Patent Publication Number: US-8116487-B2

Title: Piezoelectric speaker and electronic apparatus with piezoelectric speaker

Description:
This application is the National Phase of PCT/JP2007/071861, filed Nov. 5, 2007, which claims priority to Japanese Patent Application No. 2006-303455, filed on Nov. 9, 2006, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates to a piezoelectric speaker using a piezoelectric element, and to an electronic apparatus equipped with the piezoelectric speaker, such as a mobile telephone, a personal digital assistant (PDA), and a portable game device. 
     BACKGROUND ART 
     A piezoelectric speaker converts an electric signal into sound using as a vibrator a piezoelectric element that deforms when a voltage is applied thereto. The sound includes an alarm, a melody, and the like, as well as voice. Hereinafter, description is made exemplifying sound as voice. 
     In a piezoelectric speaker, a circular piezoelectric element is attached to a diaphragm held in the piezoelectric speaker. When an electric signal is applied to the piezoelectric element, the piezoelectric element deforms in response to the applied electric signal. The electric signal to be applied has a voltage and a current changed variously depending upon an input voice signal, and thus the deformation of the piezoelectric element results in deformation motion in accordance with a change in input voice signal. The deformation motion of the piezoelectric element is transmitted to the diaphragm with the piezoelectric element attached thereto, and the vibration of the diaphragm vibrates surrounding air, whereby voice is generated. 
     It is desired that the diaphragm of the piezoelectric speaker have a largest possible vibration plane and be capable of vibrating freely with a largest possible amplitude. This is because, if the diaphragm is capable of vibrating largely due to a large vibration plane, voice with a large volume under a large sound pressure can be generated. Further, if the diaphragm is capable of vibrating freely, the deformation motion of the piezoelectric element is converted into vibration motion efficiently, which enables a signal input to the piezoelectric element to be reproduced more accurately. 
     Regarding a method of supporting a diaphragm, various methods are proposed. More specifically, a support member, a support structure, and the like of the diaphragm are proposed (see, for example, Patent Document 1: JP 2005-130156 A, Patent Document 2: JP 2001-119795 A, Patent Document 3: JP 10-164694 A). 
     Referring to  FIG. 1 , an example of a support structure of the diaphragm will be described. 
     In the support structure of the diaphragm shown in  FIG. 1 , a diaphragm  11  is supported between a housing  12  and a structural part (a cover  13 ) inside the piezoelectric speaker via spacers  14  and  15 . By using members having appropriate elasticity such as silicone rubber, the diaphragm  11  is held at the structural part while keeping an appropriate degree of freedom. 
     However, when only a degree of freedom of vibration motion is pursued in the support structure of the diaphragm of the piezoelectric speaker, there arises a problem in that the sound characteristics of the piezoelectric speaker may be degraded partially. Specifically, problems such as the degradation in reproduction characteristics at a time of rising during the start of a signal input and the degradation in reproduction characteristics of a signal with a small sound pressure are caused. 
     Those problems are caused by the fact that the motion of the diaphragm for generating voice is reciprocating motion in a direction perpendicular to the vibration plane, whereas the deformation of the piezoelectric element is extension and contraction motion parallel to the plane of the diaphragm, and thus, those motions are in completely different directions. 
     Hereinafter, the mechanism of converting the extension and contraction motion of the piezoelectric element into the reciprocating motion in the direction perpendicular to the diaphragm in the piezoelectric speaker will be described. 
     When a voice signal is input to the piezoelectric speaker, an electric signal is applied to the piezoelectric element, and the piezoelectric element extends. When the piezoelectric element extends, the diaphragm is also extended along therewith. 
     Here, if the diaphragm is capable of extending by an extension amount or more of the piezoelectric element and is supported in an ideal state, and there is no factor for blocking the extension of the diaphragm, the diaphragm is extended freely in parallel with the vibration plane by the extension amount of the piezoelectric element. However, because the diaphragm is supported by the support structure, a reaction force is generated from the support structure when the diaphragm extends to some degree, whereby the extension is blocked. When the extension of the diaphragm is blocked, the motion of the diaphragm of spreading in a direction parallel to the vibration plane loses a place to go, with the result that the diaphragm starts bending. Due to the bending of the diaphragm, the extension of the piezoelectric element is converted into the motion in a direction perpendicular to the diaphragm. Once the diaphragm starts bending, the motion of the diaphragm gains momentum as the bending motion, and the extension and contraction motion of the piezoelectric element thereafter is converted into the bending motion of the diaphragm to become vibration motion. 
     In the piezoelectric speaker, the extension and contraction motion of the piezoelectric element is converted into the bending motion of the diaphragm through the above-mentioned process, whereby the diaphragm is vibrated finally. Further, in order for the diaphragm to start the vibration motion, a “trigger” for the diaphragm to initially start the bending motion is required. 
     The above-mentioned point will be described by way of a specific example shown in  FIGS. 2A to 2D .  FIGS. 2A to 2D  are views showing that the piezoelectric speaker in a silent state (no input state) vibrates the diaphragm to start generating voice in four stages in accordance with a time passage. 
     The piezoelectric speaker includes a piezoelectric element  21 , a diaphragm  22 , and holding members  23 ,  24 ,  25 , and  26 . The piezoelectric element  21  is attached to the diaphragm  22  in intimate contact therewith so that the extension and contraction motion thereof is transmitted to the diaphragm  22 . The holding members  23  to  26  are members having appropriate elasticity, such as silicone rubber, and each one end thereof is attached and fixed to a housing or a cover portion (hereinafter, described as a housing) (not shown). Further, each of the holding members  23  to  26  is bonded to the diaphragm  22  at the other end thereof. That is, the holding members  23  to  26  are positioned between the diaphragm  22  and the housing, thereby holding and fixing the diaphragm  22  to the housing. Simultaneously, due to the elasticity of the holding members  23  to  26 , the holding and fixing of the diaphragm  22  is rendered flexible connection, thereby ensuring the degree of freedom at which the diaphragm  22  performs vibration motion to such a degree as to generate voice. 
       FIG. 2A  shows a piezoelectric speaker in an initial state, which is stopped in a silent state, with no voice signal input thereto. Next,  FIG. 2B  shows a state in which a voice signal starts being input to the piezoelectric speaker and the piezoelectric element  21  starts extending.  FIGS. 2B and 2C  show that the piezoelectric element  21  extends and the diaphragm  22  also extends along therewith. In the stages shown in  FIGS. 2B and 2C , the extension of the diaphragm  22  is absorbed by the deformation of the holding members  23  to  26 . However, as the extension of the diaphragm  22  proceeds from  FIG. 2B  to  FIG. 2C , reaction forces RF 1  and RF 2  from the holding members  23  to  26  also increase. In the stage shown in  FIG. 2D , the reaction forces RF 1  and RF 2  from the holding members  23  to  26  become larger than the force by which the diaphragm  22  tries to extend, and the force of the diaphragm  22  of trying to extend, which is transmitted from the piezoelectric element  21 , loses a place to go. The force in the extension direction, which has lost a place to go, causes the diaphragm  22  to bend so as to swell a center portion thereof, and escapes in a perpendicular direction. Thus, the diaphragm  22  starts bending. Thereafter, the extension and contraction motion of the piezoelectric element  21  is continuously converted into the bending motion of the diaphragm  22  to become the vibration motion of the diaphragm  22 , whereby voice starts being generated. 
     As described above, in order for the extension and contraction motion of the piezoelectric element  21  to be converted into the bending and vibration motions of the diaphragm  22 , the above-mentioned “trigger” for starting the conversion of the extension motion into the bending motion of the diaphragm  22  is required. Then, in order to allow the bending motion to start from the early stage in which the diaphragm  22  starts the extension operation, a structure of holding the diaphragm  22  with a strong binding force is desired. As the structure of holding the diaphragm  22  with a strong binding force, for example, the elasticity of the holding members  23  to  26  is reduced to be hard holding members in the example shown in  FIGS. 2A to 2D . This is because the hard holding member with a small elasticity generates a large reaction force in response to even small deformation, and starts the bending of the diaphragm  22  in the stage in which the extension of the diaphragm  22  is small. 
     However, the support structure for holding the diaphragm  22  with a strong binding force, the “trigger” is obtained in the early stage, and the bending motion is started quickly. On the other hand, however, the motion of the diaphragm  22  after the start of the bending motion is also blocked, which impairs the sound characteristics of the piezoelectric speaker. 
     Conversely, when the degree of freedom of the vibration motion of the diaphragm  22  is enhanced considering the sound characteristics, the start of the bending motion in the diaphragm  22  is blocked. Specifically, the start of the vibration of the diaphragm  22  is delayed, whereby the start of the reproduction of voice at a time of the start of the input of a voice signal is delayed. That is, the rising of the voice reproduction becomes dull. Further, when the input of a voice signal is performed with a small volume and sound pressure (amplitude) from the start to the end, the deformation of the piezoelectric element  21  also becomes a small extension and contraction motion. Consequently, the bending motion of the diaphragm  22  does not start until the end, and voice may not be generated until the end. 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     This invention provides a piezoelectric speaker in which bending motion of a diaphragm can be started quickly and motion of the diaphragm after the start of the bending motion is not blocked. 
     Means to Solve the Problem 
     This invention is applicable to a piezoelectric speaker including a piezoelectric element that deforms in response to an input signal, a diaphragm that vibrates due to the deformation of the piezoelectric element to generate sound, and a resilient portion that holds at least a part of an outer edge of the diaphragm. 
     According to an aspect of this invention, the piezoelectric speaker further includes a lock portion that comes into contact with a vibration plane of the diaphragm to lock a part of the diaphragm. The lock portion is in a lock state capable of coming into contact with the diaphragm to block extension of the diaphragm in a state in which the input signal is not input to the piezoelectric speaker, and the diaphragm vibrates while being held by the resilient portion when the lock state is released immediately after an input of the input signal. 
     The following can be taken as a specific example of the lock portion. 
     The lock portion includes a ring body which has magnetic property with at least a part of an outer edge thereof being held by the resilient portion and which is capable of pressing a vicinity of the outer edge of the diaphragm from one vibration plane side thereof, and a magnetic body which is held by the resilient portion and is capable of coming into contact with and leaving from another vibration plane in the vicinity of the outer edge of the diaphragm. The ring body and the magnetic body attract each other with a magnetic force and sandwich the diaphragm to obtain the lock state, and the lock state is released when the ring body and the magnetic body are detached from each other due to a vibration of the diaphragm immediately after the input of the input signal. 
     The magnetic body preferably includes a plurality of rubber magnets divided in a plural number in an outer peripheral direction of the diaphragm. Further, the resilient portion is preferably made of a plurality of pressure-sensitive adhesive tapes divided in a plural number in the outer peripheral direction of the diaphragm, each of the pressure-sensitive adhesive tapes being attached to respective outer peripheral ends of the ring body, the diaphragm, and the rubber magnets, each of the pressure-sensitive adhesive tapes being preferably attached so that there is looseness between an adhesion attachment portion of the ring body and an adhesion attachment portion of the diaphragm, and between the adhesion attachment portion of the diaphragm and an adhesion attachment portion of the rubber magnets. Still further, the diaphragm preferably includes at least one of a concave portion and a convex portion that are engaged with the magnetic body in a portion with which the magnetic body comes into contact. 
     Effect of the Invention 
     In the piezoelectric speaker of this invention, in an initial state before an input of a signal, a part of the diaphragm is locked by the lock portion, and after the signal is input and the diaphragm starts bending motion, the lock is released and the diaphragm is vibrated in a free state. Thus, the quick start of the bending motion of the diaphragm and the vibration of the diaphragm with a high degree of freedom and a large amplitude and a large vibration plane can be satisfied. This is because in the initial state, the diaphragm obtains a reaction force with respect to the extension operation of the diaphragm due to the lock by the lock portion to start a bending motion quickly, and after the start of the bending motion, the lock by the lock portion is released to eliminate binding, whereby the amplitude and the vibration plane spread. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing a configuration of a conventional piezoelectric speaker. 
         FIG. 2A  is a view illustrating an operation of the conventional piezoelectric speaker. 
         FIG. 2B  is a view illustrating an operation of the conventional piezoelectric speaker continued from  FIG. 2A . 
         FIG. 2C  is a view illustrating an operation of the conventional piezoelectric speaker continued from  FIG. 2B . 
         FIG. 2D  is a view illustrating an operation of the conventional piezoelectric speaker continued from  FIG. 2C . 
         FIG. 3  is a block diagram showing a configuration of a mobile telephone equipped with a piezoelectric speaker according to this invention. 
         FIG. 4  is an exploded view schematically showing a piezoelectric speaker according to a first embodiment of this invention. 
         FIG. 5  is a cross-sectional view showing a partially enlarged piezoelectric speaker according to the first embodiment of this invention. 
         FIG. 6A  is a cross-sectional view illustrating an operation of the piezoelectric speaker according to the first embodiment of this invention. 
         FIG. 6B  is a cross-sectional view illustrating an operation of the piezoelectric speaker according to the first embodiment continued from  FIG. 6A . 
         FIG. 6C  is a cross-sectional view illustrating an operation of the piezoelectric speaker according to the first embodiment continued from  FIG. 6B . 
         FIG. 6D  is a cross-sectional view illustrating an operation of the piezoelectric speaker according to the first embodiment continued from  FIG. 6C . 
         FIG. 6E  is a cross-sectional view illustrating an operation of the piezoelectric speaker according to the first embodiment continued from  FIG. 6D . 
         FIG. 7  is a cross-sectional view showing a partially enlarged piezoelectric speaker according to a second embodiment of this invention. 
         FIG. 8  is a cross-sectional view showing a partially enlarged piezoelectric speaker according to a third embodiment of this invention. 
         FIG. 9A  is an enlarged cross-sectional view showing a part of a piezoelectric speaker according to a fourth embodiment of this invention in the absence of an input of a signal. 
         FIG. 9B  is an enlarged cross-sectional view showing a part of the piezoelectric speaker according to the fourth embodiment of this invention in the presence of an input of a signal. 
         FIG. 10  is a cross-sectional view showing a partially enlarged piezoelectric speaker obtained by combining the second to fourth embodiments. 
     
    
    
     BEST MODE FOR EMBODYING THE INVENTION 
     This invention will be described in detail by way of embodiments with reference to the drawings. 
       FIG. 3  shows a configuration of a mobile telephone  30  as an example of an electronic apparatus having a piezoelectric speaker  38  according to an embodiment of this invention. 
     The mobile telephone  30  includes an antenna portion  31 , a wireless portion  32 , a display portion  33 , a central processing unit (CPU)  34 , a drive portion  35 , an input portion  36 , a memory  37 , and the piezoelectric speaker  38 . In the mobile telephone  30 , the piezoelectric speaker  38  is operated, for example, as follows. 
     The wireless portion  32  receives a radio signal via the antenna portion  31 . When the radio signal is, for example, an incoming signal, the CPU  34  displays information on the incoming on the display portion  33  and reads sound data on an incoming sound and an incoming melody stored in the memory  37 . The CPU  34  inputs the read sound data in the drive portion  35 . The drive portion  35  drives the piezoelectric speaker  38  in accordance with the input sound data, and the piezoelectric speaker  38  generates a sound corresponding to the sound data. 
     First Embodiment 
     Referring to  FIG. 4 , the configuration of a piezoelectric speaker according to a first embodiment of this invention will be described.  FIG. 4  shows portions, particularly, corresponding to the structure of holding a diaphragm in a piezoelectric speaker according to the first embodiment schematically in an exploded view. 
     In  FIG. 4 , a piezoelectric speaker  40  includes a piezoelectric element  42 , a diaphragm  43 , a fixing ring  44 , a plurality of pressure-sensitive adhesive tapes (resilient portions)  45 , and a plurality of rubber magnets  46 . The piezoelectric speaker  40  is formed of a combination of the respective sites as represented by arrows of  FIG. 4 . That is, the piezoelectric element  42  is attached to the center of one surface of the diaphragm  43  in contact therewith. Next, the pressure-sensitive adhesive tapes  45  are attached to each outer peripheral end of the diaphragm  43 , the fixing ring  44 , and the rubber magnets  46 . This fixes (locks) the outer edge of the diaphragm  43  so that it is sandwiched between the fixing ring  44  and the plurality of rubber magnets  46 . 
     The fixing ring  44  is made of, for example, metal and has magnetic property. The fixing ring  44  has an integrated structure, whereas the pressure-sensitive adhesive tapes  45  and the rubber magnets  46  are divided in a plural number along the circumference of the fixing ring  44 . Further, the pressure-sensitive adhesive tapes  45  have not only an appropriate elasticity but also looseness on the adhesion attachment surface. The rubber magnets  46  are divided in a plural number along the circumference of the fixing ring  44  and held by the pressure-sensitive adhesive tapes  45  with an elasticity, whereby the operation as described later can be performed. In  FIG. 4 , wiring and the like for applying an electric signal to the piezoelectric element  42  are omitted. Further, similarly to the subsequent figures, unless otherwise required for description, wiring and the like for applying an electric signal to the piezoelectric element  42  are omitted. 
       FIG. 5  is a partial cross-sectional view of the assembled piezoelectric speaker  40  shown in  FIG. 4 .  FIG. 5  shows a partially enlarged outer peripheral portion of the piezoelectric speaker  40  shown in  FIG. 4 . In  FIG. 5 , components corresponding to those in  FIG. 4  are denoted by the same reference numerals as those in  FIG. 4 . 
     As shown in  FIG. 5 , the pressure-sensitive adhesive tapes  45  are attached to each outer peripheral end of the fixing ring  44 , the diaphragm  43 , and the rubber magnets  46 . Reference numeral  53  denotes adhesion attachment portions of the pressure-sensitive adhesive tapes  45 , and the pressure-sensitive adhesive tapes  45  are attached to the fixing ring  44 , the diaphragm  43 , and the rubber magnets  46 , respectively, via three adhesion attachment portions  53 . Further, the pressure-sensitive adhesive tapes  45  have an appropriate elasticity, and hence gap (play) is present between the fixing ring  44  and the diaphragm  43 , and between the diaphragm  43  and the rubber magnets  46  due to a flexure denoted by reference numeral  52 . The gap is formed due to the flexure  52 , but the fixing ring  44  with magnetic property and the rubber magnets  46  attract each other by a magnetic force indicated by arrows  51  and are fixed (locked) with the diaphragm  43  interposed therebetween. Therefore, the diaphragm  43  is desirably formed of a material that does not interrupt the attraction by the magnetic force between the fixing ring  44  and the rubber magnets  46 . 
     Here, the holding members of the diaphragm in the conventional piezoelectric speakers correspond to the pressure-sensitive adhesive tapes  45 . That is, the conventional support structure is a structure of supporting the diaphragm only with the holding members corresponding to the pressure-sensitive adhesive tapes  45 . In contrast, the piezoelectric speaker according to the first embodiment additionally includes the fixing ring  44  and the rubber magnets  46 , and hence, additionally has a structure of fixing (locking) the diaphragm  43  by sandwiching it between the fixing ring  44  and the rubber magnets  46 , unlike the conventional support structure. The piezoelectric speaker assembled as represented by arrows in  FIG. 4  is attached to a part of a housing of the piezoelectric speaker via a double-sided tape attached to an upper portion of the fixing ring  44 , for example. Alternatively, the whole assembled piezoelectric speaker may be bonded to and housed in a frame formed by molding with an adhesive, and fixed to a part of a housing. 
     Next, the operation of the piezoelectric speaker  40  according to the first embodiment will be described with reference to  FIGS. 6A to 6E . Also in the following, the case where a voice signal is given as an input signal will be described.  FIGS. 6A  to  6 E are cross-sectional views in 5 stages showing the silent stationary state (no input state) of the piezoelectric speaker  40 , i.e., the initial state to the state where the piezoelectric speaker  40  vibrates continuously. Further, in  FIGS. 6A to 6E , the components corresponding to those in  FIG. 4  are denoted by the same reference numerals as those in  FIG. 4 . 
     The piezoelectric speaker  40  shown in  FIG. 6A  shows the same initial state as that of the piezoelectric speaker  40  shown in  FIG. 5 . That is, the piezoelectric speaker  40  in this stage is in a stationary state or a silent state. At a time shown in  FIG. 6B , an electric signal corresponding to a voice signal is applied to the piezoelectric element  42 , and the piezoelectric element  42  starts extending. The diaphragm  43  is fixed under the condition of being sandwiched between the fixing ring  44  and the rubber magnets  46 ; therefore, the extension amount of the diaphragm  43  is smaller than that in the case where the diaphragm  43  is held and fixed only with the pressure-sensitive adhesive tapes  45 , which generates reaction forces RF 1 , RF 2  that try to push back the extension. That is, the diaphragm  43  is sandwiched and fixed (locked) by the fixing ring  44  and the rubber magnets  46 , whereby the extension of the diaphragm  43  is blocked in an early stage immediately after the application of an electric signal to the piezoelectric element  42 , which functions as a “trigger” for converting the extension of the diaphragm  43  to a bending motion. Then, at a time shown in  FIG. 6C , the diaphragm  43  starts the bending motion. In this manner, the combination of the fixing ring  44  and the plurality of rubber magnets  46  has a lock function of blocking the extension of the diaphragm  43  immediately after the application of an electric signal, and hence the combination may be called a lock portion. 
     Next, when the electric signal applied to the piezoelectric element  42  changes to contract the piezoelectric element  42 , the diaphragm  43  has already started bending and obtained an impetus of a vibration motion, and hence the diaphragm  43  bends downward as shown in  FIG. 6D . The diaphragm  43  bends downward, and hence the fixing ring  44  and the rubber magnets  46  that attract each other due to a magnetic force are detached from each other. When the fixing ring  44  and the rubber magnets  46  are detached to be in a free state, as shown in  FIG. 6E , the diaphragm  43  starts a free vibration (bending) motion only with the binding of the elasticity of the pressure-sensitive adhesive tape  45  continuously, thereby generating a voice. That is, the diaphragm  43  is put in a sound state.  FIG. 6E  shows the diaphragm  43  bending upward in the figure and the diaphragm  43  bending downward in the figure so that they are overlapped for ease of understanding, thereby showing the vibration (bending) motion of the diaphragm  43  schematically. 
     As shown in  FIG. 6E , the diaphragm  43  is bound by only the elasticity of the pressure-sensitive adhesive tape  45 , and hence the binding is weaker than that in the states shown in  FIGS. 6A to 6C , that is, the state where the diaphragm  43  is sandwiched and fixed between the fixing ring  44  and the rubber magnets  46 . 
     Further, in the states shown in  FIGS. 6A to 6C , the vibration of the diaphragm  43  is bound by edges on an inner diameter side of the ring-shaped fixing ring  44  and the rubber magnets  46  as represented by binding points  61  shown in  FIG. 6A . Due to the binding by the binding points  61 , the vibration of the diaphragm  43  is limited. However, in the state shown in  FIG. 6E , the sandwiching (holding) and fixing by the fixing ring  44  and the rubber magnets  46  are released, and hence the diaphragm  43  becomes capable of performing the vibration (bending) motion over the entire surface of the diaphragm  43  with adhesion attachment points  62  with respect to the pressure-sensitive adhesive tapes  45  as pivots. That is, in the state shown in  FIG. 6E , the vibration motion can be performed over the vibration plane wider than that in the state shown in  FIG. 6A . Therefore, when the piezoelectric speaker  40  is released from the sandwiched state between the fixing ring  44  and the rubber magnets  46 , the piezoelectric speaker  40  becomes capable of generating a voice due to the vibration over the wide vibration plane with a higher degree of freedom. 
     When the application of the electric signal to the piezoelectric element  42  is finished, and the vibration of the diaphragm  43  converges to finish the generation of a voice, the fixing ring  44  and the rubber magnets  46  sandwich and fix the diaphragm  43  again due to the elasticity of the pressure-sensitive adhesive tapes  45  and the magnetic force between the fixing ring  44  and the rubber magnets  46 , whereby the state returns to the initial state shown in  FIG. 6A . 
     As described above, before the diaphragm  43  starts the bending motion, the fixing ring  44  and the rubber magnets  46  sandwich and fix the diaphragm  43 , thereby giving the reaction forces for starting the bending motion to the diaphragm  43  that tries to extend along with the application of an electric signal as the “trigger”. Therefore, even in the early stage of an input of a voice signal to the piezoelectric speaker  40 , or to the input of a small voice signal to the piezoelectric speaker  40 , the piezoelectric speaker  40  can start the bending motion to generate a voice. Further, after the start of the bending motion, the sandwiching of the diaphragm  43  by the fixing ring  44  and the rubber magnets  46  is released, and the diaphragm  43  can vibrate with a larger degree of freedom, that is, with a weak binding force and a large amplitude by a larger vibration plane. 
     Further, by variously combining elements such as the number of the rubber magnets  46 , i.e., how many rubber magnets  46  divided in an arc shape fix the outer edge of the diaphragm  43 , or the magnetic force and weight of the rubber magnets  46 , and further the elasticity of the pressure-sensitive adhesive tape  45 , the operation and timing of a switching between the fixed state and the released state of the diaphragm  43  can be variously changed. 
     Second Embodiment 
       FIG. 7  shows a second embodiment of this invention.  FIG. 7  is a view corresponding to  FIG. 4 , and shows the piezoelectric speaker  40  in a cross-section with an outer edge enlarged partially. In  FIG. 7 , components corresponding to those shown in  FIG. 4  are denoted by the same reference numerals as those in  FIG. 4 . 
     In the second embodiment, a convex portion  71  is provided at the diaphragm  43 . The convex portion  71  is provided so as to correspond to the position where the rubber magnet  46  is in contact with the diaphragm  43  when the diaphragm  43  is sandwiched and fixed. The convex portion  71  blocks more exactly the motion of the diaphragm  43  that tries to extend to allow a reaction force to be generated, and allows the diaphragm  43  to start a bending motion, whereby the fixed state (lock state) is released. 
     Further, a slope angle of the convex portion  71  may be varied variously with respect to a contact portion (edge on an inner diameter side) of the rubber magnet  46 . For example, if the cross-sectional shape of the convex portion  71  is a triangle as shown in  FIG. 7 , when the diaphragm  43  extends in a sandwiched and fixed state, it is possible to obtain the function of pushing up the diaphragm  43  perpendicularly with respect to the vibration plane. 
     Note that, the convex portion  71  may be provided in a ring shape so as to form a complete continuous circle on the vibration plane of the diaphragm  43 . Alternatively, the convex portions  71  may be provided discontinuously in portions corresponding to the respective edges of the plurality of rubber magnets  46  in contact with the diaphragm  43 . 
     Third Embodiment 
       FIG. 8  shows a third embodiment of this invention. In the same way as in  FIG. 7 ,  FIG. 8  is also a view corresponding to  FIG. 4 , and shows the piezoelectric speaker  40  in a cross-section with an outer edge enlarged partially. In  FIG. 8 , the components corresponding to those shown in  FIG. 4  are denoted by the same reference numerals as those in  FIG. 4 . 
     In the third embodiment, a concave portion  81  is provided in place of the convex portion  71  in the second embodiment. The concave portion  81  is provided at a position corresponding to an edge of the rubber magnet  46  in the sandwiched and fixed state of the diaphragm  43 . Therefore, in the same way as in the second embodiment, the concave portion  81  blocks more exactly the motion of the diaphragm  43  that tries to extend to allow a reaction force to be generated, and allows the diaphragm  43  to start a bending motion, thereby releasing the fixed state (lock state). In the same way as in the second embodiment, a slope angle of the concave portion  81  may be changed variously with respect to an edge of the rubber magnet  46  in a sandwiched and fixed state. For example, as shown in  FIG. 8 , if the cross-sectional shape of the concave portion  81  is a triangle, when the diaphragm  43  extends in a sandwiched and fixed state, it is possible to obtain the function of pushing up the diaphragm  43  perpendicularly with respect to the vibration plane. In addition, the concave portion  81  can be realized by simple processing of cutting away the vibration plane of the diaphragm  43  partially to form a groove. 
     Fourth Embodiment 
       FIGS. 9A and 9B  show a fourth embodiment of this invention.  FIGS. 9A and 9B  are views corresponding to  FIG. 4 , and show the piezoelectric speaker  40  in a cross-section with an outer edge enlarged partially. In  FIGS. 9A and 9B , the components corresponding to those in  FIG. 4  are denoted by the same reference numerals as those in  FIG. 4 . Note that, the diaphragm  43  with the concave portion  81  described in  FIG. 8  is shown; however, the diaphragm  43  may have the convex portion  71  described in  FIG. 7 . 
     Hereinafter, the fourth embodiment will be described based on the difference between the fourth embodiment and the first to third embodiments. 
     In the piezoelectric speaker  40  in the fourth embodiment, the fixing ring  44  and the pressure-sensitive adhesive tape  45  are not used, and the diaphragm  43  is held by a tubular housing  114  of the piezoelectric speaker by a gathered edge (resilient portion)  111 . The gathered edge  111  has an elasticity, which makes it possible for the diaphragm  43  to move freely. A plurality of arc-shaped holding portions  112  are used in place of the fixing ring  44  and the rubber magnets  46  that sandwich and fix the diaphragm  43  in an initial state of the first to third embodiments. The plurality of holding portions  112  have a substantially ring shape as a whole after assembly in the same way as in the rubber magnets  46  described in  FIG. 4 . In the fourth embodiment, the holding portions  112  are not required to be a magnet as a whole. Instead, the material for the holding portions  112  may have sufficient stiffness and appropriate weight (lightness) with respect to the mechanism and operation described later. The holding portions  112  are not required to have magnetic property as a whole, but the holding portions  112  have a magnet portion  113  at an end opposed to the housing  114 . Further, an electromagnetic portion  115  is provided at a position of the housing  114  opposed to the magnet portion  113 . The electromagnetic portion  115  generates a magnetic force in a direction repelling the magnet portion  113 . The holding portion  112  is a swing body capable of swinging with respect to a fulcrum  110 . The housing  114  is provided with a spring portion (elongation spring)  116  symmetric with respect to the electromagnet portion  115  with the fulcrum  110  placed therebetween. The spring portion  116  is biased in a direction of allowing the holding portion  112  to approach the diaphragm  43 . The electromagnet portion  115 , the spring portion  116 , the magnet portion  113 , and the fulcrum  110  constitute one rotary electromagnetic switch  120 . 
     The electromagnetic switch  120  is operated by an electromagnetic switch driving signal S sd  from a control portion  118 . The control portion  118  generates an electric signal that drives the piezoelectric element  42  in the same way as in the drive portion  35  of  FIG. 3 , and further generates the driving signal S sd  of the electromagnetic switch  120  in the fourth embodiment. More specifically, the control portion  118  receives a voice signal  119  and outputs the above-mentioned electromagnetic switch driving signal S sd  and a piezoelectric driving signal S pd . The piezoelectric driving signal S pd  output from the control portion  118  is input to the piezoelectric element  42 . 
     Next, the operation of the piezoelectric speaker according to the fourth embodiment will be described. 
       FIG. 9A  shows an initial state of the piezoelectric speaker  40 . In the initial state, the electromagnetic switch  120  is not activated, the holding portion  112  is pressed against the diaphragm  43  by the spring portion  116 , and the edge of the holding portion  112  is placed at a first position in which the edge is fitted in the concave portion  81 . More specifically, the electromagnetic switch  120  is in a lock state capable of blocking the extension of the diaphragm  43  immediately after the input of a voice signal. 
     When the voice signal  119  for generating a voice is input to the control portion  118 , the control portion  118  outputs the piezoelectric element driving signal S pd  in accordance with the input voice signal  119 . The output piezoelectric element driving signal S pd  is applied to the piezoelectric element  42 . 
     Due to the application of the piezoelectric element driving signal S pd , the piezoelectric element  42  starts extending. When the diaphragm  43  tries to extend along with the extension of the piezoelectric element  42 , a reaction force is generated in a contact portion between the concave portion  81  and the edge of the holding portion  112  with respect to the extension of the diaphragm  43 , which functions as a “trigger” to allow the diaphragm  43  to start a bending motion. 
     When the diaphragm  43  starts a bending motion, the control portion  118  activates the electromagnetic switch  120  with the electromagnetic switch driving signal S sd . When the electromagnetic switch  120  is activated, as shown in  FIG. 9B , the electromagnet portion  115  is excited, and the electromagnet portion  115  and the magnet portion  113  repel each other with a force stronger than that of the bias force of the spring portion  116 . Thus, the edge of the holding portion  112  swings to a second position so as to be away from the diaphragm  43  with respect to the fulcrum  110 . Consequently, the diaphragm  43  is released from the held and fixed state (lock state) made by the holding portion  112 , and starts a vibration motion in a free state bound by only the gathered edge  111 . Thus, in the fourth embodiment, a combination of the control portion  118  and the electromagnetic switch  120  functions as a lock portion having a lock function of blocking the extension of the diaphragm  43  immediately after the application of an electric signal. 
     When the generation of a voice is completed, the control portion  118  detects the completion of the generation of a voice from the voice signal  119 . When the control portion  118  detects the completion of the generation of a voice, the control portion  118  allows the piezoelectric element driving signal S pd  to converge in accordance with the voice signal  119 . Further, when the vibration of the diaphragm  43  converges, the control portion  118  turns off the electromagnetic switch  120 , and returns the piezoelectric speaker  40  to the state shown in  FIG. 9A  that is an initial state. 
     As described above, in the fourth embodiment, the use of the electromagnetic switch  120  allows the control portion  118  to control the holding, fixing, and releasing operations of the diaphragm  43  by the holding portion  112  electrically. Thus, the holding, fixing, and releasing timings of the diaphragm  43  by the holding portion  112  can be controlled more suitably and more minutely. If the control portion  118  is advanced using a CPU and a digital logic circuit, more complicated control of the holding, fixing, and releasing operations of the diaphragm  43  by the holding portion  112  can be performed. For example, the holding, fixing, and releasing operations and the operation timing of the diaphragm  43  by the holding portion  112  may be switched minutely in accordance with the amplitude, frequency, continuation time of a signal, and other signal properties of the voice signal  119  to be input. For example, the control portion  118  may release the diaphragm  43  from a lock state after a predetermined period from the input of a voice signal or when the input voice signal satisfies a predetermined condition. The predetermined condition in this case can be considered to be, for example, that the voice signal has an amplitude larger than a predetermined amplitude. 
     In the fourth embodiment, the holding portion  112  is biased to the first position where the holding portion  112  comes into contact with the diaphragm  43  using the spring portion  116  by an extension coil spring, and the holding portion  112  is swung to the second position by the reaction force between the magnet portion  113  and the electromagnet portion  115 . However, the fourth embodiment may have the following configuration. A compression coil spring is provided in place of the electromagnet portion  115  and the magnet portion  113 , whereby the holding portion  112  is swung to the first position with a tensile force of the compression coil spring. On the other hand, an electromagnet portion is provided in the housing  114 , and a magnet portion or magnetic body is provided in the holding portion  112 , respectively, in place of the spring portion  116 . Then, the electromagnet is excited immediately after the input of a voice signal to generate an attraction force therebetween, whereby the holding portion  112  is swung to the second position. 
     In the first embodiment, the case has been described in which the rubber magnet  46  comes into contact with the vibration plane of the diaphragm  43  at an edge portion (see  FIGS. 5 and 6 ). However, the rubber magnet  46  does not necessarily come into contact with the vibration plane  43  at an edge. For example, the rubber magnet  46  may be provided with a contact plane so that the contact portion between the rubber magnet  46  and the diaphragm  43  becomes a plane, instead of the edge. Further, in order to increase a friction force, the material and surface shape of the contact plane between the rubber magnet  46  and the diaphragm  43  may be varied. For example, the contact plane may be made of a material with a large friction coefficient such as silicone rubber, and the surface shape of the contact plane may be a shape provided with grooves or cut-in, such as a tread pattern of a tire and a sole pattern of shoes, or a shape such as a file. 
     In the first to third embodiments, the pressure-sensitive adhesive tape  45  is used, but it is not necessary to use a pressure-sensitive adhesive tape as long as the material has an elasticity. For example, those which achieve the free vibration motion of the diaphragm  43  due to the material or structure can be used as in the gathered edge  111  in the fourth embodiment. 
     Further, in the first to third embodiments, the fixing ring  44  and the rubber magnets  46  attract each other with a magnetic force while sandwiching the diaphragm  43 , thereby sandwiching the diaphragm  43  to fix it. However, the fixing ring  44  and the rubber magnets  46  do not necessarily have a structure of sandwiching the diaphragm  43 . Even if the fixing ring  44  and the rubber magnets  46  do not sandwich the diaphragm  43 , the rubber magnets  46  only need to hold and fix the diaphragm  43  in an initial state, give a reaction force to the extension operation of the diaphragm  43 , and promote the start of a bending operation. 
     Further, in the second to fourth embodiments, the convex portion  71  and the concave portion  81  are provided on the diaphragm  43 , but the convex portion  71  and the concave portion  81  may be combined. That is, the first to fourth embodiments can be combined appropriately. 
       FIG. 10  shows an example of a combination of the second to fourth embodiments. The piezoelectric speaker  40  shown in  FIG. 10  uses the gathered edge  111  in place of the pressure-sensitive adhesive tape  45 , has a configuration of holding and fixing the diaphragm  43  with the holding portion  112  instead of the configuration of sandwiching the diaphragm  43  between the fixing ring  44  and the rubber magnets  46 , and is provided with a combination of the convex portion  71  and the concave portion  81 . In the piezoelectric speaker  40  shown in  FIG. 10 , the entire housing  114 , or a part of the position of the housing  114  corresponding to the magnet portion  113  has magnetic property. Then, the magnet portion  113  and the portion of the housing  114  having magnetic property attract each other with a magnetic force, whereby the holding portion  112  swings, whereby the diaphragm  43  is held and fixed by the holding portion  112 . When the diaphragm  43  is bent downward immediately after the application of an electric signal, the housing  114  and the magnet portion  113  attracting each other with a magnetic force are detached from each other. When the magnet portion  113  is detached from the housing  114  to release the diaphragm  43 , the diaphragm  43  starts a free vibration (bending) motion continuously, bound by only the elasticity of the gathered edge  111 . 
     In the above description, a mobile telephone is exemplified as electronic equipment with the piezoelectric speaker of this invention, but this invention is also applicable to portable electronic equipment such as a PDA and a portable game appliance. 
     As described above, the piezoelectric speaker and the electronic equipment provided with a piezoelectric speaker according to this invention sandwiches (holds) and fixes the diaphragm in an initial state and releases the sandwiching (holding) and fixing of the diaphragm after an electric signal is applied and the diaphragm starts a bending motion. This enables the start of the rapid bending motion of the diaphragm, and the vibration of the diaphragm with a large amplitude and a large vibration plane. 
     According to this invention, a piezoelectric speaker has an effect of enabling the generation of a voice at the start of an input of a voice signal and at a rising of the voice signal, the generation of a voice due to the vibration of a diaphragm with a large amplitude and a large vibration plane, and the generation of a minute voice signal, and electronic equipment provided with a piezoelectric speaker having the effect is realized. Needles to say, the function is completely the same even with a sound signal such as an alarm and an incoming melody, as well as a voice signal.