Patent Publication Number: US-2021176566-A1

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2019-219414, filed on Dec. 4, 2019, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to a display device. 
     Description of the Background 
     A display device including a display panel and an actuator, and having a function of vibrating the display panel by controlling the actuator is disclosed in Korean Patent Publication No. 10-2018-0077582A. 
     In order to improve a sense of realism or the like, a display device, which emits a sound by allowing a display panel itself to function as a speaker, is being considered. However, although a display panel is allowed to function as a speaker through a structure disclosed in Korea Patent Publication No. 10-2018-0077582A, sound quality may not be sufficient. 
     SUMMARY 
     Accordingly, the present disclosure is directed to improving sound quality in a display device which emits a sound from a display panel. 
     According to an aspect of the present disclosure, there is provided a display device including a piezoelectric element vibrated according to input audio signals, a display panel configured to display an image, and an elastic member configured to connect a portion of the piezoelectric element and the display panel so as to transmit a vibration of the piezoelectric element to the display panel. 
     According to another aspect of the present disclosure, there is provided a display device including a piezoelectric element which includes a first vibration part and a second vibration part extending in different directions when viewed from above and is vibrated according to input audio signals, a display panel configured to display an image, and an elastic member configured to connect a portion of the piezoelectric element and the display panel so as to transmit a vibration of the piezoelectric element to the display panel. 
     According to still another aspect of the present disclosure, there is provided a display device including a plurality of piezoelectric elements vibrated according to input audio signals, and a display panel configured to display an image, wherein each of the plurality of piezoelectric elements is connected to the display panel to transmit a vibration to the display panel, and a first piezoelectric element and a second piezoelectric element among the plurality of piezoelectric elements have different frequency characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of the disclosure, illustrate implementations of the disclosure and together with the description serve to explain the principles of aspects of the disclosure. 
         FIG. 1  is a schematic block diagram of a display device according to a first aspect. 
         FIG. 2  is a plan view illustrating a schematic configuration of a piezoelectric element according to the first aspect. 
         FIG. 3  is a cross-sectional view illustrating the schematic configuration of the piezoelectric element according to the first aspect. 
         FIG. 4  is a cross-sectional view illustrating a structure of the piezoelectric element according to the first aspect in more detail. 
         FIG. 5  is a schematic view illustrating deformation of the piezoelectric element to which a voltage is applied according to the first aspect. 
         FIG. 6  is a schematic view illustrating deformation of the piezoelectric element to which a voltage is applied according to the first aspect. 
         FIG. 7  is a cross-sectional view illustrating a structure of a piezoelectric element according to a comparative example. 
         FIG. 8  is a schematic view illustrating a vibration model according to the comparative example. 
         FIG. 9  is a schematic view illustrating a vibration model according to the first aspect. 
         FIG. 10  is a plan view illustrating a schematic configuration of a piezoelectric element according to a second aspect. 
         FIG. 11  is a cross-sectional view illustrating a structure of a piezoelectric element according to a third aspect. 
         FIG. 12  is a plan view illustrating a schematic configuration of a piezoelectric element according to a fourth aspect. 
         FIG. 13  is a cross-sectional view illustrating the schematic configuration of the piezoelectric element according to the fourth aspect. 
         FIG. 14  is a cross-sectional view illustrating a structure of the piezoelectric element according to the fourth aspect in more detail. 
         FIG. 15  is a plan view illustrating a schematic configuration of a piezoelectric element according to a fifth aspect. 
         FIG. 16  is a plan view illustrating a schematic configuration of a piezoelectric element according to a sixth aspect. 
         FIG. 17  is a plan view illustrating a schematic configuration of a piezoelectric element according to a seventh aspect. 
         FIG. 18  is a plan view illustrating the layout of a piezoelectric element according to an eighth aspect. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, aspects according to the present disclosure will be described in detail with reference to the accompanying drawings. In each drawing, the same numerals are assigned to the elements having the common function, and redundant descriptions thereof will be omitted or simplified. 
     &lt;First Aspect&gt; 
       FIG. 1  is a schematic block diagram of a display device  1  according to a first aspect. The display device  1  of the present aspect may be, for example, used for an image output device of a computer, a television receiver, a smartphone, a game console, and the like, but the present disclosure is not particularly limited thereto. 
     As shown in  FIG. 1 , the display device  1  includes a piezoelectric element  10 , a display panel  20 , an elastic member  30 , a first controller  40 , a second controller  50 , a data driving circuit  60 , and a gate driving circuit  70 . The display device  1  is a device that displays an image on the display panel  20  on the basis of input RGB data or the like and generates a sound based on an input audio signal or the like. 
     The display panel  20  includes a plurality of pixels P arranged to form a plurality of rows and a plurality of columns. The display device  1  may be, for example, an organic light-emitting diode (OLED) display using an OLED as a light-emitting element of the pixel P. When the display device  1  is capable of displaying a color image, the pixel P may be a subpixel that displays any one of a plurality of colors (for example, red (R), green (G), and blue (B)) constituting the color image. 
     The piezoelectric element  10  is an element that is displaced by an inverse piezoelectric effect when a voltage based on an input audio signal is applied. The piezoelectric element  10  may be, for example, an element such as a bimorph element or a unimorph element, which is flexural-displaced according to a voltage. Since the input audio signal is usually an alternating current (AC) voltage, the piezoelectric element  10  functions as a vibration element that vibrates according to the input audio signal. 
     The elastic member  30  is a member made of a material having elasticity. As a material of the elastic member  30 , a material such as rubber having an elastic modulus smaller than that of the piezoelectric element  10  and the display panel  20  is typically used. A portion of the piezoelectric element  10  and a portion of the display panel  20  are connected by the elastic member  30 . As a result, a vibration of the piezoelectric element  10  is transmitted to the display panel  20 , and the display panel  20  emits a sound based on the input audio signal. 
     A host system  2  is a system including a device or a plurality of devices for controlling the display device  1  by supplying an image signal (for example, RGB data), an audio signal, and timing signals (vertical synchronization signal, horizontal synchronization signal, data enable signal, and the like). The host system  2  may be, for example, a television system, a set-top box, a navigation system, an optical disk player, a computer, a home theater system, a video phone system, or the like. Furthermore, the display device  1  and the host system  2  may be an integrated device or may be separate devices. 
     The first controller  40  supplies a voltage to the piezoelectric element  10  on the basis of the audio signal and the timing signal input from the host system  2 . 
     The second controller  50  controls the data driving circuit  60  and the gate driving circuit  70  on the basis of the image data and the timing signals input from the host system  2 . The data driving circuit  60  supplies a data voltage or the like to the plurality of pixels P through a driving line  61  disposed for each column of the plurality of pixels P. The gate driving circuit  70  supplies control signals to the plurality of pixels P through a driving line  71  disposed for each row of the plurality of pixels P. Furthermore, each of the driving line  61  and the driving line  71  may be provided with a plurality of lines. 
     Each of the first controller  40 , the second controller  50 , the data driving circuit  60 , and the gate driving circuit  70  may be provided as one or more semiconductor integrated circuits. In addition, some or all of the first controller  40 , the second controller  50 , the data driving circuit  60 , and the gate driving circuit  70  may be integrally provided as one semiconductor integrated circuit. 
       FIG. 2  is a plan view illustrating a schematic configuration of the piezoelectric element  10  according to the first aspect.  FIG. 3  is a schematic cross-sectional view illustrating the configuration of the piezoelectric element  10  according to the first aspect. The layout of the piezoelectric element  10  will be described with reference to  FIGS. 2 and 3 . In  FIG. 2 , a rectangular outline of the display panel  20  schematically shows an exterior of the display panel  20 . 
     As shown in  FIG. 3 , a surface of the display panel  20  on which an image is displayed is an image display surface  20   a , and a surface thereof opposite to the image display surface  20   a  is a rear surface  20   b . In this case,  FIG. 2  is a plan view of the display panel  20  viewed from the rear surface  20   b .  FIG. 2  illustrates coordinate axes in which a horizontal direction of the image display surface  20   a  corresponds to an x-axis, a vertical direction of the image display surface  20   a  corresponds to a z-axis, and a depth direction of the image display surface  20   a  corresponds to a y-axis. Furthermore, a direction from the rear surface  20   b  toward the image display surface  20   a  is a forward direction of the y-axis.  FIG. 3  is a cross-sectional view taken along line A-A′ in  FIG. 2 . 
     The piezoelectric element  10  has a flat plate shape. As shown in  FIG. 2 , the piezoelectric element  10  has a rectangular shape having a long side direction (z direction in the drawing) and a short side direction (x direction in the drawing) when viewed from above. As a result, deformation such as bending occurs when viewed from a cross section (line A-A′) in the long side direction. The piezoelectric element  10  is disposed such that the long side direction is perpendicular to an end of the display panel  20 . 
     The elastic member  30  is connected at a position including a center of the piezoelectric element  10  in the long side direction. Since the center of the piezoelectric element  10  in the long side direction becomes a center of a vibration, the vibration is efficiently transmitted to the display panel  20 . 
     As shown in  FIG. 3 , the piezoelectric element  10  has a first main surface  10   a  and a second main surface  10   b . The elastic member  30  connects the first main surface  10   a  of the piezoelectric element  10  and the rear surface  20   b  of the display panel  20 . As described above, the piezoelectric element  10  and the elastic member  30  are disposed on the rear surface  20   b  of the display panel  20  so as to not interfere with when a user views the image display surface  20   a.    
     The elastic member  30  is connected only to a portion of the first main surface  10   a  of the piezoelectric element  10 . Since both ends of the piezoelectric element  10  in the long side direction are in a floating state, a displacement of a flexural vibration is great in the both ends in the long side direction, and thus, the vibration of the piezoelectric element  10  is difficult to disrupt. 
       FIG. 4  is a cross-sectional view illustrating a structure of the piezoelectric element  10  according to the first aspect in more detail. Although the direction of  FIG. 4  is different from that of  FIG. 3 , like  FIG. 3 ,  FIG. 4  is a cross-sectional view along line A-A′ of  FIG. 2 . In addition,  FIG. 4  schematically illustrates a connection relationship between electrodes included in the piezoelectric element  10  using a circuit diagram in order to describe a method of inputting an audio signal to the piezoelectric element  10 . 
     The piezoelectric element  10  shown in  FIG. 4  has a structure referred to as a bimorph in which two piezoelectric layers are stacked. The piezoelectric element  10  includes electrodes  101 ,  103  and  105  and piezoelectric layers  102  and  104 . The electrode  101  (first electrode) installed at a side closest to the display panel  20  is connected to the elastic member  30 . The electrode  101  and the electrode  103  (second electrode) are disposed such that the piezoelectric layer  102  (first piezoelectric layer) is interposed therebetween in a thickness direction thereof. The electrode  103  and the electrode  105  (third electrode) are disposed such that the piezoelectric layer  104  (second piezoelectric layer) is interposed therebetween in a thickness direction thereof. Arrows shown inside the piezoelectric layers  102  and  104  indicate polarization directions of the piezoelectric layers  102  and  104 . That is, the polarization directions of the piezoelectric layer  102  and the piezoelectric layer  104  are the same. Furthermore, lines for applying voltages to the electrodes may be connected to the electrodes  101 ,  103  and  105  through soldering or the like, but the illustration of the lines is omitted in  FIG. 4 . 
     Since a voltage applied to the piezoelectric element  10  is based on an audio signal, the voltage may be considered as an AC voltage according to a frequency of a sound to be generated. In  FIG. 4 , the AC voltage is illustrated by circuit symbol V of an AC power source. One terminal of the AC power source V is connected to the electrodes  101  and  105 , and the other terminal thereof is connected to the electrode  103 . In other words, voltages having the same phase are applied to the electrode  101  and the electrode  105 , voltages having opposite phases are applied to the electrode  101  and the electrode  103 , and voltages having opposite phases are also applied to the electrode  103  and the electrode  105 . Accordingly, voltages in opposite directions are applied to the piezoelectric layer  102  and the piezoelectric layer  104 . 
     A material of the piezoelectric layers  102  and  104  is not particularly limited, but a material having excellent piezoelectricity such as lead zirconate titanate may be used because the material may increase a displacement amount. In addition, although not shown in the configuration of  FIG. 4 , an outer periphery of the piezoelectric element  10  may be covered with an insulator such as a resin in order to avoid a short circuit with other members. 
       FIGS. 5 and 6  are schematic views illustrating deformation of the piezoelectric element  10  to which a voltage is applied according to the first aspect. As shown in  FIG. 4 , the polarization directions of the piezoelectric layers  102  and  104  are the same direction, and voltages in opposite directions are applied to the piezoelectric layers  102  and  104 . Accordingly, expansion and contraction directions of the piezoelectric layer  102  and the piezoelectric layer  104  are opposite. 
     As shown in  FIG. 5 , at a timing when the piezoelectric layer  102  is deformed so as to contract in a lateral direction, the piezoelectric layer  104  is deformed to expand in the lateral direction. Accordingly, an end of the piezoelectric element  10  is bent in a direction toward the display panel  20 . In this case, the display panel  20  is deformed by receiving stress in a direction toward the piezoelectric element  10 . 
     As shown in  FIG. 6 , at a timing when the piezoelectric layer  102  is deformed to expand in the short side direction, the piezoelectric layer  104  is deformed to contract in the short side direction. Accordingly, the end of the piezoelectric element  10  is bent in a direction away from the display panel  20 . In this case, the display panel  20  is deformed by receiving stress in a direction away from the piezoelectric element  10 . 
     When an AC voltage based on an audio signal is applied to the piezoelectric element  10 , the state of  FIG. 5  and the state of  FIG. 6  are alternately repeated at a frequency of a sound. As described above, a vibration of the piezoelectric element  10  is transmitted to the display panel  20 , and the display panel  20  vibrates. Accordingly, since a sound based on an audio signal is emitted from the display panel  20 , the display panel  20  functions as a speaker. 
     In the present aspect, effects obtained by connecting the portion of the piezoelectric element  10  and the display panel  20  through the elastic member  30  will be described in more detail with reference to  FIGS. 7 to 9 .  FIG. 7  is a cross-sectional view illustrating a configuration of a piezoelectric element  10  according to a comparative example.  FIG. 8  is a schematic view illustrating a vibration model according to the comparative example.  FIG. 9  is a schematic view illustrating a vibration model according to the first aspect. 
     As shown in  FIG. 7 , in the comparative example, a front surface of the piezoelectric element  10  is connected directly to a display panel  20 . Even in such a configuration, a displacement of the piezoelectric element  10  is transmitted to the display panel  20  to allow the display panel  20  to function as a speaker. 
     In the vibration model according to the comparative example, as shown in  FIG. 8 , springs S 1  and S 2  are connected to both ends of a material point showing the piezoelectric element  10  and the display panel  20 . The piezoelectric element  10  having mass m 1  and the display panel  20  having mass m 2  are directly connected. 
     The spring S 1  having a spring constant k 1  is connected to the piezoelectric element  10 , and the spring S 2  having a spring constant k 2  is connected to the display panel  20 . The spring S 1  is a model of elasticity of the piezoelectric element  10 . The spring S 2  is a model of the display panel  20  itself or a member such as a housing for restricting the display panel  20 . In addition, both ends in the present vibration model are typically fixed ends. 
     In the vibration model of the comparative example, the piezoelectric element  10  and the display panel  20  may be replaced with one material point having mass m 1 +m 2 . When a voltage is applied to the piezoelectric element  10 , a force generated by the piezoelectric element  10  vibrates the entirety of both of the piezoelectric element  10  and the display panel  20  having the mass m 1 +m 2 . Here, since the display panel  20  is much larger than the piezoelectric element  10 , the mass m 1 +m 2  is much greater than the mass m 1 . Since the force generated by the piezoelectric element  10  is applied to an object having very large mass, acceleration of the piezoelectric element  10  and the display panel  20  due to the force is small. Accordingly, a displacement amount of the piezoelectric element  10  and the display panel  20  is not so large, and in the configuration of the comparative example, sound pressure of a sound emitted from the display panel  20  may not be sufficient. 
     On the other hand, in the vibration model according to the present aspect, as shown in  FIG. 9 , a spring S 3  having a spring constant k 3  is connected between material points showing the piezoelectric element  10  and the display panel  20 . The spring S 3  is a model of elasticity of the elastic member  30 . The piezoelectric element  10  having mass m 1  and the display panel  20  having mass m 2  are connected through the spring S 3 . 
     In the vibration model of the present aspect, the piezoelectric element  10  and the display panel  20  are independently displaced. A force generated when a voltage is applied to the piezoelectric element  10  vibrates the piezoelectric element  10  having the mass m 1 . Since mass of an object to which a force is applied is small as compared with the case of the comparative example, acceleration of the piezoelectric element  10  due to the force is greater than that of the comparative example. As a result, the piezoelectric element  10  resonates in a large displacement range. Since the displacement of the piezoelectric element  10  is gradually transmitted to the display panel  20  through the spring S 3 , the displacement is difficult to disturb due to the mass of the display panel  20  as in the case of the comparative example. Therefore, in the present aspect, when compared to the case of the comparative example, the displacement can be increased, and the sound pressure can be improved. 
     As described above, according to the present aspect, when the display panel  20  functions as a speaker, the display device  1 , which is capable of improving sound quality by improving sound pressure of a sound generated by the display panel  20 , is provided. 
     In addition, in the present aspect, a vibration source is the piezoelectric element  10 , but a sound source is the display panel  20  having large mass and a low natural frequency. Therefore, when compared to a configuration in which a sound is generated directly from the piezoelectric element  10  or a configuration in which a sound is generated from a member having a high natural frequency such as a configuration in which the piezoelectric element  10  is connected to a small diaphragm separate from the display panel  20 , it is possible to improve sound pressure in bass. 
     &lt;Second Aspect&gt; 
     In the present aspect, a modified example of the layout of the piezoelectric element  10  according to the first aspect will be described. Since the basic configuration of a display device  1 , the structure of a piezoelectric element  10 , and the like are the same as those of the first aspect, descriptions thereof will be omitted. 
       FIG. 10  is a plan view illustrating a schematic configuration of the piezoelectric element  10  according to a second aspect. As shown in  FIG. 10 , in the present aspect, the piezoelectric element  10  is disposed such that a long side direction of the piezoelectric element  10  is not perpendicular to any one of ends of a display panel  20 . 
     When the piezoelectric element  10  has a rectangular shape, in a distribution of vibrations generated in the display panel  20 , a main component of the vibration is a component directed in the long side direction of the piezoelectric element  10 . As in the first aspect, when the long side direction of the piezoelectric element  10  is perpendicular to the end of the display panel  20 , a vibration generated by the piezoelectric element  10  and a vibration reflected from the end of the display panel  20  may be reinforced to generate a resonance. The resonance may cause noise. On the other hand, in the present aspect, since the long side direction of the piezoelectric element  10  is not perpendicular to any one of the ends of the display panel  20 , a resonance due to the above-described factors is difficult to generate, thereby reducing noise. 
     Accordingly, according to the present aspect, the same effect as in the first aspect is obtained, and simultaneously, noise due to a resonance generated by reflection from an end surface of the display panel  20  is reduced, thereby providing the display device  1  having improved sound quality. 
     In addition, since vibration reinforcement occurs in the configuration of the first aspect, in some cases, sound pressure improvement, frequency characteristic adjustment, or the like are possible by using the vibration reinforcement. Therefore, according to design conditions such as required characteristics and design constraints, in some cases, it is desirable to make the long side direction of the piezoelectric element  10  perpendicular to the end of the display panel  20  as in the first aspect. 
     &lt;Third Aspect&gt; 
     In the present aspect, a modified example of the cross-sectional structure of the piezoelectric element  10  according to the first aspect will be described. Since the basic configuration of a display device  1 , the structure of a piezoelectric element  10 , and the like are the same as those of the first aspect, descriptions thereof will be omitted. 
       FIG. 11  is a cross-sectional view illustrating the structure of the piezoelectric element  10  according to a third aspect. The position of a cross section is the same as that in  FIGS. 3 and 4 . 
     The piezoelectric element  10  includes electrodes  111 ,  113 ,  115  and  117 , piezoelectric layers  112  and  116 , and an insulating layer  114 . The electrode  111  (first electrode) installed at a side closest to a display panel  20  is connected to an elastic member  30 . The electrode  111  and the electrode  113  (second electrode) are disposed such that the piezoelectric layer  112  (first piezoelectric layer) is interposed therebetween in a thickness direction thereof. The electrode  115  (third electrode) and the electrode  117  (fourth electrode) are disposed such that the piezoelectric layer  116  (second piezoelectric layer) is interposed therebetween in a thickness direction thereof. The insulating layer  114  is disposed between the electrode  113  and the electrode  115 . The insulating layer  114  is a layer that secures an insulating property between the electrode  113  and the electrode  115 . Arrows shown inside the piezoelectric layers  112  and  116  indicate polarization directions of the piezoelectric layers  112  and  116 . That is, the polarization directions of the piezoelectric layer  112  and the piezoelectric layer  116  are opposite. 
     One terminal of a current power source V showing a voltage based on an audio signal is connected to the electrodes  111  and  115 , and the other terminal thereof is connected to the electrodes  113  and  117 . In other words, voltages having the same phase are applied to the electrode  111  and the electrode  115 . Voltages having a phase opposite to that of the voltages input to the electrode  111  and the electrode  115  are applied to the electrode  113  and the electrode  117 . Accordingly, voltages in the same direction are applied to the piezoelectric layer  112  and the piezoelectric layer  116 . 
     Even in the present aspect, when one of the two piezoelectric layers contracts in a lateral direction, a flexural vibration occurs in the same manner as in the first aspect. Therefore, even in the present aspect, the same effect as in the case of the first aspect can be obtained. As described above, the structure of the piezoelectric element  10 , the structure of the electrodes, and the like are not limited to the structures of the first aspect, and various structures can be applied. 
     For example, the piezoelectric element  10  may have a structure referred to as a unimorph in which one piezoelectric layer, a pair of electrodes with the piezoelectric layer interposed therebetween, and a vibration plate are stacked. However, in order to improve the conversion efficiency between a voltage and a displacement, it is desirable to adopt a bimorph structure as shown in  FIG. 4 or 11 . 
     &lt;Fourth Aspect&gt; 
     In the present aspect, a modified example of the structure of the piezoelectric element  10  according to the first aspect will be described. Since the basic configuration and the like of a display device  1  are the same as those of the first aspect, descriptions thereof will be omitted. 
       FIG. 12  is a plan view illustrating a schematic configuration of a piezoelectric element  10  according to a fourth aspect.  FIG. 13  is a cross-sectional view illustrating the schematic configuration of the piezoelectric element  10  according to the fourth aspect. The layout of the piezoelectric element  10  will be described with cross-reference to  FIGS. 12 and 13 . 
     As shown in  FIG. 12 , the piezoelectric element  10  of the present aspect includes a first vibration part  12  and a second vibration part  14  extending in different directions when viewed from above. A configuration of the first vibration part  12  is the same as that of the piezoelectric element  10  of the first aspect. The first vibration part  12  has a rectangular shape having a long side direction (z direction in the drawing) and a short side direction (x direction in the drawing) when viewed from above. The second vibration part  14  extends in a direction different from that of the first vibration part  12 . That is, the second vibration part  14  has a rectangular shape having a long side direction (x direction in the drawing) and a short side direction (z direction in the drawing) when viewed from above. The long side direction of the first vibration part  12  and the long side direction of the second vibration part  14  are perpendicular to each other. In addition, the long side direction of the first vibration part  12  and the long side direction of the second vibration part  14  are both perpendicular to an end of a display panel  20 . 
     As shown in  FIG. 13 , the first vibration part  12  has a first main surface  12   a  and a second main surface  12   b . An elastic member  30  connects the first main surface  12   a  of the first vibration part  12  and a rear surface  20   b  of the display panel  20 . The elastic member  30  is connected only to a portion of the first main surface  12   a  of the first vibration part  12 . As described above, the piezoelectric element  10  and the elastic member  30  are disposed on the rear surface  20   b  of the display panel  20  so as to not interfere with when a user views an image display surface  20   a.    
     The second vibration part  14  is connected only to a portion of the second main surface  12   b  of the first vibration part  12 . Thus, both ends of the first vibration part  12  in the long side direction are in a floating state, and also, both ends of the second vibration part  14  in the long side direction are in a floating state. When both ends of the piezoelectric element  10  in the long side direction are in a floating state, a displacement of a flexural vibration is great in the both ends in the long side direction, and thus, the vibration of the piezoelectric element  10  is difficult to disrupt. 
       FIG. 14  is a cross-sectional view illustrating the structure of the piezoelectric element  10  according to the fourth aspect in more detail. Although the direction of  FIG. 14  is different from that of  FIG. 13 , like  FIG. 13 ,  FIG. 14  is a cross-sectional view along line B-B′ of  FIG. 12 . In addition,  FIG. 14  schematically illustrates a connection relationship between electrodes included in the piezoelectric element  10  using a circuit diagram in order to describe a method of inputting an audio signal to the piezoelectric element  10 . 
     The piezoelectric element  10  shown in  FIG. 14  has a structure in which two bimorphs are stacked. The piezoelectric element  10  includes the first vibration part  12  and the second vibration part  14 . The structure of the first vibration part  12  is the same as that of the piezoelectric element  10  of the first aspect. In  FIG. 14 , an insulating layer  120  is provided between the first vibration part  12  and the second vibration part  14  but is not essential. 
     The second vibration part  14  includes electrodes  121 ,  123 , and  125  and piezoelectric layers  122  and  124 . The electrode  121  installed at a side closest to the first vibration part  12  is connected to the insulating layer  120 . The electrodes  121  and  123  are disposed such that the piezoelectric layer  122  is interposed therebetween in a thickness direction thereof. The electrodes  123  and  125  are disposed such that the piezoelectric layer  124  is interposed therebetween in a thickness direction thereof. Arrows shown inside the piezoelectric layers  122  and  124  indicate polarization directions of the piezoelectric layers  122  and  124 . That is, the polarization directions of the piezoelectric layer  122  and the piezoelectric layer  124  are the same. 
     One terminal of an AC power source V showing a voltage based on an audio signal is connected to the electrodes  101 ,  105 ,  121 , and  125 , and the other terminal thereof is connected to the electrodes  103  and  123 . In other words, voltages having the same phase are applied to the electrodes  101 ,  105 ,  121 , and  125 . Voltages having a phase opposite to that of the voltages input to the electrodes  101 ,  105 ,  121 , and  125  are applied to the electrodes  103  and  123 . Accordingly, voltages in the same direction are applied to the piezoelectric layers  102 ,  104 ,  122 , and  124 . 
     Even in both of the first vibration part  12  and the second vibration part  14 , when one of the two piezoelectric layers contracts in a lateral direction, the other thereof expands in the lateral direction. Accordingly, both the first vibration part  12  and the second vibration part  14  are flexural-vibrated in the same manner as in the first aspect. Further, by setting the polarization direction and the voltage direction as described above, the first vibration part  12  and the second vibration part  14  vibrate in the same phase according to an audio signal. As a result, a vibration generated by the first vibration part  12  and a vibration generated by the second vibration part  14  are reinforced to improve vibration efficiency. 
     According to the present aspect, similarly to the first aspect, the display device  1 , which is capable of improving sound quality by improving sound pressure of a sound generated by the display panel  20 , is provided. In addition, since the piezoelectric element  10  of the present aspect uses two vibration parts, sound pressure can be further improved as compared with the configuration of the first aspect in which one vibration part is provided. 
     Furthermore, in the first aspect, since only one vibration part is provided, a vibration distribution is concentrated in the long side direction of the piezoelectric element  10 , that is, is concentrated one-dimensionally. Therefore, a resonance is likely to occur in the display panel  20 , and noise caused by the resonance may be increased. On the other hand, in the present aspect, since the piezoelectric element  10  includes the first vibration part  12  and the second vibration part  14  extending in different directions, a vibration distribution is two-dimensional, and it is difficult for the vibration distribution to be concentrated on a specific part. Therefore, it is difficult for a resonance to occur in the display panel  20 . Accordingly, in the present aspect, noise due to a resonance in the display panel  20  is reduced, and the display device  1  having improved sound quality is provided. 
     &lt;Fifth Aspect&gt; 
     In the present aspect, a modified example of the layout of the piezoelectric element  10  according to the fourth aspect will be described. Since the basic configuration of a display device  1 , the structure of a piezoelectric element  10 , and the like are the same as those of the fourth aspect, descriptions thereof will be omitted. 
       FIG. 15  is a plan view illustrating a schematic configuration of the piezoelectric element  10  according to a fifth aspect. As shown in  FIG. 15 , in the present aspect, the piezoelectric element  10  is disposed such that long side directions of a first vibration part  12  and a second vibration part  14  are not perpendicular to any one of ends of a display panel  20 . 
     As described in the description of the second aspect, vibrations generated by the first vibration part  12  and the second vibration part  14  and vibration reflected from the end of the display panel  20  may be reinforced to generate a resonance. The resonance may cause noise. On the contrary, in the present aspect, long side directions of the first vibration part  12  and the second vibration part  14  are not perpendicular to any one of the ends of the display panel  20 , and thus a resonance is difficult to generate, thereby reducing noise. 
     Therefore, according to the present aspect, the same effect as in the fourth aspect is obtained, and concurrently, noise, which is caused by a resonance generated due to reflection from an end surface of the display panel  20 , is reduced, thereby providing a display device  1  having improved sound quality. 
     In addition, in the configuration of the fourth aspect, in some cases, a design for sound pressure improvement, frequency characteristic adjustment, or the like is possible by using vibration reinforcement. Therefore, according to design conditions such as required characteristics and design constraints, in some cases, it is desirable to make the long side direction of the piezoelectric element  10  perpendicular to the end of the display panel  20  as in the fourth aspect. 
     &lt;Sixth Aspect&gt; 
     In the present aspect, a modified example of the structure of the piezoelectric element  10  according to the fourth aspect will be described. The basic configuration and the like of a display device  1  are the same as those of the fourth aspect, and thus descriptions thereof will be omitted. 
       FIG. 16  is a plan view illustrating a schematic configuration of a piezoelectric element  10  according to a sixth aspect. As shown in  FIG. 16 , the piezoelectric element  10  of the present aspect includes a first vibration part  12 , a second vibration part  14 , and a third vibration part  16  extending in different directions when viewed from above. Since only the number of layers is increased by adding piezoelectric layers and electrodes corresponding to the third vibration part  16  to the structure of  FIG. 14 , descriptions of a cross-sectional structure of the piezoelectric element  10  will be omitted. 
     In the present aspect, similarly to the first aspect, a display device  1  capable of improving sound pressure of a sound generated by the display panel  20  is provided. In addition, since the piezoelectric element  10  of the present aspect uses three vibration parts, sound pressure can be further improved as compared with the configuration of the fourth aspect having two vibration parts. As described above, the number of vibration parts is not limited to one or two and may be three or more. As the number of vibration parts is increased, sound pressure is improved. 
     In addition, in the present aspect, a vibration distribution is more uniform two-dimensionally as compared with the configuration of the fourth aspect. Therefore, a vibration is more difficult to generate in the display panel  20 . Accordingly, according to the present aspect, noise due to a resonance in the display panel  20  is further reduced, and the display device  1  having improved sound quality is provided. 
     &lt;Seventh Aspect&gt; 
     In the present aspect, a modified example of the structure of the piezoelectric element  10  according to the first aspect will be described. The basic configuration and the like of a display device  1  are the same as those of the first aspect, and thus descriptions thereof will be omitted. 
       FIG. 17  is a plan view illustrating a schematic configuration of a piezoelectric element  10  according to a seventh aspect. The piezoelectric element  10  has a flat plate shape. As shown in  FIG. 17 , the piezoelectric element  10  of the present aspect has a circular shape when viewed from above. An elastic member  30  is connected at a position including a center of a circle of the piezoelectric element  10 . Since a cross-sectional structure of the piezoelectric element  10  is the same as that in  FIGS. 3 and 4 , descriptions thereof will be omitted. 
     In the present aspect, similarly to the first aspect, a display device  1  capable of improving sound pressure of a sound generated by a display panel  20  is provided. 
     In addition, in the present aspect, since the piezoelectric element  10  has the circular shape, the vibration distribution is two-dimensionally uniform. Therefore, a resonance is difficult to generate in the display panel  20  for the same reason as in the case of the fourth aspect. Therefore, according to the present aspect, noise caused by a resonance in the display panel  20  is reduced, thereby providing the display device  1  having improved sound quality. 
     &lt;Eighth Aspect&gt; 
     In the present aspect, a modified example of the structure of the piezoelectric element  10  according to the fifth aspect will be described. The basic configuration and the like of a display device  1  are the same as those of the fifth aspect, and thus descriptions thereof will be omitted. 
       FIG. 18  is a plan view illustrating the layout of a piezoelectric element  10  according to an eighth aspect. As shown in  FIG. 18 , in the present aspect, a plurality of piezoelectric elements  10  are disposed on a display panel  20 . The plurality of piezoelectric elements  10  are arranged in a matrix in an x direction and a z direction. The piezoelectric element  10  is the same as that of the fifth aspect. The plurality of piezoelectric elements  10  are disposed on the display panel  20 , and thus, sound pressure of a sound generated by the display panel  20  is improved as compared to the case of one piezoelectric element. 
     The display panel  20  includes a region R 1  and a region R 2 . The piezoelectric element  10  (first piezoelectric element) in the region R 1  and the piezoelectric element  10  (second piezoelectric element) in the region R 2  have different frequency characteristics. For example, the frequency characteristics of the two piezoelectric elements are made to be different from each other, thereby reducing the bias of a frequency characteristic of a sound generated by the display panel  20 . 
     Since the piezoelectric element  10  has a natural frequency due to a shape or the like thereof, the piezoelectric element  10  may have a biased frequency characteristic such as sound pressure at a specific frequency being increased. When the characteristics of all the piezoelectric elements  10  of the display panel  20  are the same, the biases of the frequency characteristics of the plurality of piezoelectric elements  10  overlap each other, and thus the frequency characteristics of the entire display panel  20  may be biased. In the present aspect, the frequency characteristic of the piezoelectric element  10  in the region R 1  and the frequency characteristic of the piezoelectric element  10  in the region R 2  are made to be different from each other and be leveled off, thereby reducing the bias of the frequency characteristic that may occur due to the above-described factors. 
     The bias of the frequency characteristic of the piezoelectric element  10  is mainly caused by a natural frequency of the piezoelectric element  10 . Therefore, it is desirable to make the natural frequency of the piezoelectric element  10  in the region R 1  different from the natural frequency of the piezoelectric element  10  in the region R 2 . For example, by making the natural frequency of the piezoelectric element  10  in the region R 2  lower than the natural frequency of the piezoelectric element  10  in the region R 1 , the region R 1  functions as a high-pitched tone generating region, and the region R 2  functions as a low-pitched tone generating region. Therefore, the display device  1  capable of generating bass and treble in balance is provided. 
     The natural frequency of the piezoelectric element  10  depends on the shape or material of the piezoelectric element  10 . Accordingly, the piezoelectric element  10  in the region R 1  and the piezoelectric element  10  in the region R 2  may have different natural frequencies by making shapes or sound speeds different from each other. As examples of material properties that affect a sound speed in a material, there are an elastic modulus and density. Therefore, it is desirable to use materials that differ in any one of a sound speed, elastic modulus, and density. In addition, when the piezoelectric element  10  in the region R 1  and the piezoelectric element  10  in the region R 2  are made to differ only in shape, a material may be used in common, which is desirable. 
     As described above, according to the present aspect, the bias of a frequency characteristic of a sound generated the display panel  20  is reduced, thereby providing the display device  1  having improved sound quality. 
     In addition, although the piezoelectric element  10  of the fifth aspect has been described as an example of the piezoelectric element  10  applicable to the present aspect, the present disclosure is not limited thereto. The piezoelectric element  10  having any structure or layout among the first to seventh aspects may be used, or a piezoelectric element having a different structure or layout from that described in the first to seventh aspects may be used. 
     &lt;Other Aspects&gt; 
     The above-described aspects are merely several exemplary aspects to which the present disclosure can be applied, and the technical scope of the present disclosure should not be construed as being limited by the above-described aspects. In addition, the present disclosure may be appropriately modified and changed to be implemented in various aspects without departing from the spirit and scope of the present disclosure. For example, it should be understood that an aspect in which some components of one aspect are added to another aspect, or an aspect in which some components of one aspect are replaced with some components of another aspect are also an aspect to which the present disclosure can be applied. 
     In the above-described aspect, the device configuration of the display device  1  or the like is merely an example and is not limited to the illustrated one. For example, the display device  1  may be not an OLED display but may be a liquid crystal display, a cathode ray tube (CRT) display, or the like. Since it is desirable that the display device  1  is a display capable of efficiently transmitting a vibration from the piezoelectric element  10  to the display panel  20 , an OLED display with few cavities is particularly desirable. 
     According to the present disclosure, sound quality can be improved in a display device which emits a sound from a display panel.