Patent Publication Number: US-2020287125-A1

Title: Piezoelectric speaker

Description:
TECHNICAL FIELD 
     The present invention relates to a piezoelectric speaker. 
     BACKGROUND ART 
     In recent years, a speaker employing a piezoelectric film (such a speaker may hereinafter be referred to as a piezoelectric speaker) is sometimes adopted as a speaker for audio equipment or for sound reduction. Piezoelectric speakers have an advantage in that they are small in volume and light. 
     Patent Literature 1 describes an exemplary piezoelectric speaker. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP S63-149997 A 
         Patent Literature 2: JP 2016-122187 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the piezoelectric speaker of Patent Literature 1, in order to ensure a sufficient volume of sound, an edge portion of a piezoelectric film is fixed by a supporting member and the piezoelectric film is kept in a curved shape. However, in view of achieving a piezoelectric speaker having fewer restrictions on its installation location, curving a piezoelectric film is desirably not essential. 
     The present invention aims to provide a piezoelectric speaker exhibiting practical acoustic characteristics even when a piezoelectric film is not kept in a curved shape. 
     Solution to Problem 
     The present invention provides a piezoelectric speaker, including: 
     a first electrode; 
     a second electrode; 
     a piezoelectric film including a piezoelectric body sandwiched by the first electrode and the second electrode; 
     a first joining layer having pressure-sensitive adhesiveness or adhesiveness; and 
     an interposed layer disposed between the piezoelectric film and the first joining layer, wherein 
     both principal surfaces of the piezoelectric film vibrate up and down as a whole. 
     Advantageous Effects of Invention 
     A piezoelectric speaker including a piezoelectric film whose both principal surfaces vibrate up and down as a whole can exhibit practical acoustic characteristics even when a piezoelectric film is not kept in a curved shape. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing a piezoelectric speaker cut along the thickness direction thereof. 
         FIG. 2  is a top view showing a piezoelectric speaker viewed from the side opposite to a first joining face. 
         FIG. 3  illustrates a piezoelectric speaker fixed to a support. 
         FIG. 4A  schematically shows how a piezoelectric speaker vibrates. 
         FIG. 4B  schematically shows how a piezoelectric speaker vibrates. 
         FIG. 5  illustrates a structure for measurement of a sample. 
         FIG. 6  is a block diagram of an output system. 
         FIG. 7  is a block diagram of an evaluation system. 
         FIG. 8A  is a table showing the results of evaluation of samples. 
         FIG. 8B  is a table showing the results of evaluation of samples. 
         FIG. 9  is a graph showing a relationship between the holding degree of an interposed layer and a frequency at which emission of sound starts. 
         FIG. 10  is a graph showing the frequency characteristics of a sample of Example 1 in terms of sound pressure level. 
         FIG. 11  is a graph showing the frequency characteristics of a sample of Example 2 in terms of sound pressure level. 
         FIG. 12  is a graph showing the frequency characteristics of a sample of Example 3 in terms of sound pressure level. 
         FIG. 13  is a graph showing the frequency characteristics of a sample of Example 4 in terms of sound pressure level. 
         FIG. 14  is a graph showing the frequency characteristics of a sample of Example 5 in terms of sound pressure level. 
         FIG. 15  is a graph showing the frequency characteristics of a sample of Example 6 in terms of sound pressure level. 
         FIG. 16  is a graph showing the frequency characteristics of a sample of Example 7 in terms of sound pressure level. 
         FIG. 17  is a graph showing the frequency characteristics of a sample of Example 8 in terms of sound pressure level. 
         FIG. 18  is a graph showing the frequency characteristics of a sample of Example 9 in terms of sound pressure level. 
         FIG. 19  is a graph showing the frequency characteristics of a sample of Example 10 in terms of sound pressure level. 
         FIG. 20  is a graph showing the frequency characteristics of a sample of Example 11 in terms of sound pressure level. 
         FIG. 21  is a graph showing the frequency characteristics of a sample of Example 12 in terms of sound pressure level. 
         FIG. 22  is a graph showing the frequency characteristics of a sample of Example 13 in terms of sound pressure level. 
         FIG. 23  is a graph showing the frequency characteristics of a sample of Example 14 in terms of sound pressure level. 
         FIG. 24  is a graph showing the frequency characteristics of a sample of Example 15 in terms of sound pressure level. 
         FIG. 25  is a graph showing the frequency characteristics of a sample of Example 16 in terms of sound pressure level. 
         FIG. 26  is a graph showing the frequency characteristics of a sample of Reference Example 1 in terms of sound pressure level. 
         FIG. 27  is a graph showing the frequency characteristics of background noise in terms of sound pressure level. 
         FIG. 28  illustrates a supporting structure of a piezoelectric film. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The following description is only illustrative of the embodiments of the present invention and has no intention to limit the present invention. 
     First Embodiment 
     A piezoelectric speaker according to a first embodiment will be described using  FIG. 1  and  FIG. 2 . A piezoelectric speaker  10  includes a piezoelectric film  35 , a first joining layer  51 , an interposed layer  40 , and a second joining layer  52 . The first joining layer  51 , the interposed layer  40 , the second joining layer  52 , and the piezoelectric film  35  are laminated in this order. 
     The piezoelectric film  35  includes a piezoelectric body  30 , an electrode  51 , and an electrode  52 . The piezoelectric film  35  has the shape of a film. 
     The piezoelectric body  30  has the shape of a film. The piezoelectric body  30  is vibrated by application of voltage. A ceramic film, a resin film, and the like can be used as the piezoelectric body  30 . Examples of the material of the piezoelectric body  30  that is a ceramic film include lead zirconate, lead zirconate titanate, lead lanthanum zirconate titanate, barium titanate, Bi-layered compounds, compounds having a tungsten bronze structure, and solid solutions of barium titanate and bismuth ferrite. Examples of the material of the piezoelectric body  30  that is a resin film include polyvinylidene fluoride and polylactic acid. The material of the piezoelectric body  30  that is a resin film may be a polyolefin such as polyethylene or polypropylene. The piezoelectric body  30  may be a non-porous body or may be a porous body. 
     The thickness of the piezoelectric body  30  is, for example, 10 μm to 300 μm and may be 30 μm to 110 μm. 
     The electrode  61  and the electrode  62  sandwich the piezoelectric body  30 . Specifically, the piezoelectric body  30  is in contact with the electrode  61  and the electrode  62 . The first electrode  61  and the second electrode  62  each have the shape of a film. The first electrode  61  and the second electrode  62  are each connected to a lead wire which is not illustrated. The first electrode  61  and the second electrode  62  can be formed on the piezoelectric body  30  by vapor deposition, plating, sputtering, or the like. A metal foil can be used as each of the first electrode  61  and the second electrode  62 . A metal foil can be stuck to the piezoelectric body  30  using a double-faced tape, a pressure-sensitive adhesive, an adhesive, or the like. Examples of the materials of the first electrode  61  and the second electrode  62  include metals, and specific examples thereof include gold, platinum, silver, copper, palladium, chromium, molybdenum, iron, tin, aluminum, and nickel. Examples of the materials of the first electrode  61  and the second electrode  62  also include carbon and electrically conductive polymers. Examples of the materials of the first electrode  61  and the second electrode  62  also include alloys of the above metals. The first electrode  61  and the second electrode  62  may include, for example, a glass component. 
     The thickness of the first electrode  61  and that of the second electrode  62  are each, for example, 10 nm to 150 μm, and may be 20 nm to 100 μm. 
     In the example in  FIG. 1  and  FIG. 2 , the first electrode  61  covers one entire principal surface of the piezoelectric body  30 . The first electrode  61  may cover only a portion of the one principal surface of the piezoelectric body  30 . The second electrode  62  covers the other entire principal surface of the piezoelectric body  30 . The second electrode  62  may cover only a portion of the other principal surface of the piezoelectric body  30 . 
     The interposed layer  40  is disposed between the piezoelectric film  35  and the first joining layer  51 . The interposed layer  40  may be a layer other than an adhesive layer and a pressure-sensitive adhesive layer, or may be an adhesive layer or a pressure-sensitive adhesive layer. In the present embodiment, the interposed layer  40  is a porous body layer and/or a resin layer. Here, the resin layer is a concept including a rubber layer and an elastomer layer. Therefore, the interposed layer  40  that is a resin layer may be a rubber layer or an elastomer layer. Examples of the interposed layer  40  that is a resin layer include an ethylene propylene rubber layer, a butyl rubber layer, a nitrile rubber layer, a natural rubber layer, a styrene-butadiene rubber layer, a silicone layer, a urethane layer, and an acrylic resin layer. Examples of the interposed layer  40  that is a porous body layer include foam layers. Specifically, examples of the interposed layer  40  that is a porous body layer and a resin layer include an ethylene propylene rubber foam layer, a butyl rubber foam layer, a nitrile rubber foam layer, a natural rubber foam layer, a styrene-butadiene rubber foam layer, a silicone foam layer, and a urethane foam layer. Examples of the interposed layer  40  that is not a porous body layer but a resin layer include acrylic resin layers. Examples of the interposed layer  40  that is not a resin layer but a porous body layer include porous metal body layers. The term “resin layer” as used herein refers to a resin-including layer. The term “resin layer” as used herein refers to a layer that may include a resin in an amount of 30% or more, in an amount of 45% or more, in an amount of 60% or more, or in an amount of 80% or more. The same applies to, for example, a rubber layer, an elastomer layer, an ethylene propylene rubber layer, a butyl rubber layer, a nitrile rubber layer, a natural rubber layer, a styrene-butadiene rubber layer, a silicone layer, an urethane layer, an acrylic resin layer, a metal layer, a resin film, and a ceramic film. The interposed layer  40  may be a blended layer including two or more materials. 
     The elastic modulus of the interposed layer  40  is, for example, 10000 N/m 2  to 10000000 N/m 2 , and may be 20000 N/m 2  to 100000 N/m 2 . 
     In an example, the pore diameter of the interposed layer  40  that is a porous body layer is 0.1 mm to 7.0 mm, and may be 0.3 mm to 5.0 mm. In another example, the pore diameter of the interposed layer  40  that is a porous body layer is, for example, 0.1 mm to 2.5 mm, and may be 0.2 mm to 1.5 mm or 0.3 mm to 0.7 mm. 
     The porosity of the interposed layer  40  that is a porous body layer is, for example, 70% to 99%, and may be 80% to 99% or 90% to 95%. 
     A known foam (for example, the foam used in Patent Literature 2) can be used as the interposed layer  40  that is a foam layer. The interposed layer  40  that is a foam layer may have an open-cell structure, a closed-cell structure, or a semi-open-/semi-closed-cell structure. The term “open-cell structure” refers to a structure having an open cell rate of 100%. The term “closed-cell structure” refers to a structure having an open cell rate of 0%. The term “semi-open-/semi-closed-cell structure” refers to a structure having an open cell rate of greater than 0% and less than 100%. The open cell rate can be calculated, for example, using the following equation after a test in which a foam layer is sunk in water: open cell rate (%)={(volume of absorbed water)/(volume of cell part)}×100. In a specific example, the “volume of absorbed water” can be obtained by sinking and leaving a foam layer in water under a reduced pressure of −750 mmHg for 3 minutes, measuring the mass of water having replaced the air in cells of the foam layer, and converting the mass of water in the cells into volume on the assumption that the density of water is 1.0 g/cm 3 . The term “volume of cell part” refers to a value calculated using the following equation: volume of cell part (cm 3 )={(mass of foam layer)/(apparent density of foam layer)}−{(mass of foam layer)/(density of material)}. The term “density of material” refers to the density of a matrix (solid, or non-hollow, body) forming the foam layer. 
     The foaming factor (the ratio between the density before foaming and that after foaming) of the interposed layer  40  that is a foam layer is, for example, 5 to 40, and may be 10 to 40. 
     The interposed layer  40  in an uncompressed state has a thickness of, for example, 0.1 mm to 30 mm, and may have a thickness of 1 mm to 30 mm, 1.5 mm to 30 mm, or 2 mm to 25 mm. The interposed layer  40  in an uncompressed state is typically thicker than the piezoelectric film  35  in an uncompressed state. The thickness of the interposed layer  40  in an uncompressed state is, for example, 3 or more times the thickness of the piezoelectric film  35  in an uncompressed state, and may be 10 or more times or 30 or more times the thickness of the piezoelectric film  35  in an uncompressed state. The interposed layer  40  in an uncompressed state is typically thicker than the first joining layer  51  in an uncompressed state. 
     The first joining layer  51  is a layer having pressure-sensitive adhesiveness or adhesiveness. In other words, the first joining layer  51  is an adhesive layer or a pressure-sensitive adhesive layer. The first joining layer  51  can be stuck to a support. In the example of  FIG. 1 , the first joining layer  51  is in contact with the interposed layer  40 . The first joining layer  51  is used to join the interposed layer  40  to a support. Specifically, the first joining layer  51  has a first joining face  17  to be stuck to a support. Examples of the first joining layer  51  include a double-faced tape including a substrate and a pressure-sensitive adhesive applied to the both sides of the substrate. Examples of the substrate of the double-faced tape used as the first joining layer  51  include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the first joining layer  51  include pressure-sensitive adhesives including an acrylic resin. The first joining layer  51  may be a layer including no substrate and formed of a pressure-sensitive adhesive. 
     The thickness of the first joining layer  51  is, for example, 0.01 mm to 1.0 mm, and may be 0.05 mm to 0.5 mm. 
     The second joining layer  52  is disposed between the interposed layer  40  and the piezoelectric film  35 . In the present embodiment, the second joining layer  52  is a layer having pressure-sensitive adhesiveness or adhesiveness. In other words, the second joining layer  52  is an adhesive layer or a pressure-sensitive adhesive layer. The second joining layer  52  is in contact with the interposed layer  40  and the piezoelectric film  35 . The second joining layer  52  joins the interposed layer  40  and the piezoelectric film  35 . Examples of the second joining layer  52  include a double-faced tape including a substrate and a pressure-sensitive adhesive applied to the both sides of the substrate. Examples of the substrate of the double-faced tape used as the second joining layer  52  include non-woven fabric. Examples of the pressure-sensitive adhesive of the double-faced tape used as the second joining layer  52  include pressure-sensitive adhesives including an acrylic resin. The second joining layer  52  may be a layer including no substrate and formed of a pressure-sensitive adhesive. 
     The thickness of the second joining layer  52  is, for example, 0.01 mm to 1.0 mm, and may be 0.05 mm to 0.5 mm. 
     In the present embodiment, the piezoelectric film  35  is integrated with the layers on the first joining face  17  side by bringing the second joining layer  52  into contact with the piezoelectric film  35 . 
     A structure in which the piezoelectric speaker  10  of  FIG. 1  is stuck to a support  80  with the aid of the first joining face  17  is shown in  FIG. 3 . In this state, voltage is applied to the piezoelectric film  35  through the lead wires. This vibrates the piezoelectric film  35 , and thus sound is emitted from the piezoelectric film  35 . In the example in  FIG. 3 , the support  80  has a flat surface, the piezoelectric speaker  10  is stuck to the flat surface, and the piezoelectric film  35  is extended flat thereon. This implementation is advantageous in that a sound wave radiated from the piezoelectric film  35  is close to a plane wave. When the support  80  has a curved surface, the piezoelectric speaker  10  may be stuck onto the curved surface. 
     The area of a surface of the support  80 , the surface facing the first joining face  17 , is typically equal to or greater than the area of the first joining face  17 . The former area is, for example, 1.0 or more times greater than the latter area, and may be 1.5 or more times or 5 or more times greater than the latter area. The support  80  typically has a high stiffness (a product of Young&#39;s modulus and the second moment of area), a high Young&#39;s modulus, and/or a great thickness, compared to the interposed layer  40 . The support  80  may have the same stiffness, Young&#39;s modulus, and/or thickness as that of the interposed layer  40 , or may have a lower stiffness, a lower Young&#39;s modulus, and/or a smaller thickness than that of the interposed layer  40 . The support  80  has a Young&#39;s modulus of, for example, 1 GPa or more, and may have a Young&#39;s modulus of 10 GPa or more or 50 GPa or more. The upper limit of the Young&#39;s modulus of the support  80  is, for example, but not particularly limited to, 1000 GPa. Since various articles can be employed as the support  80 , it is difficult to define the range of the thickness thereof. The thickness of the support  80  is, for example, 0.1 mm or more, and may be 1 mm or more, 10 mm or more, or 50 mm or more. The upper limit of thickness of the support  80  is, for example, but not particularly limited to, 1 m. The position and/or the shape of the support  80  typically does not vary depending on the piezoelectric speaker  10 . The support  80  is typically produced on the assumption that the support  80  is not bent. 
     The piezoelectric speaker  10  fixed to the support  80  can be used as an acoustic speaker and as a speaker for sound reduction. 
     In the piezoelectric speaker  10 , both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole. The piezoelectric speaker  10  as described above can exhibit practical acoustic characteristics even when the piezoelectric film  35  is not kept in a curved shape. 
     Specifically, both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole when voltage is applied between the first electrode  61  and the second electrode  62  with the entire first joining face  17  stuck to the support  80 , the entire first joining face  17  being a principal surface of the first joining layer  51 , the principal surface being opposite to the interposed layer  40 . That a principal surface vibrates up and down as a whole means that the principal surface vibrates so as to be displaced to two opposite sides in a thickness direction of the piezoelectric film  35  with reference to the position at which the principal surface lies when voltage is not applied between the first electrode  61  and the second electrode  62 . 
     That “both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole” is a concept including not only the case where the completely entire regions of both principal surfaces vibrate but also the case where substantially entire regions of both principal surfaces vibrate. Specifically, that “both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole” means that a region accounting for 90% or more of the area of one surface (a supported face  16 ) of both principal surfaces vibrates up and down and a region accounting for 90% or more of the area of the other surface of both principal surfaces vibrates up and down. A region accounting for 95% or more of the area of the one surface may vibrate up and down, or a region accounting for 100% of the area of the one surface may vibrate up and down. A region accounting for 95% or more of the area of the other surface may vibrate up and down, or a region accounting for 100% of the area of the other surface may vibrate up and down. 
     An outer edge of the one surface of the piezoelectric film  35  has a closed outline. Typically, a portion accounting for 90% or more of the closed outline vibrates up and down. A portion accounting for 95% or more of the closed outline may vibrate up and down, or a portion accounting for 100% of the closed outline may vibrate up and down. 
     An outer edge of the other surface of the piezoelectric film  35  has a closed outline. Typically, a portion accounting for 90% or more of the closed outline vibrates up and down. A portion accounting for 95% or more of the closed outline may vibrate up and down, or a portion accounting for 100% of the closed outline may vibrate up and down. 
     Typically, the piezoelectric speaker  10  does not include a fixing member (such as a screw or a self-tightening member tightened by swaging) that fixes a portion of the piezoelectric film  35  to the support  80  and that allows the portion to function as a node of vibration of the piezoelectric film  35  when voltage is applied between the first electrode  61  and the second electrode  62  with the entire first joining face  17  stuck to the support  80 . 
     In a non-limiting embodiment, the principal surface on the piezoelectric film  35  side of the interposed layer  40  vibrates up and down as a whole. Specifically, the principal surface on the piezoelectric film  35  side of the interposed layer  40  vibrates up and down as a whole when voltage is applied between the first electrode  61  and the second electrode  62  with the entire first joining face  17  stuck to the support  80 . 
     That “the principal surface on the piezoelectric film  35  side of the interposed layer  40  vibrates up and down as a whole” is a concept including not only the case where the completely entire region of the principal surface vibrates but also the case where a substantially entire region of the principal surface vibrates. Specifically, that “the principal surface on the piezoelectric film  35  side of the interposed layer  40  vibrates up and down as a whole” means that a region accounting for 90% or more of the area of the principal surface vibrates up and down. A region accounting for 95% or more of the area of the principal surface may vibrate up and down, or a region accounting for 100% of the area of the principal surface may vibrate up and down. 
     How the piezoelectric film  35  vibrates will be described with reference to  FIGS. 4A and 4B . In  FIGS. 4A and 4B , a reference position RP represents the position of the supported face  16  of the piezoelectric film  35  in a resting state. The supported face  16  refers to the principal surface on the interposed layer  40  side of the piezoelectric film  35 . The first joining layer  51  and the second joining layer  52  are not illustrated in  FIGS. 4A and 4B . In the description using  FIGS. 4A and 4B , the term “thickness direction” refers to a thickness direction of the piezoelectric film  35  in a resting state, and the term “in-plane direction” refers to a direction perpendicular to the thickness direction. 
       FIG. 4A  shows the piezoelectric film  35  vibrating at a relatively low frequency. 
     In the example shown in  FIG. 4A , the piezoelectric film  35  is in the state shown on the left side of the block arrow at a certain moment. In this state, a central region, which is defined in the in-plane direction, of the supported face  16  of the piezoelectric film  35  is displaced to the support  80  side with respect to the reference position RP, and an outer region of the supported face  16  of the piezoelectric film  35  is displaced to the side opposite to the support  80  with respect to the reference position RP. A principal surface of the piezoelectric film  35 , the principal surface being opposite to the supported face  16 , is also displaced in the same manner with reference to the position of the principal surface in a resting state. 
     In the example shown in  FIG. 4A , the piezoelectric film  35  is in the state shown on the right side of the block arrow at a different moment. In this state, the central region, which is defined in the in-plane direction, of the supported face  16  of the piezoelectric film  35  is displaced to the side opposite to the support  80  with respect to the reference position RP, and the outer region of the supported face  16  of the piezoelectric film  35  is displaced to the support  80  side with respect to the reference position RP. A principal surface of the piezoelectric film  35 , the principal surface being opposite to the supported face  16 , is also displaced in the same manner with reference to the position of the principal surface in a resting state. 
       FIG. 4B  shows the piezoelectric film  35  vibrating at a relatively high frequency. 
     In the example shown in  FIG. 4B , the piezoelectric film  35  is in the state shown on the left side of the block arrow at a certain moment. In this state, a portion where the supported face  16  is displaced to the side opposite to the support  80  with respect to the reference position RP, a portion where the supported face  16  is displaced to the support  80  side with respect to the reference position RP, a portion where the supported face  16  is displaced to the side opposite to the support  80  with respect to the reference position RP, a portion where the supported face  16  is displaced to the support  80  side with respect to the reference position RP, and a portion where the supported face  16  is displaced to the side opposite to the support  80  with respect to the reference position RP are arranged in this order along the in-plane direction of the piezoelectric film  35 . A principal surface of the piezoelectric film  35 , the principal surface being opposite to the supported face  16 , is also displaced in the same manner with reference to the position of the principal surface in a resting state. 
     In the example shown in  FIG. 4B , the piezoelectric film  35  is in the state shown on the right side of the block arrow at a different moment. In this state, a portion where the supported face  16  is displaced to the support  80  side with respect to the reference position RP, a portion where the supported face  16  is displaced to the side opposite to the support  80  with respect to the reference position RP, a portion where the supported face  16  is displaced to the support  80  side with respect to the reference position RP, a portion where the supported face  16  is displaced to the side opposite to the support  80  with respect to the reference position RP, and a portion where the supported face  16  is displaced to the support  80  side with respect to the reference position RP are arranged in this order along the in-plane direction of the piezoelectric film  35 . A principal surface of the piezoelectric film  35 , the principal surface being opposite to the supported face  16 , is also displaced in the same manner with reference to the position of the principal surface in a resting state. 
     In the embodiments shown in  FIGS. 4A and 4B , the principal surface on the piezoelectric film  35  side of the interposed layer  40  also vibrates up and down as a whole. 
     The piezoelectric film  35  whose both principal surfaces vibrate up and down as a whole is advantageous in that the piezoelectric film  35  exhibits practical acoustic characteristics. In the present embodiment, the interposed layer  40  is disposed between the piezoelectric film  35  and the first joining layer  51 . The presence of the interposed layer  40  is thought to contribute to allowing the piezoelectric film  35  to exhibit practical acoustic characteristics. Hereinafter, the piezoelectric speaker  10  according to the present embodiment, including the interposed layer  40 , will be further described. 
     In the present embodiment, the interposed layer  40  prevents difficulty in emitting lower-frequency sound in the audible range from increasing as a result of sticking the first joining face  17  of the piezoelectric speaker  10  to the support  80 . It is likely that lower-frequency sound in the audible range is easily generated from the piezoelectric film  35  owing to the interposed layer  40  adequately holding one principal surface of the piezoelectric film  35 , although the detail of the effect needs to be studied in the future. 
     It is thought that adequate holding, which is mentioned above, is achieved by appropriate selection of the holding degree of the interposed layer  40 . The interposed layer  40  has a holding degree of, for example, 5×10 8  N/m 3  or less. The interposed layer  40  has a holding degree of, for example, 1×10 4  N/m 3  or more. The interposed layer  40  preferably has a holding degree of 2×10 8  N/m 3  or less and more preferably 1×10 5  to 5×10 7  N/m 3 . The holding degree (N/m 3 ) of the interposed layer  40  is a value obtained by dividing a product of the elastic modulus (N/m 2 ) of the interposed layer  40  and the surface filling factor of the interposed layer  40  by the thickness (m) of the interposed layer  40 , as represented by the following equation. The surface filling factor of the interposed layer  40  is the filling factor (a value obtained by subtracting the porosity from 1) of the principal surface on the piezoelectric film  35  side of the interposed layer  40 . When pores of the interposed layer  40  are evenly distributed, the surface filling factor can be regarded as equal to a three-dimensionally determined filling factor of the interposed layer  40 . 
       Holding degree (N/m 3 )=Elastic modulus (N/m 2 )×Surface filling factor÷Thickness (m)
 
     The holding degree can be considered to be a parameter representing the degree of holding the piezoelectric film  35  by means of the interposed layer  40 . The above equation indicates that the greater the elastic modulus of the interposed layer  40  is, the greater the degree of holding becomes. The above equation indicates that the greater the surface filling factor of the interposed layer  40  is, the greater the degree of holding becomes. The above equation indicates that the smaller the thickness of the interposed layer  40  is, the greater the degree of holding becomes. Although the relationship between the holding degree of the interposed layer  40  and sound generated from the piezoelectric film  35  needs to be studied in the future, it is likely that an excessively great holding degree prevents the piezoelectric film  35  from deforming, which is necessary to emit lower-frequency sound. On the other hand, when the holding degree is excessively small, it is likely that the piezoelectric film  35  does not sufficiently deform in its thickness direction and extends and contracts only in its in-plane direction (the direction perpendicular to the thickness direction) and thus generation of lower-frequency sound is prevented. It is thought that since the holding degree of the interposed layer  40  is set within an adequate range, extension and contraction of the piezoelectric film  35  in the in-plane direction is adequately converted into deformation thereof in the thickness direction and that results in appropriate bending of the piezoelectric film  35  as a whole and makes it easy to generate lower-frequency sound. 
     As can be understood from the above description, there may be a layer other than the interposed layer  40  between the piezoelectric film  35  and the first joining face  17 . The other layer is, for example, a second pressure-sensitive adhesive layer  52 . 
     The support  80  may have a greater holding degree than that of the interposed layer  40 . In this case as well, lower-frequency sound can be generated from the piezoelectric film  35  because of the contribution of the interposed layer  40 . The support  80  may have the same holding degree as that of the interposed layer  40 , or may have a smaller holding degree than that of the interposed layer  40 . The holding degree (N/m 3 ) of the support  80  is a value obtained by dividing a product of the elastic modulus (N/m 2 ) of the support  80  and the surface filling factor of the support  80  by the thickness (m) of the support  80 . The surface filling factor of the support  80  is the filling factor (a value obtained by subtracting the porosity from 1) of the principal surface on the piezoelectric film  35  side of the support  80 . 
     In the piezoelectric speaker  10  of the present embodiment, both the interposed layer  40  and the first joining layer  51  support the entire supported face  16 . 
     That “both the interposed layer  40  and the first joining layer  51  support the entire supported face  16 ” is a concept including not only the case where both the interposed layer  40  and the first joining layer  51  support the completely entire region of the supported face  16  but also the case where both the interposed layer  40  and the first joining layer  51  support a substantially entire region of the supported face  16 . Specifically, that “both the interposed layer  40  and the first joining layer  51  support the entire supported face  16 ” means that the interposed layer  40  supports a region accounting for 90% or more of the area of the supported face  16  and the first joining layer  51  supports a region accounting for 90% or more of the area of the supported face  16 . The interposed layer  40  may support a region accounting for 95% or more of the area of the supported face  16 , or the interposed layer  40  may support a region accounting for 100% of the area of the supported face  16 . The first joining layer  51  may support a region accounting for 95% or more of the area of the supported face  16 , or the first joining layer  51  may support a region accounting for 100% of the area of the supported face  16 . 
     The above piezoelectric speaker  10  is easily stuck to the support  80 . The above piezoelectric speaker  10  can exhibit practical acoustic characteristics when stuck to the support  80 . 
     In the present embodiment, when voltage is applied between the first electrode  61  and the second electrode  62  with the first joining layer  51  stuck to the support  80 , the interposed layer  40  and the first joining layer  51  withstand vibration of the piezoelectric film  35  and support the entire supported face  16 . 
     In the present embodiment, the interposed layer  40  is joined to the entire supported face  16 . Specifically, the second joining layer  52  joins the entire supported face  16  to the interposed layer  40 . More specifically, the second joining layer  52  is in contact with the entire supported face  16 . When voltage is applied between the first electrode  61  and the second electrode  62  with the first joining layer  51  stuck to the support  80 , the piezoelectric film  35  vibrates with the entire supported face  16  joined to the interposed layer  40  with the aid of the second joining layer  52 . 
     The piezoelectric speaker  10  according to the present invention can be interpreted as follows. 
     A piezoelectric speaker  10 , including: 
     a first electrode  61 ; 
     a second electrode  62 ; 
     a piezoelectric film  35  including a piezoelectric body  30  sandwiched by the first electrode  61  and the second electrode  62 ; 
     a first joining layer  51  having pressure-sensitive adhesiveness or adhesiveness; and 
     an interposed layer  40  disposed between the piezoelectric film  35  and the first joining layer  51 , wherein 
     when a principal surface of the piezoelectric film  35 , the principal surface on a side of the interposed layer  40 , is defined as a supported face  16 , the interposed layer  40  is joined to the entire supported face  16 . 
     The piezoelectric speaker  10  as described above can exhibit practical acoustic characteristics even when the piezoelectric film  35  is not kept in a curved shape. 
     That “the interposed layer  40  is joined to the entire supported face  16 ” is a concept including not only the case where the interposed layer  40  is joined to the completely entire region of the supported face  16  but also the case where the interposed layer  40  is joined to a substantially entire region of the supported face  16 . Specifically, that “the interposed layer  40  is joined to the entire supported face  16 ” means that the interposed layer  40  is joined to a region accounting for 90% or more of the area of the supported face  16 . The interposed layer  40  may be joined to a region accounting for 95% or more of the area of the supported face  16 , or may be joined to a region accounting for 100% of the supported face  16 . Likewise, in the context of the sentence “the second joining layer  52  joins the entire supported face  16  to the interposed layer  40 ” as well, “the entire supported face  16 ” can be interpreted as a region accounting for 90% or more of the area of the supported face  16 , a region accounting for 95% or more of the area of the supported face  16 , or a region accounting for 100% of the area of the supported face  16 . 
     Likewise, that “the second joining layer  52  is in contact with the entire supported face  16 ” is a concept including not only the case where the second joining layer  52  is in contact with the completely entire region of the supported face  16  but also the case where the second joining layer  52  is in contact with a substantially entire region of the supported face  16 . Specifically, that “the second joining layer  52  is in contact with the entire supported face  16 ” means that the second joining layer  52  is in contact with a region accounting for 90% or more of the area of the entire supported face  16 . The second joining layer  52  may be in contact with a region accounting for 95% or more of the area of the entire supported face  16 , or may be in contact with a region accounting for 100% of the area of the entire supported face  16 . 
     As can be understood from the above description, in the piezoelectric speaker  10 , the interposed layer  40  may be disposed on a region accounting for 90% or more of the area of the piezoelectric film  35 , on a region accounting for 95% or more of the area of the piezoelectric film  35 , or on a region accounting for 100% of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. The first joining layer  51  may be disposed on a region accounting for 90% or more of the area of the piezoelectric film  35 , on a region accounting for 95% or more of the area of the piezoelectric film  35 , or on a region accounting for 100% of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. Furthermore, the second joining layer  52  may be disposed on a region accounting for 90% or more of the area of the piezoelectric film  35 , on a region accounting for 95% or more of the area of the piezoelectric film  35 , or on a region accounting for 100% of the area of the piezoelectric film  35  when the piezoelectric film  35  is viewed in plan. 
     When the interposed layer  40  is a porous body, the rate of the region where the interposed layer  40  is disposed is not defined from a microscopical perspective in consideration of pores in the porous structure of the interposed layer  40 , but rather from a relatively macroscopic perspective. For example, when the piezoelectric film  35  and the interposed layer  40  that is a porous body are plate-like bodies having the same outline in plan, the interposed layer  40  is described as being disposed on a region accounting for 100% of the area of the piezoelectric film  35 . 50% or more of a principal surface  15  of the piezoelectric speaker  10 , the principal surface  15  being opposite to the first joining face  17 , can be composed of the piezoelectric film  35 . 75% or more of the principal surface  15  may be composed of the piezoelectric film  35 , or the entire principal surface  15  may be composed of the piezoelectric film  35 . 
     In the present embodiment, layers located between the piezoelectric film  35  and the first joining face  17  and adjacent to each other are joined together. The location between the piezoelectric film  35  and the first joining face  17  includes the piezoelectric film  35  and the first joining face  17 . Specifically, the first joining layer  51  and the interposed layer  40  are joined together, the interposed layer  40  and the second joining layer  52  are joined together, and the second joining layer  52  and the piezoelectric film  35  are joined together. This allows the piezoelectric film  35  to be stably disposed regardless of the orientation in which the piezoelectric speaker  10  is attached to the support  80 . This also makes it easy to attach the piezoelectric speaker  10  to the support  80 . Moreover, because of the contribution of the interposed layer  40 , sound is emitted from the piezoelectric film  35  regardless of the orientation in which the piezoelectric speaker  10  is attached. Thus, in the present embodiment, the combination of these allows achievement of the piezoelectric speaker  10  of high usability. 
     In the present embodiment, the first joining layer  51  has a uniform thickness. The interposed layer  40  has a uniform thickness. The piezoelectric film  35  has a uniform thickness. Their having uniform thicknesses is often advantageous from various points of view, for example, in view of storage of the piezoelectric speaker  10 , the usability thereof, and control of sound emitted from the piezoelectric film  35 . “Having a uniform thickness” refers to, for example, having the smallest thickness which is 70% or more and 100% or less of the largest thickness. The smallest thickness of each of the first joining layer  51 , the interposed layer  40 , and the piezoelectric film  35  may be 85% or more and 100% or less of the largest thickness. The smallest thickness of each of the first joining layer  51 , the interposed layer  40 , and the piezoelectric film  35  may be 90% or more and 100% or less of the largest thickness. “Having a uniform thickness” also refers to having a uniform thickness in a resting state. 
     In the example shown in  FIG. 1 , the first joining face  17  of the first joining layer  51  is a bear surface. The first joining face  17  may be covered by a release layer capable of being separated from the first joining face  17 . Typically, the release layer covers a region accounting for 100% of the area of the first joining face  17 . The release layer, for example, includes a film and a release agent applied to the principal surface on the first joining face  17  side of the film. Examples of the film of the release layer include paper and resin films. Examples of the release agent of the release layer include polymers having a long-chain alkyl group, fluorine-containing compounds and polymers, and silicone polymers. The piezoelectric film  35  and the interposed layer  40  are allowed to be fixed to the support  80  by sticking the first joining face  17  from which the release layer has been removed to the support  80 . Such a sticking procedure is less troublesome to a person who does the procedure. 
     Resin is a material less likely to be cracked than, for example, ceramics. In a specific example, the piezoelectric body  30  of the piezoelectric film  35  is a resin film and the interposed layer  40  is a resin layer not functioning as a piezoelectric film. This specific example is advantageous in that the piezoelectric speaker  10  is cut, for example, with scissors or by hand without cracking the piezoelectric body  30  or the interposed layer  40 . Additionally, in this specific example, the piezoelectric body  30  or the interposed layer  40  is unlikely to be cracked by bending the piezoelectric speaker  10 . Moreover, that the piezoelectric body  30  is a resin film and the interposed layer  40  is a resin layer is advantageous in that the piezoelectric speaker  10  is fixed onto a curved surface without cracking the piezoelectric body  30  or the interposed layer  40 . 
     In the example in  FIG. 1 , the piezoelectric film  35 , the interposed layer  40 , the first joining layer  51 , and the second joining layer  52  are each a rectangle having a short side and a long side when viewed in plan. The piezoelectric film  35 , the interposed layer  40 , the first joining  51 , and the second joining layer  52  each may be, for example, a square, a circle, or an oval. 
     The piezoelectric speaker may also include a layer other than the layers shown in  FIG. 1 . 
     EXAMPLES 
     The present invention will be described in detail using Examples. It should be noted that Examples given below are only illustrative of the present invention and do not limit the present invention. 
     Example 1 
     A first joining face  17  of a piezoelectric speaker  10  was stuck to a supporting member fixed. A structure in which the supporting member is used as a support  80  as in  FIG. 3  was thus produced. Specifically, a 5-mm-thick stainless steel plate (SUS plate) was used as the supporting member. A 0.16-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric both sides of which were impregnated with an acrylic adhesive was used as the first joining layer  51 . A 3-mm-thick closed-cell foam obtained by foaming a mixture including ethylene propylene rubber and butyl rubber by a foaming factor of about  10  was used as the interposed layer  40 . A 0.15-mm-thick pressure-sensitive adhesive sheet (double-faced tape) including non-woven fabric as a substrate to the both sides of which a pressure-sensitive adhesive including a solventless acrylic resin was applied was used as the second joining layer  52 . A polyvinylidene fluoride film on each side of which a copper electrode (including nickel) was vapor-deposited (total thickness: 33 μm) was used as the piezoelectric film  35 . The first joining layer  51 , the interposed layer  40 , the second joining layer  52 , and the piezoelectric film  35  of Example 1 each have dimensions of 37.5 mm long by 37.5 mm wide when viewed in plan, each have the shape of a plate which is neither divided nor frame-shaped, and have outlines overlapping when viewed in plan. (The same applies to Examples and Reference Example described later.) The supporting member  80  has dimensions of 50 mm long by 50 mm wide when viewed in plan and covers the entire first joining layer  51 . A sample of Example 1 having a structure as shown in  FIG. 3  was produced in this manner. 
     Example 2 
     A 3-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about  10  was used as an interposed layer  40 . This foam includes sulfur. Except for that, a sample of Example 2 was produced in the same manner as in Example 1. 
     Example 3 
     A 5-mm-thick foam formed of the same material and having the same configuration as those of the interposed layer  40  of Example 2 was used as an interposed layer  40  in Example 3. Except for that, a sample of Example 3 was produced in the same manner as in Example 2. 
     Example 4 
     A 10-mm-thick foam formed of the same material and having the same configuration as those of the interposed layer  40  of Example 2 was used as an interposed layer  40  in Example 4. Except for that, a sample of Example 4 was produced in the same manner as in Example 2. 
     Example 5 
     A 20-mm-thick foam formed of the same material and having the same configuration as those of the interposed layer  40  of Example 2 was used as an interposed layer  40  in Example 5. Except for that, a sample of Example 5 was produced in the same manner as in Example 2. 
     Example 6 
     A 20-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about 10 was used as an interposed layer  40 . This foam does not include sulfur and is more flexible than the foams used as the interposed layers  40  of Examples 2 to 5. Except for that, a sample of Example 6 was produced in the same manner as in Example 1. 
     Example 7 
     A 20-mm-thick semi-open-/semi-closed-cell foam obtained by foaming a mixture including ethylene propylene rubber by a foaming factor of about 20 was used as an interposed layer  40 . Except for that, a sample of Example 7 was produced in the same manner as in Example 1. 
     Example 8 
     A porous metal body was used as an interposed layer  40 . This porous metal body is made of nickel and has a pore diameter of 0.9 mm and a thickness of 2.0 mm. A pressure-sensitive adhesive layer same as a first joining layer  51  as used in Example 1 was used as a second joining layer  52 . Except for those, a sample of Example 8 was produced in the same manner as in Example 1. 
     Example 9 
     A 3-mm-thick substrate-less pressure-sensitive adhesive sheet formed of an acrylic pressure-sensitive adhesive was used as an interposed layer  40 . Except for that, a sample of Example 9 was produced in the same manner as in Example 8. 
     Example 10 
     A 5-mm-thick urethane foam was used as an interposed layer  40 . Except for that, a sample of Example 10 was produced in the same manner as in Example 8. 
     Example 11 
     A 10-mm-thick urethane foam was used as an interposed layer  40 . This urethane foam has a smaller pore diameter than that of the urethane foam used as the interposed layer  40  of Example 10. Except for that, a sample of Example 11 was produced in the same manner as in Example 8. 
     Example 12 
     A 5-mm-thick closed-cell acrylonitrile butadiene rubber foam was used as an interposed layer  40 . Except for that, a sample of Example 12 was produced in the same manner as in Example 8. 
     Example 13 
     A 5-mm-thick closed-cell ethylene propylene rubber foam was used as an interposed layer  40 . Except for that, a sample of Example 13 was produced in the same manner as in Example 8. 
     Example 14 
     A 5-mm-thick closed-cell foam in which natural rubber and styrene-butadiene rubber are blended was used as an interposed layer  40 . Except for that, a sample of Example 14 was produced in the same manner as in Example 8. 
     Example 15 
     A 5-mm-thick closed-cell silicone foam was used as an interposed layer  40 . Except for that, a sample of Example 15 was produced in the same manner as in Example 8. 
     Example 16 
     A 10-mm-thick foam formed of the same materials and having the same configuration as those of the interposed layer  40  of Example 1 was used as an interposed layer  40 . A pressure-sensitive adhesive sheet same as the one used as the second joining layer  52  in Example 1 was used as a second joining layer  52 . A 35-μm-thick resin sheet including a corn-derived polylactic acid as a main raw material was used as a piezoelectric body  30  of a piezoelectric film  35 . A first electrode  61  and a second electrode  62  of the piezoelectric film  35  are each formed of a 0.1-μm-thick aluminum film and were formed by vapor deposition. A piezoelectric film  35  having a total thickness of 35.2 μm was thus obtained. Except for those, a sample of Example 16 was produced in the same manner as in Example 1. 
     Reference Example 1 
     A piezoelectric film  35  as used in Example 1 was employed as a sample of Reference Example 1. In Reference Example 1, the sample was placed on a board parallel to the ground without being adhered to the board. 
     The methods for evaluation of the samples according to Examples and Reference Example are as follows. 
     &lt;Thickness of Interposed Layer (Uncompressed State)&gt; 
     The thickness of each of the interposed layers was measured using a thickness gauge. 
     &lt;Elastic Modulus of Interposed Layer&gt; 
     A small piece was cut out from each of the interposed layers. The small piece was subjected to a compression test at ordinary temperature using a tensile tester (“RSA-G2” manufactured by TA Instruments). A stress-strain curve was thus obtained. The elastic modulus was calculated from the initial slope of the stress-strain curve. 
     &lt;Pore Diameter of Interposed Layer&gt; 
     An enlarged image of each of the interposed layers was obtained using a microscope. The average of the pore diameters of the interposed layer was determined by image analysis of the enlarged image. The average thus determined was employed as the pore diameter of the interposed layer. 
     &lt;Porosity of Interposed Layer&gt; 
     A small rectangular cuboid piece was cut out from each of the interposed layers. The apparent density was determined from the volume and the mass of the small rectangular cuboid piece. The apparent density was divided by the density of a matrix (solid, or non-hollow, body) forming the interposed layer. The filling factor was thus calculated. Then, the filling factor was subtracted from 1. The porosity was thus obtained. 
     &lt;Surface Filling Factor of Interposed Layer&gt; 
     For Examples 2 to 15, the filling factor calculated as above is employed as the surface filling factor. For Examples 1 and 16, the surface filling factor is 100% because the interposed layers have a surface skin layer. 
     &lt;Frequency Characteristics of Sample in Terms of Sound Pressure Level&gt; 
     A structure for measurement of the samples of Examples 1 to 16 is shown in  FIG. 5 . An electrically conductive copper foil tape  70  (CU-35C manufactured by 3M) having dimensions of 70 μm thick by 5 mm long by 70 mm wide was attached to a corner of each side of the piezoelectric film  35 . An alligator clip  75  with a cover was attached to each of the electrically conductive copper foil tapes  70 . The electrically conductive copper foil tapes  70  and the alligator clips  75  with covers compose a portion of an electrical pathway used for application of AC voltage to the piezoelectric film  35 . 
     A structure for measurement of the sample of Reference Example 1 is based on the structure of  FIG. 5 . Specifically, as in  FIG. 5 , an electrically conductive copper foil tape  70  was attached to a corner of each side of the piezoelectric film  35 , and an alligator clip  75  with a cover was attached to each of the tapes  70 . The resulting assembly was placed on a board parallel to the ground without being adhered to the board. 
     Block diagrams for measurement of the acoustic characteristics of the samples are shown in  FIG. 6  and  FIG. 7 . Specifically, an output system is shown in  FIG. 6 , and an evaluation system is shown in  FIG. 7 . 
     In the output system shown in  FIG. 6 , a personal computer (a personal computer may hereinafter be simply described as a PC)  401  for audio output, an audio interface  402 , a speaker amplifier  403 , a sample  404  (any of the piezoelectric speakers of Examples and Reference Example) were connected in this order. The speaker amplifier  403  was also connected to an oscilloscope  405  so that output from the speaker amplifier  403  to the sample  404  could be monitored. 
     WaveGene was installed in the PC  401  for audio output. WaveGene is free software for generation of a test audio signal. QUAD-CAPTURE manufactured by Roland Corporation was used as the audio interface  402 . The sampling frequency of the audio interface  402  was set to 192 kHz. A-924 manufactured by Onkyo Corporation was used as the speaker amplifier  403 . DP02024 manufactured by Tektronix, Inc. was used as the oscilloscope  405 . 
     In the evaluation system shown in  FIG. 7 , a microphone  501 , an acoustic evaluation apparatus (PULSE)  502 , and a PC  503  for acoustic evaluation were connected in this order. 
     Type 4939-C-002 manufactured by Bruel &amp; Kjaer Sound &amp; Vibration Measurement A/S was used as the microphone  501 . The microphone  501  was disposed 1 m away from the sample  404 . Type 3052-A-030 manufactured by Bruel &amp; Kjaer Sound &amp; Vibration Measurement A/S was used as the acoustic evaluation apparatus  502 . 
     The output system and the evaluation system were configured in the above manners. AC voltage was applied from the PC  401  for audio output to the sample  404  via the audio interface  402  and the speaker amplifier  403 . Specifically, a test audio signal whose frequency sweeps from 100 Hz to 100 kHz in 20 seconds was generated using the PC  401  for audio output. During this, voltage output from the speaker amplifier  403  was monitored using the oscilloscope  405 . Additionally, sound generated from the sample  404  was evaluated using the evaluation system. A test for measurement of the sound pressure frequency characteristics was performed in this manner. 
     The details of the output system and evaluation system settings are as follows. 
     [Output System Settings]
     Frequency range: 100 Hz to 100 kHz   Sweep time: 20 seconds   Effective voltage: 10 V   Output waveform: sine curve   

     [Evaluation System Settings]
     Measurement time: 22 seconds   Peak hold   Measurement range: 4 Hz to 102.4 kHz   Number of lines: 6400   

     &lt;Determination of Frequency at Which Emission of Sound Starts&gt; 
     The lower end of a frequency domain (exclusive of a sharp peak portion in which a frequency range where the sound pressure level is maintained higher than that of background noise by +3 dB or more falls within ±10% of a peak frequency (a frequency at which the sound pressure level reaches a peak)) where the sound pressure level is higher than that of background noise by 3 dB or more was determined as a frequency at which emission of sound starts. 
     The evaluation results for Examples 1 to 16 and Reference Example 1 are shown in  FIG. 8A  to  FIG. 26 . The frequency characteristics of background noise in terms of sound pressure level are shown in  FIG. 27 . E 1  to E 16  in  FIG. 9  correspond to Examples 1 to 16. 
     &lt;Vibration Evaluation&gt; 
     Furthermore, voltage of 10 V at 2000 Hz was applied to the sample  404  of Example 1 using the output system described with reference to  FIG. 6  to vibrate the sample  404 . For the sample  404  vibrating, displacement of the piezoelectric film  35  in the thickness direction was measured by laser irradiation in a non-contact manner using a laser doppler vibrometer (PSV-400) manufactured by Polytec Japan. The measurement result indicates that both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole. 
     The samples  404  of Examples 2 to 7 were also subjected to the measurement in the same manner. The measurement results for the samples  404  of Examples 2 to 7 also indicate that both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole. 
     The samples  404  of Examples 1 to 7 were subjected to the measurement in the same manner, except that the frequency of voltage was changed from 2000 Hz to another frequency in the audible range. The results of this measurement for the samples  404  of Examples 1 to 7 also indicate that both principal surfaces of the piezoelectric film  35  vibrate up and down as a whole. 
     The vibration evaluation reveals that both principal surfaces of the piezoelectric film  35  can actually vibrate up and down as a whole as schematically shown in  FIGS. 4A and 4B . 
     [Piezoelectric Film-Supporting Structure and Degree of Freedom of Vibration] 
       FIG. 3  is referred back to for an exemplary piezoelectric speaker-supporting structure of the present invention. In the piezoelectric speaker  10 , the entire surface of the piezoelectric film  35  is fixed to the support  80  with the pressure-sensitive adhesive layers  51  and  52  and the interposed layer  40  therebetween. 
     It is also conceivable that a portion of the piezoelectric film  35  is supported to be spaced away from the support  80  in order to prevent the support  80  from hindering vibration of the piezoelectric film  35 . An exemplary supporting structure based on this design concept is shown in  FIG. 28 . In a hypothetical piezoelectric speaker  108  shown in  FIG. 28 , a frame  88  supports a peripheral portion of the piezoelectric film  35  at a position distant from the support  80 . 
     It is easy to ensure a sufficient volume of sound emitted from a piezoelectric film already curved and fixed in one direction. Therefore, it is conceivable that, for example, in the piezoelectric speaker  108 , a nonuniformly thick interposition having a convex upper surface is disposed in a space  48  surrounded by the piezoelectric film  35 , the frame  88 , and the support  80  and a central portion of the piezoelectric film  35  is pushed upward. However, such an interposition is not joined to the piezoelectric film  35  so as not to hinder vibration of the piezoelectric film  35 . Therefore, even with the interposition disposed in the space  48 , it is only the frame  88  that supports the piezoelectric film  35  so as to determine vibration of the piezoelectric film  35 . 
     As described above, the piezoelectric speaker  108  shown in  FIG. 28  employs the supporting structure locally supporting the piezoelectric film  35 . On the other hand, as shown in  FIG. 3 , the piezoelectric film  35  of the piezoelectric speaker  10  is not supported at a particular portion. Unexpectedly, the piezoelectric speaker  10  exhibits practical acoustic characteristics in spite of the fact that the entire surface of the piezoelectric film  35  is fixed to the support  80 . Specifically, in the piezoelectric speaker  10 , even a peripheral portion of the piezoelectric film  35  possibly vibrates up and down. The piezoelectric film  35  can vibrate up and down as a whole. Therefore, compared to the piezoelectric speaker  108 , the piezoelectric speaker  10  has a higher degree of freedom of vibration and is relatively advantageous in achieving good sound emission characteristics.