Patent Publication Number: US-2018041839-A1

Title: Piezoelectric speaker

Description:
TECHNICAL FIELD 
     The present invention relates to piezoelectric speakers. 
     BACKGROUND ART 
     There is a piezoelectric speaker including a piezoelectric element that vibrates by an input of an electrical signal, and a vibrating body to which the piezoelectric element is joined through a joining material. 
     For example, Patent Literature 1 discloses a piezoelectric speaker in which a joining material has a protruding portion that protrudes from an outer edge of a piezoelectric element when a vibrating body is seen in a planar view. At least a part of the protruding portion has a wavy shape. Hereby, frequency characteristics of a sound pressure can be flattened. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1 
     International Patent Publication No. WO2014/045645 
     SUMMARY OF INVENTION 
     Technical Problem 
     By the way, there is required a speaker that reproduces sound in a high frequency domain. However, the piezoelectric speaker disclosed in Patent Literature 1 cannot maintain good sound pressure characteristics in the high frequency domain in some cases. 
     The present invention provides a piezoelectric speaker that has good frequency characteristics of a sound pressure in the high frequency domain. 
     Solution to Problem 
     A piezoelectric speaker according to the present invention includes: a piezoelectric element; and a metal vibration part to which the piezoelectric element is made to adhere through an adhesive part. In the piezoelectric speaker, the piezoelectric element is a substantially rectangular plate, the metal vibration part includes a substantially rectangular plate-shaped part that is vibrated by the piezoelectric element, and a frequency of a natural vibration mode of the piezoelectric element and a frequency of a natural vibration mode of the metal vibration part are set to be different from each other. 
     According to such a configuration, the piezoelectric speaker has good frequency characteristics of a sound pressure in a high frequency domain. 
     In addition, a relation between an area Ap of the piezoelectric element and an area Am of the rectangular plate-shaped part of the metal vibration part may satisfy 1.1≦Am/Ap≦10. 
     In addition, the adhesive part may be an elastic body. 
     In addition, a mechanical quality factor Qm of a vibrating body in which the piezoelectric element and the adhesive part are integrated with each other may satisfy Qm≦5.0. 
     In addition, the piezoelectric speaker further includes a case at which the metal vibration part is provided, the case having a sound emitting hole, and the sound emitting hole may have a horn shape. 
     In addition, the rectangular plate-shaped part may have a frequency adjusting hole. 
     In addition, the piezoelectric speaker further includes a case, and the metal vibration part may be made to adhere to the case through an elastic body. 
     A plurality of the piezoelectric elements may be made to adhere to the metal vibration part through the adhesive part. 
     Frequencies of natural vibration modes of the plurality of piezoelectric elements may be different from each other. 
     The metal vibration part may have one metal plate, and the plurality of piezoelectric elements may be made to adhere to the metal plate through the adhesive part. 
     The plurality of piezoelectric elements may be attached to the same surface of the metal plate. 
     The piezoelectric speaker may further include a case, and an electromagnetic speaker arranged inside the case. 
     The piezoelectric element may be arranged inside the case. 
     A mounting surface of the electromagnetic speaker and a mounting surface of the piezoelectric element may be the same surface of the case. 
     The piezoelectric element may be arranged outside the case, and the mounting surface of the electromagnetic speaker and the mounting surface of the piezoelectric element may be opposed surfaces of the case. 
     The metal vibration part may serve as a side plate or a back plate of the case. 
     The piezoelectric speaker further includes a cover that covers an opening of the case, and the metal vibration part may be fixed to the case or the cover through an elastic member. 
     The metal vibration part may include a metal plate having a thickness of 10 to 300 μm. 
     A piezoelectric speaker according to the other aspect of the present invention includes: a housing that has a front plate having a sound emitting hole, a back plate opposed to the front plate, and side plates between the front plate and the back plate; an electromagnetic speaker provided inside the housing; and a piezoelectric element attached to the housing. 
     The piezoelectric element is fixed to the housing through an adhesive part, and the adhesive part may be an elastic body. 
     The piezoelectric element may be arranged inside the housing. 
     A mounting surface of the electromagnetic speaker and a mounting surface of the piezoelectric element may be the same surface of the housing. 
     The piezoelectric element may be arranged outside the housing, and the mounting surface of the electromagnetic speaker and the mounting surface of the piezoelectric element may be opposed surfaces of the housing. 
     The mounting surface of the piezoelectric element may be a metal plate. 
     The metal plate may be fixed to the side plate, the front plate, or the back plate through an elastic member. 
     A thickness of the metal plate may be 10 to 300 μm. 
     The side plate, the front plate, or the back plate may serve as the mounting surface of the piezoelectric element, and may include a metal material and a resin material. 
     Advantageous Effects of Invention 
     According to the present invention, there can be provided a piezoelectric speaker that has good frequency characteristics of a sound pressure in a high frequency domain. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a piezoelectric speaker according to an embodiment 1; 
         FIG. 2  is a cross-sectional view of the piezoelectric speaker according to the embodiment 1; 
         FIG. 3  is a bottom view of a main portion of the piezoelectric speaker according to the embodiment 1; 
         FIG. 4  is a graph showing a sound pressure with respect to a frequency; 
         FIG. 5  is a graph showing a sound pressure with respect to a frequency; 
         FIG. 6  is a bottom view of a main portion of a piezoelectric speaker according to an embodiment 2; 
         FIG. 7A  is a cross-sectional view of a modified example 1 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 7B  is a cross-sectional view of the modified example 1 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 7C  is a cross-sectional view of the modified example 1 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 7D  is a cross-sectional view of the modified example 1 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 8A  is a cross-sectional view of a modified example 2 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 8B  is a cross-sectional view of the modified example 2 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 8C  is a cross-sectional view of the modified example 2 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 8D  is a cross-sectional view of the modified example 2 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 9A  is a cross-sectional view of a modified example 3 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 9B  is a cross-sectional view of the modified example 3 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 9C  is a cross-sectional view of the modified example 3 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 9D  is a cross-sectional view of the modified example 3 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 10A  is a cross-sectional view of a modified example 4 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 10B  is a cross-sectional view of the modified example 4 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 10C  is a cross-sectional view of the modified example 4 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 10D  is a cross-sectional view of the modified example 4 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 11A  is a cross-sectional view of a modified example 5 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 11B  is a cross-sectional view of the modified example 5 of the main portion of the piezoelectric speaker according to the embodiment 2; 
         FIG. 12A  is an exploded perspective view of a piezoelectric speaker according to an embodiment 3; 
         FIG. 12B  is an exploded perspective view of a modified example of the piezoelectric speaker according to the embodiment 3; 
         FIG. 12C  is an exploded perspective view of a modified example of the piezoelectric speaker according to the embodiment 3; 
         FIG. 13  is a graph showing a sound pressure with respect to a frequency of an Example of the piezoelectric speaker according to the embodiment 1; 
         FIG. 14  is a graph showing a sound pressure with respect to a frequency of a related speaker; 
         FIG. 15  is a bottom view of a related piezoelectric speaker; 
         FIG. 16  is a graph showing a sound pressure with respect to a frequency of the related piezoelectric speaker; 
         FIG. 17  is a cross-sectional view of a piezoelectric speaker according to an embodiment 4; 
         FIG. 18  is a bottom view of a main portion of the piezoelectric speaker according to the embodiment 4; 
         FIG. 19  is a graph showing a sound pressure with respect to a frequency of the piezoelectric speaker according to the embodiment 4; 
         FIG. 20  is a perspective view showing a configuration of a piezoelectric speaker according to an embodiment 5; 
         FIG. 21  is a cross-sectional view of a main portion of the piezoelectric speaker according to the embodiment 5; 
         FIG. 22  is a graph showing a sound pressure with respect to a frequency of the piezoelectric speaker according to the embodiment 5; 
         FIG. 23  is a cross-sectional view of a main portion of a piezoelectric speaker according to a modified example 6 of the embodiment 5; 
         FIG. 24  is a cross-sectional view of a main portion of a piezoelectric speaker according to a modified example 7 of the embodiment 5; and 
         FIG. 25  is a cross-sectional view of a main portion of a piezoelectric speaker according to a modified example 8 of the embodiment 5. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     A piezoelectric speaker according to an embodiment 1 will be explained with reference to  FIGS. 1 to 5 .  FIG. 1  is a perspective view of the piezoelectric speaker according to the embodiment 1.  FIG. 2  is a cross-sectional view of the piezoelectric speaker according to the embodiment 1.  FIG. 3  is a bottom view of a main portion of the piezoelectric speaker according to the embodiment 1.  FIGS. 4 and 5  are graphs each showing a sound pressure with respect to a frequency. 
     As shown in  FIGS. 1 to 3 , a piezoelectric speaker  100  includes: a cover  5 ; a case  6 ; and a piezoelectric vibration unit  7 . 
     The cover  5  is in a plate shape that has a sound emitting hole  5   a  in a center thereof. The sound emitting hole  5   a  penetrates through the cover  5 , and a cross-sectional shape of the sound emitting hole  5   a  becomes larger as it goes toward an outside of the piezoelectric speaker  100 . The sound emitting hole  5   a , for example, has a horn shape. The case  6  is a rectangular parallelepiped housing that includes an opening  6   a  in one surface thereof. Note that the case  6  may be a frame-shaped body, and that the frame shape is a rectangular shape, for example, a substantially quadrangular shape, a substantially oblong shape, a substantially square shape, and a substantially trapezoidal shape. The opening  6   a  is closed by the cover  5 . The case  6  equipped with the cover  5  has a width Lx, a depth Ly, and a height Lz. The width Lx is, for example, 10 to 20 mm, the depth Ly is, for example, 5 to 10 mm, and the height Lz is, for example, 2 to 10 mm. 
     The piezoelectric vibration unit  7  is made to adhere to an inner principal surface of the cover  5  through an adhesive part  4 . Specifically, the piezoelectric vibration unit  7  is made to adhere to the inner principal surface of the cover  5  so as to close the sound emitting hole  5   a.    
     The adhesive part  4  may just be a viscoelastic body, a viscous body, or a plate-shaped body or a band-shaped body having an adhesive property on both-side principal surfaces, while having a predetermined elastic coefficient. The adhesive part  4  may just be an elastic body. As the adhesive part  4 , there is included, for example, a plate-shaped body or synthetic resin, such as silicone resin or epoxy resin that is formed using a double-sided tape. The adhesive part  4  preferably includes a material having such mechanical properties that vibrations of the piezoelectric vibration unit  7  are maintained to have magnitude required as a piezoelectric speaker. When the piezoelectric speaker  100  is seen from the cover  5  side, the adhesive part  4  may be a frame-shaped body not exposed from the sound emitting hole  5   a . The adhesive part  4  is arranged so as to cover an outer edge  2   h  of a metal diaphragm  2 . When the piezoelectric speaker  100  is seen from the cover  5  side, the outer edge  2   h  is covered with the cover  5 . In addition, the adhesive part  4  preferably has the predetermined elastic coefficient since an apparent mechanical quality factor Qm 21  (mentioned later) of the metal diaphragm  2  can be decreased. 
     The piezoelectric vibration unit  7  includes: a piezoelectric element  1 ; the metal diaphragm  2 ; and an adhesive part  3 . The piezoelectric element  1  is made to adhere to the metal diaphragm  2  through the adhesive part  3 . The piezoelectric element  1  is a vibrator that includes a substantially rectangular plate including a single ceramics plate. Note that the piezoelectric element  1  may be a stacked type, a bimorph type, and a unimorph type. The piezoelectric element  1  is electrically connected to an amplifier (illustration is omitted) etc., and vibrates by supply of an electrical signal for reproducing sound. 
     The metal diaphragm  2  is a substantially rectangular plate (it may be referred to as a rectangular plate-shaped part) that has a larger area than the piezoelectric element  1 . The metal diaphragm  2 , for example, includes steel and a copper alloy. As the steel and the copper alloy, there are included, for example, stainless steel, brass, phosphor bronze. The metal diaphragm  2  vibrates by vibrations of the piezoelectric element  1 . 
     The adhesive part  3  includes the same type of material as the adhesive part  4 . The metal diaphragm  2 , for example, has a thickness of 0.5 to 1.5 mm. A size, a shape, a material, etc. of the metal diaphragm  2  are decided so that a natural vibration mode of the metal diaphragm  2  and a natural vibration mode of the piezoelectric element  1  may be set to have different frequencies. In other words, either one of the frequency (a resonance frequency) of the natural vibration mode of the metal diaphragm  2  and the frequency of the natural vibration mode of the piezoelectric element  1  is higher. 
     (Area Ratio) 
     Subsequently, there will be explained a relation between an area of the piezoelectric element  1  and an area of the metal diaphragm  2 . 
     A relational expression of an area Ap of the piezoelectric element  1  and an area Am of the metal diaphragm  2  is determined by using the following Formula 1. 
       1.1≦ Am/Ap≦ 10  (Formula 1)
 
     Hereby, the natural vibration mode of the metal diaphragm  2  and the natural vibration mode of the piezoelectric element  1  are set to have different frequencies more reliably. For example, as shown in  FIG. 4 , the natural vibration mode of the metal diaphragm  2  ranges from 10 to 20 kHz, the natural vibration mode of the piezoelectric element  1  is approximately 30 kHz, and thus they are set to have different frequencies. 
     Here, since the frequency of the natural vibration mode of the metal diaphragm  2  and the frequency of the natural vibration mode of the piezoelectric element  1  are different from each other, an amplitude when the piezoelectric vibration unit  7  vibrates the metal diaphragm  2  is almost the same as amplitudes corresponding to an elastic coefficient of the metal diaphragm  2  and an elastic coefficient of the piezoelectric element  1 , respectively, or rarely exceeds the amplitudes corresponding to the elastic coefficient of the metal diaphragm  2  and the elastic coefficient of the piezoelectric element  1 , respectively. In addition, even if the piezoelectric vibration unit  7  vibrates the metal diaphragm  2  almost to elastic limits of the metal diaphragm  2  and the piezoelectric element  1 , a total harmonic distortion is hard to be large, and harsh sound is hard to occur. 
     An SN ratio SN 1 , i.e. a relational expression between a sound pressure SP 1  and a total harmonic distortion THD 1 , is determined using the following Formula 2. 
         SN 1= SP 1− THD 1  (Formula 2)
 
     For example, as shown in  FIG. 4 , the SN ratio SN 1  at 40 kHz is determined using Formula 2, and it is approximately 60 dB sq1. 
     Since the frequency of the natural vibration mode of the metal diaphragm  2  and the frequency of the natural vibration mode of the piezoelectric element  1  are different from each other, the total harmonic distortion can be suppressed from increasing, and thereby sound can be reproduced with a high SN ratio at a target frequency. 
     Further, a frequency band of the natural vibration mode of the metal diaphragm  2  is cut using a filter circuit, such as a high-pass filter, and thereby only a reproduction frequency range of the high SN ratio can be used. Note that in a case where the filter circuit, such as the high-pass filter, is used, a rigidity k 2  of the metal diaphragm  2  is desirably 5 to 30, and a thickness t 2  [mm] of the metal diaphragm  2  is desirably 0.05 to 0.3. 
     Mechanical Quality Factor Qm of Metal Diaphragm 
     Subsequently, the mechanical quality factor Qm of a metal diaphragm will be explained. 
     Although the metal diaphragm  2  has a unique mechanical quality factor Qm 20 , the metal diaphragm  2  has adhered to the cover  5  through the adhesive part  4 , and thus the apparent mechanical quality factor Qm 21  of the metal diaphragm  2  is lower than the unique mechanical quality factor Qm 20 . The apparent mechanical quality factor Qm 21  of the metal diaphragm  2  may be referred to as the mechanical quality factor Qm 21  of a vibrating body in which the metal diaphragm  2  and the adhesive part  4  are integrated with each other. Materials and shapes of the piezoelectric element  1 , the metal diaphragm  2 , and the adhesive part  3  are desirably set so that the apparent mechanical quality factor Qm 21  of the metal diaphragm  2  can satisfy the following Formula 3. 
         Qm 21≦5.0  (Formula 3)
 
     Formula 3 is preferably satisfied since a sound pressure characteristic curve is flattened. 
     In addition, the materials and the shapes of the piezoelectric element  1 , the metal diaphragm  2 , and the adhesive part  3  are desirably set so that the apparent mechanical quality factor Qm 21  of the metal diaphragm  2  can satisfy Formula 3 and the following Formula 4. 
         Qm 21≧0.5  (Formula 4)
 
     In addition, since the piezoelectric element  1  has adhered to the metal diaphragm  2  through the adhesive part  3 , a band of a frequency becomes wide. Here, the rigidity k 2  of the metal diaphragm  2  is preferably 5 to 20, and the metal diaphragm  2  is desirably, for example, a plate including brass or phosphor bronze. 
     Since the apparent mechanical quality factor Qm 21  of the metal diaphragm  2  is low, and the piezoelectric element  1  has adhered to the metal diaphragm  2  through the adhesive part  3 , sound can be reproduced in a wide frequency band, and with a flat sound pressure characteristic curve. A sound pressure characteristic curve of one example of the piezoelectric speaker  100  was measured, and the sound pressure characteristic curve was shown in  FIG. 5 . 
     (Comparative Example “Electromagnetic-Type Speaker”) 
     By the way, as shown in  FIG. 14 , a sound pressure and a total harmonic distortion with respect to a frequency were measured using one example of an electromagnetic-type speaker that vibrates a diaphragm by supplying an electrical signal to a voice coil to thereby generate a magnetic moment. An SN ratio SN 2  of this one example was approximately 50 kHz, which is smaller compared with the SN ratio SN 1  of one example of the piezoelectric speaker  100 . The electromagnetic-type speaker reproduces sound having a high frequency of equal to or higher than 20 kHz using the voice coil. In that case, an electric power given by the rise of an impedance in the high frequency is converted into heat instead of an audio signal. Accordingly, the electromagnetic-type speaker is considered to be difficult to achieve a high sound pressure and a high SN ratio, compared with the piezoelectric speaker  100 . 
     (Comparative Example “Circular-Type Speaker”) 
     In addition, a sound pressure with respect to a frequency was measured using one example of a piezoelectric vibration unit  907  shown in  FIG. 15 . The piezoelectric vibration unit  907  includes a piezoelectric element  901  and a metal diaphragm  902 . 
     The piezoelectric element  901  has the same configuration as the piezoelectric element  1  (refer to  FIG. 2 ) except for being a disc-shaped body. The metal diaphragm  902  has the same configuration as the metal diaphragm  2  (refer to  FIG. 2 ) except for being a disc-shaped body. The piezoelectric vibration unit  907  is arranged inside a cover  905  (illustration is omitted) and the case  6  (refer to  FIG. 2 ), and thereby a piezoelectric speaker  900  (illustration is omitted) is formed. Note that the cover  95  has the same configuration as the cover  5  except for having a sound emitting hole with a circular cross section. A piezoelectric element having a diameter of 20 mm and a thickness of 0.1 mm was used as the piezoelectric element  901 , and a metal diaphragm including stainless steel having a diameter of 25 mm and a thickness of 0.1 mm was used as the metal diaphragm  902 . As shown in  FIG. 16 , a mechanical quality factor Qm 91  of a resonance frequency was equal to or more than 10. A sound pressure characteristic curve shown in  FIG. 16  has less flat portions, i.e. has more portions with rise and fall, compared with the sound pressure curve shown in  FIG. 5 . That is, the piezoelectric speaker  900  is difficult to obtain a flat sound pressure characteristic curve compared with the piezoelectric speaker  100 . 
     Here, a metal vibration part having a rectangular shape has more different natural vibration modes depending on directions of its principal surface compared with a metal vibration part having a circular shape. Note that the principal surface of the metal vibration part having the rectangular shape, for example, has a Y direction and an X direction as shown in  FIG. 3 . Therefore, the mechanical quality factor Qm is low. Further, a frequency can be easily adjusted by adjusting sizes of the metal vibration part and the piezoelectric element. 
     Hereinbefore, according to the piezoelectric speaker according to the embodiment 1, sound can be reproduced with good sound pressure characteristics in the high frequency domain. For example, reproduced sound has a high sound pressure and a high S/N ratio in a high frequency range, for example, from 20 to 70 kHz. In addition, the sound pressure characteristic curve is flat, and the frequency band of the piezoelectric speaker is wide. 
     Embodiment 2 
     A piezoelectric speaker according to an embodiment 2 will be explained with reference to  FIG. 6 .  FIG. 6  is a bottom view of a main portion of the piezoelectric speaker according to the embodiment 2. In the following explanation, explanation of the same configuration as the piezoelectric speaker according to the embodiment 1 is appropriately omitted, and different configurations are explained. Note that modified examples 1 to 5 of the piezoelectric speaker according to the embodiment 2, a piezoelectric speaker according to an embodiment 3, and modified examples thereof, which will be mentioned later, are similarly explained. 
     As shown in  FIG. 6 , a piezoelectric speaker  200  (illustration is omitted) has the same configuration as the piezoelectric speaker  100  except for the piezoelectric vibration unit  7 . The piezoelectric speaker  200  includes a piezoelectric vibration unit  207 . The piezoelectric vibration unit  207  has the same configuration as the piezoelectric vibration unit  7  except for the metal diaphragm  2 . The piezoelectric vibration unit  207  includes a metal diaphragm  22 . The metal diaphragm  22  has the same configuration as the metal diaphragm  2  except for having frequency adjusting holes  22   b  near four corners. An effective length of the metal diaphragm  22  and a width of the metal diaphragm  22  can be adjusted by changing the number and a size of the frequency adjusting holes  22   b . Hereby, a frequency can be easily adjusted. 
     The above-described frequency adjusting method by the change in the number and the size of the frequency adjusting holes  22   b  can vibrate the metal diaphragm more easily, compared with a frequency adjusting method for adjusting the frequency by providing an additional member at the metal diaphragm. In addition, according to the above-described frequency adjusting method by the change in the number and the size of the frequency adjusting holes  22   b , even if the piezoelectric speaker  200  is placed on an electromagnetic speaker, esp. a diaphragm thereof, the piezoelectric speaker  200  hardly cuts off reproduced sound by the electromagnetic speaker. In addition, the frequency adjusting hole  22   b  is formed by using etching processing or press working. Accordingly, the above-described frequency adjusting method by the number and the size of the frequency adjusting holes  22   b  can be carried out at low cost. 
     Hereinbefore, according to the above-described piezoelectric speaker according to the embodiment 2, sound can be reproduced with good sound pressure characteristics in a high frequency domain similarly to the piezoelectric speaker according to the embodiment 1. Further, since the metal diaphragm having the frequency adjusting hole is used, a frequency can be easily adjusted. 
     Modified Example 1 of Piezoelectric Vibration Unit 
     Next, there will be explained a modified example 1 of the piezoelectric vibration unit  207  of the piezoelectric speaker  200  according to the embodiment 2 with reference to  FIGS. 6 and 7A to 7D .  FIGS. 7A to 7D  are cross-sectional views of the modified example 1 of the main portion of the piezoelectric speaker according to the embodiment 2. 
     As shown in  FIGS. 6 and 7A , there is a piezoelectric vibration unit  217  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  217  has the same configuration as the piezoelectric vibration unit  207  except for having holders  9 . The piezoelectric vibration unit  217  includes the holders  9 , and ends of the metal diaphragm  22  have adhered to the holders  9  through the adhesive parts  3 . In addition, the metal diaphragm  22  is held by the holders  9 . The holder  9  is a wall body that extends from a bottom of the case  6  (refer to  FIG. 2 ) toward the metal diaphragm  22 . The holders  9  are arranged to cover the surroundings of the piezoelectric element  1  so that neither water nor foreign substances may attach to the piezoelectric element  1 . Since the piezoelectric vibration unit  217  has the holders  9 , it suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . 
     Meanwhile, as shown in  FIG. 7B , there is a piezoelectric vibration unit  227  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  227  has a metal diaphragm  32  having the same shape as a shape in which the metal diaphragm  22  and the holders  9  are integrated with each other. 
     Since in the piezoelectric vibration unit  227 , a body  32   a  (it may be referred to as a substantially rectangular plate-shaped part) and holding parts  32   b  are integrated with each other, the piezoelectric vibration unit  227  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since in the piezoelectric vibration unit  227 , the body  32   a  and the holding parts  32   b  are integrated with each other, the piezoelectric vibration unit  227  can be manufactured at low cost. 
     Further, as shown in  FIG. 7C , there is a piezoelectric vibration unit  237  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  237  has a metal vibration part  42 . The metal vibration part  42  has the same configuration as the metal diaphragm  32  (refer to  FIG. 7B ) except for having a bottom  42   c . The metal vibration part  42  includes: a body  42   a ; holding parts  42   b ; and the bottom  42   c . The body  42   a  has the same configuration as the body  32   a , and the holding part  42   b  has the same configuration as the holding part  32   b . The bottom  42   c  is integrated with the holding parts  42   b , and is a plate-shaped body that is opposed to the body  42   a.    
     Since in the piezoelectric vibration unit  237 , the body  42   a , the holding parts  42   b , and the bottom  42   c  are integrated with each other, the piezoelectric vibration unit  237  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since in the piezoelectric vibration unit  237 , the body  42   a , the holding parts  42   b , and the bottom  42   c  are integrated with each other, the piezoelectric vibration unit  237  has a high rigidity. 
     Further, as shown in  FIG. 7D , there is a piezoelectric vibration unit  247  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  247  has the same configuration as the piezoelectric vibration unit  227  (refer to  FIG. 7B ) except for including a bottom plate  8 . The piezoelectric vibration unit  247  includes the bottom plate  8 . The bottom plate  8  is provided under lower ends of the holding parts  32   b , and is a plate-shaped body that is opposed to the body  32   a . An outer edge of the bottom plate  8  and the lower ends of the holding parts  32   b  may be installed so as to abut against each other. Since the piezoelectric vibration unit  247  has the bottom plate  8 , it further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  227 . In addition, since the piezoelectric vibration unit  247  has the bottom plate  8 , it has a higher rigidity compared with the piezoelectric vibration unit  227  (refer to  FIG. 7B ). 
     Modified Example 2 of Piezoelectric Vibration Unit 
     Next, there will be explained a modified example 2 of the piezoelectric vibration unit  207  of the piezoelectric speaker  200  according to the embodiment 2 with reference to  FIGS. 8A to 8D .  FIGS. 8A to 8D  are cross-sectional views of the modified example 2 of the main portion of the piezoelectric speaker according to the embodiment 2. 
     As shown in  FIG. 8A , there is a piezoelectric vibration unit  317  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  317  has the same configuration as the piezoelectric vibration unit  217  except for a metal diaphragm  52  and stepped holders  19 . The piezoelectric vibration unit  317  includes the metal diaphragm  52  and the stepped holders  19 , and the metal diaphragm  52  has adhered to the stepped holders  19  through the adhesive parts  3 . In addition, the metal diaphragm  52  is held by the stepped holders  19 . The stepped holder  19  is a wall body that extends from the bottom of the case  6  (refer to  FIG. 2 ) toward the metal diaphragm  52 , and stepwisely bends in the middle. The stepped holders  19  are arranged to cover the surroundings of the piezoelectric element  1  so that neither water nor foreign substances may attach to the piezoelectric element  1 . 
     Since the piezoelectric vibration unit  317  has the stepped holders  19 , it suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since the piezoelectric vibration unit  317  has the stepped holders  19 , it has a higher pressure resistance compared with the piezoelectric vibration unit  217 . 
     Meanwhile, as shown in  FIG. 8B , there is a piezoelectric vibration unit  327  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  327  has a metal diaphragm  62  having the same shape as a shape in which the metal diaphragm  52  and the stepped holders  19  are integrated with each other, similarly to the piezoelectric vibration unit  227  (refer to  FIG. 7B ). 
     Since in the piezoelectric vibration unit  327 , a body  62   a  (it may be referred to as a substantially rectangular plate-shaped part) and holding parts  62   b  are integrated with each other, the piezoelectric vibration unit  327  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  317  (refer to  FIG. 8A ). In addition, since in the piezoelectric vibration unit  327 , the body  62   a  and the holding parts  62   b  are integrated with each other, the piezoelectric vibration unit  327  can be manufactured at lower cost compared with the piezoelectric vibration unit  317 . 
     In addition, as shown in  FIG. 8C , there is a piezoelectric vibration unit  337  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  337  has a metal vibration part  72 . The metal vibration part  72  has the same configuration as the metal diaphragm  62  (refer to  FIG. 8B ) except for having a bottom  72   c . The metal vibration part  72  includes: a body  72   a ; holding parts  72   b ; and the bottom  72   c . The body  72   a  has the same configuration as the body  62   a , and the holding part  72   b  has the same configuration as the holding part  62   b . The bottom  72   c  is integrated with the holding parts  72   b , and is a plate-shaped body that is opposed to the body  72   a.    
     Since in the piezoelectric vibration unit  337 , the body  72   a , the holding parts  72   b , and the bottom  72   c  are integrated with each other, the piezoelectric vibration unit  337  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  317 . In addition, since in the piezoelectric vibration unit  337 , the body  72   a , the holding parts  72   b , and the bottom  72   c  are integrated with each other, the piezoelectric vibration unit  337  has a higher rigidity compared with the piezoelectric vibration unit  317 . 
     In addition, as shown in  FIG. 8D , there is a piezoelectric vibration unit  347  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  347  has the same configuration as a unit in which the bottom plate  8  is added to the piezoelectric vibration unit  327  (refer to  FIG. 8B ). 
     Since the piezoelectric vibration unit  347  has the bottom plate  8 , it further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  327  (refer to  FIG. 8B ). In addition, since the piezoelectric vibration unit  247  has the bottom plate  8 , it has a higher rigidity compared with the piezoelectric vibration unit  327 . 
     Modified Example 3 of Piezoelectric Vibration Unit 
     Next, there will be explained a modified example 3 of the piezoelectric vibration unit  207  of the piezoelectric speaker  200  according to the embodiment 2 with reference to  FIGS. 9A to 9D .  FIGS. 9A to 9D  are cross-sectional views of the modified example 3 of the main portion of the piezoelectric speaker according to the embodiment 2. 
     As shown in  FIG. 9A , there is a piezoelectric vibration unit  417  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  417  has the same configuration as the piezoelectric vibration unit  217  (refer to  FIG. 7A ) except for having a metal diaphragm  82 . The piezoelectric vibration unit  417  includes the metal diaphragm  82 , and the metal diaphragm  82  includes a body  82   a , and gripped portions  82   d  that extend from ends of the body  82   a . The body  82   a  has the same configuration as the metal diaphragm  22 , and the ends of the body  82   a  have adhered to the holders  9  through the adhesive parts  3 . The gripped portions  82   d  extend toward side walls of the case  6 . By the way, the piezoelectric vibration unit  417  is mounted in the case  6 , and thereby the piezoelectric speaker  200  can be assembled. Here, since the gripped portion  82   d  has a shape that extends from the end of the body  82   a , it is easy to grip. In addition, the shape of the gripped portion  82   d  may be changed as needed, in order to make the piezoelectric vibration unit  417  easy to mount in the case  6 . 
     Since the piezoelectric vibration unit  417  has the metal diaphragm  82  and the holders  9 , it suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since the piezoelectric vibration unit  417  has the metal diaphragm  82 , it can be mounted more easily compared with the piezoelectric vibration unit  217  (refer to  FIG. 7A ). 
     Meanwhile, as shown in  FIG. 9B , there is a piezoelectric vibration unit  427  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  427  has a metal diaphragm  92  having the same shape as a shape in which the metal diaphragm  82  and the holders  9  are integrated with each other. 
     Since in the piezoelectric vibration unit  427 , a body  92   a , holding parts  92   b , and gripped portions  92   d  are integrated with each other, the piezoelectric vibration unit  427  further suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  417  (refer to  FIG. 9A ). In addition, since in the piezoelectric vibration unit  427 , the body  92   a , the holding parts  92   b , and the gripped portions  92   d  are integrated with each other, the piezoelectric vibration unit  427  can be manufactured at lower cost compared with the piezoelectric vibration unit  417  (refer to  FIG. 9A ). 
     In addition, as shown in  FIG. 9C , there is a piezoelectric vibration unit  437  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  437  has a metal vibration part  102 . The metal vibration part  102  has the same configuration as the metal diaphragm  92  (refer to  FIG. 9B ) except for having a bottom  102   c . The metal vibration part  102  includes: a body  102   a ; holding parts  102   b ; and the bottom  102   c . The body  102   a  has the same configuration as the body  92   a , and the holding part  102   b  has the same configuration as the holding part  92   b . The bottom  102   c  is integrated with the holding parts  102   b , and is a plate-shaped body that is opposed to the body  102   a.    
     Since in the piezoelectric vibration unit  437 , the body  102   a , the holding parts  102   b , and the bottom  102   c  are integrated with each other, the piezoelectric vibration unit  437  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  417 . In addition, since in the piezoelectric vibration unit  437 , the body  102   a , the holding parts  102   b , and the bottom  102   c  are integrated with each other, the piezoelectric vibration unit  437  has a higher rigidity compared with the piezoelectric vibration unit  417 . 
     In addition, as shown in  FIG. 9D , there is a piezoelectric vibration unit  447  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  447  has the same configuration as a unit in which the bottom plate  8  is added to the piezoelectric vibration unit  427  (refer to  FIG. 9B ). 
     Since the piezoelectric vibration unit  447  has the bottom plate  8 , similarly to the piezoelectric vibration unit  247  (refer to  FIG. 7D ), the piezoelectric vibration unit  447  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  427  (refer to  FIG. 9D ). In addition, since the piezoelectric vibration unit  447  has the bottom plate  8 , it has a higher rigidity compared with the piezoelectric vibration unit  427 . 
     Modified Example 4 of Piezoelectric Vibration Unit 
     Next, there will be explained a modified example 4 of the piezoelectric vibration unit  207  of the piezoelectric speaker  200  according to the embodiment 2 with reference to  FIGS. 10A to 10D .  FIGS. 10A to 10D  are cross-sectional views of the modified example 4 of the main portion of the piezoelectric speaker according to the embodiment 2. 
     As shown in  FIG. 10A , there is a piezoelectric vibration unit  517  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  517  has the same configuration as the piezoelectric vibration unit  217  except for having tapered holders  29  instead of the holders  9 . The piezoelectric vibration unit  517  includes the tapered holders  29 , and ends of the metal diaphragm  22  have adhered to the tapered holders  29  through the adhesive parts  3 . In addition, the metal diaphragm  22  is held by the tapered holders  29 . The tapered holder  29  is a wall body that extends from the bottom of the case  6  (refer to  FIG. 2 ) toward the metal diaphragm  22 . The tapered holder  29  has a tapered shape, which is a shape whose cross-sectional area becomes larger toward the metal diaphragm  22  from the bottom of the case  6 . More specifically, the tapered shape inclines on the piezoelectric element  1  side. The holders  9  are arranged to cover the surroundings of the piezoelectric element  1  so that neither water nor foreign substances may attach to the piezoelectric element  1 . 
     Since the piezoelectric vibration unit  517  has the tapered holders  29 , it suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . 
     Meanwhile, as shown in  FIG. 10B , there is a piezoelectric vibration unit  527  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  527  has a metal diaphragm  112  having the same shape as a shape in which the metal diaphragm  22  and the tapered holders  29  are integrated with each other, similarly to the piezoelectric vibration unit  227  (refer to  FIG. 7B ). 
     Since in the piezoelectric vibration unit  527 , a body  112   a  and holding parts  12   b  are integrated with each other, the piezoelectric vibration unit  527  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  517  (refer to  FIG. 10A ). In addition, since in the piezoelectric vibration unit  527 , the body  112   a  and the holding parts  12   b  are integrated with each other, it can be manufactured at lower cost compared with the piezoelectric vibration unit  517 . 
     In addition, as shown in  FIG. 10C , there is a piezoelectric vibration unit  537  that is a modified example of the piezoelectric vibration unit  207 . 
     The piezoelectric vibration unit  537  has a metal vibration part  122 . The metal vibration part  122  has the same configuration as the metal diaphragm  112  (refer to  FIG. 10B ) except for having a bottom  122   c . The metal vibration part  122  includes: a body  122   a ; holding parts  122   b ; and the bottom  122   c . The body  122   a  has the same configuration as the body  112   a , and the holding part  122   b  has the same configuration as the holding part  112   b . The bottom  122   c  is integrated with the holding parts  122   b , and is a plate-shaped body that is opposed to the body  122   a.    
     Since in the piezoelectric vibration unit  537 , the body  12   a , the holding parts  122   b , and the bottom  122   c  are integrated with each other, the piezoelectric vibration unit  537  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  517  (refer to  FIG. 10A ). In addition, since in the piezoelectric vibration unit  537 , the body  102   a , the holding parts  102   b , and the bottom  102   c  are integrated with each other, the piezoelectric vibration unit  537  has a higher rigidity compared with the piezoelectric vibration unit  517 . 
     In addition, as shown in  FIG. 10D , there is a piezoelectric vibration unit  547  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  547  has the same configuration as a unit in which the bottom plate  8  is added to the piezoelectric vibration unit  527  (refer to  FIG. 10B ). 
     Since the piezoelectric vibration unit  547  has the bottom plate  8 , similarly to the piezoelectric vibration unit  247  (refer to  FIG. 7D ), the piezoelectric vibration unit  547  further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 , compared with the piezoelectric vibration unit  527  (refer to  FIG. 10D ). In addition, since the piezoelectric vibration unit  547  has the bottom plate  8 , it has a higher rigidity compared with the piezoelectric vibration unit  527 . 
     Modified Example 5 of Piezoelectric Vibration Unit 
     Next, there will be explained a modified example 5 of the piezoelectric vibration unit  207  of the piezoelectric speaker  200  according to the embodiment 2 with reference to  FIGS. 11A and 11B .  FIGS. 11A and 11B  are cross-sectional views of the modified example 5 of the main portion of the piezoelectric speaker according to the embodiment 2. 
     As shown in  FIG. 11A , there is a piezoelectric vibration unit  637  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  637  has the same configuration as the piezoelectric vibration unit  237  (refer to  FIG. 7C ) except for having a metal vibration part  142 . The piezoelectric vibration unit  637  includes the metal vibration part  142 , and the metal vibration part  142  has the same configuration as the metal vibration part  42  (refer to  FIG. 7C ) except for having air holes  142   e . The air holes  142   e  are installed in a body  142   a , and are connected to a pressure adjusting unit (illustration is omitted). The pressure adjusting unit is, for example, a compressor. In the metal vibration part  142 , pressure adjusting gas is supplied or discharged through the air holes  142   e , and thereby a pressure of an inner space of the metal vibration part  142  is kept constant. 
     Since in the piezoelectric vibration unit  637 , the body  142   a , holding parts  142   b , and a bottom  142   c  are integrated with each other, and the pressure of the inner space is kept constant, the piezoelectric vibration unit  637  further suppresses water and foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since in the piezoelectric vibration unit  637 , the body  142   a , the holding parts  142   b , and the bottom  142   c  are integrated with each other, the piezoelectric vibration unit  637  has a high rigidity. 
     Meanwhile, as shown in  FIG. 11B , there is a piezoelectric vibration unit  647  that is a modified example of the piezoelectric vibration unit  207 . The piezoelectric vibration unit  647  has the same configuration as the piezoelectric vibration unit  247  (refer to  FIG. 7D ) except for a metal diaphragm  132 . The piezoelectric vibration unit  647  includes the metal diaphragm  132 , and the metal diaphragm  132  has the same configuration as the metal diaphragm  32  (refer to  FIG. 7D ) except for having air holes  132   e . The air holes  132   e  are installed in a body  132   a , and are connected to a pressure adjusting unit (illustration is omitted). The pressure adjusting unit is, for example, a compressor. In the metal diaphragm  132 , pressure adjusting gas is supplied or discharged through the air holes  132   e , and thereby a pressure of an inner space of the metal diaphragm  132  is kept constant. 
     Since the piezoelectric vibration unit  647  has the metal diaphragm  132  and the bottom plate  8 , and keeps constant the pressure of the inner space, it further suppresses the water and the foreign substances having entered from the frequency adjusting hole  22   b  etc. from coming into contact with the piezoelectric element  1 . In addition, since the piezoelectric vibration unit  647  has the bottom plate  8 , it has a higher rigidity compared with the piezoelectric vibration unit  227  (refer to  FIG. 7B ). 
     Embodiment 3 
     Next, a piezoelectric speaker according to an embodiment 3 will be explained with reference to  FIG. 12A .  FIG. 12A  is an exploded perspective view of a modified example of the piezoelectric speaker according to the embodiment 3. The piezoelectric speaker according to the embodiment 3 has the same configuration as the piezoelectric speaker  100  according to the embodiment 1, except for the metal diaphragm  2  (refer to  FIG. 2 ), the adhesive part  4 , and the cover  5 . 
     As shown in  FIG. 12A , a piezoelectric speaker  300  includes: a metal diaphragm  152 ; a cover  15 ; and a case  16 . The metal diaphragm  152  has the same configuration as the metal diaphragm  2  (refer to  FIG. 2 ) except for being integrated with the cover  15 . The cover  15  has the same configuration as the cover  5  (refer to  FIG. 2 ) except for being integrated with the metal diaphragm  152 . The integrated cover  15  and metal diaphragm  152 , for example, can be obtained by raising of one plate material. Accordingly, since the cover  15  and the metal diaphragm  152  can be integrally manufactured by performing one processing of an integrated material, material cost and processing cost can be reduced. Note that unlike the piezoelectric speaker  100  (refer to  FIG. 2 ), the piezoelectric speaker  300  does not include the adhesive part  4 . The case  16  is an oblong frame-shaped body. Note that the piezoelectric speaker  300  may include the case  6  (refer to  FIG. 2 ) instead of the case  16 . 
     Hereinbefore, according to the piezoelectric speaker according to the embodiment 3, a cover and a metal diaphragm are integrated with each other, whereby material cost and processing cost can be reduced omitting an adhesive part, and thereby the piezoelectric speaker can be manufactured at low cost. 
     Modified Example 
     Next, there will be explained modified examples of the piezoelectric speaker  300  according to the embodiment 3.  FIGS. 12B and 12C  are exploded perspective views of the modified examples of the piezoelectric speaker according to the embodiment 3. 
     As shown in  FIG. 12B , there is a piezoelectric speaker  400  that is the modified example of the piezoelectric speaker  300 . The piezoelectric speaker  400  has the same configuration as the piezoelectric speaker  300  except for a cover and a case. A cover  25  has the same configuration as the cover  15  except for including locking pieces  25   f . A case  26  has the same configuration as the case  16  except for including locking holes  26   g . The cover  25  includes the locking pieces  25   f , and the case  26  includes the locking holes  26   g . The locking pieces  25   f  are installed at places corresponding to a vicinity of an outer edge of the cover  25 , specifically, a vicinity of a center of each side of a shape of the cover  25 , i.e. a rectangle. The locking pieces  25   f  extend toward the case  16  side. The locking pieces  25   f  are, for example, formed using press working after the metal diaphragm  252  is formed using raising. The locking holes  26   g  are provided so as to correspond to the locking pieces  25   f  in a contact surface of the cover  25  that comes into contact with the case  26 . The locking pieces  25   f  are inserted into the locking holes  26   g , thereby the locking pieces  25   f  and the locking holes  26   g  are locked to each other, and the cover  25  is fixed to the case  26 . 
     As shown in  FIG. 12C , there is a piezoelectric speaker  500  that is the modified example of the piezoelectric speaker  300 . The piezoelectric speaker  500  has the same configuration as the piezoelectric speaker  400  (refer to  FIG. 12B ) except for a metal diaphragm. A metal diaphragm  352  has the same configuration as the metal diaphragm  252  except for including frequency adjusting holes  352   e . The metal diaphragm  352  includes the frequency adjusting holes  352   e . The frequency adjusting holes  352   e  are installed at places corresponding to locking pieces  35   f . In other words, the frequency adjusting holes  352   e  are installed at the places corresponding to a vicinity of an outer edge of the metal diaphragm  352 , specifically, a vicinity of a center of each side of a shape of a cover  35 , i.e. a rectangle. An effective length and a width of the metal diaphragm  352  are changed by changing the number, positions, and a size of the frequency adjusting holes  352   e , and thereby a frequency can be adjusted. 
     EXAMPLES 
     Next, Examples 1 and 2 of the piezoelectric speaker according to the embodiment 1 will be explained using  FIG. 13 .  FIG. 13  is a graph showing a sound pressure with respect to a frequency of the Example of the piezoelectric speaker according to the embodiment 1. 
     In the Examples 1 and 2, there was used a piezoelectric speaker having the same configuration as the piezoelectric speaker  100  according to the embodiment 1. Specifically, in the Examples 1 and 2, additionally, a plate including brass and having a thickness of 1 mm was used as the metal diaphragm  2  (refer to  FIG. 2 ). In addition, in the Example 1, a double-sided tape was used as the adhesive part  4  (refer to  FIG. 2 ), and in the Example 2, an epoxy resin body formed by curing an epoxy resin agent was used as the adhesive part  4  (refer to  FIG. 2 ). The double-sided tape used in the Example 1 is a band-shaped base material having a predetermined elastic modulus, an adhesive is applied to both-side principal surfaces of the base material, and thus the principal surfaces have an adhesive property. 
     In addition, the base material has a lower elastic coefficient compared with epoxy resin. 
     Sound was reproduced at 2 Vpp (peak to peak), and a sound pressure with respect to a frequency was measured in the Examples 1 and 2. The results were shown in  FIG. 13 . 
     As shown in  FIG. 13 , in the Example 1, the sound pressure reaches a maximum value in a frequency domain of 20 to 30 kHz. A sound pressure characteristic curve of the Example 1 falls within a predetermined range of approximately 79 to 93 dB sq1 in a frequency domain of 20 to 100 kHz. That is, in the Example 1, sound can be reproduced with stable sound pressures in the frequency domain of 20 to 100 kHz. 
     In addition, in the Example 2, the sound pressure reaches a maximum value in a frequency domain of approximately 30 kHz. In the Example 1, rise of the sound pressure tends to be earlier compared with the Example 2. It is considered that this is because the double-sided tape used as an adhesive part in the Example 1 is harder compared with the epoxy resin body formed by curing the epoxy resin. 
     A sound pressure characteristic curve of the Example 2 falls within the predetermined range of approximately 79 to 93 dB sq1 in a frequency domain of approximately 25 to 100 kHz. That is, also in the Example 2, sound can be reproduced with stable sound pressures in the frequency domain of 20 to 100 kHz. 
     Note that there is also considered an Example using a silicon resin body formed by curing a silicon adhesive as the adhesive part  4  (refer to  FIG. 2 ). In this Example, it is expected that a sound pressure characteristic curve similar to those of the Examples 1 and 2 is obtained. 
     By the way, human beings are supposed to be unable to hear sound of a frequency higher than 20 kHz. Therefore, it is considered that such reproduction of the sound of the high frequency does not seemingly contribute to increase in quality of sound output by a speaker. However, sound of a fine signal can also be reproduced by reproducing the sound of the high frequency. Hereby, such reproduction of the sound of the high frequency can contribute to the increase in quality of the sound output by the speaker. 
     Embodiment 4 
     A speaker unit  700  according to the embodiment will be explained using  FIGS. 17 and 18 .  FIG. 17  is an XZ cross-sectional view showing a configuration of the speaker unit  700 .  FIG. 18  is a bottom view showing a configuration of a main portion of the speaker unit  700 . In the embodiment, two piezoelectric vibration units  7   a  and  7   b  are arranged in the case  6 . Note that since basic configurations other than the two piezoelectric vibration units  7   a  and  7   b  are similar to those of the speaker units  100 ,  200 ,  300 ,  400 , and  500  of the above-described embodiments, explanation thereof is appropriately omitted. For example, there can be used the case  6 , the adhesive part  3 , the metal diaphragm  2 , etc. that have similar configurations shown in  FIGS. 1 and 3 . 
     The piezoelectric vibration units  7   a  and  7   b  are housed in the case  6 . The piezoelectric vibration unit  7   a  has: a piezoelectric element  1   a ; an adhesive part  3   a ; and the metal diaphragm  2 . Similarly to the embodiment 1, the piezoelectric element  1   a  is made to adhere to the metal diaphragm  2  through the adhesive part  3   a . The piezoelectric vibration unit  7   b  has: a piezoelectric element  1   b ; an adhesive part  3   b ; and the metal diaphragm  2 . Similarly to the embodiment 1, the piezoelectric element  1   b  is made to adhere to the metal diaphragm  2  through the adhesive part  3   b.    
     The metal diaphragm  2  is in common in the two piezoelectric vibration units  7   a  and  7   b . That is to say, the metal diaphragm  2  has one metal plate, and the piezoelectric elements  1   a  and  1   b  are attached to the one metal plate. The piezoelectric elements  1   a  and  1   b  are attached to the same surface of the metal diaphragm  2 . Specifically, the piezoelectric elements  1   a  and  1   b  are attached to a surface of the metal diaphragm  2  on an opposite side of the sound emitting hole  5   a  side. When a voltage is supplied to the piezoelectric elements  1   a  and  1   b , the piezoelectric elements  1   a  and  1   b  are distorted. Hereby, the metal diaphragm  2  vibrates, and sound is generated from the sound emitting hole  5   a.    
     The two piezoelectric elements  1   a  and  1   b  are arranged side by side in an X direction. That is to say, the piezoelectric element  1   a  is arranged on a +X side of the piezoelectric element  1   b . The piezoelectric elements  1   a  and  1   b  overlap with the sound emitting hole  5   a  in an XY planar view. Further, parts of the piezoelectric elements  1   a  and  1   b  protrude from the sound emitting hole  5   a . The piezoelectric elements  1   a  and  1   b  each have a substantially rectangular shape in the XY planar view. 
     The two piezoelectric elements  1   a  and  1   b  have different sizes in the XY planar view. Specifically, the two piezoelectric elements  1   a  and  1   b  have different widths in the X direction. Note that the two piezoelectric elements  1   a  and  1   b  have the same width in a Y direction. The two piezoelectric elements  1   a  and  1   b  differ in frequency of natural vibration modes. That is to say, a resonance frequency of the piezoelectric element  1   a  is different from that of the piezoelectric element  1   b . In addition, the frequencies of the natural vibration modes of the piezoelectric elements  1   a  and  1   b  are different from the frequency of the natural vibration mode of the metal diaphragm  2 . 
     In the embodiment, the two piezoelectric elements  1   a  and  1   b  having the different resonance frequencies are connected to the metal diaphragm  2  through the adhesive parts  3   a  and  3   b . In doing so, a high sound pressure and a high SN ratio can be obtained also in a high frequency domain of 5 to 50 kHz. Accordingly, a high-performance speaker unit can be realized with simple structure. In the high frequency domain, while the SN ratio in a general electromagnetic speaker is 45 dB, the SN ratio of 60 dB can be achieved in the piezoelectric speaker unit  700 . 
     Frequency characteristics of a sound pressure of the piezoelectric speaker unit  700  are shown in  FIG. 19 . In  FIG. 19 , the frequency characteristics of the sound pressure of the piezoelectric speaker unit  700  are shown as an Example. In addition, in  FIG. 19 , frequency characteristics in a case of using a dynamic speaker (an electromagnetic speaker) and an LPF (Low Pass Filter) are shown as a comparative example 1, and frequency characteristics of a piezoelectric speaker unit having one piezoelectric element are shown as a comparative example 2. Compared with the comparative examples 1 and 2, the piezoelectric speaker unit  700  can obtain a high sound pressure also in a high frequency domain of not less than 5 kHz. 
     The piezoelectric elements  1   a  and  1   b  having different dimensions from each other differ in resonance frequency. Additionally, flatness of the sound pressure frequency characteristics can be optimized by a combination of respective shapes of the rectangular piezoelectric elements  1   a  and  1   b  and the metal diaphragm  2 . Note that although the two piezoelectric elements  1   a  and  1   b  are provided in the above explanation, three or more piezoelectric elements can be provided. That is to say, a plurality of piezoelectric elements  1  may just be made to adhere to the metal diaphragm  2  through the adhesive part  3 . 
     A frequency domain in which the mode of the metal diaphragm  2  does not rise is preferably matched with the resonance frequencies of the piezoelectric elements  1   a  and  1   b . Further, the resonance frequency Qm of the piezoelectric element is preferably set in a range of 1.0 to 5.0 by using an elastic body for the adhesive part  3 . Hereby, sound can be reproduced in a wide frequency band, and with a flat sound pressure characteristic curve. 
     Embodiment 5 
     A piezoelectric speaker unit  800  according to the embodiment will be explained using  FIGS. 20 and 21 .  FIG. 20  is a perspective view showing an appearance of the piezoelectric speaker unit  800 .  FIG. 21  is an XY plan view showing a configuration in an internal space of a housing  820  of the piezoelectric speaker unit  800 . In the embodiment, an electromagnetic speaker  810  is provided inside the case  6  of  FIG. 21 . In addition, the piezoelectric element  1  is provided outside the case  6 . Note that explanation of configurations similar to the above-described embodiments 1 to 4 is appropriately omitted. 
     The housing  820  has a box shape. For example, the housing  820  has the case  6  and the cover  5 . The case  6  includes side plates  6   d  and a back plate  6   e . The back plate  6   e  is opposed to the cover  5 . The cover  5  and the back plate  6   e  are flat plates parallel to each other. The cover  5 , the side plates  6   d , and the back plate  6   e  are preferably rectangular metal plates, respectively. Further, the cover  5  has the sound emitting hole  5   a . A cross-sectional shape of the sound emitting hole  5   a  is a tapered shape that becomes larger toward an outside similarly to the configuration shown in  FIG. 2 . 
     Note that the embodiment is explained assuming as a front side a side on which the sound emitting hole  5   a  is provided. The back plate  6   e  is arranged to be opposed to the cover  5 . The side plates  6   d  are arranged between the cover  5  and the back plate  6   e . That is to say, the side plates  6   d  connect the cover  5  and the back plate  6   e . Here, since outer shapes of the cover  5  and the back plate  6   e  are substantially rectangular shapes in the XY planar view, the case  6  has the four side plates  6   d . That is to say, the side plates  6   d  are arranged at each end side of the substantially rectangular cover  5  and back plate  6   e , respectively. The opposed two side plates  6   d  are in parallel to each other. The adjacent two side plates  6   d  are perpendicular to each other. 
     Assume the internal space of the housing  820  as an air chamber  6   f . That is to say, the space defined by the cover  5 , the back plate  6   e , and the side plates  6   d  serves as the air chamber  6   f . Specifically, the rectangular parallelepiped space surrounded by the cover  5 , the back plate  6   e , and the four side plates  6   d  serves as the air chamber  6   f . The air chamber  6   f  is communicated with an outer space through the sound emitting hole  5 . The cover  5  and the back plate  6   e  are arranged to be opposed to each other through the air chamber  6   f . Accordingly, the cover  5  serves as a front plate for defining the air chamber  6   f.    
     Note that parts or all of the cover  5 , the back plate  6   e , and the side plates  6   d  may be integrally formed. For example, the back plate  6   e  and the side plates  6   d  may be integrally formed similarly to the case  6  shown in the embodiment 1. Additionally, the cover  5  may be removable as the cover  5  of the Embodiment 1. As a matter of course, components other than the cover  5  may be removable. 
     The electromagnetic speaker  810  is arranged in the air chamber  6   f . The electromagnetic speaker  801  is attached to the one side plate  6   d  in  FIG. 21 . Specifically, the electromagnetic speaker  810  is installed on a surface (hereinafter referred to as an inner surface) of the air chamber  6   f  side of the side plate  6   d  of a −Y side. The electromagnetic speaker  810  has: a diaphragm; a voice coil; a permanent magnet; etc. The voice coil and the diaphragm vibrate by supplying a current to the voice coil. Hereby, the electromagnetic speaker  810  generates sound. Here, the electromagnetic speaker  810  generates the sound toward the sound emitting hole  5   a.    
     The piezoelectric element  1  is provided outside the case  6 . The piezoelectric element  1  is made to adhere to the side plate  6   d  of the case  6  through the adhesive part  3 . The adhesive part  3  is an elastic body similarly to the above. Here, the piezoelectric element  1  is attached to a surface (hereinafter referred to as an outer surface) of the side plate  6   d  on an opposite side of the air chamber  6   f  side. The inner surface of the one side plate  6   d  serves as a mounting surface of the electromagnetic speaker  810 , and the outer surface thereof serves as a mounting surface of the piezoelectric element  1 . As described above, the piezoelectric element  1  is arranged on the one surface (the outer surface) of the opposed two surfaces of the side plate  6   d , and the electromagnetic speaker  810  is arranged on the other surface (the inner surface) thereof. In other words, the mounting surface of the piezoelectric element  1  and the mounting surface of the electromagnetic speaker  801  serve as opposed surfaces of the case  6 . 
     The electromagnetic speaker  810  is fixed to the case  6  in the piezoelectric speaker unit  800  according to the embodiment. Both the electromagnetic speaker  810  and the piezoelectric element  1  mounted at the case  6  vibrate. A frequency of a natural vibration mode of the electromagnetic speaker  810  and the frequency of the natural vibration mode of the piezoelectric element  1  are different from each other. Accordingly, a high sound pressure and a high SN ratio can be realized also in a high frequency region. Sound reproduction in a wide band of 100 Hz to 100 kHz can be made by the configuration of the embodiment. 
     The side plate  6   d  serving as the mounting surface on which the piezoelectric element  1  is mounted is preferably formed of a metal plate. That is to say, the side plates  6   d , the adhesive part  3 , and the piezoelectric element  1  are included in the piezoelectric vibration unit  7 . In doing so, the side plate  6   d  functions as the metal vibration part  2  of the embodiment 1 etc. Consequently, the high sound pressure and the high SN ratio can be realized in the high frequency region similarly to the embodiment 1. Note that the piezoelectric vibration unit  7  does not close the sound emitting hole  5   a  in the embodiment. 
     Note that the side plate  6   d  serving as the mounting surface on which the piezoelectric element  1  is mounted is preferably formed of a metal plate having a thickness of 10 to 300 μm. In doing so, a higher sound pressure and a higher SN ratio can be realized also in the high frequency region. 
       FIG. 22  is a graph showing frequency characteristics of a sound pressure of the piezoelectric speaker unit  800  according to the embodiment. In  FIG. 22 , the sound pressure frequency characteristics in a configuration in which only the electromagnetic speaker  810  is mounted are shown as “electromagnetic”. The sound pressure frequency characteristics in a configuration in which only the piezoelectric element  1  is mounted are shown as “piezoelectric”. The sound pressure frequency characteristics of the electromagnetic speaker  810  in which both the piezoelectric element  1  and the electromagnetic speaker  810  are mounted are shown as “electromagnetic+piezoelectric”. As shown in  FIG. 22 , in a case where both the piezoelectric element  1  and the electromagnetic speaker  810  are mounted, reproduction with a high sound pressure can be made also in a frequency of not less than 20 kHz. A high sound pressure and a high SN ratio can be realized also in a high frequency region by the configuration of the embodiment. 
     Modified Example 6 
     A modified example 6 of the embodiment 5 will be explained using  FIG. 23 .  FIG. 23  is an XY cross-sectional view showing a main portion of the piezoelectric speaker unit  800  according to the modified example 6. In the modified example 6, a position of the piezoelectric element  1  is different from the configuration of the embodiment 5. Specifically, the piezoelectric element  1  is arranged in the housing  820 . Note that since a basic configuration of the piezoelectric speaker unit  800  is the same as the above, explanation thereof is appropriately omitted. 
     In the embodiment, the piezoelectric element  1  is arranged in the air chamber  6   f . That is to say, the piezoelectric element  1  is attached to the inner surface of the side plate  6   d  through the adhesive part  3 . In the embodiment, the inner surface of the side plate  6   d  of the −Y side serves as the mounting surface of the piezoelectric element  1 . Accordingly, the electromagnetic speaker  810  and the piezoelectric element  1  are installed on the same surface (the inner surface) of the side plate  6   d . The mounting surface of the piezoelectric element  1  and the mounting surface of the electromagnetic speaker  801  are the same surface of the case  6 . 
     Also in the modified example 6, both the electromagnetic speaker  810  and the piezoelectric element  1  mounted in the case  6  vibrate. The frequency of the natural vibration mode of the electromagnetic speaker  810  and the frequency of the natural vibration mode of the piezoelectric element  1  are different from each other. Further, in the modified example 6, vibrations of the electromagnetic speaker  810  and the piezoelectric element  1  are mixed in the air chamber  6   f  and subsequently, the mixed vibrations are emitted from the sound emitting hole  5   a . Accordingly, a high sound pressure and a high SN ratio can be realized also in a high frequency region. Sound reproduction in the wide band of 100 Hz to 100 kHz can be made by the configuration of the embodiment. 
     The side plate  6   d  serving as the mounting surface on which the piezoelectric element  1  is mounted is preferably formed of a metal plate. That is to say, the side plates  6   d , the adhesive part  3 , and the piezoelectric element  1  are included in the piezoelectric vibration unit  7 . In doing so, the high sound pressure and the high SN ratio can be realized in the high frequency region similarly to the embodiment 1. 
     Note that the side plate  6   d  serving as the mounting surface on which the piezoelectric element  1  is mounted is preferably formed of a metal plate having a thickness of 10 to 300 μm. In doing so, a higher sound pressure and a higher SN ratio can be realized in the high frequency region. 
     Modified Example 7 
     A modified example 7 of the embodiment 5 will be explained using  FIG. 24 .  FIG. 24  is a YZ cross-sectional view showing a main portion of the piezoelectric speaker unit  800  according to the modified example 7. In the modified example 7, positions of the piezoelectric element  1  and the electromagnetic speaker  810  are different from the configuration of the embodiment 5. Note that since the basic configuration of the piezoelectric speaker unit  800  is the same as the above, explanation thereof is appropriately omitted. 
     As shown in  FIG. 23 , the electromagnetic speaker  810  is attached to the back plate  6   e . Specifically, the electromagnetic speaker  810  is fixed to an inner surface of the back plate  6   e . Accordingly, the electromagnetic speaker  810  is arranged in the air chamber  6   f . The electromagnetic speaker  810  generates sound toward the sound emitting hole  5   a.    
     The piezoelectric element  1  is made to adhere to the back plate  6   e  and the cover  5 . Specifically, the adhesive part  3  including an elastic body is provided on both surfaces of the piezoelectric element  1 . A back surface of the piezoelectric element  1  is made to adhere to the back plate  6   e  through the adhesive part  3 . The back plate  6   e  serving as the mounting surface of the piezoelectric element  1  is preferably a metal plate having a thickness of 10 to 300 μm. A front surface of the piezoelectric element  1  is made to adhere to the metal diaphragm  2  through the adhesive part  3 . 
     The adhesive part  4  is provided on a front surface of the metal diaphragm  2 . Additionally, the metal diaphragm  2  is made to adhere to the cover  5  through the adhesive part  4 . The adhesive part  4  is attached to the outer edge  2   h  of the metal diaphragm  2 . Accordingly, when the piezoelectric speaker  100  is seen from the cover  5  side, the outer edge  2   h  is covered with the cover  5 . In addition, the adhesive part  4  is provided except for a portion corresponding to the sound emitting hole  5   a  of the cover  5 . Accordingly, when the piezoelectric speaker  100  is seen from the cover  5  side, the metal diaphragm  2  can be seen from the sound emitting hole  5   a . The metal diaphragm  2  serving as the mounting surface of the piezoelectric element  1  is preferably a metal plate having a thickness of 10 to 300 μm. 
     As described above, in the modified example 7, the adhesive part  3  is provided on the front surface and the back surface of the piezoelectric element  1 . That is to say, the piezoelectric element  1  is sandwiched by the two adhesive parts  3 . Additionally, both surfaces of the piezoelectric element  1  are fixed to the housing  820  through the adhesive parts  3 . The back plate  6   e , the adhesive part  3 , the piezoelectric element  1 , the adhesive part  3 , and the metal diaphragm  2  are included in the piezoelectric vibration unit  7 . 
     Also in the modified example 7, both the electromagnetic speaker  810  and the piezoelectric element  1  mounted in the case  6  vibrate. The frequency of the natural vibration mode of the electromagnetic speaker  810  and the frequency of the natural vibration mode of the piezoelectric element  1  are different from each other. Further, in the modified example, vibrations of the electromagnetic speaker  810  and the piezoelectric element  1  are mixed in the air chamber  6   f  and subsequently, the mixed vibrations are emitted from the sound emitting hole  5   a . Sound reproduction in the wide band of 100 Hz to 100 kHz can be made by the configuration of the embodiment. 
     The metal diaphragm  2  serving as the mounting surface of the piezoelectric element  1  is fixed to the other member (the cover  5 ) through the adhesive part  4  that is an elastic body. Consequently, good characteristics can be obtained as in the above-described embodiments. In addition, a surface on which the metal diaphragm  2  is mounted is not limited to the cover  5 . The metal plate (the metal vibration part  2 ) may be fixed to the side plate  6   d  or the back plate  6   e  through the adhesive part  3  that is the elastic body. 
     Modified Example 8 
     A modified example 8 of the embodiment 5 will be explained using  FIG. 25 .  FIG. 25  is a YZ cross-sectional view showing a main portion of the piezoelectric speaker unit  800  according to the modified example 8. In the modified example 8, a configurations of the back plate  6   e  is different from the configuration of the modified example 7. Note that since the basic configuration of the piezoelectric speaker unit  800  is the same as the above, explanation thereof is appropriately omitted. 
     Although the back plate  6   e  is the metal plate in the modified example 7, some parts of the back plate  6   e  are resin  6   g  in the modified example 8. That is to say, the back plate  6   e  includes a metal material and a resin material. That is to say, some parts of the back plate  6   e  are formed of the resin material, and a remaining portion thereof is formed of the metal material. Consequently, since some of the back plate  6   e  serving as the mounting surface are formed of the resin  6   g  as described above, the back plate  6   e  is partially a metal plate. The side plate  6   d , the cover  5 , or the back plate  6   e  serves as the mounting surface of the piezoelectric element  1 , the metal material and the resin material are included, and thereby good characteristics can be obtained. 
     Note that although in the embodiment 5, and the modified examples 6 to 8 thereof, the piezoelectric element  1  is fixed to the case  6  through the adhesive part  3  that is the elastic body, the piezoelectric element  1  may be fixed to the case  6  without the elastic body. 
     Although the side plate  6   d  is used for the mounting surface of the piezoelectric element  1  in the embodiment 5 and the modified example 6, and the back plate  6   e  is used therefor in the modified examples 7 and 8, the mounting surface of the piezoelectric element  1  is not particularly limited. Further, the piezoelectric element  1  may be attached to an outside surface of the housing  820 . 
     In addition, the configuration of the embodiment 4 and the configuration of the embodiment 5 may be combined with each other. In this case, the electromagnetic speaker  810  is arranged inside the case  6 , and the two or more piezoelectric elements  1  are mounted at the case  6 . 
     The piezoelectric speakers according to the above-described embodiments 1 to 5 can be used incorporated in various apparatuses. For example, the above-described piezoelectric speakers can be used as a high frequency speaker that is incorporated in PCs (personal computers), tablet PCs, next-generation 4K televisions, next-generation 8K televisions, and in-vehicle and non-portable high-resolution audios. 
     Particularly, along with the expansion of digital sound, information on sound source sampling frequency in music reproduction, and the number of bits, there are growing needs for a speaker reproducible with a high sound pressure and a high SN ratio in a high frequency of 20 to 70 kHz. Although a high frequency of not less than 20 kHz is supposed to be inaudible to human beings, actually, even the high frequency being reproducible leads to even fine signals being reproducible. Increase in quality of a sound source enables to contribute to increase in quality of a speaker output. Note that in the above explanation, the embodiments are explained, omitting wires etc. that are connected to the piezoelectric element and the electromagnetic speaker. 
     Hereinbefore, although the present invention has been explained in the context of the above-described embodiments and Examples, it is not limited only to the configurations of the above-described embodiments and Examples. It is needless to say that the present invention includes various deformations, modifications, and combinations that can be made by those skilled in the art within the scope of the invention of claims of CLAIMS in the present application. 
     This application claims priority based on Japanese Patent Application No. 2015-24041 filed on Feb. 10, 2015, and Japanese Patent Application No. 2015-106550 filed on May 26, 2015, and the entire disclosure thereof is incorporated herein. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  200 ,  300 ,  400 , and  500  piezoelectric speaker 
           7 ,  207 ,  217 ,  227 ,  237 ,  247 ,  317 ,  327 ,  337 ,  347 ,  417 ,  427 ,  437 ,  447 ,  517 ,  527 ,  537 ,  547 ,  637 , and 
           647  piezoelectric vibration unit 
           1  piezoelectric element 
           2 ,  22 ,  32 ,  42 ,  52 ,  62 ,  72 ,  82 ,  102 ,  112 ,  132 ,  142 ,  152 ,  252 , and  352  metal diaphragm (metal vibration part) 
           12   a ,  32   a ,  42   a ,  72   a ,  82   a ,  102   a , and  132   a  body 
           3  adhesive part