JP2001008290A - Piezoelectric speaker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a reproduction frequency band of voice wide by energizing an electrode provided by forming a ceramics piezoelectric element into a dome shape. SOLUTION: Expansion and contraction of a ceramics piezoelectric element 1 is converted into vertical vibration of the entire piezoelectric element, and a sound wave is obtained by holding a peripheral end part of the ceramics piezoelectric element 1 on a frame 2. In addition, the cross-sectional shape of the ceramics piezoelectric element 1 in the vertical direction is defined as a structure, on which plural circular arcs with different radius are piled. Thus, since resonance frequency is inversely proportional to the radius, a curved surface of the dome-shaped piezoelectric element is constituted by continuing curved surfaces with respective different radii and the resonance frequency becomes hard to appear. Namely, a peak due to primary resonance or a peak due to higher-order resonance are not generated, therefore, a frequency characteristic becomes flatter and making the band become wide. In addition, influences of the resonance frequency due to a metal plate and inconvenient vibration in the shape of a diaphragm are not generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric speaker used for audio equipment and the like.

[0002]

2. Description of the Related Art The structure of a conventional piezoelectric speaker will be described. The piezoelectric speaker shown in FIG. 5 is obtained by bonding a metal plate 11 to one surface of a thin ceramic piezoelectric element 1 and is called a unimorph type. Further, the one in which the ceramic piezoelectric elements 1 are bonded to both surfaces of the metal plate 11 is called a bimorph type. In any case, the sound wave is obtained by bending the metal plate 11 by expansion and contraction of the ceramic piezoelectric element 1, and the bimorph type has higher efficiency.

Another conventional example shown in FIG. 6 has a configuration having a conical diaphragm. The conical diaphragm 12 has a peripheral end supported by the frame 2 and a top connected to the center of the piezoelectric element 1 bonded to the metal plate 11. Therefore, the vertical vibration of the central portion of the piezoelectric element 1 due to the expansion and contraction of the piezoelectric element 1 and the bending of the metal plate 11 is transmitted to the conical diaphragm 12 to emit acoustic sound.

Another conventional example shown in FIG.
Reference numeral 2 denotes a configuration including a dome portion 12a called an inner semi-dome and an outer cone portion 12b. This diaphragm 12
The outer peripheral portion of the dome portion 12a is bonded to the piezoelectric element 1, and the outer peripheral portion of the cone portion 12b is bonded and fixed to the frame 2. Therefore, in this piezoelectric speaker, the piezoelectric element 1 is configured to hang down by the diaphragm 12, and can perform expansion and contraction movement in a substantially free state.

Further, as another prior art, there has been proposed a configuration in which a polymer piezoelectric film sheet made of an organic material is used in a curved state, and a sound wave is obtained by converting an extension displacement of the sheet into a vertical displacement.

[0006]

However, the following problems are pointed out in the above conventional example.

[0007] In the conventional example shown in FIG.
A highly efficient sound wave can be obtained at a resonance frequency determined by the shape, and the structure is simple and preferable for emitting a single sound. However, since the reproduction frequency band is narrow, it is extremely unsuitable as a speaker for reproducing a wideband sound.

Further, in the conventional example shown in FIG. 6, although the reproduction band is slightly widened due to the acoustic reflection from the conical diaphragm 12, the influence of the resonance frequency by the metal plate 11 cannot be ignored. Therefore, it does not have characteristics enough to be used as a wideband speaker.

In the case of the conventional example shown in FIG.
Since the expansion and contraction occur at the boundary between the second dome portion 12a and the cone portion 12b, the mutual expansion and contraction motion is reversed. That is, when the dome portion 12a expands, the cone portion 12b contracts, so that the motion is in the opposite phase. Therefore, the output sound pressure level is extremely low, and the peak / dip in the frequency characteristic becomes large.

[0010] In the case of a conventional structure using a polymer piezoelectric film sheet, a piezoelectric element made of an organic material has a significantly smaller piezoelectric constant than a ceramic element. Therefore, the efficiency of the obtained speaker is also insufficient.

Further, since the vibration of the polymer piezoelectric film sheet has directionality due to the piezoelectric effect, the rolling direction of the sheet is significantly different from the rolling direction of the sheet. For this reason, the sheet cannot be formed into a spherical shell or a dome shape, but can be formed only into a cylindrical shape or “kamaboko” by bending a sheet.

In addition, as a conventional technique using a piezoelectric element made of an organic material, a technique using a composite of a ceramic material and an organic material has been proposed. This is because the flexibility of the organic material is given to the ceramic material so that the degree of freedom of the shape is given. However, there is a drawback that the procedure of compounding the ceramic material is extremely complicated. Further, by combining with an organic material which does not originally have piezoelectricity, the piezoelectric characteristics are reduced, and a practical speaker cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a piezoelectric speaker having a wide sound reproduction frequency band.

[0014]

In order to achieve the above object, a piezoelectric speaker according to the present invention forms a ceramic piezoelectric element in a dome shape, forms an electrode on the dome, and expands and contracts by energizing to obtain a sound wave. Configuration. Therefore, since there is no need to provide a metal plate as in the related art, the influence of the resonance frequency due to metal does not occur. Also, there is no danger that undesired vibrations will occur due to the unique shape of the metal.

[0015] The peripheral end of the piezoelectric element is held by a frame. Thereby, expansion and contraction of the ceramic piezoelectric element is converted into vertical vibration of the entire piezoelectric element, and a sound wave is obtained.

The vertical cross section of the ceramic piezoelectric element has a configuration in which a plurality of circular arcs having different radii are continuously formed. When the thickness t of the spherical piezoelectric element is sufficiently smaller than the radius a, the resonance frequency fr of the piezoelectric element is expressed by the following equation (1).

[0017]

(Equation 1)

In the above equation (1), E, ρ and σ are the Young's modulus, density and Poisson's ratio of the piezoelectric ceramic, respectively.

That is, the resonance frequency fr is inversely proportional to the radius a. Therefore, when the curved surface of the dome-shaped piezoelectric element is constituted by a plurality of continuous spherical surfaces having different radii as in the present invention, the resonance frequency is less likely to appear.

The vibrating body is configured to be polarized in the thickness direction and extend and contract in the plane direction. According to this, since a voltage is applied in the thickness direction of the piezoelectric element, manufacturing is easy. Further, since the vibration direction of the piezoelectric element is orthogonal to the voltage application direction, the resonance frequency can be reduced.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the piezoelectric speaker according to the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view in the vertical direction of the piezoelectric speaker of the present embodiment. 1 is a ceramic piezo element formed in a dome shape, 2 is a frame for holding the peripheral end of the piezo element 1, and 3 are electrodes (not shown) formed on both sides of the piezo element 1.
Is a lead wire for applying a signal voltage to.

A method for manufacturing the ceramic piezoelectric element 1 will be described. PZ as a piezoelectric ceramic material
It is desirable to use a material having a high electromechanical coupling coefficient such as T-based piezoelectric ceramics. The mixture of the powdered piezoelectric ceramic material and the organic binder is sandwiched and pressed with a mold having a desired shape, and fired at 1200 ° C. Thereby, a dome-shaped piezoelectric element base is obtained.

An electrode is formed by applying a silver paste on both the front and back surfaces of the substrate and firing it. As the electrode, a general electrode material such as palladium, platinum, gold, and copper can be used in addition to silver, and sputtering or plating can be used as a forming method. Further, a polarization process is performed in the thickness direction to obtain a ceramic piezoelectric element.

FIG. 2 is an explanatory view showing a cross section in the vertical direction of the ceramic piezoelectric element 1 of the present embodiment. This cross section forms an arc with a radius of 7 mm near the top of the dome,
The vicinity of the bottom forms an arc having a radius of 12.5 mm, and these are continuous.

FIG. 3 is a frequency characteristic diagram of the piezoelectric speaker of this embodiment. As a comparative example, FIG. 4 is a frequency characteristic diagram of a piezoelectric speaker in which the cross section of the piezoelectric element is formed only by an arc having a radius of 12.5 mm. In the case of the comparative example shown in FIG.
A peak due to primary resonance occurs near z and a sharp peak occurs due to higher resonance around 30 kHz. On the other hand, it is recognized that the frequency characteristics of the present embodiment are flatter and the band is broadened.

In this embodiment, as shown in FIG. 2, the cross section of the piezoelectric element 1 is constituted by two kinds of arcs, but may be constituted by a plurality of arcs. Of course.

[0027]

As described above, the piezoelectric speaker of the present invention has a wide sound reproduction frequency band because there is no influence of the resonance frequency due to the metal plate and no inconvenient vibration due to the unique shape of the diaphragm. Become.

In particular, since the dome-shaped curved surface is formed by combining spherical surfaces having different radii, the influence of the resonance frequency of the dome-shaped ceramic piezoelectric element itself is less likely to appear.

Further, since a voltage is applied in the thickness direction of the piezoelectric element, manufacturing is easy. Also, since the vibration direction of the piezoelectric element is orthogonal to the voltage application direction,
The resonance frequency can be lowered.

[Brief description of the drawings]

FIG. 1 is a sectional view of a piezoelectric speaker according to the present invention.

FIG. 2 is an explanatory diagram showing a cross section of a ceramic piezoelectric element in the piezoelectric speaker.

FIG. 3 is a frequency characteristic diagram of the piezoelectric speaker according to the present invention.

FIG. 4 is a frequency characteristic diagram of a piezoelectric speaker as a comparative example.

FIG. 5 is a schematic view showing an example of a conventional piezoelectric speaker.

FIG. 6 is a schematic view showing another example of a conventional piezoelectric speaker.

FIG. 7 is a schematic diagram showing still another example of a conventional piezoelectric speaker.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ceramic piezoelectric element 2 Frame 3 Lead wire

Claims (4)

[Claims] 1. A piezoelectric speaker characterized in that sound waves are obtained by energizing a ceramic piezoelectric element formed in a dome shape through electrodes. 2. The piezoelectric speaker according to claim 1, wherein a peripheral end of the ceramic piezoelectric element is held by a frame. 3. The piezoelectric speaker according to claim 1, wherein a vertical cross-sectional shape of the ceramic piezoelectric element is formed by connecting a plurality of arcs each having a different radius. 4. The piezoelectric speaker according to claim 1, wherein the ceramic piezoelectric element is configured to be polarized in a thickness direction and expandable and contractable in a plane direction.