Patent Publication Number: US-9837597-B2

Title: Piezoelectric sound-generating body and electronic device using the same

Description:
This application is the U.S. National Phase under 35 U.S.C. §371 of International Application PCT/JP2012/082740, filed Dec. 18, 2012, which claims priority to Japanese Patent Applications No. 2011-278536, filed Dec. 20, 2011 and No. 2012-270854, filed Dec. 11, 2012. The International Application was published under PCT Article 21(2) in a language other than English. 
     TECHNICAL FIELD 
     The present invention relates to a piezoelectric sound-generating body and electronic device using the same, and more specifically to improving a piezoelectric sound-generating body in a manner suitable for installation in small devices, etc. 
     BACKGROUND ART 
     Mobile phones, smartphones, etc., are offering not only telephone functions but also more functions as personal digital assistants in recent years. In terms of the size of devices, there is a strong demand for smaller, thinner and lighter devices, which in turn is generating a greater demand for smaller, thinner and lighter components used for such devices. Speakers are facing the same demand, and piezoelectric speakers that utilize the expanding/contracting displacement of piezoelectric elements in 31 directions to provide enhanced displacement amplification based on flexural displacement are used in mobile devices as they can easily be made thinner while ensuring high sound pressures. In addition, piezoelectric speakers are suitable components of mobile devices for which battery life is important, because these voltage-driven speakers consume less power than dynamic speakers. 
     These piezoelectric speakers are formed by a laminate comprising up to eight layers or so to particularly reduce the driving voltage, which speakers are attached to a metal plate or other shim plate. Here, a piezoelectric speaker constituted by only one laminate piezoelectric body attached to a metal plate is called the unimorph type, while a piezoelectric speaker constituted by laminate piezoelectric bodies polarized in opposite directions, each attached on either side of a metal plate, is called the bimorph type. These unimorph and bimorph piezoelectric speakers are based on the technology described in Patent Literature 1 below, for example. A bimorph piezoelectric speaker may be achieved with only one element, without using a metal plate, by polarizing the top half and bottom half of a laminate piezoelectric element in opposite directions. This one-piece bimorph element offers relatively high efficiency in terms of flexural displacement because it has no extra structure such as a metal plate. 
     PRIOR ART LITERATURE 
     Patent Literature 
     Japanese patent Laid-open No. 2003-259488 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Piezoelectric speakers are capacitive elements that, from the viewpoint of effective power consumption, consume much less power than dynamic speakers as mentioned above and thus allow batteries to last longer. However, their current increases at certain frequencies, especially around 10 to 20 kHz near the upper end of the audible spectrum, as the impedance drops. Despite low effective power consumption, such increase in current gives rise to a problem of heat generating in areas subject to higher resistance, such as where conductive wires constituting the speaker are connected. Generated heat accelerates the deterioration of piezoelectric elements, potentially causing their characteristics to deteriorate before the design life is reached. Also, thick conductive wires, etc., must be used to accommodate large current that may flow in the speaker driving circuit, which in turn presents a problem in that mobile devices, etc., cannot be made smaller. 
     The present invention focuses on the points made above and its object is to provide a piezoelectric sound-generating body whose current is kept low without affecting the amount of displacement of the element, thus preventing deterioration characteristics and allowing for size reduction. Another object is to provide an electronic device utilizing the aforementioned piezoelectric sound-generating body. 
     Means for Solving the Problems 
     A piezoelectric sound-generating body conforming to the present invention uses a piezoelectric driving element constituted by a laminate of multiple piezoelectric layers, wherein such piezoelectric sound-generating body is characterized in that: an electrode layer is formed between the multiple piezoelectric layers; the piezoelectric layer in the area associated with the smallest displacement of the piezoelectric driving element is the thickest; and the other piezoelectric layers become gradually thinner in the lamination direction from the thickest piezoelectric layer. 
     Another piezoelectric sound-generating body conforming to the present invention comprises a support plate supporting a bimorph piezoelectric driving element constituted by a laminate of four or more piezoelectric layers of an even number contributing to displacement, wherein such piezoelectric sound-generating body is characterized in that: an electrode layer is formed between the multiple piezoelectric layers; piezoelectric layers of the same number above and below the center in the lamination direction are polarized in the opposite directions; the piezoelectric layers become gradually thinner upward and downward in the lamination direction from the center; and the piezoelectric layers at the same position in the laminate above and below the center, being the base point, have the same thickness. 
     Yet another piezoelectric sound-generating body is a unimorph type made by attaching to one main side of a support plate a piezoelectric driving element constituted by a laminate of two or more piezoelectric layers contributing to displacement, wherein such piezoelectric sound-generating body is characterized in that: an electrode layer is formed between the multiple piezoelectric layers; and the piezoelectric layers become gradually thinner in the lamination direction from the piezoelectric layer on the support plate side. 
     An electronic device conforming to the present invention is characterized in that it utilizes any one of the piezoelectric sound-generating bodies mentioned above. The aforementioned and other objects, characteristics and benefits of the present invention are made clear by the detailed explanations provided below as well as the drawings attached hereto. 
     Effects of the Invention 
     According to the present invention, a piezoelectric sound-generating body using a piezoelectric driving element constituted by a laminate of multiple piezoelectric layers is formed in such a way that the piezoelectric layer in the area associated with the smallest displacement is the thickest and the other piezoelectric layers become gradually thinner toward the outer side. This way, the capacity can be reduced and current can be kept low without affecting the amount of displacement of the element. As a result, failures due to heat generation can be prevented, while size reduction also becomes possible because there is no longer a need to use thick conductive wires for the driving circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Drawings illustrating Example 1 of the present invention, where (A) is a section view showing the laminate structure of a piezoelectric driving element, (B) and (C) are each a section view showing the piezoelectric driving element in flexed state, while (D) and (E) are each a drawing showing an example of a frame to support the piezoelectric driving element. 
         FIG. 2  Drawings illustrating the formation of electrode layers of a bimorph piezoelectric driving element constituted by a laminate of four to eight layers, where (A- 1 ) through (A- 3 ) show an electrode formation at the time of polarization operation, while (B- 1 ) through (B- 3 ) show an electrode formation at the time of driving. 
         FIG. 3  Plan views each showing an internal electrode pattern of a piezoelectric driving element conforming to the present invention. 
         FIG. 4  Drawing explaining the definition of the thickness of piezoelectric driving element that takes into account the thickness of internal electrode layers. 
         FIG. 5  Drawings illustrating other examples of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Example 1 
     The best modes for carrying out the present invention are explained in detail below based on examples.  FIG. 1  (A) is a section view showing the laminate structure of a piezoelectric driving element, (B) and (C) are each a section view showing the piezoelectric driving element in flexed state, while (D) and (E) are each a drawing showing an example of a frame to support the piezoelectric driving element.  FIG. 2  provides drawings illustrating the formation of electrode layers of a bimorph piezoelectric driving element constituted by a laminate of four to eight layers, where (A- 1 ) through (A- 3 ) show an electrode formation at the time of polarization operation, while (B- 1 ) through (B- 3 ) show an electrode formation at the time of driving.  FIG. 3  provides plan views each showing an internal electrode pattern of a piezoelectric driving element conforming to the present invention.  FIG. 4  is a drawing explaining the definition of the thickness of piezoelectric driving element that takes into account the thickness of internal electrode layers. A piezoelectric sound-generating body  10  in this example is utilized, for example, as a speaker for personal digital assistants, representative forms of which include mobile phones and smartphones. 
     As shown in  FIG. 1  (A) and (D), the piezoelectric driving element  10  used for the piezoelectric sound-generating body in this example is a bimorph type whose overall shape is roughly a rectangle. The piezoelectric driving element  10  is constituted by six piezoelectric layers  20 ,  22 ,  24 ,  30 ,  32 ,  34 , electrode layers  40 ,  42 ,  44 ,  52 ,  54  provided in between these piezoelectric layers, and electrode layers  46 ,  56  formed on the laminate surface. In this example, three piezoelectric layers are formed above and also below the electrode layer  40  at the center in the thickness direction. The piezoelectric layers  20 ,  22  and  24  form a top laminate piezoelectric body  12 , while the piezoelectric layers  30 ,  32  and  34  form a bottom laminate piezoelectric body  14 . 
     In this example, the piezoelectric layers  20 ,  30  in the areas associated with the smallest displacement (smallest expansion and contraction in the lateral direction) of the piezoelectric driving element  10  are formed the thickest. Then, the piezoelectric layers  20 ,  22  and  24  become gradually thinner in this order, while the piezoelectric layers  30 ,  32  and  34  also become gradually thinner in this order. The piezoelectric layers  20  and  30  have the same thickness, piezoelectric layers  22  and  32  have the same thickness, and piezoelectric layers  24  and  34  have the same thickness. In other words, the thickness of each piezoelectric layer is set in such a way to achieve a vertically symmetrical layer structure and thicknesses with reference to the electrode layer  40  used as the plane of symmetry. This means that, when adopting the bimorph structure as is the case in this example, there are always four or more piezoelectric layers (piezoelectric layers contributing to displacement) of an even number that constitute the piezoelectric driving element. The thickness ratios of piezoelectric layers are explained in detail later. 
     The piezoelectric driving element  10  can be produced with a normal method comprising forming PZT or other piezoelectric sheets, printing a paste containing electrodes on the sheets and stacking/pressure-bonding the printed sheets, and then sintering the stacked/pressure-bonded sheets at a specified temperature. The element dimensions in planar directions are not specified in any way, but a circle of approx. 20 to 25 mm in diameter or rectangle of approx. 15 to 20 mm per side is desired when use of the element for normal mobile devices is assumed. In this example, the element is rectangular. In the example of  FIG. 1  (A), the piezoelectric layer  34 , electrode layer  54 , piezoelectric layer  32 , electrode layer  52 , piezoelectric layer  30 , electrode layer  40 , piezoelectric layer  20 , electrode layer  42 , piezoelectric layer  22 , electrode layer  44 , and piezoelectric layer  24  are stacked in this order from the bottom. The outermost electrode layers  46 ,  56  may be formed by printing a paste and sintering it simultaneously with the laminate, just as the internal electrode layers are formed, or by applying and baking a paste after the laminate has been sintered. Alternatively, they may be formed by deposition, sputtering, plating, or other low-temperature process. 
     Next, the piezoelectric layers  20  through  24 ,  30  through  34  of the laminate thus formed are impressed with polarization voltage using the electrode layers  40  through  46 ,  52  through  56 , and polarized as specified. For instance, in the example shown in  FIG. 2  (A- 2 ), the electrode layers  42  and  46  are connected by a side electrode  62  as a positive electrode pattern, while the electrode layers  52  and  56  are connected by a side electrode  64  as a negative electrode pattern. Additionally, the electrode layers  40 ,  44  and  54  are connected by a side electrode  60  as a common pattern. Examples of these positive electrode pattern, negative electrode pattern, and common pattern are shown in  FIG. 3  (A) through (C). The side electrodes  60 ,  62 ,  64  are formed, for example, by a method of applying a paste on the side face of the laminate or by a method that employs deposition, sputtering, plating, or other low-temperature process. Or, instead of connecting the electrode layers via their exterior side faces, it is possible to use the through hole method of making holes in the piezoelectric sheets and interconnecting the electrode layers when the paste is printed, or any other conventional method, to connect the electrode layers. 
     When the piezoelectric driving element  10  has a four-layer structure, on the other hand, the electrode layer  42  provides a positive electrode pattern, while the electrode layer  52  provides a negative electrode pattern, as shown in  FIG. 2  (A- 1 ). In addition, the electrode layers  40 ,  44  and  54  are connected by the side electrode  60  as a common pattern. When the piezoelectric driving element  10  has an eight-layer structure, the electrode layers  42  and  46  are connected by the side electrode  62  as a positive electrode pattern, while the electrode layers  52  and  56  are connected by the side electrode  64  as a negative electrode pattern, as shown in  FIG. 2  (A- 3 ). Additionally, the electrode layers  40 ,  44 ,  48 ,  54 , and  58  are connected by a side electrode  68  as a common pattern. 
     Sintering of the laminate and formation of the electrodes are followed by polarization. A voltage equal to or greater than the coercive electric field of the material is applied as the polarization voltage, but the voltage applied must be appropriate for the thickest layer. Also, the temperature may be raised to lower the voltage at the time of polarization. Polarization is implemented based on three poles associated with positive voltage, negative voltage, and common voltage of 0 V, respectively, using the positive electrode pattern, negative electrode pattern, and common pattern, as shown in  FIG. 2  (A- 1 ) through (A- 3 ). At this time, the positive voltage and negative voltage must be the same and applied simultaneously. If the voltages are different or not applied simultaneously, the element may deform abnormally and crack due to stress. When the polarization is complete, the positive electrode and negative electrode are connected as one electrode, as shown in  FIG. 2  (B- 1 ) through (B- 3 ). In the example of the four-layer structure shown in  FIG. 2  (B- 1 ), the electrode layers  42  and  52  are connected by a side electrode  66 . In the example of the six-layer structure shown in  FIG. 2  (B- 2 ) and example of the eight-layer structure shown in  FIG. 2  (B- 3 ), the electrode layers  42 ,  46 ,  52 , and  56  are connected by a side electrode  50 . 
     Then, signals are input to these connected electrodes and common electrode to cause the top half and bottom half of the piezoelectric driving element  10  to expand and contract in opposite directions, thereby producing flexural displacement. In the example of the six-layer structure in  FIG. 1  (A), the polarization direction of the piezoelectric layers  30 ,  32 ,  34  constituting the bottom laminate piezoelectric body  14  is opposite the polarization direction of the piezoelectric layers  20 ,  22 ,  24  constituting the top laminate piezoelectric body  12 . On the other hand, audio signals and other driving voltages are applied to the electrode layers  42 ,  46 ,  52 , and  56 , while the remaining electrode layers  40 ,  44 ,  54  are connected to ground. Accordingly, the laminate piezoelectric body  12  expands and contracts in the direction of arrow FA opposite to the direction of arrow FC in which the laminate piezoelectric body  14  expands and contracts. In other words, the laminate piezoelectric body  14  contracts in the direction of arrow FC when the piezoelectric body  12  expands in the direction of arrow FA, as shown in  FIG. 1  (B). On the other hand, the laminate piezoelectric body  14  expands in the direction of arrow FC when the piezoelectric body  12  contracts in the direction of arrow FA, as shown in  FIG. 1  (C). As a result, the entire element vibrates in the direction of arrow FB. 
     The overall thickness of the piezoelectric driving element  10  is 50 to 200 μm. If the thickness is smaller than this range, insufficient strength is produced to overcome air or the rigidity of a support plate  70  described layer, thus preventing the element from displacing fully. If the thickness is greater than this range, on the other hand, the piezoelectric driving element  10  cannot also displace fully, due to excessive rigidity of the element itself. While the example in  FIG. 1  (A) shows six piezoelectric layers, any even number of layers greater than four is acceptable and the four-layer structure shown in  FIGS. 2  (A- 1 ) and (B- 1 ) or eight-layer structure shown in  FIGS. 2  (A- 3 ) and (B- 3 ) may be adopted. In any event, the layers are stacked symmetrically above and below the center in the thickness direction (electrode layer  40  in this example). 
     The thickness ratios of multiple piezoelectric layers can be given by Equation 1 below when flexural displacement is assumed and the total number of layers is given by 2n (n is a natural number) from the amount of expansion/contraction required of each layer as calculated from the radius of curvature:
 
√{square root over (n)}−√{square root over (n−1)};√{square root over (n−1)}−√{square root over (n−2)}; . . . √{square root over (2)}−√{square root over (1)}:1:1:√{square root over (2)}−√{square root over (1)};√{square root over (3)}−√{square root over (2)}; . . . √{square root over (n−1)}−√{square root over (n−2)};√{square root over (n)}−√{square root over (n−1)}  [Equation 1]
 
     When Equation 1 above is used, the thickness ratios of piezoelectric bodies are √2−1:1:1:√2−1 from the bottom layer when there are four layers (n=2). They are √3−√2:√2−1:1:1:√2−1:√3−√2 when there are six layers (n=3), and 2−√3:√3−√2:√2−1:1:1:√2−1:√3−√2:2−√3 when there are eight layers (n=4). Note that an acceptable margin of error for the thickness of each layer is up to ±10% of the above ratio. It has been shown that, when the thicknesses of respective layers having these ideal thickness ratios are added up, the total layer thickness is expressed by the relationship of 2×t dmost ×(√n), where t dmost  indicates the thickness of the thickest piezoelectric layer among the piezoelectric layers contributing to displacement and the number of piezoelectric layers contributing to displacement is given by 2n. In other words, when the thickness of the thickest piezoelectric layer among the piezoelectric layers contributing to displacement is given by t dmost , the thickness from the base point to the nth layer (n is a natural number) satisfies t dmost ×(√n), where the base point represents the boundary surface between the thickest piezoelectric layer and the center electrode layer. Since the piezoelectric driving element in this example is of the bimorph structure, the piezoelectric layer thickness of the element as a whole is twice that, or specifically 2×t dmost ×(√n) as mentioned above. 
     However, the actual laminate must have an electrode layer formed between layers. These electrodes must be formed simultaneously as the ceramics (piezoelectric layers) are sintered, and therefore use silver, platinum, palladium, or alloy thereof that does not melt but is only sintered at the sintering temperature of the ceramics. Unlike the piezoelectric layers, the electrode layers do not deform under voltage and thus Equation 1 above is modified according to the presence of electrode layers. Given this inhibition of the amount of displacement of the piezoelectric driving element  10  according to the presence of electrode layers, the electrode layers should be as thin as possible, such as 1 to 2 μm when the printing method is used. In addition, having more layers means a higher electrode ratio, so the practical number of piezoelectric layers to be stacked is no more than eight. Also, at least four layers are required because if there are only two layers, there is no gradient or difference in layer thickness. 
     Equation 1 above can be modified to account for greater overall thickness and higher bending rigidity by giving the thickness of this electrode layer by t ie , thickness of the thickest piezoelectric layer by t dmost , and ratio A of the thickness of this electrode layer to the thickness of the thickest piezoelectric layer by (t ie /t dmost ) but such modified equation cannot be solved analytically. When the Young&#39;s modulus of the electrode material is assumed as 50 to 150 GPa, overall thickness of the piezoelectric driving element  10  is assumed as 50 to 200 μm, and maximum electrode thickness is assumed as 5 μm; however, the equation can be calculated approximately. When there are four piezoelectric layers, optimum characteristics can be achieved by adjusting the thicknesses of piezoelectric layers  32 ,  30 ,  20 , and  22  to the ratios given by Equation 2 below:
 
√{square root over (2)}−1−A:1−4A:1−4A:√{square root over (2)}−1−A  [Equation 2]
 
     Similarly, when there are six piezoelectric layers, the thickness ratios of piezoelectric layers  34 ,  32 ,  30 ,  20 ,  22 , and  24  that provide optimum characteristics are given by Equation 3 below:
 
√{square root over (3)}−√{square root over (2)}−A:√{square root over (2)}−1−2A:1−4A:1−4A:√{square root over (2)}−1−2A:√{square root over (3)}−√{square root over (2)}−A  [Equation 3]
 
     Furthermore, when there are eight piezoelectric layers, the thickness ratios of piezoelectric layers  36 ,  34 ,  32 ,  30 ,  20 ,  22 ,  24 , and  26  that provide optimum characteristics are given by Equation 4 below: 
     
       
         
           
             
               
                 
                   
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     The effects of the present invention can be demonstrated so long as the margin of error of the thickness of each piezoelectric layer is within ±10%. Since this example applies to the bimorph type, however, each layer on the outer side must be thinner than other layer present on the inner side of it. If this condition is not met, the element capacity will increase and driving current will rise, thereby preventing the desired effects from manifesting. 
     It has been shown that, when the electrode layer thickness is also considered, as mentioned above, the thickness from the base point to the nth layer is expressed by the relationship of t dmost ×(√n)+Σt ie (n−1), where t dmost  represents the thickness of the thickest piezoelectric layer among the piezoelectric layers contributing to displacement and the base point represents the boundary surface between the thickest piezoelectric layer and the center electrode layer. 
     This is explained in concrete terms by referring to  FIG. 4 .  FIG. 4  shows the top laminate piezoelectric body  12  side of the piezoelectric driving element  10 . The piezoelectric layer  20  contacting the center electrode layer  40  contributes most to displacement. In this example, the thickness from the base point (boundary surface between the piezoelectric layer  20  and electrode layer  40 ) to the first piezoelectric layer is t dmost  when the above equation of t dmost  (√n) is applied. Next, the thickness to the second layer (n=2) is t dmost ×(√2)+t ie (1), which corresponds to t dmost ×(√2) plus the thickness t ie (1) of the electrode layer  42 . Furthermore, the thickness to the third layer (n=3) is t dmost ×(√3)+t ie (1) t ie (2), after adding the thickness of the second electrode layer  44 . In other words, because the (n−1) number of electrode layers are present in between the n number of piezoelectric layers, the thickness from the aforementioned base point to the nth layer can be expressed by t dmost ×(√n)+Σt ie (n−1) by adding up their thicknesses. 
     As shown in  FIG. 1  (D), the aforementioned piezoelectric driving element  10  is attached to the support plate  70 . The softest possible material is used for the support plate  70 . For example, rubber and urethane are suitable. The thickness of the support plate  70  is in a range of 50 to 200 μm similar to that of the piezoelectric driving element  10 . If the thickness of the plate is smaller than this range, the element cannot be supported fully and may be damaged as it vibrates; if the thickness of the plate is greater than this range, on the other hand, vibration of the element is inhibited and the sound pressure will drop. The support plate  70  to which the piezoelectric driving element  10  has been attached is then attached to a frame made of metal, plastic, etc., and the electrodes are connected to a terminal strip, etc., to obtain a piezoelectric sound-generating body. Here, lead wires, etc., may be used or conductive paste or other material that hardens under heat may be used. 
     The aforementioned frame may be a simple frame shape with an opening  82  like a frame  80  shown in  FIG. 1  (D), or it may be shaped as a lid. However, a sufficient clearance must be provided between the top of the lid on one hand and the element and vibration plate on the other to prevent contact due to vibration. For example, a frame  90  shown in  FIG. 1  (E) is of the lid type mentioned above, having a sufficient space  92  not to inhibit the element from vibrating as well as multiple sound-emitting holes  96  provided in a bottom surface  94  of the lid. The piezoelectric sound-generating body thus obtained has 50 to 60% lower current than a piezoelectric sound-generating body constituted by a simple laminate of piezoelectric bodies of an identical thickness, while the sound pressure is the same, and consequently heat generation at connection points can be suppressed and use of small, low-cost drive circuit components becomes possible. 
     Table 1 below lists the sound pressure (average of sound pressures at 800, 1000, 1500, and 2000 Hz) and current in driven state of each speaker produced in this method. The elements tested were 14×18 mm in size, each attached to the support plate  70  using a 100 μm thick elastomer and to the lid-shaped metal frame  90  as shown in  FIG. 1  (E). Examples 1 through 4 based on different numbers of layers and layer formations were produced and tested. As Comparative Examples 1 through 6, speakers were produced in the same manner using elements whose layer thickness formation was outside the scope of the present invention, and tested in the same way, the results of which are shown in Table 1. 
                                                 TABLE 1                                           Current when           Number       Electrode   Element   Average sound   10 kHz sine           of   Layer formation   layer thickness   thickness   pressure level   wave is input           layers   (μm)   (μm)   (μm)   (dB)   (mA)                                                                Example 1   4   12:24:24:12   2   72   97.0   220       Comparative   4   18:18:18:18   2   72   96.9   380       Example 1       Comparative   4   8:28:28:8   2   72   96.8   420       Example 2       Example 2   4   16:32:32:16   3   96   96.9   280       Comparative   4   24:24:24:24   3   96   96.7   490       Example 3       Comparative   4   12:36:36:12   3   96   97.0   540       Example 4       Example 3   6   11:12:31:31:12:11   2   108   97.0   600       Comparative   6   18:18:18:18:18:18   2   102   96.8   820       Example 5       Example 4   8   15:17:20:48:48:20:17:15   2   200   96.8   860       Comparative   8   25:25:25:25:25:25:25:25   2   200   96.9   1240       Example 6                    
As is evident from the results of Examples 1 through 4 and Comparative Examples 1 through 6 in Table 1, the elements within the scope of the present invention had sufficiently small current, while those outside the scope of the present invention had large current and could not achieve desired effects.
 
     As explained, Example 1 involves a piezoelectric sound-generating body using a bimorph piezoelectric driving element  10  constituted by a laminate of multiple piezoelectric layers, where the piezoelectric layer at the center where the amount of displacement is the smallest is made the thickest. It also has the same number of layers above and below the center in the thickness direction and a vertically symmetrical layer structure, and its piezoelectric layers become gradually thinner from the center toward the outer side. This way, the capacity can be reduced and current can be kept low even when high frequency signals are input, without affecting the amount of displacement of the element. As a result, failures due to heat generation can be prevented, while size reduction also becomes possible because there is no longer a need to use thick conductive wires for the driving circuit. 
     &lt;Variation Example 1&gt; . . . Next, Variation Example 1 of this example is explained by referring to  FIG. 5  (A). While the piezoelectric driving element  10  shown in  FIG. 1  (A) has its top laminate piezoelectric body  12  and bottom laminate piezoelectric body  14  formed with the electrode layer  40  sandwiched in between, a piezoelectric driving element  100  shown in  FIG. 5  (A) has its laminate piezoelectric bodies  12 ,  14  formed symmetrically with an inactive layer (non-polarizing layer)  102  other than an electrode layer sandwiched in between. In this case, too, effects similar to those described in the aforementioned example can be achieved.
 
&lt;Variation Example 2&gt; . . . Next, Variation Example 2 of this example is explained by referring to  FIG. 5  (B). While the piezoelectric driving element  10  shown in  FIG. 1  (A) is of the bimorph type not using any shim plate, a constitution where laminate piezoelectric bodies  12 ,  14  are attached at the top and bottom of a metal plate or other shim plate  112  may also be adopted, as is the case of a piezoelectric driving element  110  shown in  FIG. 5  (B). In this case, a piezoelectric sound-generating body can be constituted by supporting the shim plate  112  with the frame  80  or  90  shown in  FIG. 1  (D) or  FIG. 1  (E) to achieve effects similar to those described in Example 1.
 
     Example 2 
     Next, Example 2 of the present invention is explained by referring to  FIG. 5  (C). While the piezoelectric driving element in Example 1 is of the bimorph type, the present invention can also be applied to the unimorph type. A piezoelectric driving element  120  shown in  FIG. 5  (C) is constituted by the laminate piezoelectric body  12  of four-layer structure attached to one main side of a vibration plate  122  made of metal material. The foregoing is then attached to the aforementioned frame  80  or  90  to constitute a piezoelectric sound-generating body. With the unimorph type like the one shown in this example, the piezoelectric layer undergoing the smallest displacement (area undergoing the smallest expansion and contraction in the lateral direction), or specifically the piezoelectric layer  20  on the vibration plate  122  side, is the thickest and the piezoelectric layers  22 ,  24 , and  26  become gradually thinner toward the upper side in the lamination direction. 
     It suffices that there are at least two piezoelectric layers, but if there are n number of layers (n is a natural number), for example, ideally the thickness ratios of piezoelectric layers correspond to the ratios given by Equation 5 below from the vibration plate  122  side toward the upper layers. Needless to say, a margin of error of up to ±10% is allowed for the ratio of each piezoelectric layer, as is the case in Example 1 above. To apply Equation 5 below, the vibration plate  122  to be used is one whose Young&#39;s modulus is 50 to 200 GPa and thickness is one half or less that of the laminate piezoelectric body  12 .
 
1:√{square root over (2)}−√{square root over (1)}:√{square root over (3)}−√{square root over (2)}: . . . √{square root over (n−1)}−√{square root over (n−2)}:√{square root over (n)}−√{square root over (n−1)}  [Equation 5]
 
     Furthermore, desirably the total layer thickness is specified as t dmost ×(√n) where t dmost  represents the thickness of the thickest piezoelectric layer among the piezoelectric layers contributing to displacement and n represents the number of piezoelectric layers contributing to displacement. 
     In addition, Equation 5 above can be modified according to the presence of electrode layers in between piezoelectric layers by defining the ratio A of the thickness of the electrode layer (t ie ) to the thickness of the thickest piezoelectric layer (t dmost ) as A=(t ie /t dmost ), as is the case in Example 1 above. For example, the equation can be calculated approximately by assuming the Young&#39;s modulus of the electrode material to be 50 to 150 GPa, total thickness of the piezoelectric driving element  120  to be 50 to 200 μm, and maximum electrode thickness to be 5 μm. When there are two piezoelectric layers, optimal characteristics can be achieved by adjusting the thicknesses of piezoelectric layers  20 ,  22  to the ratios given by Equation 6 below:
 
1−4A:√{square root over (2)}−1−A  [Equation 6]
 
     Similarly, when there are three piezoelectric layers, the thickness ratios of piezoelectric layers  20 ,  22 ,  24  that provide optimal characteristics are given by Equation 7 below:
 
1−4A:√{square root over (2)}−1−2A:√{square root over (3)}−√{square root over (2)}−A  [Equation 7]
 
     Furthermore, when there are four piezoelectric layers, the thickness ratios of piezoelectric layers  20 ,  22 ,  24 ,  26  that provide optimal characteristics are given by Equation 8 below: 
     
       
         
           
             
               
                 
                   1 
                   - 
                   
                     4 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       A 
                       : 
                       
                         
                           2 
                         
                         - 
                         1 
                         - 
                         
                           
                             3 
                             2 
                           
                           ⁢ 
                           
                             A 
                             : 
                             
                               
                                 
                                   3 
                                 
                                 - 
                                 
                                   2 
                                 
                                 - 
                                 
                                   A 
                                   2 
                                 
                               
                               : 
                               
                                 2 
                                 - 
                                 
                                   3 
                                 
                                 - 
                                 
                                   A 
                                   4 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     The effects of the present invention can be demonstrated so long as the margin of error of the thickness of each piezoelectric layer is within ±10%. Since this example applies to the bimorph type, however, each layer on the outer side must be thinner than the piezoelectric layer  20  present on the vibration plate  122  side of it. If this condition is not met, the element capacity will increase and driving current will rise, thereby preventing the desired effects from manifesting. So long as the foregoing is met, effects similar to those described in Example 1 can be achieved even when the unimorph type is used as in this example. When the thickness of the electrode layer is considered, the thickness from the base point (boundary surface between the vibration plate  122  and piezoelectric layer  20  in this example) to the nth layer is expressed by t dmost  ×(√n)+Σt ie (n−1) as described above in Example 1. 
     The present invention is not limited to the aforementioned examples in any way, and various changes can be added to the extent that doing so does not deviate from the key points of the present invention. For example, the following are also permitted as included in the scope of the present invention: (1) The shape of the piezoelectric driving element as shown in the above examples is an example and it can be changed to a circle, etc., as deemed appropriate if necessary. (2) The dimensions of the piezoelectric driving element in planar directions as shown in the above examples are also an example and the design can be changed as deemed appropriate if necessary. (3) The material shown in the above examples is also an example and any of various known materials can be used. (4) The mechanism to support the piezoelectric driving element using the support plate  70  and frame  80  or  90  as shown in Example 1 above is also an example and the design can be changed as deemed appropriate to the extent that similar effects are achieved. (5) The lamination method of the piezoelectric driving element as shown in Example 1 above is also an example and it can be changed as deemed appropriate if necessary. In the case of a bimorph type of four-layer structure whose piezoelectric layers  20 , on the center side are roughly twice as thick as the piezoelectric layers  22 ,  32  on the outer side, two of the piezoelectric sheets used as the piezoelectric layer  22  or  32  are stacked on top of each other to form the piezoelectric layer  20  or  30 , as shown in  FIG. 5  (D). Manufacturing becomes easy when the thickness of each piezoelectric layer can be aligned by adjusting the number of sheets to be stacked. (6) While the examples above were explained based on a speaker to be installed in a mobile phone, etc., the present invention can be applied as a piezoelectric sound-generating body used for any of various known electronic devices such as a receiver for mobile phones. 
     INDUSTRIAL FIELD OF APPLICATION 
     According to the present invention, a piezoelectric sound-generating body using a piezoelectric driving element constituted by a laminate of multiple piezoelectric layers is formed in such a way that the piezoelectric layer in the area associated with the smallest displacement is the thickest and the piezoelectric layers become gradually thinner toward the outer side. This way, the capacity can be reduced and current can be kept low without affecting the amount of displacement of the element, and as this prevents failures and allows for size reduction, the present invention can be applied to a piezoelectric sound-generating body installed in electronic devices, etc. In particular, the present invention is suitable for mobile electronic devices, etc., representative examples of which include mobile phones and smartphones. 
     DESCRIPTION OF THE SYMBOLS 
       10 : Piezoelectric driving element,  12 ,  14 : Laminate piezoelectric body,  20  to  26 ,  30  to  36 : Piezoelectric layer,  40  to  46 ,  52  to  58 : Electrode layer,  50 ,  60  to  68 : Side electrode,  70 : Support plate,  80 ,  90 : Frame,  82 : Opening,  92 : Space,  94 : Bottom surface,  96 : Sound emitting hole,  100 : Piezoelectric driving element,  102 : Inactive layer,  110 : Piezoelectric driving element,  112 : Shim plate (support plate),  120 : Piezoelectric driving element,  122 : Vibration plate