JPH0753000B2 - Piezoelectric speaker - Google Patents

Description

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

2. Description of the Related Art In recent years, almost all audio equipment has been digitized and the sound quality has been dramatically improved. Such a diaphragm is required to be less deformed by an external force and to have less sound distortion, and to have a clear reproduced sound with a wide reproduction range. For that purpose, it is light and has excellent elastic modulus and rigidity. Things are required. When this is summarized as a condition for specific physical property values, the Young's modulus (E) is large.

 The density (ρ) is small.

 The sound velocity (sound velocity V) is high.

 The internal vibration loss (tan δ) must be appropriate.

And so on. However, there is a relationship of V = √E / ρ between V, E and ρ. Of course, in addition to these conditions, ease of processing,
It goes without saying that solderability, rust resistance, and stability against external conditions such as heat and humidity are also important.

On the other hand, a piezoelectric buzzer or speaker using a piezoelectric element made of ceramic as a sound source (electrical-acoustic conversion element) has come to be widely used. The reason is that it is thin, it is easy to reduce the price, and it consumes low power. In a ceramic (piezoelectric) speaker, an electric signal is applied to a ceramic thin film to generate vibration, and the vibration is transmitted to a diaphragm in close contact with the ceramic plate to generate sound in the audible region. As a diaphragm material, brass plates, iron plates, stainless steel, iron-nickel alloys, aluminum, plastics, etc. have been conventionally used as the material.

However, it is hard to say that most diaphragm materials currently used for piezoelectric speakers have sufficient performance from the viewpoint of high-fidelity audio. For example,
The characteristics of plastic such as Young's modulus, density and speed of sound are not sufficient as a diaphragm, and the frequency characteristics are extremely inferior especially in the high frequency band, and the sound quality is clear as a diaphragm of a high frequency speaker (tweeter). It's hard to get.
In addition, even with aluminum or the like, it is difficult to obtain a sufficiently high sound velocity due to its high density. Metals such as boron and beryllium have a high sound velocity (12 km / sec) and are said to be excellent diaphragm materials for tweeters. However, in the case of these metals, since the internal loss of vibration is small, a high frequency vibration phenomenon or the like is likely to occur, and only a metallic high pitched sound is generated. In fact, it is clear that the current piezoelectric speaker has not yet reached the point where it competes with the conventional voice coil type (electric type) cone or flat type speaker in consideration of sound quality.

The present invention aims to provide a novel speaker that overcomes the drawbacks of the conventional diaphragm material in the piezoelectric speaker as described above, has excellent high frequency characteristics, and is capable of reproducing a good tone color. Is.

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems of the diaphragm for the piezoelectric speaker, the present inventors have found that it is difficult to further improve the characteristics with the conventional diaphragm material. The conclusion was reached. Then, as a result of measuring the characteristics by using the novel graphite film for the diaphragm for the piezoelectric speaker, the inventors obtained the knowledge that they have excellent vibration characteristics, and arrived at the present invention.

That is, according to the present invention, in order to realize the hi-fi characteristic,
A piezoelectric film is used as a part or all of the diaphragm of the piezoelectric speaker element.

With the above configuration, in the present invention, by utilizing the excellent physical property values of the novel graphite film and using it as a part or the whole of the diaphragm of the piezoelectric speaker, the reproduction range is remarkably higher than that of the conventional diaphragm. A piezoelectric speaker having a wide and clear high-fidelity sound quality can be manufactured.

Examples Hereinafter, examples of the present invention will be described in detail.

The basic structure of the present invention is shown in FIGS. 1 and 2. Fig. 1 shows what is called a unimorph-type sounding body.
An aluminum plate is attached to a 100-micron ceramic thin film, and the graphite film according to the present invention is evenly adhered thereto. The graphite coating thickness was 5 to 25 microns, preferably 10 microns for 100 micron ceramic plates. The ceramics used are those conventionally known, for example, PbTiO ₃ -PbZrO _3, with a dielectric constant of typically 5000 and an electromechanical coupling coefficient of 50 to 70. The electrodes are taken out from the graphite film or by applying silver paste to ceramics. or,
FIG. 2 shows a bimorph-type sounding body, which is made by sandwiching a metal plate with piezoelectric ceramics, and a graphite film is adhered to the outside of the ceramics.

The graphite film used in the present invention consists of high-performance pyrographite obtained by pyrolyzing a condensation polymer invented by the same inventors (Murakami et al., Applied, Physical Letters; Appl. Phys. Lett. 48). Volume, Issue 23,
(1594, 1986) This is obtained by heat-treating a film of a heat-resistant, condensation-type polymer such as polyoxadiazole, polyimide, or polyamide-imide in an inert gas at a temperature of about 2800 ° C or higher and graphitizing it. Is. Pyrographite obtained from such special polymer raw materials is
The ab plane of graphite is perfectly oriented along the film surface, and the film has a structure close to that of a single crystal. Table 1 shows the material constants of the graphite film (10 μm thick) used in the present invention as a diaphragm in comparison with the conventionally known materials. From this, it is clear that graphite used in the present invention is far superior in E and V to all the other materials, and it can be said that it is most suitable as a speaker diaphragm having good sound quality especially in the high frequency range. .

On the other hand, although a graphite powder molding such as the conventionally known Grafoil (Union Carbide) is effective in improving sound quality in principle, it is difficult to manufacture a thin film of 100 microns or less. Since the sound velocity is on the order of 5 km / sec, it has been clarified that it does not reach the graphite film of the present invention.

Next, the inventors conducted a study on a speaker configuration that maximizes the excellent acoustic characteristics of the piezoelectric speaker device having the graphite diaphragm of the present invention. As a result, the flat panel type speaker having the configuration shown in FIG. It has been found that the performance as a speaker is further improved. That is, the third
The honeycomb core material shown in the figure is a honeycomb structure with a thickness of 1 to 2 mm and a diameter of 20 to 100 mm, which is formed by molding a resin such as epoxy. Usually, acoustic diaphragms are attached to both surfaces as skin materials, It is magnetically excited by an electric motor (voice coil). In the present invention, a graphite film is attached as a skin material on both sides of this honeycomb core material,
Furthermore, it has been verified that it is optimal to have a structure in which the piezoelectric ceramic plate is in close contact with the graphite film.

Examples of the present invention will be described in more detail below.

<Example 1> Piezoelectric ceramic material PCM-33A (Matsushita Electronic Components Co., Ltd.)
We manufactured a speaker using a 25 μm thick graphite film as a diaphragm. The shape and characteristics of the ceramics were as follows.

Shape: φ23 5 × 100μm Capacitance: ~ 110nF (at1kHz) Resonance frequency: ~ 90kHz Resonance resistance: ~ 1Ω Graphite film is made by firing Kapton polyimide with a thickness of 50μm at a pressure of several 100g / cm ² at 3000 ℃. The shape was φ23 × 25 μm. A silver paste was applied to both sides of the ceramic plate, and a graphite film was adhered to one side to form a unimorph type speaker.

When a 3V (rms) sine wave is applied as an input, it changes from 5k to 5k
An almost flat sound pressure output of 40 dB was obtained for the frequency of 0 kHz.

Next, in the unimorph type speaker having the configuration shown in FIG. 1, a 0.07 mm iron-nickel alloy was used as a metal diaphragm, and a graphite film having a thickness of 12 μm was laminated. For the same input voltage, the output increased to 70dB and the high frequency limit frequency was 35kHz. It was confirmed that providing a graphite film on a part of the diaphragm not only enabled the generation of high-pitched sounds, but also that the sound quality was soft and clear.

Example 2 An example of a bimorph-type speaker having the configuration of FIG. 2 in which another graphite film is used for the same piezoelectric ceramic as in Example 1 will be shown. The dimensions of the ceramics and the first and first vibrating plates (brass plate) are φ14mm × 100μm and φ15mm × 5, respectively.
The thickness was 0 μm, and a graphite film of φ13 mm × 11 μm was adhered on both sides with an epoxy resin. The graphite film was treated with polyparaphenylene oxadiazole (manufactured by Furukawa Electric Co., Ltd., thickness: 25 μm) in a hot press furnace at 2800 ° C for 10 kg / cm ^{2 for} 1 hour, and further in an ultrahigh temperature furnace at 3000 ° C for 3 hours (150 g / cm ² ) It was obtained by processing. Wiring was performed so that a voltage was applied in parallel to the two ceramic plates, and the frequency characteristics of the speaker were measured. 1kHz to 25kHz for 1.5V input voltage
It was found that an output of 80 dB was obtained almost flat over the period.

Example 3 A graphite film (thickness: 6.5 μm) obtained by graphitizing a polyimide or polyamide resin film (15 μm) on both surfaces of a honeycomb-shaped core material made of Micar epoxy having a diameter of 28 mm and a thickness of 1.5 mm at 3000 ° C. ) Was glued. Next, the ceramic vibration plate (diameter 16
mm) was adhered to the graphite film with silver paste (DuPont 4929). As shown in FIG. 3, the lead wire was taken out from the silver paste layer formed on the other surface of the ceramic substrate and the graphite in contact with the ceramic diaphragm, and an AC voltage was applied to the lead wire to evaluate the frequency characteristics, acoustic output and sound quality. 1.5V
For the sine wave input (1Hz to 100kHz), an almost constant output was obtained between 5kHz and 75kHz, and its magnitude was about 100dB. Moreover, the sound quality was not damaged at all as compared with the case of the electric speaker.

EFFECTS OF THE INVENTION In summary, the present invention provides a high-quality speaker that uses a graphite film as part or all of the diaphragm of a piezoelectric speaker element, has excellent frequency characteristics, and has ultra-high fidelity characteristics.

The piezoelectric speaker device obtained by the present invention is not only used as a high-fidelity audio speaker, but also contributes to a dramatic improvement in the sound quality of a computer, a microcomputer device, a melody output, or a telephone receiver.

Further, the graphite film diaphragm used in the piezoelectric speaker of the present invention is not limited to the materials listed in the examples, and a high-quality thin film graphite obtained by pyrolyzing and graphitizing a polymer can be used. The same applies to all.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a piezoelectric speaker element (unimorph type) in one embodiment of the present invention, FIG. 2 is a conceptual diagram of a piezoelectric speaker element (bimorph type) in another embodiment of the present invention, and FIG. It is a conceptual diagram of a flat plate type hi-fi speaker using the piezoelectric speaker element of the present invention. 11 …… Piezoelectric ceramics, 12 …… Metal plate, 13 …… Graphite thin film, 14 …… Electrode, 15 …… Lead wire, 21 …… Piezoelectric ceramics, 22 …… Metal plate, 23 …… Graphite thin film, 24 …… Lead wire, 31 …… Piezoelectric ceramics, 32 ……
Honeycomb cone, 33 ... Graphite thin film, 34 ... Electrode, 35 ... Lead wire.

Claims (3)

[Claims] 1. A piezoelectric speaker comprising a piezoelectric vibrating element and a vibrating plate in close contact therewith, wherein the vibrating plate has a graphite film as a part or the whole thereof. 2. A honeycomb type core material having a thickness of 1 to 2 mm and a diameter of 20 to 100 mm obtained by molding a resin such as epoxy has graphite coatings on both sides, and a piezoelectric vibrating element is adhered to one side of the core. Piezoelectric speaker characterized by having. 3. The graphite film is obtained by graphitizing a condensation polymer selected from polyoxadiazole, polyimide, and polyamide at a temperature of 2800 ° C. or higher. The piezoelectric speaker described.