Electroacoustic transducer

Abstract

Provided is an electroacoustic transducer capable of obtaining sufficient sound pressure even when a small piezoelectric film is used. The present invention includes a foam body that has a curved plane on the surface and has a hollow part that is drilled from the surface, and a piezoelectric film that is disposed in the foam body.

Description

electroacoustic transducer

[0001] The present invention relates to an electro-acoustic transducer.

[0002] It has been proposed to use a piezoelectric film in which a piezoelectric layer is sandwiched between an electrode layer and a protective layer as a thin speaker.

[0003] For example, Patent Document 1 describes an electro-acoustic transducer having a piezoelectric film having two opposing main surfaces that expand and contract depending on the state of an electric field, a viscoelastic support that is placed in close contact with one of the main surfaces of the piezoelectric film, and a pressing member that presses the piezoelectric film against the viscoelastic support, thereby holding at least a portion of the viscoelastic support in a thinned state, wherein, in a cross section in a predetermined direction perpendicular to the main surfaces of the piezoelectric film, the piezoelectric film has a flat portion that is held substantially linearly by the surface of the viscoelastic support in at least a portion excluding the pressing portion that is pressed by the pressing member, and an inclined portion that is connected to the pressing portion and the flat portion and extends in a direction intersecting the pressing portion.

[0004] JP 2014-017799 A

[0005] The present inventors have considered using piezoelectric films in earphones, headphones, and the like. When using piezoelectric films in earphones, headphones, and the like, they need to be smaller than when used in speakers, etc. In other words, the area of ​​the piezoelectric film needs to be smaller. However, it has been found that it is difficult to obtain sufficient sound pressure when the area of ​​the piezoelectric film is reduced.

[0006] An object of the present invention is to solve the problems of the prior art and to provide an electroacoustic transducer that can obtain sufficient sound pressure even when a small piezoelectric film is used.

[0007] In order to solve the above-mentioned problems, the present invention has the following configurations. [1] An electroacoustic transducer comprising a foam having a curved surface and a hollow portion drilled from the surface, and a piezoelectric film disposed on the foam. [2] The electroacoustic transducer according to [1], in which the piezoelectric film is disposed on the surface of the foam. [3] The electroacoustic transducer according to [1] or [2], in which the piezoelectric film is disposed on the inner circumferential surface of the hollow portion of the foam. [4] The electroacoustic transducer according to any one of [1] to [3], in which the piezoelectric film is disposed on both the surface of the foam and the inner circumferential surface of the hollow portion. [5] The electroacoustic transducer according to any one of [1] to [4], in which the foam has a cylindrical shape or a spherical shape with a hollow portion formed therein. [6] The electroacoustic transducer according to any one of [1] to [5], in which the cross-section of the opening of the hollow portion has an equivalent circle diameter of 3 mm to 30 mm. [7] The electroacoustic transducer according to any one of [1] to [6], wherein the ratio of the outer diameter of the foam to the circle-equivalent diameter of the cross section of the opening of the hollow portion is 10 to 1.1. [8] The electroacoustic transducer according to any one of [1] to [7], wherein the hollow portion penetrates the foam, and the diameter of the hollow portion at one surface of the foam is larger than the diameter at the other surface. [9] The electroacoustic transducer according to any one of [1] to [8], wherein the foam is a foamed plastic or a foam made primarily of starch.

[10] The electroacoustic transducer according to any one of [1] to [9], wherein the foam has an expansion ratio in the range of 10 to 60.

[11] The electroacoustic transducer according to any one of [1] to

[10] , wherein the piezoelectric film has a piezoelectric layer made of a polymer composite piezoelectric containing piezoelectric particles in a matrix containing a polymer material, and electrode layers provided on both sides of the piezoelectric layer.

[12] The electroacoustic transducer according to

[11] , wherein the foam is spherical, and the piezoelectric film is molded into a convex shape to match the shape of the foam.

[0008] According to the present invention, it is possible to provide an electroacoustic transducer that can obtain sufficient sound pressure even when a small piezoelectric film is used.

[0009] 10 is a perspective view conceptually showing an example of an electroacoustic transducer of the present invention. A cross-sectional view taken along line A-A of the electroacoustic transducer shown in FIG. 1. A cross-sectional view taken along line B-B of the electroacoustic transducer shown in FIG. 2. A cross-sectional view conceptually showing another example of an electroacoustic transducer of the present invention. A cross-sectional view conceptually showing another example of an electroacoustic transducer of the present invention. A cross-sectional view conceptually showing another example of an electroacoustic transducer of the present invention. A cross-sectional view conceptually showing another example of an electroacoustic transducer of the present invention. A perspective view conceptually showing another example of an electroacoustic transducer of the present invention. A view of the electroacoustic transducer shown in FIG. 8 as seen from the direction of arrow C. A cross-sectional view taken along line D-D of the electroacoustic transducer shown in FIG. 9. A cross-sectional view illustrating an example of the configuration of a piezoelectric film. A conceptual diagram illustrating an example of a method for producing a piezoelectric film. A conceptual diagram illustrating an example of a method for producing a piezoelectric film. A conceptual diagram illustrating an example of a method for producing a piezoelectric film.

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The electroacoustic transducer of the present invention will be described in detail below with reference to preferred embodiments shown in the accompanying drawings.

[0011] The following description of the constituent elements may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0012] [Electroacoustic Transducer] The electroacoustic transducer of the present invention is an electroacoustic transducer comprising: a foam having a curved surface on the surface and a hollow portion formed from the surface; and a piezoelectric film disposed on the foam.

[0013] Fig. 1 is a perspective view conceptually showing an example of an electroacoustic transducer of the present invention. Fig. 2 is a cross-sectional view taken along line A-A of the electroacoustic transducer of Fig. 1. Fig. 3 is a cross-sectional view taken along line B-B of the electroacoustic transducer of Fig. 2.

[0014] The electroacoustic transducer 100 shown in FIGS. 1 to 3 includes a foam member 102 a, a piezoelectric film 10 , and an adhesive tape 106 .

[0015] The foam 102a is a member made of a foam material, has a curved surface 104a on at least a portion of its surface (outer surface), and has a hollow portion 103a drilled from the surface of the foam 102a. In the illustrated example, the hollow portion 103a has an opening cross-sectional shape (see FIG. 3) that is approximately circular and has a constant diameter.

[0016] In the illustrated example, the cross section of the opening of the hollow portion 103a is substantially circular, but this is not limited to this and various shapes such as elliptical, polygonal, or irregular shapes may be used.

[0017] 1 to 3, the foam 102a has a generally cylindrical shape, with the bottom and top surfaces each protruding outward in a generally spherical crown shape. The foam 102a has a hollow portion 103a that penetrates from the bottom surface to the top surface.

[0018] The shape of the foam 102a is not limited to a substantially cylindrical shape, but may be any cylindrical shape. For example, the outer shape of the cross section perpendicular to the height direction may be various shapes such as an ellipse, a polygon, or an irregular shape. Furthermore, although the foam 102a has a bottom surface and a top surface each protruding outward in a substantially spherical crown shape, the shape is not limited to this. For example, the bottom surface and the top surface may be substantially flat.

[0019] Examples of foam materials that can be used to form the foam 102a include various foam plastics such as polystyrene foam, styrene board, etc. Foam materials whose main ingredient is starch such as cornstarch, cork board, etc. can also be used.

[0020] The piezoelectric film 10 includes, for example, a piezoelectric layer having piezoelectric properties, electrode layers provided on both sides of the piezoelectric layer, and a protective layer provided on the electrode layer. When a voltage is applied to the electrode layers (electrode pair) sandwiching the piezoelectric layer, the piezoelectric layer expands and contracts in response to the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer) contracts in the thickness direction. At the same time, due to the Poisson's ratio, the piezoelectric film 10 also expands and contracts in the plane direction. When the piezoelectric film 10 is held in a curved state, this expansion and contraction in the plane direction becomes vibration in the thickness direction, generating sound. In other words, the electroacoustic transducer 100 vibrates in response to the magnitude of the voltage (drive voltage) applied to the piezoelectric film 10, generating sound in response to the drive voltage applied to the piezoelectric film 10. Details of the piezoelectric film 10 will be described later.

[0021] The piezoelectric film 10 is arranged so as to be wrapped around the peripheral surface 104a of the foam 102a. That is, the piezoelectric film 10 is arranged on the curved peripheral surface 104a of the foam 102a. The bottom and top ends of the piezoelectric film 10 are fixed to the foam 102a with adhesive tape 106.

[0022] In the illustrated example, the piezoelectric film 10 is wrapped around the entire circumferential area of ​​the peripheral surface 104a of the foam 102a. However, the present invention is not limited to this, and the piezoelectric film 10 may be wrapped around only a portion of the circumferential area of ​​the peripheral surface 104a of the foam 102a.

[0023] As mentioned above, when a piezoelectric film is used in earphones, headphones, etc., it needs to be smaller than when it is used in speakers, etc. However, if the area of ​​the piezoelectric film is small, it is difficult to obtain sufficient sound pressure.

[0024] In contrast, the electroacoustic transducer 100a of the present invention shown in Figures 1 to 3 includes a foam 102a having a curved surface 104a on its surface and a hollow portion 103a drilled from the surface, and a piezoelectric film 10 disposed on the curved surface 104a of the foam 102a. In this electroacoustic transducer 100a, when a voltage is applied to the piezoelectric film 10, the piezoelectric film 10 expands and contracts in the circumferential direction of the foam 102a, exciting a concentric vibration mode in the foam 102a, resulting in the emission of sound waves into the hollow portion 103a. The vibration also vibrates the foam 102a, emitting sound waves. The sound waves pass through the hollow portion 103a and are emitted to the outside through the opening of the hollow portion 103a. Since the direction of sound wave propagation is restricted by the hollow portion 103a, the sound pressure in the direction of sound wave propagation can be improved. In other words, the vibration energy of the piezoelectric film is efficiently collected in the hollow portion 103a by the ring-shaped structure and converted into sound waves. Therefore, when the electro-acoustic transducer 100a is used in earphones, headphones, etc., by positioning the opening of the hollow portion 103a toward the ear, a sufficient volume can be provided to the user. At the same time, the hollow structure of the foam 102a also allows external environmental sounds to be taken in.

[0025] Furthermore, by using foam 102a, the foam 102a itself functions as a vibration transmission medium while maintaining its shape as a support for the piezoelectric film 10, and the vibration energy emitted by the piezoelectric film 10 can be propagated to the hollow portion 103a without attenuation (with reduced attenuation).

[0026] From the viewpoint of maintaining the shape of the support, vibrating itself, and propagating sound waves with high efficiency, the expansion ratio of foam 102a is preferably 10 to 60, more preferably 20 to 50, and even more preferably 30 to 40. The expansion ratio refers to the weight per unit volume, with 1 liter / kg being taken as 1.

[0027] Although there are no particular limitations on the circle-equivalent diameter of the opening cross section of hollow section 103a drilled in foam 102a, from the viewpoint of radiating acoustic energy into the auditory canal without loss, it is preferably 2 mm to 30 mm, more preferably 3 mm to 20 mm, and even more preferably 5 mm to 10 mm. Note that if the opening cross section of hollow section 103a has a shape other than a circle, the circle-equivalent diameter may be within the above range.

[0028] Furthermore, from the viewpoint of allowing foam 102a to vibrate itself while maintaining its shape and propagating vibrations with high efficiency, the ratio D0 / D1 of the outer diameter (equivalent circle diameter) D0 of foam 102a to the equivalent circle diameter D1 of the opening cross section of hollow section 103a when viewed in a cross section perpendicular to the drilling direction of hollow section 103a (see Figure 3) is preferably 10 to 1.1 times, more preferably 5 to 1.3 times, and even more preferably 3 to 1.5 times.

[0029] From the viewpoint of radiating acoustic energy to the ear canal without loss, the length of hollow portion 103a in the drilling direction is preferably 5 mm to 40 mm, more preferably 10 mm to 30 mm, and even more preferably 15 mm to 20 mm.

[0030] There is no particular limitation on the length of the piezoelectric film 10 in the drilling direction of the hollow portion 103 a. From the viewpoint of radiating acoustic energy into the auditory canal without loss, the length of the piezoelectric film 10 is preferably 5 mm to 40 mm, more preferably 10 mm to 30 mm, and even more preferably 15 mm to 20 mm.

[0031] In addition, in the example shown in Figures 1 to 3, one piezoelectric film 10 is arranged on the peripheral surface 104a of the foam 102a, but this is not limited to this, and two or more piezoelectric films 10 may be arranged on the peripheral surface 104a of the foam 102a.

[0032] 1 to 3, the end of the piezoelectric film 10 is fixed to the foam 102a with adhesive tape 106, but this is not limiting. For example, the piezoelectric film 10 and the foam 102a may be attached with an adhesive layer or a sticky layer. Furthermore, this is not limiting, either, to a configuration in which only the end of the piezoelectric film 10 is fixed to the foam 102a, and the entire surface of the piezoelectric film 10 facing the foam 102a may be attached to the foam 102a.

[0033] 2, the hollow portion 103a is configured to penetrate the foam 102a, but is not limited to this. The hollow portion may be non-penetrating.

[0034] Fig. 4 shows a cross-sectional view of another example of an electroacoustic transducer of the present invention. The electroacoustic transducer 100b shown in Fig. 4 has a foam 102b and a piezoelectric film 10. Note that the adhesive tape that secures the piezoelectric film 10 to the foam 102b is not shown. This also applies to electroacoustic transducers 100c to 100f, which will be described later. Furthermore, the electroacoustic transducer 100b shown in Fig. 4 has a configuration similar to that of the electroacoustic transducer 100a shown in Fig. 2, except for the shape of the hollow portion 103b of the foam 102b. Therefore, in the following description, the same components are denoted by the same reference numerals, and the different components will be mainly described.

[0035] The foam 102b is a member made of a foam material, has a curved surface 104a on at least a portion of its surface (outer surface), and has a hollow portion 103b drilled from the surface of the foam 102b. The hollow portion 103b is drilled from the upper surface of the foam 102b to a predetermined depth. In other words, the hollow portion 103b is a non-through hole.

[0036] Even in electroacoustic transducer 100b in which hollow portion 103b is a non-through hole, a portion of the sound waves emitted by piezoelectric film 10 and foam 102b propagate through foam 102b, reach hollow portion 103b, pass through hollow portion 103a, and are radiated to the outside from the opening of hollow portion 103b. At that time, the traveling direction of the sound waves is restricted by hollow portion 103b, so that the sound pressure in the traveling direction of the sound waves can be improved.

[0037] Here, when the hollow portion 103a penetrates the foam 102a, as in the electro-acoustic transducer 100a, when used in earphones, headphones, etc., the user can fully hear external sounds through the hollow portion 103a.

[0038] On the other hand, when the hollow portion 103b does not penetrate the foam 102b, as in the electro-acoustic transducer 100b, the sound waves that propagate through the foam 102b and reach the hollow portion 103b are radiated to the outside from one opening in the hollow portion 103b, thereby further improving the sound pressure in the direction of travel of the sound waves.

[0039] The preferred ranges for the circle-equivalent diameter of the opening cross section of hollow portion 103b when hollow portion 103b is non-penetrating and the ratio D0 / D1 of the outer diameter (circle-equivalent diameter) D0 of foam 102b to the circle-equivalent diameter D1 of the opening cross section of hollow portion 103b are the same as those for the case where hollow portion 103a is penetrating.

[0040] When the hollow portion 103b is not through-hole, there is no particular limit to the depth of the hollow portion 103b, but from the viewpoint of radiating acoustic energy into the ear canal without loss, it is preferably 2 mm to 30 mm, more preferably 3 mm to 20 mm, and even more preferably 5 mm to 10 mm.

[0041] 2, the electroacoustic transducer 100a has a configuration in which the piezoelectric film 10 is disposed on the surface (circumferential surface) 104a of the foam 102a, but is not limited to this. The piezoelectric film 10 may also be disposed on the inner circumferential surface of the hollow portion 103a.

[0042] 5 and 6 are cross-sectional views showing other examples of the electroacoustic transducer of the present invention.

[0043] The electroacoustic transducer 100c shown in Fig. 5 includes a foam 102a and a piezoelectric film 10. The electroacoustic transducer 100c shown in Fig. 5 has the same configuration as the electroacoustic transducer 100a shown in Fig. 2, except that the piezoelectric film 10 is disposed on the inner circumferential surface of the hollow portion 103a of the foam 102a.

[0044] In the electroacoustic transducer 100c shown in FIG. 5, the piezoelectric film 10 is arranged in a curved manner along the inner circumferential surface of the hollow portion 103a.

[0045] In this manner, when the piezoelectric film 10 is disposed on the inner peripheral surface of the hollow portion 103a, when the piezoelectric film 10 vibrates, the piezoelectric film 10 expands and contracts in the circumferential direction of the foam 102a, exciting a vibration mode in the foam 102a in the concentric direction, resulting in the emission of sound waves into the hollow portion 103a. The vibration also causes the foam 102a to vibrate and emit sound waves. A portion of the sound waves passes through the hollow portion 103a and is emitted to the outside from the opening of the hollow portion 103a, thereby further improving the sound pressure in the direction of sound wave propagation.

[0046] In the example shown in Figure 5, one piezoelectric film 10 is arranged on the inner surface of the hollow portion 103a, but this is not limited to this, and two or more piezoelectric films 10 may be arranged on the inner surface of the hollow portion 103a.

[0047] The electroacoustic transducer 100d shown in Fig. 6 includes a foam 102a and two piezoelectric films 10a and 10b. The electroacoustic transducer 100d shown in Fig. 6 has the same configuration as the electroacoustic transducer 100a shown in Fig. 2, except that it further includes a piezoelectric film 10b disposed in a hollow portion 103a.

[0048] In the electroacoustic transducer 100d shown in Figure 6, the piezoelectric film 10a is arranged so as to be wrapped around the peripheral surface 104a of the foam 102a, and the piezoelectric film 10b is arranged so as to be curved along the inner peripheral surface of the hollow portion 103a of the foam 102a.

[0049] In this way, even when piezoelectric films are disposed on the peripheral surface 104a of the foam 102a and the inner peripheral surface of the hollow portion 103a, when each piezoelectric film expands and contracts in the circumferential direction, the foam 102a vibrates concentrically to emit sound. A portion of the sound waves emitted by the foam 102a passes through the hollow portion 103a and is radiated to the outside from the opening of the hollow portion 103a, thereby further increasing the sound pressure in the direction of propagation of the sound waves.

[0050] Here, there are no particular limitations on the lengths (lengths in the drilling direction of hollow portion 103a) of piezoelectric film 10a disposed on peripheral surface 104a of foam 102a and piezoelectric film 10b disposed on the inner peripheral surface of hollow portion 103a. From the viewpoint of radiating acoustic energy into the auditory canal without loss, the ratio of the length of piezoelectric film 10b to the length of piezoelectric film 10a is preferably 0.3 to 1.0, more preferably 0.5 to 1.0, and even more preferably 0.7 to 1.0.

[0051] In the electroacoustic transducer 100d shown in Fig. 6, the same signal may be applied to the two piezoelectric films 10a and 10b, or different signals may be applied thereto. For example, by applying a mid-low frequency signal to the piezoelectric film with a larger area and a high frequency signal to the piezoelectric film with a smaller area, an electroacoustic transducer can be obtained that can obtain higher sound pressure over a wider frequency band. Alternatively, an electroacoustic transducer may be provided with a noise-canceling function that reduces noise by applying an audio signal to be reproduced to one piezoelectric film and applying a signal that is out of phase with the surrounding environmental sound (noise) to the other piezoelectric film.

[0052] In addition, in the examples shown in Figures 5 and 6, a piezoelectric film is arranged in a hollow portion 103a that penetrates the foam 102a, but even in a configuration using a foam 102b having a non-penetrating hollow portion 103b, as in the example shown in Figure 4, a piezoelectric film may be arranged on the inner surface of the hollow portion 103b.

[0053] 2, the diameter (circle-equivalent diameter) of the cross section of the opening of the hollow portion 103a is constant, but is not limited to this. The diameter (circle-equivalent diameter) of the cross section of the opening of the hollow portion 103a may vary gradually.

[0054] Fig. 7 shows a cross-sectional view of another example of an electroacoustic transducer of the present invention. The electroacoustic transducer 100e shown in Fig. 7 includes a foam 102c and a piezoelectric film 10. The electroacoustic transducer 100e shown in Fig. 7 has the same configuration as the electroacoustic transducer 100a shown in Fig. 2, except for the shape of the hollow portion 103c formed in the foam 102c.

[0055] In the electroacoustic transducer 100e shown in Figure 7, the hollow portion 103c drilled in the foam 102c has a diameter (circle equivalent diameter) on one surface, i.e., diameter D3 of one opening, which is larger than the diameter (circle equivalent diameter) on the other surface, i.e., diameter D2 of the other opening, and the diameter gradually increases from one surface to the other surface.

[0056] In this way, the electro-acoustic transducer 100e, in which the diameter D of one opening of the hollow portion 103c drilled in the foam 102c is larger than the diameter D of the other opening, can collect more sound waves at the opening with the larger diameter D than at the opening with the smaller diameter D. Therefore, when the electro-acoustic transducer 100e is used in earphones, headphones, etc., energy efficiency can be improved by positioning the opening with the larger diameter D so that it faces the user's ear.

[0057] The ratio D3 / D2 of the diameter D3 at one opening of the hollow portion 103c to the diameter D2 at the other opening is preferably 5.0 to 2.0, more preferably 5.0 to 3.0, and even more preferably 5.0 to 4.0.

[0058] As shown in FIG. 7, even in the case where the diameter of the hollow portion 103c changes from one surface to the other, a piezoelectric film may be disposed on the circumferential surface of the hollow portion 103c.

[0059] 4, even when the hollow portion 103b is not through-hole, the diameter (circle-equivalent diameter) of the opening cross section of the hollow portion 103b may vary in the drilling direction of the hollow portion 103b. That is, the diameter at the opening position of the hollow portion 103b may be different from the diameter at the bottom position. In this case, it is preferable that the diameter at the opening position of the hollow portion 103b is larger than the diameter at the bottom position.

[0060] In the example shown in FIGS. 1 to 3, the foam 102a has a substantially cylindrical shape, but is not limited to this and can have any shape as long as it has a hollow portion.

[0061] Fig. 8 shows a perspective view of another example of an electroacoustic transducer of the present invention. Fig. 9 shows a view of the electroacoustic transducer of Fig. 8 as seen from the direction of arrow C. Fig. 10 shows a cross-sectional view of the electroacoustic transducer of Fig. 9 taken along line DD.

[0062] The electroacoustic transducer 100f shown in FIGS. 8 to 10 includes a foam member 102d and a piezoelectric film 10c.

[0063] The foam 102d is made of the same foam material as the foam 102a, and has a substantially spherical outer shape. The foam 102d has a hollow portion 103d that penetrates the foam 102d.

[0064] A piezoelectric film 10c is disposed on a part of the spherical surface of the surface 104b of the foam 102d.

[0065] Like the piezoelectric film 10 described above, the piezoelectric film 10c has a piezoelectric layer with piezoelectric properties, and when a voltage is applied, the piezoelectric layer expands and contracts in response to the applied voltage, vibrating in the thickness direction and generating sound.

[0066] The piezoelectric film 10c is formed in a convex shape that is roughly spherical and conforms to the spherical surface 104b of the foam 102d, and its ends are fixed to the foam 102d by, for example, adhesive tape.

[0067] 1 to 3, the piezoelectric film 10 is curved along the circumferential surface 104a of the substantially cylindrical foam body 102a. In other words, the piezoelectric film 10 is curved in one direction.

[0068] 8 to 10, the piezoelectric film 10c is curved along the surface 104b of the substantially spherical foam 102d. In other words, the piezoelectric film 10c is curved in multiple directions.

[0069] Thus, even in the electroacoustic transducer 100f having a configuration in which the foam 102d is substantially spherical and the piezoelectric film 10c is molded into a substantially spherical crown-like convex shape to match the shape of the foam 102d, when a voltage is applied to the piezoelectric film 10c, the piezoelectric film 10c expands and contracts in the circumferential direction of the foam 102d, exciting a concentric vibration mode in the foam 102d, resulting in the emission of sound waves into the hollow portion 103d. The vibration causes the foam 102d to vibrate and emit sound waves. A portion of the sound waves passes through the hollow portion 103d and is emitted to the outside from the opening of the hollow portion 103d. This further improves the sound pressure in the direction of sound wave propagation.

[0070] In addition, when the piezoelectric film 10c is curved in multiple directions, as in the examples shown in Figures 8 to 10, the piezoelectric film 10c is preferably a piezoelectric film having a piezoelectric layer made of a polymer composite piezoelectric material containing piezoelectric particles in a matrix containing a polymer material, as described below, and electrode layers provided on both sides of the piezoelectric layer. A piezoelectric layer made of a polymer composite piezoelectric material can be polarized in the thickness direction, not in the plane direction. Therefore, a piezoelectric film having a piezoelectric layer made of a polymer composite piezoelectric material does not have in-plane anisotropy in its piezoelectric properties, and when a driving voltage is applied, it expands and contracts isotropically in all directions in the plane direction. Therefore, even when the piezoelectric film 10c is used while being curved in multiple directions, it can expand and contract (vibrate) appropriately when a voltage is applied, and can generate sound.

[0071] 8 to 10, the foam has a generally spherical shape, but is not limited thereto and may have, for example, a generally ellipsoidal shape. Also, in the examples shown in Fig. 8 to 10, the foam has a generally spherical shape, but is not limited thereto and may have a partially spherical shape (a spherical indentation).

[0072] <Piezoelectric Film> The piezoelectric film will be described below. In the following description, the piezoelectric films 10, 10a, 10b, and 10c will be collectively referred to as the piezoelectric film 10 unless there is a need to distinguish between them.

[0073] In the present invention, there are no particular limitations on the piezoelectric film 10, and any conventionally known piezoelectric film can be used as appropriate. For example, as shown in Fig. 11, the piezoelectric film 10 includes a piezoelectric layer 20, which is a sheet-like material having piezoelectricity, a first electrode layer 24 laminated on one side of the piezoelectric layer 20, a first protective layer 28 laminated on the side of the first electrode layer 24 opposite the piezoelectric layer 20, a second electrode layer 26 laminated on the other side of the piezoelectric layer 20, and a second protective layer 30 laminated on the side of the second electrode layer 26 opposite the piezoelectric layer 20. That is, the piezoelectric film 10 has a configuration in which the piezoelectric layer 20 is sandwiched between electrode layers, and a protective layer is laminated on the side of the electrode layer that is not in contact with the piezoelectric layer.

[0074] In the present invention, various known piezoelectric layers can be used as the piezoelectric layer 20. In the present invention, the piezoelectric layer 20 is preferably a polymer composite piezoelectric material that contains piezoelectric particles 36 in a matrix 34 containing a polymer material, as conceptually shown in Fig. 11 .

[0075] Furthermore, it is preferable to use a polymer material that has viscoelasticity at room temperature as the material for the matrix 34 (which also serves as a matrix and binder) of the polymer composite piezoelectric material that constitutes the piezoelectric layer 20. In this specification, "room temperature" refers to a temperature range of about 0 to 50°C.

[0076] Fig. 11 shows an enlarged view of a portion of the piezoelectric film 10. The piezoelectric film 10 shown in Fig. 11 includes a piezoelectric layer 20, which is a sheet-like material having piezoelectricity, a first electrode layer 24 laminated on one side of the piezoelectric layer 20, a first protective layer 28 laminated on the side of the first electrode layer 24 opposite the piezoelectric layer 20, a second electrode layer 26 laminated on the other side of the piezoelectric layer 20, and a second protective layer 30 laminated on the side of the second electrode layer 26 opposite the piezoelectric layer 20. That is, the piezoelectric film 10 has a configuration in which the piezoelectric layer 20 is sandwiched between electrode layers, and a protective layer is laminated on the side of the electrode layer that is not in contact with the piezoelectric layer.

[0077] Here, it is preferable that the polymer composite piezoelectric material (piezoelectric material layer 20) satisfies the following requirements.

[0078] (i) Flexibility: For example, when a portable piezoelectric material is held loosely bent like a newspaper or magazine, it is constantly subjected to relatively slow, large external bending deformation at frequencies of a few Hz or less. If the polymer composite piezoelectric material is too hard, a correspondingly large bending stress will be generated, which may cause cracks at the interface between the polymer matrix and the piezoelectric particles, eventually leading to destruction. Therefore, polymer composite piezoelectric materials must be moderately flexible. Furthermore, if the strain energy can be diffused to the outside as heat, stress can be alleviated. Therefore, the loss tangent of the polymer composite piezoelectric material must be moderately large.

[0079] (ii) Sound Quality Speakers vibrate piezoelectric particles at frequencies in the audio band between 20 Hz and 20 kHz, and the resulting vibration energy causes the entire polymer composite piezoelectric material (piezoelectric film) to vibrate as a unit, reproducing sound. Therefore, polymer composite piezoelectric materials must have an appropriate hardness to increase the efficiency of vibration energy transmission. Furthermore, if the frequency characteristics of a speaker are smooth, the amount of change in sound quality when the lowest resonance frequency changes with changes in curvature will also be small. Therefore, the loss tangent of a polymer composite piezoelectric material must be appropriately large.

[0080] In summary, the polymer composite piezoelectric material is required to be hard against vibrations of 20 Hz to 20 kHz, and soft against vibrations of a few Hz or less. The loss tangent of the polymer composite piezoelectric material is also required to be appropriately large for vibrations of all frequencies below 20 kHz.

[0081] Generally, polymer solids have a viscoelastic relaxation mechanism, and large-scale molecular motion is observed as a decrease (relaxation) in the storage modulus (Young's modulus) or a maximum (absorption) in the loss modulus with increasing temperature or decreasing frequency. Among these, relaxation caused by micro-Brownian motion of molecular chains in the amorphous region is called primary dispersion, and a very large relaxation phenomenon is observed. The temperature at which this primary dispersion occurs is the glass transition point (Tg), where the viscoelastic relaxation mechanism is most pronounced. In a polymer composite piezoelectric (piezoelectric layer 20), by using a polymer material with a glass transition point at room temperature—in other words, a polymer material with viscoelasticity at room temperature—as the matrix, a polymer composite piezoelectric that behaves hard in response to vibrations of 20 Hz to 20 kHz and soft in response to slow vibrations of a few Hz or less is realized. In particular, in order to favorably exhibit this behavior, it is preferable to use a polymer material with a glass transition point at room temperature, i.e., 0 to 50°C, as the matrix of the polymer composite piezoelectric.

[0082] Various known polymeric materials having viscoelasticity at room temperature can be used. Preferably, a polymeric material is used that has a maximum value of loss tangent Tanδ of 0.5 or more at a frequency of 1 Hz in a dynamic viscoelasticity test at room temperature, i.e., 0 to 50°C. This alleviates stress concentration at the interface between the polymer matrix and the piezoelectric particles at the maximum bending moment when the polymer composite piezoelectric element is slowly bent by an external force, and high flexibility can be expected.

[0083] Furthermore, the polymer material having viscoelasticity at room temperature preferably has a storage modulus (E') at a frequency of 1 Hz measured by dynamic viscoelasticity measurement of 100 MPa or more at 0° C. and 10 MPa or less at 50° C. This reduces the bending moment that occurs when the polymer composite piezoelectric body is slowly bent by an external force, and allows the body to behave rigidly against acoustic vibrations of 20 Hz to 20 kHz.

[0084] Furthermore, it is more preferable for a polymer material that has viscoelasticity at room temperature to have a relative dielectric constant of 10 or more at 25°C. This allows a higher electric field to be applied to the piezoelectric particles in the matrix when a voltage is applied to the polymer composite piezoelectric, and a large amount of deformation can be expected. However, on the other hand, in consideration of ensuring good moisture resistance, it is also preferable for the polymer material to have a relative dielectric constant of 10 or less at 25°C.

[0085] Examples of polymeric materials that satisfy these conditions and have viscoelasticity at room temperature include cyanoethylated polyvinyl alcohol (cyanoethylated PVA), polyvinyl acetate, polyvinylidene chloride-co-acrylonitrile, polystyrene-vinyl polyisoprene block copolymer, polyvinyl methyl ketone, and polybutyl methacrylate. Commercially available products such as Hybrar 5127 (manufactured by Kuraray Co., Ltd.) can also be suitably used as these polymeric materials. Of these, it is preferable to use a material having a cyanoethyl group as the polymeric material, and cyanoethylated PVA is particularly preferable.

[0086] As a polymeric material having viscoelasticity at room temperature, it is preferable to use a polymeric material having a cyanoethyl group, and it is particularly preferable to use cyanoethylated PVA. That is, in the present invention, it is preferable to use a polymeric material having a cyanoethyl group, and it is particularly preferable to use cyanoethylated PVA, as the matrix 34 of the piezoelectric layer 20. In the following description, the above-mentioned polymeric materials, typified by cyanoethylated PVA, will also be collectively referred to as "polymeric materials having viscoelasticity at room temperature."

[0087] These polymeric materials having viscoelasticity at room temperature may be used alone or in combination (mixture) of two or more kinds.

[0088] The matrix 34 using such a polymeric material having viscoelasticity at room temperature may contain a plurality of polymeric materials in combination as needed. That is, in addition to a viscoelastic material such as cyanoethylated PVA, other dielectric polymeric materials may be added to the matrix 34 as needed for the purpose of adjusting the dielectric properties and mechanical properties.

[0089] Examples of the dielectric polymer material that can be added include fluorine-based polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, and polyvinylidene fluoride-tetrafluoroethylene copolymer, vinylidene cyanide-vinyl acetate copolymer, cyanoethyl cellulose, cyanoethyl hydroxysucrose, cyanoethyl hydroxycellulose, cyanoethyl hydroxypullulan, cyanoethyl methacrylate, cyanoethyl acrylate, Examples of suitable dielectric polymers include polymers having a cyano or cyanoethyl group, such as cyanoethyl hydroxyethyl cellulose, cyanoethyl amylose, cyanoethyl hydroxypropyl cellulose, cyanoethyl dihydroxypropyl cellulose, cyanoethyl hydroxypropyl amylose, cyanoethyl polyacrylamide, cyanoethyl polyacrylate, cyanoethyl pullulan, cyanoethyl polyhydroxymethylene, cyanoethyl glycidol pullulan, cyanoethyl sucrose, and cyanoethyl sorbitol, as well as synthetic rubbers such as nitrile rubber and chloroprene rubber. Among these, polymeric materials having a cyanoethyl group are preferred. Furthermore, the matrix 34 of the piezoelectric layer 20 is not limited to one type of dielectric polymeric material, and multiple types may be added.

[0090] In addition to the dielectric polymer material, in order to adjust the glass transition temperature Tg, thermoplastic resins such as vinyl chloride resin, polyethylene, polystyrene, methacrylic resin, polybutene, and isobutylene, and thermosetting resins such as phenol resin, urea resin, melamine resin, alkyd resin, and mica may be added to the matrix 34. Furthermore, in order to improve adhesion, tackifiers such as rosin ester, rosin, terpene, terpene phenol, and petroleum resin may be added.

[0091] When a material other than a polymeric material having viscoelasticity such as cyanoethylated PVA is added to the matrix 34 of the piezoelectric layer 20, the amount of the material added is not particularly limited, but is preferably 30 mass % or less in terms of the ratio to the matrix 34. This allows the properties of the polymeric material to be expressed without impairing the viscoelastic relaxation mechanism in the matrix 34, thereby achieving favorable results in terms of increasing the dielectric constant, improving heat resistance, and improving adhesion between the piezoelectric particles 36 and the electrode layer.

[0092] The piezoelectric layer 20 is a layer made of a polymer composite piezoelectric material containing piezoelectric particles 36 in such a matrix 34. The piezoelectric particles 36 are dispersed in the matrix 34. Preferably, the piezoelectric particles 36 are dispersed uniformly (approximately uniformly) in the matrix 34.

[0093] The piezoelectric particles 36 are made of ceramic particles having a perovskite or wurtzite crystal structure. Examples of ceramic particles that make up the piezoelectric particles 36 include lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), barium titanate (BaTiO), zinc oxide (ZnO), and a solid solution of barium titanate and bismuth ferrite (BiFe) (BFBT).

[0094] There are no limitations on the particle size of the piezoelectric particles 36, and it may be selected appropriately depending on the size and intended use of the piezoelectric film 10. The particle size of the piezoelectric particles 36 is preferably 1 to 10 μm. By setting the particle size of the piezoelectric particles 36 within this range, favorable results can be obtained in that the piezoelectric film 10 can achieve both high piezoelectric properties and flexibility.

[0095] The piezoelectric particles 36 in the piezoelectric layer 20 may be dispersed uniformly and regularly in the matrix 34, or may be dispersed irregularly in the matrix 34 as long as they are dispersed uniformly.

[0096] In the piezoelectric film 10, there are no limitations on the quantitative ratio of the matrix 34 to the piezoelectric particles 36 in the piezoelectric layer 20, and this may be set appropriately depending on the size and thickness in the plane direction of the piezoelectric film 10, the intended use, the required properties, and the like. The volume fraction of the piezoelectric particles 36 in the piezoelectric layer 20 is preferably 30 to 80%, more preferably 50% or more, and therefore even more preferably 50 to 80%. By setting the quantitative ratio of the matrix 34 to the piezoelectric particles 36 within the above range, favorable results can be obtained, such as the ability to achieve both high piezoelectric properties and flexibility.

[0097] In the piezoelectric film 10, there are no particular limitations on the thickness of the piezoelectric layer 20, and it may be set appropriately depending on the characteristics required of the piezoelectric film 10. A thicker piezoelectric layer 20 is advantageous in terms of rigidity, such as the stiffness of the sheet-like material, but the voltage (potential difference) required to expand or contract the piezoelectric film 10 by the same amount increases. The thickness of the piezoelectric layer 20 is preferably 10 to 300 μm, more preferably 20 to 200 μm, and even more preferably 30 to 150 μm. By setting the thickness of the piezoelectric layer 20 within the above range, favorable results can be obtained in terms of ensuring both rigidity and appropriate flexibility.

[0098] Furthermore, the piezoelectric layer 20 is preferably polarized (poled) in the thickness direction. When a voltage is applied to the electrode layers (electrode pair) sandwiching the piezoelectric layer 20, the piezoelectric particles 36 in the piezoelectric layer 20 expand and contract in the polarization direction in response to the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer 20) contracts in the thickness direction. At the same time, due to the Poisson's ratio, the piezoelectric film 10 also expands and contracts in the plane direction. This allows the piezoelectric film 10 to exhibit piezoelectric performance.

[0099] In the present invention, the piezoelectric layer 20 is not limited to the polymer composite piezoelectric material containing piezoelectric particles 36 in a matrix 34 made of a polymer material that has viscoelasticity at room temperature, such as cyanoethylated PVA, as described above. In other words, in the piezoelectric film 10 of the present invention, various known piezoelectric layers can be used as the piezoelectric layer.

[0100] Examples that can be used include a polymer composite piezoelectric material containing similar piezoelectric particles 36 in a matrix containing a dielectric polymer material such as the above-mentioned polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, and vinylidene fluoride-trifluoroethylene copolymer, a piezoelectric layer made of polyvinylidene fluoride, a piezoelectric layer made of a fluororesin other than polyvinylidene fluoride, and a piezoelectric layer formed by laminating a film made of poly-L-lactic acid and a film made of poly-D-lactic acid. However, as described above, a polymer composite piezoelectric material containing piezoelectric particles 36 in a matrix 34 made of a polymer material having viscoelasticity at room temperature, such as the above-mentioned cyanoethylated PVA, is preferably used because it behaves hard with respect to vibrations of 20 Hz to 20 kHz and soft with respect to slow vibrations of a few Hz or less, thereby achieving excellent acoustic properties and providing excellent flexibility.

[0101] 11 , the piezoelectric film 10 has a first electrode layer 24 on one surface of the piezoelectric layer 20, with a first protective layer 28 thereon, and a second electrode layer 26 on the other surface of the piezoelectric layer 20, with a second protective layer 30 thereon. Here, the first electrode layer 24 and the second electrode layer 26 form an electrode pair.

[0102] That is, the piezoelectric film 10 has a configuration in which both sides of the piezoelectric layer 20 are sandwiched between an electrode pair, i.e., a first electrode layer 24 and a second electrode layer 26, and this laminate is sandwiched between a first protective layer 28 and a second protective layer 30. In this way, in the piezoelectric film 10, the region sandwiched between the first electrode layer 24 and the second electrode layer 26 expands and contracts in response to an applied voltage. Note that the terms "first" and "second" in the electrode layer and protective layer are used for convenience in explaining the piezoelectric film 10. Therefore, the terms "first" and "second" in the present invention have no technical meaning and are unrelated to the actual state of use.

[0103] In addition to these layers, the piezoelectric film 10 of the present invention may have, for example, an adhesive layer for adhering the electrode layer to the piezoelectric layer 20, and an adhesive layer for adhering the electrode layer to the protective layer. The adhesive may be an adhesive or a pressure-sensitive adhesive. The adhesive may also suitably be the same material as the polymer material of the piezoelectric layer 20 excluding the piezoelectric particles 36, i.e., the matrix 34. The adhesive layer may be provided on both the first electrode layer 24 side and the second electrode layer 26 side, or on only one of the first electrode layer 24 side and the second electrode layer 26 side.

[0104] In the piezoelectric film 10, the first protective layer 28 and the second protective layer 30 cover the first electrode layer 24 and the second electrode layer 26 and also serve to impart appropriate rigidity and mechanical strength to the piezoelectric layer 20. That is, in the piezoelectric film 10, the piezoelectric layer 20, which is composed of a matrix 34 and piezoelectric particles 36, exhibits excellent flexibility with respect to slow bending deformation, but may lack rigidity or mechanical strength depending on the application. To compensate for this, the first protective layer 28 and the second protective layer 30 are provided in the piezoelectric film 10. The first protective layer 28 and the second protective layer 30 have the same configuration but differ in their location. Therefore, in the following description, when there is no need to distinguish between the first protective layer 28 and the second protective layer 30, both components will be collectively referred to as the protective layer.

[0105] The protective layer is not limited and may be made of various sheet-like materials, and various resin films are preferred examples thereof. Among these, resin films made of polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polyphenylene sulfite (PPS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyimide (PI), polyethylene naphthalate (PEN), triacetyl cellulose (TAC), cyclic olefin resins, etc. are preferred because of their excellent mechanical properties and heat resistance.

[0106] There is no limitation on the thickness of the protective layer. The first protective layer 28 and the second protective layer 30 are basically the same in thickness, but may be different in thickness. If the protective layer is too rigid, not only will it restrict the expansion and contraction of the piezoelectric layer 20, but it will also impair flexibility. Therefore, except in cases where mechanical strength or good handleability as a sheet-like material is required, a thinner protective layer is more advantageous.

[0107] In the piezoelectric film 10, favorable results can be obtained in terms of ensuring both rigidity and appropriate flexibility, as long as the thickness of the protective layer is no more than twice the thickness of the piezoelectric layer 20. For example, when the thickness of the piezoelectric layer 20 is 50 μm and the protective layer is made of PET, the thickness of the protective layer is preferably no more than 100 μm, more preferably no more than 50 μm, and even more preferably no more than 25 μm.

[0108] In the piezoelectric film 10, a first electrode layer 24 is formed between the piezoelectric layer 20 and the first protective layer 28, and a second electrode layer 26 is formed between the piezoelectric layer 20 and the second protective layer 30. The first electrode layer 24 and the second electrode layer 26 are provided to apply a voltage to the piezoelectric layer 20 (piezoelectric film 10).

[0109] The first electrode layer 24 and the second electrode layer 26 are basically the same except for their positions. Therefore, in the following description, when there is no need to distinguish between the first electrode layer 24 and the second electrode layer 26, both components will be collectively referred to as electrode layers.

[0110] In the present invention, there are no limitations on the material for forming the electrode layer, and various conductors can be used. Specific examples include metals such as carbon, palladium, iron, tin, aluminum, nickel, platinum, gold, silver, copper, titanium, chromium, and molybdenum, alloys thereof, laminates and composites of these metals and alloys, and indium tin oxide. Alternatively, conductive polymers such as PEDOT / PPS (polyethylenedioxythiophene-polystyrene sulfonate) are also exemplified. Among these, copper, aluminum, gold, silver, platinum, and indium tin oxide are preferred examples for the electrode layer. Among these, copper is more preferred from the viewpoints of conductivity, cost, flexibility, and the like.

[0111] There are also no limitations on the method for forming the electrode layer, and various known methods can be used, such as vapor phase deposition methods (vacuum film formation methods) such as vacuum deposition and sputtering, film formation by plating, and methods of attaching foils formed from the above materials.

[0112] Among these, thin films of copper, aluminum, etc. formed by vacuum deposition are particularly suitable for use as electrode layers because they can ensure the flexibility of the piezoelectric film 10. Among these, thin films of copper formed by vacuum deposition are particularly suitable.

[0113] There is no limit to the thickness of the electrode layer. The first electrode layer 24 and the second electrode layer 26 are basically the same in thickness, but may be different in thickness. Here, as with the protective layer described above, if the electrode layer is too rigid, it not only restricts the expansion and contraction of the piezoelectric layer 20, but also impairs flexibility. Therefore, the thinner the electrode layer, the more advantageous it is, as long as the electrical resistance does not become too high.

[0114] In the piezoelectric film 10, it is preferable that the product of the thickness and Young's modulus of the electrode layer be less than the product of the thickness and Young's modulus of the protective layer, since this does not significantly impair flexibility. For example, in the case of a combination in which the protective layer is made of PET (Young's modulus: approximately 6.2 GPa) and the electrode layer is made of copper (Young's modulus: approximately 130 GPa), if the protective layer is 25 μm thick, the thickness of the electrode layer is preferably 1.2 μm or less, more preferably 0.3 μm or less, and particularly preferably 0.1 μm or less.

[0115] As described above, the piezoelectric film 10 has a structure in which a piezoelectric layer 20, which is formed by dispersing piezoelectric particles 36 in a matrix 34 containing a polymer material, is sandwiched between a first electrode layer 24 and a second electrode layer 26, and this laminate is further sandwiched between a first protective layer 28 and a second protective layer 30. In such a piezoelectric film 10, a maximum value of loss tangent (Tan δ) at a frequency of 1 Hz as measured by dynamic viscoelasticity measurement preferably exists at room temperature, and more preferably a maximum value of 0.1 or greater exists at room temperature. This allows the strain energy to be effectively diffused to the outside as heat, even when the piezoelectric film 10 is subjected to a relatively slow, large bending deformation from the outside at a frequency of a few Hz or less, thereby preventing cracks from occurring at the interface between the polymer matrix and the piezoelectric particles.

[0116] The piezoelectric film 10 preferably has a storage modulus (E') at a frequency of 1 Hz measured by dynamic viscoelasticity measurement of 10 to 30 GPa at 0°C and 1 to 10 GPa at 50°C. This condition also applies to the piezoelectric layer 20. This allows the piezoelectric film 10 to have a large frequency dispersion in the storage modulus (E') at room temperature. In other words, it behaves hard against vibrations of 20 Hz to 20 kHz and soft against vibrations of a few Hz or less.

[0117] The piezoelectric film 10 has a thickness multiplied by a storage modulus (E') at a frequency of 1 Hz measured by dynamic viscoelasticity measurement of 1.0×10 5 ~2.0 x 10 6 N / m, 1.0 x 10 at 50°C 5 ~1.0 x 10 6 N / m. This condition also applies to the piezoelectric layer 20. This allows the piezoelectric film 10 to have appropriate rigidity and mechanical strength without impairing its flexibility and acoustic properties.

[0118] Furthermore, the piezoelectric film 10 preferably has a loss tangent (Tan δ) of 0.05 or more at 25°C and a frequency of 1 kHz in a master curve obtained from dynamic viscoelasticity measurement. This condition also applies to the piezoelectric layer 20. This makes the frequency characteristics of a speaker using the piezoelectric film 10 smooth, and the lowest resonance frequency f ０ It is also possible to reduce the amount of change in sound quality when the

[0119] In the present invention, the storage modulus (Young's modulus) and loss tangent of the piezoelectric film 10, the piezoelectric layer 20, etc. may be measured by a known method. For example, a dynamic viscoelasticity measuring device DMS6100 manufactured by SII NanoTechnology, Inc. may be used. Examples of measurement conditions include a measurement frequency of 0.1 Hz to 20 Hz (0.1 Hz, 0.2 Hz, 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, 10 Hz, and 20 Hz), a measurement temperature of −50° C. to 150° C., a temperature rise rate of 2° C. / min. (in a nitrogen atmosphere), a sample size of 40 mm×10 mm (including the clamping area), and a chuck distance of 20 mm.

[0120] In the electro-acoustic transducer 100, an external power supply (signal source) is connected to the first electrode layer 24 and the second electrode layer 26 of the piezoelectric film 10 to apply a drive voltage that expands and contracts the piezoelectric film 10, i.e., to supply drive power (drive signal). There are no restrictions on the external power supply (signal source), and it may be a DC power supply or an AC power supply. The drive voltage may also be set appropriately depending on the thickness and material of the piezoelectric layer 20 of the piezoelectric film 10, so that the piezoelectric film 10 can be properly driven.

[0121] There are no limitations on the method for drawing electrodes from the first electrode layer 24 and the second electrode layer 26, and various known methods can be used. Examples include a method of connecting a conductor such as copper foil to the first electrode layer 24 and the second electrode layer 26 to draw the electrodes to the outside, and a method of forming through holes in the first protective layer 28 and the second protective layer 30 using a laser or the like and filling the through holes with a conductive material to draw the electrodes to the outside. Suitable examples of electrode drawing methods include the methods described in Japanese Patent Application Laid-Open No. 2014-209724 and Japanese Patent Application Laid-Open No. 2016-015354.

[0122] <Method for Manufacturing Piezoelectric Film> Hereinafter, an example of a method for manufacturing the piezoelectric film 10 will be described with reference to FIGS.

[0123] First, a sheet-like material 11a is prepared, in which a first electrode layer 24 is formed on the surface of a first protective layer 28, as shown in Fig. 12. Further, a sheet-like material 11c is prepared, in which a second electrode layer 26 is formed on the surface of a second protective layer 30, as conceptually shown in Fig. 14.

[0124] The sheet-like material 11a may be produced by forming a copper thin film or the like as the first electrode layer 24 on the surface of the first protective layer 28 by vacuum deposition, sputtering, plating, or the like. Similarly, the sheet-like material 11c may be produced by forming a copper thin film or the like as the second electrode layer 26 on the surface of the second protective layer 30 by vacuum deposition, sputtering, plating, or the like. Alternatively, a commercially available product having a copper thin film or the like formed on a protective layer may be used as the sheet-like material 11a and / or the sheet-like material 11c. The sheet-like material 11a and the sheet-like material 11c may be of the same type or different types.

[0125] If the protective layer is very thin and difficult to handle, a protective layer with a separator (temporary support) may be used as needed. The separator may be made of PET or the like having a thickness of 25 to 100 μm. The separator may be removed after the electrode layer and the protective layer are thermocompression bonded.

[0126] 13, a coating material (coating composition) that will become the piezoelectric layer 20 is applied onto the first electrode layer 24 of the sheet-like material 11a, and then cured to form the piezoelectric layer 20. In this way, a piezoelectric laminate 11b is produced in which the sheet-like material 11a and the piezoelectric layer 20 are laminated together.

[0127] Various methods can be used to form the piezoelectric layer 20, depending on the material used to form the piezoelectric layer 20. As an example, a polymer material such as the cyanoethylated PVA described above is first dissolved in an organic solvent, and then piezoelectric particles 36 such as PZT particles are added and stirred to prepare a coating material. There are no limitations on the organic solvent, and various organic solvents such as dimethylformamide (DMF), methyl ethyl ketone (MEK), and cyclohexanone can be used. After preparing the sheet material 11a and preparing the coating material, the coating material is cast (applied) onto the sheet material 11a, and the organic solvent is evaporated and dried. This produces a piezoelectric laminate 11b, as shown in FIG. 13, which has a first electrode layer 24 on a first protective layer 28 and a piezoelectric layer 20 laminated on the first electrode layer 24.

[0128] There are no limitations on the method for casting the paint, and all known methods (applicators) such as a bar coater, a slide coater, a doctor knife, etc. Alternatively, if the polymer material is heat-meltable, the polymer material may be heated and melted, and piezoelectric particles 36 may be added to produce a melt, which may then be extruded into a sheet shape onto the sheet-like material 11a shown in Fig. 12 by extrusion molding or the like, and cooled to produce the piezoelectric laminate 11b shown in Fig. 13.

[0129] As described above, in the piezoelectric layer 20, in addition to the polymer material having viscoelasticity at room temperature, a polymer piezoelectric material such as PVDF (polyvinylidene fluoride) may be added to the matrix 34. When adding such a polymer piezoelectric material to the matrix 34, the polymer piezoelectric material to be added may be dissolved in the coating material. Alternatively, the polymer piezoelectric material to be added may be added to a polymer material that has been heated and melted and has viscoelasticity at room temperature, and then the resulting mixture may be heated and melted.

[0130] After the piezoelectric layer 20 is formed, it may be subjected to calendering as necessary. Calendering may be performed once or multiple times. As is well known, calendering is a process in which a surface to be treated is heated and pressed with a heating press or a heating roller, etc., to perform flattening or the like.

[0131] Next, the piezoelectric layer 20 of the piezoelectric laminate 11b is subjected to polarization (poling). The piezoelectric layer 20 may be polarized before the calendaring process, but is preferably polarized after the calendaring process. There are no limitations on the method for polarizing the piezoelectric layer 20, and known methods can be used. Examples include electric field poling, in which a DC electric field is directly applied to the target to be polarized, and corona poling. When electric field poling is performed, the second electrode layer 26 may be formed before the polarization process, and the electric field poling may be performed using the first electrode layer 24 and the second electrode layer 26. Furthermore, in the piezoelectric film 10 of the present invention, polarization is performed in the thickness direction of the piezoelectric layer 20, rather than in the plane direction.

[0132] Next, as shown in Fig. 14, the previously prepared sheet material 11c is laminated on the piezoelectric layer 20 side of the polarization-treated piezoelectric laminate 11b, with the second electrode layer 26 facing the piezoelectric layer 20. Furthermore, this laminate is thermocompression-bonded using a heating press and a heating roller, etc., with the first protective layer 28 and the second protective layer 30 sandwiched between them, thereby bonding the piezoelectric laminate 11b and the sheet material 11c together, thereby producing the piezoelectric film 10 as shown in Fig. 11. Alternatively, the piezoelectric laminate 11b and the sheet material 11c may be bonded together using an adhesive, and preferably further compressed to produce the piezoelectric film 10. The adhesive used in this process may be the same material as the matrix of the piezoelectric layer 20.

[0133] The piezoelectric film 10 may be manufactured using cut sheets 11a and 11c, or may be manufactured using a roll-to-roll process.

[0134] The piezoelectric film thus produced may be cut into desired shapes for various applications. The piezoelectric film 10 thus produced is polarized in the thickness direction, not in the plane direction, and exhibits significant piezoelectric properties without the need for stretching after polarization. Therefore, the piezoelectric film 10 has no in-plane anisotropy in its piezoelectric properties, and when a driving voltage is applied, it expands and contracts isotropically in all directions in the plane direction.

[0135] 8 to 10 , when the piezoelectric film 10c is configured to have a shape that conforms to the surface of the substantially spherical foam 102d, it is preferable to heat-compression-mold the piezoelectric film 10 manufactured as described above to form convex portions that protrude from one main surface of the piezoelectric film 10 and are recessed from the other main surface. The heat-compression-mold method is not particularly limited, and various known resin film processing methods can be used. As an example, convex portions of the desired shape can be formed by heat-compression-mold-mold-piezoelectric film 10 using a molding device having a mold shaped to correspond to the shape of the convex portions to be molded.

[0136] The electroacoustic transducer of the present invention has been described in detail above, but the present invention is not limited to the above examples, and various improvements and modifications may be made within the scope of the gist of the present invention.

[0137] For example, the electroacoustic transducer of the present invention can be used as a microphone. In particular, when the electroacoustic transducer is brought into contact with a sound source, vibrations can be efficiently transmitted to the piezoelectric film and converted into an electric signal. Specifically, for example, when a person speaks while holding the electroacoustic transducer against their throat, vibrations can be efficiently converted into an electric signal.

[0138] 6, when the electroacoustic transducer has two or more piezoelectric films, at least one piezoelectric film may be used as a microphone and at least one other piezoelectric film may be used for sound generation. In this case, for example, the transducer may be used as a noise canceling device that reduces noise by detecting environmental sound (external sound) with one piezoelectric film as a microphone and reproducing a sound in the opposite phase to the detected environmental sound with another piezoelectric film.

[0139] The present invention will be described in more detail below with reference to specific examples of the present invention. However, the present invention is not limited to these examples, and the materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention.

[0140] Example 1 <Preparation of Piezoelectric Film> A piezoelectric film was prepared using the method shown in Figures 12 to 14. First, cyanoethylated PVA (CR-V, manufactured by Shin-Etsu Chemical Co., Ltd.) was dissolved in dimethylformamide (DMF) using the following composition ratio. PZT particles were then added to this solution as piezoelectric particles using the following composition ratio, and the mixture was stirred using a propeller mixer (2000 rpm) to prepare a coating material for forming a piezoelectric layer: 300 parts by weight of PZT particles, 30 parts by weight of cyanoethylated PVA, and 70 parts by weight of DMF. The PZT particles used were prepared by sintering commercially available PZT raw material powder at 1000 to 1200°C, followed by crushing and classifying the powder to an average particle size of 2 μm.

[0141] Meanwhile, sheets 11a and 11b were prepared by vacuum-depositing a 0.1 μm-thick copper thin film on a 4 μm-thick PET film. In this example, the first and second electrode layers were 0.3 μm-thick copper-deposited thin films, and the first and second protective layers were 4 μm-thick PET films. To facilitate easy handling during the process, the PET films were equipped with 50 μm-thick separators (PET temporary support). After thermocompression bonding of sheet 11c, the separators were removed from each protective layer.

[0142] The paint for forming the piezoelectric layer prepared previously was applied to the first electrode layer (copper vapor-deposited thin film) of the sheet-like material 11a using a slide coater. The paint was applied so that the thickness of the coating film after drying would be 50 μm. The sheet-like material with the paint applied was then heated and dried on a hot plate at 120°C to evaporate the DMF. This produced a piezoelectric laminate 11b having a copper first electrode layer on a PET first protective layer, and a 50 μm-thick piezoelectric layer (polymer composite piezoelectric layer) on top of that.

[0143] The produced piezoelectric layer was subjected to polarization treatment in the thickness direction.

[0144] A sheet-like material 11c, which was a PET film with a copper thin film deposited thereon, was laminated on the polarization-treated piezoelectric laminate, with the second electrode layer (the copper thin film side) facing the piezoelectric layer. Next, the laminate of the piezoelectric laminate and the sheet-like material was thermocompression-bonded at a temperature of 120°C using a laminator, thereby adhering and bonding the piezoelectric layer and the second electrode layer to produce a piezoelectric film as shown in FIG.

[0145] <Preparation of Foam> Expanded polystyrene balls (Φ50) manufactured by Daiso Industries Co., Ltd. were used as foams, and were processed into a cylindrical shape having an outer diameter of 40 mm, a height of 40 mm, and a through-hole (hollow portion) with an inner diameter of 20 mm.

[0146] <Preparation of Electroacoustic Transducer> A piezoelectric film was cut into a size of 20 mm x 120 mm, and an adhesive tape of the same size (70410 manufactured by TESA) was wound around and fixed to the periphery of a foam to prepare an electroacoustic transducer.

[0147] Example 2 An electroacoustic transducer was produced in the same manner as in Example 1, except that a piezoelectric film was cut into a size of 20 mm x 60 mm and placed on the inner circumferential surface of the hollow portion of a foam.

[0148] [Example 3] An electroacoustic transducer was fabricated in the same manner as in Example 1, except that a piezoelectric film was cut to a size of 20 mm x 120 mm and wrapped around the circumferential surface of a foam, and a piezoelectric film was cut to a size of 20 mm x 60 mm and placed on the inner circumferential surface of the hollow portion of the foam.

[0149] [Example 4] An electroacoustic transducer was produced in the same manner as in Example 1, except that the foam was processed into a cylindrical shape with a bottom, an outer diameter of 40 mm, a height of 40 mm, an inner diameter of 20 mm, and a depth of 30 mm, each having a non-through hole (hollow portion).

[0150] [Example 5] An electroacoustic transducer was produced in the same manner as in Example 1, except that the foam was processed into a cylindrical shape having a through hole (hollow portion) with an outer diameter of 40 mm, a height of 40 mm, and an inner diameter of 30 mm on one surface and 15 mm on the other surface. In Table 1, through holes with a varying inner diameter are referred to as tapered through holes.

[0151] [Example 6] An electroacoustic transducer was produced in the same manner as in Example 1, except that the foam was formed into a spherical shape with a diameter of 45 mm and processed into a shape having a through-hole (hollow portion) with a diameter of 20 mm, and the piezoelectric film was molded as follows.

[0152] The produced piezoelectric film was molded into a convex shape (spherical crown shape) by heat compression molding using a hot press machine (AH-2003 manufactured by AS ONE Corporation) in a mold having a shape corresponding to the shape to be molded.

[0153] The piezoelectric film produced in Example 1 was cut into a size of 20 mm x 120 mm, and the cut piece was wrapped around a cylindrical foam having an outer diameter of 40 mm and a height of 40 mm using TESA tape to produce an electroacoustic transducer. That is, the electroacoustic transducer was produced in the same manner as in Example 1, except that the foam did not have through-holes.

[0154] [Evaluation] A sine sweep signal with a frequency of 100 Hz to 20 kHz and an applied voltage of 30 Vrms was input to the piezoelectric film of the electroacoustic transducers of the examples and comparative examples, and the end face of the foam of each sample where the hollow portion had an opening was placed against an artificial ear (TYPE2015E, manufactured by ACO Co., Ltd.) to measure the average sound pressure. Note that for Comparative Example 1, the surface of the foam not covered by the piezoelectric film was placed against the artificial ear to measure the average sound pressure. The results are shown in Table 1.

[0155]

[0156] From Table 1, it can be seen that the examples of the present invention have higher sound pressure than the comparative examples.

[0157] Furthermore, a comparison between Example 1 and Example 2 reveals that it is preferable to place the piezoelectric film on the outer peripheral surface of the foam, because it is thought that a configuration in which the piezoelectric film is placed on the outer peripheral surface of the foam makes it easier to obtain the vibration amplification effect of the foam.

[0158] Furthermore, a comparison between Example 1 and Example 3 reveals that it is more preferable to place the piezoelectric film on both the outer peripheral surface of the foam and the inner peripheral surface of the hollow portion.

[0159] Furthermore, a comparison between Example 1 and Example 4 reveals that when the hollow portion is a non-through hole, the sound pressure can be improved more than when the hollow portion is a through hole.

[0160] Furthermore, a comparison between Example 1 and Example 5 reveals that when the hollow portion is a tapered through-hole, the sound pressure can be improved more than when the opening diameter of the through-hole is constant.

[0161] Furthermore, a comparison between Example 1 and Example 6 reveals that a cylindrical foam shape is more preferable than a spherical one. This is because, when the outer diameter and height of a cylindrical foam are the same, and the diameter and length of the hollow portion (through hole) of a cylindrical foam are approximately the same as those of a spherical foam, the maximum distance from the piezoelectric film to the hollow portion is greater in the spherical foam. From the above, the effects of the present invention are clear.

[0162] 10, 10a to 10c Piezoelectric film 11a, 11c Sheet-like material 11b Piezoelectric laminate 20 Piezoelectric layer 24 First electrode layer 26 Second electrode layer 28 First protective layer 30 Second protective layer 34 Matrix 36 Piezoelectric particles 100a to 100f Electroacoustic transducer 102a to 102d Foam 103a to 103d Hollow portion 104a to 104b Surface (curved portion) 106 Adhesive tape

Claims

1. An electroacoustic transducer comprising: a foam having a curved surface and a hollow portion drilled from the surface; and a piezoelectric film disposed on the foam.

2. The electro-acoustic transducer of claim 1, wherein said piezoelectric film is disposed on said surface of said foam.

3. The electroacoustic transducer according to claim 1, wherein the piezoelectric film is disposed on an inner peripheral surface of the hollow portion of the foam.

4. The electro-acoustic transducer according to claim 1, wherein the piezoelectric film is disposed on the surface of the foam and on the inner peripheral surface of the hollow portion.

5. The electroacoustic transducer according to claim 1, wherein the foam has a cylindrical shape or a spherical shape with the hollow portion formed therein.

6. An electroacoustic transducer according to claim 1, wherein the hollow portion has an equivalent circle diameter of an opening cross section of 3 mm to 30 mm.

7. The electroacoustic transducer according to claim 1, wherein the ratio of the outer diameter of said foam to the equivalent circle diameter of the cross section of the opening of said hollow portion is 10 to 1.

1.

8. The electro-acoustic transducer according to claim 1, wherein the hollow portion penetrates the foam, and the diameter of the hollow portion at one surface of the foam is larger than the diameter at the other surface.

9. The electroacoustic transducer of claim 1, wherein the foam is a foamed plastic or a foam based on starch.

10. The electroacoustic transducer according to claim 1, wherein the foam has an expansion ratio in the range of 10 to 60 times.

11. An electro-acoustic transducer according to any one of claims 1 to 10, wherein the piezoelectric film has a piezoelectric layer made of a polymer composite piezoelectric containing piezoelectric particles in a matrix containing a polymer material, and electrode layers provided on both sides of the piezoelectric layer.

12. The electroacoustic transducer according to claim 11, wherein said foam is spherical, and said piezoelectric film is molded into a convex shape to match the shape of said foam.

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