Patent Publication Number: US-7586241-B2

Title: Electroacoustic transducer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of International Application No. PCT/JP2006/315587, filed Aug. 7, 2006, which claims priority to Japanese Patent Application No. JP2005-339033, filed Nov. 24, 2005, the entire contents of each of these applications being incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to electroacoustic transducers used as sounders such as speakers, and, more particularly, to an electroacoustic transducer having a cantilever support structure in which a piezoelectric element is supported at one end portion thereof. 
   BACKGROUND OF THE INVENTION 
   In recent years, electroacoustic transducers such as a speaker and a buzzer using a piezoelectric effect are widely used. For example, the following Patent Document 1 discloses a sounder  101  illustrated in an elevational cross-sectional view in  FIG. 11 . The sounder  101  includes a box  102 . One end portion of a piezoelectric vibrating element  103  is coupled to the inner wall of the box  102 . The piezoelectric vibrating element  103  has a configuration in which electrodes  103   a  and  103   b  are provided on opposite surfaces of a piezoelectric ceramic plate. By applying an alternating electrical field to the piezoelectric ceramic plate from the electrodes  103   a  and  103   b , the piezoelectric ceramic plate is polarized so that vibration thereof is excited. 
   In the sounder  101 , the piezoelectric vibrating element  103  is supported at one end portion thereof, and the other end portion of the piezoelectric vibrating element  103  is a free end portion. That is, since the piezoelectric vibrating element  103  is supported in the cantilever manner, the free end portion of the piezoelectric vibrating element  103  can be significantly displaced. Accordingly, a high sound pressure can be obtained. 
   On the other hand, the following Patent Document 2 discloses a piezoelectric ceramic speaker illustrated in  FIG. 12 . In a piezoelectric ceramic speaker  111 , one end portion of a piezoelectric vibrating element  113  is coupled to a frame member  112 . The piezoelectric vibrating element  113  is supported at one end portion thereof in the cantilever manner, and the other end portion thereof is a free end portion. On the side of the free end portion, the center of a cone-shaped diaphragm  114  is fixed to the piezoelectric vibrating element  113 . Accordingly, at the time of bending vibration of the piezoelectric vibrating element  113 , the cone-shaped diaphragm  114  coupled to the free end portion of the piezoelectric vibrating element  113  vibrates. As a result, a high sound pressure can be obtained. 
   Patent Document 1: Japanese Unexamined Utility Model Application Publication No. 63-191800 
   Patent Document 2: Japanese Utility Model Registration No. 3068450 
   In the sounder  101 , the peripheral portion of the piezoelectric vibrating element  103  excluding the portion coupled to the box  102  is exposed in the box  102 . Accordingly, when the piezoelectric vibrating element  103  vibrates, the air pressure on the side of one surface of the piezoelectric vibrating element  103  and the air pressure on the side of the other surface of the piezoelectric vibrating element  103  cancel each other. As a result, sound in a low frequency range cannot be obtained. That is, a high sound pressure cannot be obtained over a wide range of frequencies. 
   On the other hand, in the piezoelectric ceramic speaker  111  disclosed in Patent Document 2, the cone-shaped diaphragm  114  capable of directly acting on the air generates an acoustic wave. Accordingly, in addition to the piezoelectric vibrating element  113 , the large cone-shaped diaphragm  114  is required. As a result, the piezoelectric ceramic speaker  111  becomes larger in size. This makes it difficult to reduce the thickness of the piezoelectric ceramic speaker  111 . Furthermore, the number of components is increased, and the number of manufacturing processes is therefore increased. This leads to the increase in cost. 
   Furthermore, natural in-plane vibration of the cone-shaped diaphragm  114  occurs. As a result, a frequency characteristic becomes deteriorated. Still furthermore, although the piezoelectric vibrating element  113  is supported at one end portion thereof in the cantilever manner, the free end portion thereof is coupled to the cone-shaped diaphragm  114 . Accordingly, there is a vibration mode in which the free end portion of the piezoelectric vibrating element  113  is hardly displaced. As a result, a sharp dip occurs in a frequency characteristic. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an electroacoustic transducer capable of obtaining a high sound pressure over a comparatively wide range of frequencies with certainty and coming down in size, in particular, in thickness without increasing the number of components and the number of manufacturing processes. 
   According to the present invention, there is provided an electroacoustic transducer including a frame having an opening, a plurality of piezoelectric elements each placed in the opening of the frame and each having one end portion coupled to the frame, and a flexible thin film that is bonded to the frame and the piezoelectric elements so as to cover at least a gap between the frame and each of the piezoelectric elements in the opening of the frame. The other end portions opposite to the end portions of the piezoelectric elements are free end portions, and face each other across a gap in the opening of the frame. 
   In an aspect of an electroacoustic transducer according to the present invention, the flexible thin film is placed so that it covers an entire area of the opening of the frame, and an entire area of one of main surfaces of each of the piezoelectric elements is bonded to the flexible thin film. 
   In another aspect of an electroacoustic transducer according to the present invention, the piezoelectric elements each has a substantially trapezoidal shape having an upper base and a lower base corresponding to the end portion coupled to the frame. 
   In still another aspect of an electroacoustic transducer according to the present invention, a length of the end portion of each of the piezoelectric elements which is coupled to the frame is smaller than a width of the opening at the gap across which the piezoelectric elements face each other. The width of the opening is a dimension in the same direction as a direction of the length of the end portion. 
   In still another aspect of an electroacoustic transducer according to the present invention, a rigid plate that is bonded to the flexible thin film at the gap across which the piezoelectric elements face each other and has a higher rigidity than that of the flexible thin film is further included. 
   In still another aspect of an electroacoustic transducer according to the present invention, a resonance frequency in a fundamental mode of at least one of the piezoelectric elements is different from that of the other piezoelectric elements. 
   In still another aspect of an electroacoustic transducer according to the present invention, a planer shape of at least one of the piezoelectric elements is different from that of the other piezoelectric elements. 
   In still another aspect of an electroacoustic transducer according to the present invention, a thickness of at least one of the piezoelectric elements is different from that of the other piezoelectric elements. 
   In still another aspect of an electroacoustic transducer according to the present invention, the frame and the piezoelectric elements are integrally formed using a single piezoelectric ceramic plate. 
   In an electroacoustic transducer according to the present invention, each of a plurality of piezoelectric elements is coupled to a frame at one end portion thereof. The other end portion of each of the piezoelectric elements is a free end portion. Thus, the piezoelectric elements are supported in the cantilever manner. Accordingly, the free end portions of the piezoelectric elements can be significantly displaced. A flexible thin film is bonded to the frame and the piezoelectric elements so that it covers at least a gap between each of the piezoelectric elements and the frame. Accordingly, the flexible thin film bonded to the piezoelectric elements capable of being significantly displaced can also be significantly displaced. Consequently, a very high sound pressure can be obtained. 
   By simply bonding the piezoelectric elements and the flexible thin film to the frame, the miniaturization of an electroacoustic transducer, in particular, the reduction in thickness of the electroacoustic transducer can be easily achieved. Furthermore, the increase in the number of components can be prevented, and the electroacoustic transducer can be easily assembled. This leads to cost reduction. 
   If the flexible thin film is placed so that it covers the entire area of the frame and the entire area of one of main surfaces of each of piezoelectric elements is bonded to the flexible thin film, the flexible thin film can be bonded to the frame so that it covers the entire area of the opening of the frame and the piezoelectric elements can be easily bonded to the flexible thin film. Accordingly, the electroacoustic transducer can be more easily manufactured, and a more inexpensive electroacoustic transducer can be provided. 
   If the piezoelectric elements each has a substantially trapezoidal shape having an upper base and a lower base corresponding to the end portion coupled to the frame, the free end portion corresponding to the upper base can be more easily displaced. Accordingly, a higher sound pressure can be obtained. 
   If the length of the end portion of each of the piezoelectric elements which is coupled to the frame is smaller than a width of the opening at the gap across which the piezoelectric elements face each other (the width of the opening is a dimension in the same direction as the length direction of the end portion), the piezoelectric elements can be easily displaced on the side of the gap across which the piezoelectric elements face each other. Accordingly, a higher sound pressure can be obtained. 
   If a rigid plate is bonded to the flexible thin film at the gap across which the piezoelectric elements face each other, the portion of the flexible thin film at the gap in which the rigid plate is disposed is displaced. As a result, a higher sound pressure can be obtained. 
   If a resonance frequency in a fundamental mode of at least one of the piezoelectric elements is different from that of the other piezoelectric elements, a high sound pressure can be obtained over a wider range of frequencies. If a planer shape of at least one of the piezoelectric elements is different from that of the other piezoelectric elements, a resonance frequency in a fundamental mode of the piezoelectric element can be easily made different from that of the other piezoelectric elements. If a thickness of at least one of the piezoelectric elements is different from that of the other piezoelectric elements, a resonance frequency in a fundamental mode of the piezoelectric element can be similarly easily made different from that of the other piezoelectric elements. 
   If the frame and the piezoelectric elements are integrally formed using a single piezoelectric ceramic plate, the number of components can be reduced and an electroacoustic transducer capable of easily coming down in size can be provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1(   a ) and  1 ( b ) are an elevational cross-sectional view and a plan view of an electroacoustic transducer according to a first embodiment of the present invention, respectively. 
       FIG. 2(   a ) is a plan view of an electroacoustic transducer according to an exemplary modification of the first embodiment of the present invention, and  FIG. 2(   b ) is a plan view of an electroacoustic transducer according to another exemplary modification of the first embodiment of the present invention. 
       FIG. 3  is a plan view describing an electroacoustic transducer according to a second embodiment of the present invention. 
       FIG. 4  is a plan view describing an electroacoustic transducer according to a third embodiment of the present invention. 
       FIGS. 5(   a ) and  5 ( b ) are an elevational cross-sectional view and a plan view describing an electroacoustic transducer according to a fourth embodiment of the present invention, respectively. 
       FIG. 6  is a schematic diagram illustrating a frequency characteristic of an electroacoustic transducer according to the fourth embodiment of the present invention. 
       FIG. 7  is a schematic diagram illustrating a frequency characteristic of an electroacoustic transducer according to the first embodiment of the present invention. 
       FIG. 8  is an elevational cross-sectional view describing an electroacoustic transducer according to a fifth embodiment of the present invention. 
       FIG. 9  is a schematic plan view describing an exemplary manufacturing method of an electroacoustic transducer according to the first embodiment of the present invention. 
       FIGS. 10(   a ) and  10 ( b ) are a plan view and a plan cross-al sectional view describing an electroacoustic transducer according to still another exemplary modification of the first embodiment of the present invention, respectively. 
       FIG. 11  is an elevational cross-sectional view describing a sounder that is an electroacoustic transducer in the related art. 
       FIG. 12  is an elevational cross-sectional view describing a piezoelectric speaker that is an electroacoustic transducer in the related art. 
   

   REFERENCE NUMERALS 
     1  electroacoustic transducer 
     2  frame member 
     3  frame member 
     4  frame 
     4   a  opening 
     5  flexible thin film 
     6  first piezoelectric element 
     6 A and  7 A piezoelectric element 
     6   a  and  7   a  end portion 
     6   b  and  7   b  end portion 
     7  second piezoelectric element 
     8  and  9  electrode film 
     8   a  and  9   a  notch 
     10  and  11  terminal electrode 
     12  electroacoustic transducer 
     13  electroacoustic transducer 
     21  electroacoustic transducer 
     22  rigid plate 
     31  electroacoustic transducer 
     32  to  35  first to fourth piezoelectric elements 
     36  rigid plate 
     41  electroacoustic transducer 
     43  and  43  piezoelectric element 
     51  electroacoustic transducer 
     52  and  53  first and second piezoelectric elements 
     71  piezoelectric ceramic plate 
     71   a  notch 
     71   b  and  71   c  side face 
     72  first piezoelectric element 
     73  second piezoelectric element 
     74  electrode film 
     75  internal electrode 
   A gap 
   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     FIGS. 1(   a ) and  1 ( b ) are an elevational cross-sectional view and a plan view of an electroacoustic transducer according to a first embodiment of the present invention, respectively. An electroacoustic transducer  1  is suitable for use as a piezoelectric speaker. The electroacoustic transducer  1  includes a frame  4  obtained by bonding a first frame member  2  and a second frame member  3  together. The frame members  2  and  3  are made of an appropriate rigid material such as metal or ceramic. In this embodiment, the frame members  2  and  3  are made of metal. 
   The frame  4  includes an opening  4   a . In the opening  4   a , a flexible thin film  5  is bonded to the frame  4  so that it covers the entire area of the opening  4   a . More specifically, the peripheral portion of the flexible thin film  5  is sandwiched and fixed between the frame members  2  and  3  so that the flexible thin film  5  covers the entire area of the opening  4   a.    
   The flexible thin film  5  is composed of a flexible thin film made of a material that is not particularly limited, but is preferably a deformable and elastic material having a large internal loss and an excellent environmental resistance such as synthetic rubber having rubber elasticity, natural rubber, or elastomer. Examples of such synthetic rubber include ethylene-butadiene rubber and styrene-butadiene rubber. 
   The thickness of the flexible thin film  5  is not particularly limited, but, in this embodiment, ranges from 30 to 100 μm. The flexible thin film  5  is required to have a certain degree of flexibility that does not restrict displacement caused by the bending vibration of a piezoelectric element that will be described later. 
   Each of a first piezoelectric element  6  and a second piezoelectric element  7  is bonded to the upper surface of the flexible thin film  5 . In this embodiment, each of the piezoelectric elements  6  and  7  is a laminated piezoelectric element obtained by forming piezoelectric ceramic layers made of piezoelectric ceramic such as lead zirconate titanate on opposite sides of a single internal electrode layer. On opposite main surfaces of this laminated piezoelectric element, electrode films are formed (not illustrated in  FIG. 1(   a )). Such a laminated piezoelectric element can be obtained using an internal electrode-ceramic integral firing technique, and is widely used in piezoelectric speakers or piezoelectric buzzers. 
   In this embodiment, the above-described internal electrode is made of an Ag—Pt alloy. The electrode films formed on the opposite main surfaces of the laminated piezoelectric element are formed by sputtering an Ni—Cu alloy. On the end face of the piezoelectric element  6 , the internal electrode thereof and the electrode films thereof are electrically connected to each other. On the end face of the piezoelectric element  7 , the internal electrode thereof and the electrode films thereof are electrically connected to each other. As illustrated in  FIG. 1(   b ), a notch  8   a  is formed near one of corners of an electrode film  8  formed on the upper surface of the first piezoelectric element  6 , and a notch  9   a  is formed near one of corners of an electrode film  9  formed on the upper surface of the second piezoelectric element  7 . Terminal electrodes  10  and  11  are provided in the notches  8   a  and  9   a , respectively. The terminal electrodes  10  and  11  extend to the end faces of the piezoelectric elements  6  and  7 , respectively, and are individually electrically connected to the internal electrodes (not illustrated) on the end faces. By applying an AC voltage between the terminal electrode  10  and the electrode film  8  and between the terminal electrode  11  and the electrode film  9 , the electroacoustic transducer  1  is driven. Each of the piezoelectric elements  6  and  7  includes two layers that are polarized the same direction of thickness. 
   One end portion  6   a  of the laminated piezoelectric element  6  and one end portion  7   a  of the laminated piezoelectric element  7  are fixed to the frame  4 , whereby the laminated piezoelectric elements  6  and  7  are supported. More specifically, as illustrated in  FIG. 1(   a ), the piezoelectric element  6  is sandwiched between the frame members  2  and  3  near the end portion  6   a  of the piezoelectric element  6  and the piezoelectric element  7  is sandwiched between the frame members  2  and  3  near the end portion  7   a  of the piezoelectric element  7 , whereby the piezoelectric elements  6  and  7  are fixed. Accordingly, the other end portions  6   b  and  7   b  opposite to the end portions  6   a  and  7   a  of the piezoelectric elements  6  and  7  are free end portions. That is, the piezoelectric elements  6  and  7  are supported in the cantilever manner. Accordingly, the end portions  6   b  and  7   b , which are free end portions, can be significantly displaced. The end portion  6   b  and the end portion  7   b  face each other with a gap therebetween. 
   Each of the piezoelectric elements  6  and  7  and the flexible thin film  5 , which are sandwiched between the frame members  2  and  3 , are bonded and fixed to each other using a known adhesive. Such an adhesive is not particularly limited. In this embodiment, however, a thermosetting silicon adhesive is used. Another adhesive such as an epoxy adhesive may be used. Alternatively, a hardening adhesive may be used instead of a thermosetting adhesive. 
   In the electroacoustic transducer  1  according to this embodiment, the piezoelectric elements  6  and  7  are used in such a manner that the bending vibrations of the piezoelectric elements  6  and  7  are in-phase with each other. Accordingly, the terminal electrodes  10  and  11  are connected to one potential, the electrode films  8  and  9  and the electrode films on the undersurfaces are connected to the other potential, and an alternating voltage is applied between the terminal electrode and the electrode film. As a result, the bending vibrations of the piezoelectric elements  6  and  7  are in-phase with each other, and the flexible thin film  5  vibrates in synchronization with the vibrations of the piezoelectric elements  6  and  7 . Accordingly, sound is produced in accordance with the difference between the air pressure on the side of the upper surface of the flexible thin film  5  and the air pressure on the side of the undersurface of the flexible thin film  5 . 
   In the electroacoustic transducer  1  that is manufactured as described previously, since the movement of the air between the side of the upper surface and the side of the undersurface of the flexible thin film  5  is interrupted, the difference between the air pressure on the side of the upper surface and the air pressure on the side of the undersurface is not canceled. Accordingly, a high sound pressure can be obtained over a wide range of frequencies. Furthermore, since the piezoelectric elements  6  and  7  are supported in the cantilever manner, the end portions  6   b  and  7   b  can be easily displaced. The flexible thin film  5  is therefore significantly displaced at a center position thereof at which the gap A is provided. Accordingly, a high sound pressure can be obtained. 
   Since the main part including the flexible thin film  5  and the piezoelectric elements  6  and  7  is obtained by simply bonding the piezoelectric elements  6  and  7  to the flexible thin film  5 , these components are on the substantially same surface. Accordingly, the thickness can be reduced. Furthermore, since it is not required to couple a cone-shaped diaphragm to the piezoelectric elements  6  and  7 , the number of components can be reduced and the manufacturing process can be simplified. Consequently, the cost of an electroacoustic transducer can be effectively reduced. 
   Since the piezoelectric elements  6  and  7  function as diaphragms, it is not required to take a natural vibration other than the natural vibrations of the piezoelectric elements  6  and  7  into consideration. Furthermore, since not only the displacements of the end portions  6   b  and  7   b  of the piezoelectric elements  6  and  7  but also the displacements of the entire surfaces of the piezoelectric elements  6  and  7  are used, a sharp dip cannot be easily occurred in a frequency characteristic. Accordingly, a flat and high sound pressure characteristic can be obtained over a wide range of frequencies. 
   In this embodiment, the flexible thin film  5  covers the entire area of the opening  4   a , but may cover at least the gap between each of the piezoelectric elements  6  and  7  and the frame  4 . That is, the peripheral portions of the piezoelectric elements  6  and  7  may be coupled to the flexible thin film  5 . The entire area of one of the main surfaces of each of the piezoelectric elements  6  and  7  may not necessarily be bonded to the flexible thin film  5  as described previously. The number of piezoelectric layers included in each of the piezoelectric elements  6  and  7  is not limited to two. Each of the piezoelectric elements  6  and  7  may include a large number of piezoelectric layers, for example, three or four piezoelectric layers. 
     FIG. 2(   a ) is a plan view describing an exemplary modification of the electroacoustic transducer  1  according to this embodiment. In an electroacoustic transducer  12  according to this exemplary modification, each of piezoelectric elements  6 A and  7 A is composed of a trapezoidal piezoelectric ceramic plate having an upper base and a lower base whose length is larger than that of the upper base. That is, on the sides of the lower bases of the piezoelectric elements  6 A and  7 A, the piezoelectric elements  6 A and  7 A are supported by the frame  4 , respectively. The upper bases of the piezoelectric elements  6 A and  7 A on the sides of tip portions thereof face each other with a gap therebetween. Except for this point, the electroacoustic transducer  12  has the same configuration as that of the electroacoustic transducer  1 . 
   As described previously, the upper base whose width is smaller than that of the lower base is on the side of tip portion of each of the piezoelectric elements  6 A and  7 A. Accordingly, on the side of the upper base, a wide gap between each of the piezoelectric elements  6 A and  7 A and the frame  4  can be obtained. Each of the piezoelectric elements  6 A and  7 A can therefore be freely and significantly displaced. Accordingly, the electroacoustic transducer  12  can easily obtain a higher sound pressure than the electroacoustic transducer  1 . 
     FIG. 2(   b ) is a plan view of another exemplary modification of the electroacoustic transducer  1 . An electroacoustic transducer  13  according to another exemplary modification has the same configuration as that of the electroacoustic transducer  1  except that the shape of an opening  4   b  of the frame  4  is different from that of the opening  4   a  illustrated in  FIG. 1(   b ). As illustrated in  FIG. 2(   b ), the width of the opening  4   b  is increased on the sides of the end portions  6   b  and  7   b  of the piezoelectric elements  6  and  7 . The width of the opening  4   b  is the dimension in the length direction of a portion of each of the piezoelectric elements  6  and  7  which is supported by the frame  4 , that is, the dimension in a direction indicated by an arrow X illustrated in  FIG. 2(   b ). In a portion in which the gap A is provided, the maximum width of the opening  4   b  is obtained. That is, the width of the opening  4   b  increases from each of the end portions of the piezoelectric elements  6  and  7  which are supported by the frame  4  to each of the end portions  6   b  and  7   b . Accordingly, the length of the portion of each of the piezoelectric elements  6  and  7  which is coupled to the frame  4 , that is, the width along the above-described X direction, is smaller than the width of the opening  4   b  at the gap A. Therefore, the end portions  6   b  and  7   b  of the piezoelectric elements  6  and  7  can be displaced faster than the end portions  6   b  and  7   b  included in the electroacoustic transducer  1 . Like the electroacoustic transducer  12 , the electroacoustic transducer  13  can easily obtain a higher sound pressure than the electroacoustic transducer  1  illustrated in  FIG. 1 . 
   From the viewpoint of capable of obtaining a high sound pressure, the electroacoustic transducers  12  and  13  are superior to the electroacoustic transducer  1  according to the first embodiment. However, in point of the simplification of a manufacturing process and the mechanical strength of the frame  4 , the electroacoustic transducer  1  is superior to the electroacoustic transducers  12  and  13 . 
     FIG. 3  is a plan view of an electroacoustic transducer according to a second embodiment of the present invention. In an electroacoustic transducer  21 , the piezoelectric elements  6  and  7  face each other with a gap therebetween in the opening  4   a  of the frame  4 . At the gap A, a rigid plate  22  having a rigidity higher than that of the flexible thin film  5  is bonded to the flexible thin film  5 . The rigid plate  22  can be made of an appropriate material having rigidity higher than that of the flexible thin film  5 . In this embodiment, the rigid plate  22  is composed of a fiber reinforced plastic plate having a thickness that is substantially the same as that of the piezoelectric elements  6  and  7 . The thickness of the rigid plate  22  is not necessarily substantially the same as that of the piezoelectric elements  6  and  7 . It is desirable that the rigid plate  22  be made of the lightest material with high rigidity. 
   The rigid plate  22  is bonded to the flexible thin film  5 . Accordingly, when the piezoelectric elements  6  and  7  are displaced, the rigid plate  22  is significantly displaced in a portion of the flexible thin film  5  in which the gap A is placed. As compared with a case in which only the flexible thin film  5  is displaced at the gap A, a higher sound pressure can be obtained since the rigid plate  22  is bonded to the flexible thin film  5 . The reason for this is that the area in which vibration is performed with the maximum displacement is increased. 
     FIG. 4  is a plan view of an electroacoustic transducer according to a third embodiment of the present invention. In an electroacoustic transducer  31 , first to fourth piezoelectric elements  32  to  35  are provided in the opening  4   a  of the frame  4 . More than two piezoelectric elements may be provided in the opening  4   a.    
   Like the piezoelectric elements  6 A and  7 A used in the electroacoustic transducer  12 , the piezoelectric elements  32  to  35  each has a substantially trapezoidal shape. Each of the piezoelectric elements  32  to  35  has an upper base at a tip portion thereof, and is fixed to the frame  4  at a lower base thereof. In this embodiment, the flexible thin film  5  is similarly fixed to the frame  4  so that it covers the entire area of the opening  4   a . In a gap region A on the side of the tip portion of each of the piezoelectric elements  32  to  35  having a substantially trapezoidal shape, a rectangular rigid plate  30  is bonded to the flexible thin film  5 . Accordingly, in this embodiment, using the rigid plate  30 , the area in which vibration is performed with the maximum displacement can also be increased and a higher sound pressure can also be obtained. 
   Furthermore, in this embodiment, a larger number of piezoelectric elements (the piezoelectric elements  32  to  35 ) are provided. Accordingly, a larger or heavier rigid plate can be disposed. As a result, the level of sound pressure can be increased. 
   In  FIG. 4 , the piezoelectric elements  32  to  35  each having a substantially trapezoidal shape are provided. When more than two piezoelectric elements are disposed, however, the planer shape thereof is not limited to trapezoid, and may be various shapes such as rectangle and triangle. 
     FIGS. 5(   a ) and  5 ( b ) are an elevational cross-sectional view and a plan view describing an electroacoustic transducer according to a fourth embodiment of the present invention, respectively. In an electroacoustic transducer  41  according to this embodiment, a first piezoelectric element  42  and a second piezoelectric element  43  each having a planer rectangular shape are used. The piezoelectric elements  42  and  43  are fixed to the frame  4  at one end portions  42   a  and  43   a  thereof, respectively, and the other end portions  42   b  and  43   b  face each other with a gap therebetween. That is, the piezoelectric elements  42  and  43  are also supported in the cantilever manner. 
   The plane area of the first piezoelectric element  42  is different from that of the second piezoelectric element  43 , and is larger than that of the second piezoelectric element  43 . Except for this point, the electroacoustic transducer  41  has the same configuration as that of the electroacoustic transducer  1 . 
   Since the area of the piezoelectric element  42  is larger than that of the piezoelectric element  43 , the resonance frequency of the fundamental wave of the piezoelectric element  42  is different from that of the piezoelectric element  43 . Accordingly, in this embodiment, a high sound pressure can be obtained over a wide range of frequencies. 
   The reason why a high sound pressure can be obtained over a wide range of frequencies when the resonance frequencies of the first piezoelectric element  42  and the second piezoelectric element  43  are different from each other will be described below. 
     FIG. 6  is a diagram illustrating a sound pressure-frequency characteristic of the electroacoustic transducer  41  in which the resonance points of the first piezoelectric element  42  and the second piezoelectric element  43  are different from each other.  FIG. 7  is a diagram illustrating a sound pressure-frequency characteristic according to the first embodiment in which the resonance points of the first piezoelectric element  6  and the second piezoelectric element  7  are the same. 
     FIGS. 6 and 7  each illustrates the result of a simulation performed without consideration of loss. Accordingly, the waveform of the peak at each resonance point is sharp. In reality, however, the waveform of the peak at each resonance point is round. 
   The larger number of resonance points are illustrated in  FIG. 6  as indicated by arrows Y and Z. Therefore, it can be understood that a high sound pressure can be obtained over a wide range of frequencies. In an electroacoustic transducer, a high sound pressure is obtained around a resonance point. Accordingly, if different resonance points are continuously obtained, a high sound pressure can be obtained over a wide range of frequencies. 
   In the electroacoustic transducer  41  illustrated in  FIGS. 5(   a ) and  5 ( b ), the area of the first piezoelectric element  42  is different from that of the second piezoelectric element  43 . On the other hand, in an electroacoustic transducer  51  illustrated in  FIG. 8 , the thickness of a piezoelectric element  52  is different from that of a second piezoelectric element  53 . That is, as illustrated in a cross-sectional view in  FIG. 8 , the thickness of a piezoelectric ceramic plate of the first piezoelectric element  52  is larger than that of a piezoelectric ceramic plate of the second piezoelectric element  53 . Accordingly, like the electroacoustic transducer  41 , the resonance frequency of a fundamental wave generated by vibration of the first piezoelectric element  52  is different from the resonance frequency of a fundamental wave generated by vibration of the second piezoelectric element  53  in the electroacoustic transducer  51 . In the electroacoustic transducer  51 , a high sound pressure can also be obtained over a wide range of frequencies. 
   Although the manufacturing method of the electroacoustic transducers  1 ,  21 ,  31 ,  41 , and  51  is not particularly limited, a method of fixing a single piezoelectric element and cutting the fixed piezoelectric element is preferably used. That is, as illustrated in a plan view in  FIG. 9 , by cutting a rectangular piezoelectric element  61  fixed to the frame  4  along broken lines B and C using a laser or mechanically cutting the piezoelectric element  61  fixed to the frame  4  along the broken lines B and C using a dicing machine, the piezoelectric elements  6  and  7  illustrated in  FIG. 1  can be obtained. In this case, the flexible thin film  5  is fixed to the frame  4  in advance. Accordingly, the above-described cutting of the piezoelectric element  61  is performed without cutting the flexible thin film  5 . 
   In the case of the above-described method of fixing the piezoelectric element  61  and then cutting the piezoelectric element  61 , only a single piezoelectric element is required so as to obtain the electroacoustic transducer  1 . Accordingly, the manufacturing process can be simplified. In a case where a piezoelectric element is fixed to the frame  4  and is then cut, misalignment of the piezoelectric elements  6  and  7  rarely occurs as compared with a case where two piezoelectric elements are prepared in advance. Accordingly, the accuracy of an electroacoustic transducer including a plurality of piezoelectric elements can be increased. 
   In an electroacoustic transducer according to the present invention, a frame and piezoelectric element may be integrated using a piezoelectric ceramic plate.  FIG. 10(   a ) is a plan view illustrating an electrode configuration of an electroacoustic transducer in which a frame and a piezoelectric element are integrated, and  FIG. 10(   b ) is a plan cross-sectional view illustrating an internal electrode of the electroacoustic transducer. 
   As illustrated in  FIG. 10(   a ), an H-shaped notch  71   a  is formed on a single ceramic plate  71 , whereby a first piezoelectric element portion  72  and a second piezoelectric element portion  73  are formed. The notch  71   a  is formed using an appropriate method such as a laser cutting or dicing. Before the notch  71   a  is formed, an electrode film  74  is formed on the upper surface of the ceramic plate  71 . The electrode film  74  extends from an edge of the ceramic plate  71  on the side of one side face  71   b  toward the other side face  71   c , but does not extend to the other side face  71 C. 
   On the undersurface of the ceramic plate  71 , an electrode film is formed using the same method as that used for the formation of the electrode film  74 . An internal electrode  75  illustrated in  FIG. 10(   b ) is formed in the ceramic plate  71  in advance. The internal electrode  75  extends from the side face  71   c  toward the other side face  71   b , but does not extend to the other side face  71   b.    
   For connection to an external portion, the electrode film  74  and the electrode film formed on the undersurface of the ceramic plate  71  are electrically connected to each other and are then connected to the external portion on the side of the side face  71   b . When electrically connecting the internal electrode  75  to an external portion, an external electrode is formed on the side of the other side face  71   c  and is then connected to the external portion. Accordingly, for example, electrical connection portions with an external portion can be integrated on the side of a supported portion of one of the first and second piezoelectric elements, and wiring of a lead wire can be simplified. 
   That is, in the electroacoustic transducer  1 , an electrical connection portion with an external portion is provided in both of the first piezoelectric elements  6  and the second piezoelectric element  7 . On the other hand, according to this embodiment, the electrical connection configuration can be simplified and design flexibility can be increased.