above, the piezoelectric diaphragm  34  includes the following components: a board  35 , formed in a rectangular shape, which has a perimeter r; a piezoelectric element  36  mounted on a portion of a top surface of the board  35 ; and a piezoelectric element  37  mounted on a portion of a bottom surface of the board  35 . The structures of the piezoelectric elements  36  and  37  are the same as those of the piezoelectric elements  16  and  17 , respectively. The third conductive portions  38  are each disposed on the board  35  in a predetermined shape, and plays a role to electrically connect the board  35  of the upper speaker circuit  30  with the board  25  of the lower speaker circuit  20 . Specifically, the third conductive portions  38  electrically connect electrodes disposed on top and bottom of each of the piezoelectric elements  36  and  37  of the upper speaker circuit  30  with respective electrodes disposed on top and bottom of each of the piezoelectric elements  26  and  27  of the lower speaker circuit  20  such that, when a voltage having polarity is applied between the first electric wiring  21   a  and the second electric wiring  21   b , the piezoelectric diaphragm  24  and the piezoelectric diaphragm  34  are displaced in directions opposite to each other. 
     As described above, according to the piezoelectric acoustic transducer  2  of the second embodiment of the present invention, the piezoelectric diaphragms of the respective two speaker circuits are electrically connected with each other via the third conductive portions  38 . Therefore, the piezoelectric diaphragm  34  of the upper speaker circuit  30  need not be supported by the outer frame, and thereby the greater displacement and the linearity of the vibration can be secured, in addition to the effects obtained by the first embodiment. 
     In the second embodiment, an example is given in which the third conductive portions  38 , which are disposed along the surfaces of the coupling members  74  and  75 , are used to electrically connect the piezoelectric diaphragms  24  and  34  with each other. The piezoelectric diaphragms  24  and  34 , however, may be electrically connected with each other through conductive portions which are provided inside of the coupling members  74  and  75  (e.g., through-holes). 
     Third Embodiment 
       FIG. 7  is an exploded view showing a structure of a piezoelectric acoustic transducer  3  according to a third embodiment of the present invention.  FIG. 8  is a cross-sectional view of the piezoelectric acoustic transducer  3 , shown in  FIG. 7 , taken along a line C-C.  FIGS. 9A and 9B  are diagrams each illustrating a vibration operation of the piezoelectric acoustic transducer  3  shown in  FIG. 7 . 
     The piezoelectric acoustic transducer  3  according to the third embodiment of the present invention includes an upper speaker circuit  10 , a lower speaker circuit  40 , a coupling member  80 , a surround  76 , an upper frame  77 , and a lower frame  81 . This piezoelectric acoustic transducer  3  is different from the piezoelectric acoustic transducer  1  described above in terms of configurations of the lower speaker circuit  40 , the coupling member  80 , and the lower frame  81 . Hereinafter, the same reference characters are given to the components that are the same as those of the piezoelectric acoustic transducer  1 , and description thereof is omitted. Different configurations are mainly described. 
     The lower speaker circuit  40  includes an outer frame portion  41  and a piezoelectric diaphragm  44 . The outer frame portion  41  is a board shaped in a rectangular frame having a perimeter R and a width w. On this outer frame portion  41 , a first electric wiring  41   a  and a second electric wiring  41   b  are formed, being insulated from each other. The piezoelectric diaphragm  44  includes the following components: a board  45 , formed in a rectangular shape, which couples the short sides of the outer frame portion  41 ; a piezoelectric element  46  mounted on a portion of a top surface of the board  45 ; and a piezoelectric element  47  mounted on a portion of a bottom surface of the board  45 . The structures of the piezoelectric elements  46  and  47  are the same as those of the piezoelectric elements  16  and  17 , respectively. This piezoelectric diaphragm  44  is connected with the outer frame portion  41  such that the piezoelectric diaphragm  44  can be curved. Typically, the outer frame portion  41  and the board  45  are integrally formed by punching a board material. 
     The lower frame  81  is formed of a substance in a rectangular frame shape, having the perimeter R and the width w. In the lower speaker circuit  40 , the bottom surface of the outer frame portion  41  is bonded with a top surface of the lower frame  81 , and a top surface of the outer frame portion  41  is bonded with the bottom surface of the upper frame  77 . Further, an electrode, disposed on bottom of the piezoelectric element  17  of the upper speaker circuit  10 , and an electrode, disposed on top of piezoelectric element  46  of the lower speaker circuit  40 , are structurally connected with each other at the center portions thereof via the coupling member  80 . Preferably, this coupling member  80  is formed of a material having rigidity lower than those of the boards  15  and  45 . The vibration operations of the piezoelectric acoustic transducer  3  having this structure are as shown in  FIGS. 9A and 9B . 
     As described above, according to the piezoelectric acoustic transducer  3  of the third embodiment of the present invention, two speaker circuits are coupled to each other merely via the coupling member  80 . Therefore, the number of components and materials cost can be reduced, in addition to the effects obtained by the first embodiment. 
     Examples are given of devices and materials used for the components of the piezoelectric acoustic transducer. 
     For the board, a general-purpose plastic material (such as a polycarbonate, a polyalylate film, a polyethylene terephthalate, or a polyimide), or a material having an insulation property, such as a liquid crystal polymer, is used. For the piezoelectric member, a piezoelectric single crystal, a piezoelectric ceramic, or a piezoelectric polymer is used. For the electrode, a membrane material which includes one of copper, aluminum, titanium, silver, and the like, or an alloy membrane material using thereof is used. For the surround, a flexible plastic material (such as polyethersulfone), a rubber polymer material (such as an SBR, an NBR, or acrylonitrile), or the like, is used. For the coupling member, the general-purpose plastic material, the rubber polymer material (such as the SBR, the NBR, or the acrylonitrile), the liquid crystal polymer, or the like, is used. 
     OTHER EMBODIMENTS 
     In the first through third embodiments described above, examples are described where each piezoelectric diaphragm has the piezoelectric elements mounted on both of the top surface and the bottom surface of the board. However, a piezoelectric diaphragm having the piezoelectric element mounted on either the top surface or the bottom surface of the board is also applicable to the piezoelectric acoustic transducer of the present invention (e.g.,  FIGS. 10A and 10B ). 
     Further, in the first through third embodiments described above, description is given of the structure having two speaker circuits coupled to each other. However, a structure having three or more speaker circuits coupled to one another is also applicable to the piezoelectric acoustic transducer of the present invention (e.g.,  FIG. 11 ). 
     [Mounting Example 1 of Piezoelectric Acoustic Transducer] 
       FIG. 12  is an external view of the piezoelectric acoustic transducers, of the present invention, which are mounted in a mobile phone terminal. In  FIG. 12 , piezoelectric acoustic transducers  103  are any of the piezoelectric acoustic transducers  1  through  3  of the present invention described above, and are disposed on both sides of a display  102  which is provided in a housing  101  of the mobile phone terminal. Sounds generated from these piezoelectric acoustic transducers  103  are emitted through sound holes  104  to the external space. 
     As described in the first through third embodiments, each of the piezoelectric acoustic transducers  103  of the present invention can achieve space-saving and high quality sound, without the necessity of an increased number of the components. Therefore, by mounting the piezoelectric acoustic transducers  103 , a mobile phone terminal achieving reduction in thickness, and high quality sound, can be readily designed. 
     [Mounting Example 2 of Piezoelectric Acoustic Transducer] 
       FIG. 13  is an external view of the piezoelectric acoustic transducers, of the present invention, which are mounted in a flat screen television. In  FIG. 13 , piezoelectric acoustic transducers  107  of the present invention are any of the piezoelectric acoustic transducers  1  through  3  of the present invention described above, and are disposed on both sides of a display  106  which is provided in a housing  105  of the flat screen television. 
     In general, a region, in which a loudspeaker is mounted in the housing  105  of the flat screen television, is very restricted and a volume of the cabinet is small. Therefore, by mounting the piezoelectric acoustic transducers  107 , the flat screen television achieving reduction in thickness, and high quality sound, can be readily designed. Particularly, the use of each of the piezoelectric acoustic transducers  107 , as a loudspeaker for bass reproduction (woofer), realizes a sense of presence of the audio-visual content to be reproduced, without increasing the installation space. 
     [Mounting Example 3 of Piezoelectric Acoustic Transducer] 
     If the piezoelectric acoustic transducer of the present invention is directly mounted in the mobile phone terminal, the flat screen television, or the like, a problem arises that the vibration caused during the operation propagates to the housing, and thereby unwanted sound (such as excitation caused by the natural vibration of the housing) is likely to occur. Therefore, in such a case, preferably, a vibration isolation and a vibration control are performed, as described in the following, at a time when the piezoelectric acoustic transducer is mounted in the housing. 
       FIG. 14  is a top view of a housing  111  of the mobile phone terminal, the flat screen television, or the like, in which the piezoelectric acoustic transducer of the present invention is mounted.  FIG. 15  is a cross-sectional view of the housing  111 , shown in  FIG. 14 , taken along a line D-D, in which the piezoelectric acoustic transducer  1  according to the first embodiment is mounted. 
     The housing  111  is a box having an opening portion  111   a , and includes projections  112  on a lower interior wall  111   c . The bottom portion of the piezoelectric acoustic transducer  1  is mounted in the housing  111  such that bottom surfaces of the lower frame  78  and the beam part  79  are mounted on the projections  112  which interpose therebetween a vibration control member  114 . The top portion of the piezoelectric acoustic transducer  1  is mounted in the housing  111  such that merely a portion, of a top surface of the surround  76 , which corresponds to the upper frame  77 , is fixed to an upper interior wall  111   b , of the housing  111 , which interposes therebetween a vibration isolation member  113 . 
     Provision of the vibration isolation member  113  can make it possible to prevent the vibration, caused by the piezoelectric acoustic transducer  1 , from propagating to a top surface of the housing  111 . Further, provision of the vibration control member  114  can make it possible to fix the frame parts of the piezoelectric acoustic transducer  1  to the housing  111  via the projections  112 , and, at the same time, to prevent the vibration, caused by the piezoelectric acoustic transducer  1 , from propagating to a bottom surface of the housing  111 . Accordingly, the occurrence of the unwanted sound, which is caused by the resonance of the housing  111 , can be prevented, in addition to the effects described above. The piezoelectric acoustic transducer  1  may be mounted in the housing  111  via one of the upper interior wall  111   b , the lower interior wall  111   c , and a side interior wall  111   d.    
     INDUSTRIAL APPLICABILITY 
     A piezoelectric acoustic transducer of the present invention is applicable to a loudspeaker, a vibrator, a sensor, a microphone, and the like, and is useful particularly to achieve both space-saving and high quality sound. 
     DESCRIPTION OF THE REFERENCE CHARACTERS 
     
         
         
           
               1 ,  2 ,  3 ,  103 ,  107  piezoelectric acoustic transducer 
               10 ,  20 ,  30 ,  40  speaker circuit 
               11 ,  21 ,  41  outer frame portion 
               11   a ,  11   b ,  21   a ,  21   b ,  41   a ,  41   b  electric wiring 
               12 ,  13 ,  22 ,  23 ,  38  conductive portion 
               14 ,  24 ,  34 ,  44  piezoelectric diaphragm 
               15 ,  25 ,  35 ,  45  board 
               16 ,  17 ,  26 ,  27 ,  36 ,  37 ,  46 ,  47  piezoelectric element 
               74 ,  75 ,  80  coupling member 
               76  surround 
               77 ,  78 ,  81  frame 
               79  beam part 
               101 ,  105 ,  111  housing 
               102 ,  106  display 
               104  sound hole 
               111   a  opening portion 
               111   b ,  111   c ,  111   d  interior wall 
               112  projection 
               113  vibration isolation member 
               114  vibration control member