JP2005311415A - Acoustic vibration generation element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acoustic vibration generation element that is shaped nearly a square, can provide a high acoustic output without distortion, provides excellent acoustic matching with a human body, and can easily be manufactured. <P>SOLUTION: The acoustic vibration generation element adopts a piezoelectric bimorph structure wherein piezoelectric ceramics rectangular plates 2 the front side and the rear side of each of which are metalized are clad to the front side and the rear side of an elastic plate 1. The acoustic vibration generation element is characterized in that at least two of the piezoelectric ceramics rectangular plates 2 or more are arranged and clad in parallel at nearly equal positions on the front side and the rear sides of the elastic plate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bone conduction speaker or the like that converts an acoustoelectric signal into acoustic vibration and propagates it to the skull or arm and listens to it with an auditory nerve, and more particularly to an acoustic vibration generating element suitable for use in a bone conduction speaker. Is.

  Conventionally, as an electro-mechanical transducer for bone conduction, the electromagnetic type is mainly used, and the driving force generated by the interaction between the magnet and the current flowing through the coil of the same principle as the dynamic speaker is changed to mechanical vibration. Several proposals have been made. However, the generated force of the electromagnetic method is electromagnetic force and requires current, but energy loss is caused by the resistance of the winding, and most of the energy supplied from the power source is lost as Joule heat and used as acoustic energy Only 1% is done. In addition, because the impedance is low in the low frequency range, the current tends to be excessive and the load on the power supply side is large. There were drawbacks.

  On the other hand, there are a few proposals for bone conduction transducers using piezoelectric elements. (For example, see Patent Documents 1 and 2) In this case, a piezoelectric unimorph element structure in which a metal plate often used as a piezoelectric sounding body and a piezoelectric material are bonded together is used. Since the frequency is 1 kHz or more, there is a drawback that reproduction in a low frequency range below the resonance frequency tends to be insufficient. In addition, since the mechanical quality factor Qm of the vibration system is high, the sound is emphasized at a specific frequency or is attenuated, so that a natural sound cannot be reproduced.

  In particular, bone conduction speakers are required to have a large acoustic output at a low voltage. For this purpose, the piezoelectric ceramic rectangular plate is made thin or a laminated structure to maintain the frequency characteristics and increase the output as described later. Although it is conceivable to increase the width of the vibrator as described above, the following problems arise.

Regarding the bimorph vibrator for generating acoustic vibrations, the basic relationship between the configuration and the acoustic performance is sufficiently theoretically expressed mathematically, and will be described below using some mathematical expressions. First, the relationship between components in relation to the number 1 for the resonance frequency f _r of the elements involved in the frequency characteristic of the acoustic output.

L in Equation 1 is the length of the bimorph vibrator, and when trying to shorten it for miniaturization, unless it is corrected by other factors, there is a tendency that the resonance frequency is suddenly raised and the frequency characteristic is raised toward the high range. Formula 1 is the same in both cases where the bimorph vibrator is supported at one end fixed and one end free cantilever, and both ends are free, and only α _n is different. As described above, α _n in the equation is a constant determined by the vibration mode, and will not be described in the following description.

  K in Equation 1 is an elastic modulus of bending determined by the thickness and elastic modulus of the piezoelectric material and shim material. This relationship is shown in Equation 2.

Here, w is the width of the element, E _s and E _c is the modulus of elasticity of the shim material and the piezoelectric material, 2t _s and t _c indicates the thickness of the shim and piezoelectric material. e represents a piezoelectric stress constant described later, and ε ^s represents a dielectric constant of the piezoelectric material. When the width w is determined, K is governed by the elastic modulus and thickness of the constituent material.

  ρS represents the weight per unit length, and is represented by Equation 3.

(Equation 3)
ρS = 2w (t _c ρ _c + t _s ρ _s )
Here, ρ _c and ρ _s indicate specific gravity of the piezoelectric material and the shim material, respectively.

Next, the source of the driving force of the acoustic vibration generating element, the bending moment M _e explained generated in response to an electric field. When a voltage is applied to the piezoelectric bimorph element, a bending moment _Me is generated, and the element is bent and vibrated. The relationship between the bending moment _Me and the constituent elements is expressed by Equation 4.

(Equation 4)
M _e = w (t _c + 2t _s ) eV
e is a piezoelectric stress constant of the piezoelectric material, indicating the degree of stress generated according to the electric field, and V indicating the voltage to be applied.

  The amplitude at the time of vibration in the free-supported form at both ends is a function of the position.

  Here, α is expressed by the angular frequency ω and the number 6 that can be defined by the components.

  Further, x is a factor of a position indicating an arbitrary point on the coordinates with the center point in the length direction of the bimorph as the origin.

From Equation 5, the greater the bending moment _Me , the greater the generated displacement, and as a result, the vibration speed and vibration acceleration, which are time derivatives thereof, increase. When the driving force in the vibration direction of the acoustic vibration generating element is expressed as Fb, the inertial force generated in each minute section obtained by the product of the weight ρSdx of the minute section dx in the length direction of the element and the acceleration α of the part is expressed in all sections. As an integrated value for, it can be obtained by Equation 7.

In Equation 7, the trigonometric and hyperbolic functions of Equation 5 are abbreviated as φ (α, x). .rho.s, K has entered in both width w form of a product, the effect of w in the portion of the route is canceled, include the width w to the number 4 of M _e. That is, the integrand of Equation 7 indicates the inertial force in each part as described above, and changes according to the vibration state and frequency, but a large amplitude is obtained as a result of a large bending moment, and a large driving force F _b. It can be explained from Equations 5 and 7 that it contributes to the generation of.

As described above, the bending moment M _e is dependent on the thickness t _c, 2t _s and the width w is in addition to e constants and the driving voltage V of the piezoelectric material. When the thickness is increased either both or one of t _c or 2t _s a large bending moment as ¥, largely on the frequency characteristics of the acoustic output for raising the resonant frequency number 2 for flexural modulus K is rapidly increased affect.

  In addition, as a measure for expanding the output power while maintaining the frequency characteristics of the acoustic characteristics, as is clear from Equation 4, an increase in the width w can also contribute to an increase in the bending moment. Interference between the resonance of the bending vibration mode and the main vibration mode in the longitudinal direction begins, and the influence of bending vibration in the width direction appears in the target frequency range, resulting in changes in the frequency characteristics and distortion of the acoustic output, resulting in natural There is a problem that it becomes impossible to reproduce a proper sound.

JP 59-140796 A JP 59-178895 A

  An object of the present invention is to provide an acoustic vibration generating element that is small, satisfies a desired frequency characteristic, and has a high output (large driving force), and that is easily matched with the human body and easy to manufacture. Is.

  According to the present invention, in an acoustic vibration generating element having a piezoelectric bimorph structure in which piezoelectric ceramic rectangular plates whose surfaces are metallized are bonded to the front and back surfaces of the elastic plate, at least two or more sheets are provided at substantially equal positions on the front and back surfaces of the elastic plate. An acoustic vibration generating element characterized in that the piezoelectric ceramic rectangular plates are aligned and bonded in parallel is obtained.

  According to the present invention, the piezoelectric ceramic rectangular plate has a structure in which a plurality of internal electrode layers and piezoelectric ceramic layers are laminated in the thickness direction and connected to a common external electrode every other layer. An acoustic vibration generating element is obtained.

  In addition, according to the present invention, an acoustic vibration generating element characterized in that the front and back surfaces or the entire surface is coated with a flexible material can be obtained.

  As described above, in a piezoelectric bimorph structure in which a piezoelectric ceramic rectangular plate and an elastic plate are bonded together, a plurality of piezoelectric ceramic rectangular plates are bonded to the front and back surfaces of one elastic plate to form a plurality of piezoelectric bimorphs. The piezoelectric element is divided at multiple locations, and the bending moment generated in the width direction is negligibly small compared to the bending moment generated in the length direction. Hateful. As a result, it is possible to provide an acoustic vibration generating element that has a vibrational mode in only one direction even with a shape close to a square, can obtain a large acoustic output with little distortion, and at the same time has good acoustic matching with the human body and is easy to manufacture.

  Furthermore, since the interval between the internal electrodes can be narrowed by making the piezoelectric ceramic rectangular plate a laminate of a plurality of internal electrodes and piezoelectric ceramic layers, the acoustic vibration generating element is driven with the same electric field strength as the piezoelectric ceramic rectangular plate. In this case, the drive voltage can be lowered.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS.

  FIG. 1 is a perspective view of an acoustic vibration generating element in which four piezoelectric ceramic rectangular plates 2 are bonded to the front and back surfaces of one elastic plate 1 according to an embodiment of the present invention. The elastic plate 1 is made of brass and has a length of 30 mm, a width of 25 mm, and a thickness of 0.1 mm. The piezoelectric ceramic rectangular plate 2 uses NEC TOKIN piezoelectric ceramics (trade name NEPEC 10), and silver electrodes are baked on the front and back surfaces of the piezoelectric ceramic rectangular plate having a length of 30 mm, a width of 8 mm, and a thickness of 0.15 mm. In a silicone oil at 100 ° C., a material polarized by applying a direct current voltage of 150 V was used and bonded together with an epoxy adhesive. The interval between the piezoelectric ceramic rectangular plates when arranged in parallel was 0.5 mm.

  FIG. 2 is a perspective view of a laminate of a plurality of internal electrodes 3 and a piezoelectric ceramic layer 4 used in different embodiments of the present invention before a silver electrode is baked on the side surface. Using the same piezoelectric ceramic powder as in the previous embodiment as a starting material, a 35 μm-thick green sheet was prepared, and an internal electrode pattern having a predetermined shape was printed on the green sheet with a silver / palladium electrode paste. Four green sheets on which internal electrodes were printed and green sheets that were not printed were thermocompression bonded and sintered at 1200 ° C., and then silver electrodes connected to the internal electrodes exposed on the two opposite side surfaces were baked. The dimensions of the laminate are the same as those of the piezoelectric ceramic rectangular plate 2 of the above-described embodiment, and this laminate is provided on the front and back surfaces of the same elastic plate as that of the above-described embodiment with 4 sheets each having an interval of 0.5 mm. To obtain an acoustic vibration generating element.

  3 is an acoustic vibration generating element in which a silicone resin 5 is coated as a flexible material on the front and back surfaces of the acoustic vibration generating element in FIG. 1, FIG. 3 (a) is a perspective view, and FIG. 3 (b) is AA. It is sectional drawing of '. The thickness of the silicone resin 5 is approximately 2 mm.

  As a comparative example, on the front and back surfaces of the same elastic plate as in the embodiment, a piezoelectric ceramic rectangular plate that is metalized on both sides with a length of 30 mm, a width of 24 mm, and a thickness of 0.15 mm with the same NEC TOKIN piezoelectric ceramic is used with an epoxy adhesive. Lamination and acoustic vibration generating elements were obtained. The acoustic vibration generating element of the comparative example is shown in FIG.

  Next, in order to quantitatively confirm the acoustic output of the acoustic vibration generating element of this embodiment and the acoustic vibration generating element of the comparative example, an artificial inner ear (Artificial Mastoid Type 4930, manufactured by B & K) is used for the auditory nerve of the human body. The acceleration and the strain at the position corresponding to were measured. The measurement results are shown in FIGS. 5 (a) and 5 (b).

  As is apparent from FIGS. 5A and 5B, in an embodiment different from the embodiment of the present invention, a substantially flat acceleration is measured from 400 Hz to 4000 Hz, which is sufficient as an acoustic vibration generating element of a bone conduction speaker. However, in the comparative example, it can be seen that a lot of unnecessary vibration is generated by coupling in the width direction and the length direction at a constant frequency interval, and the sound output has a lot of distortion.

FIG. 3 is a perspective view of an acoustic vibration generating element in which four piezoelectric ceramic rectangular plates are bonded to the front and back surfaces of one elastic plate. The perspective view before baking a silver electrode on a side surface with the laminated body of a some internal electrode and a piezoelectric ceramic layer. FIG. 3A is a perspective view, and FIG. 3B is a cross-sectional view taken along line A-A ′. FIG. 3A is an acoustic vibration generating element in which a silicone resin is coated on the front and back surfaces of the acoustic vibration generating element of FIG. The figure which shows the acoustic vibration generating element of a comparative example. The figure which shows the measurement result of the acceleration and distortion of an acoustic vibration generating element. FIG. 5A is a diagram showing a measurement result of acceleration, and FIG. 5B is a diagram showing a measurement result of strain.

Explanation of symbols

1 Elastic plate 2 Piezoelectric ceramic rectangular plate 3 Internal electrode 4 Piezoelectric ceramic layer 5 Silicone resin

Claims (3)

  In an acoustic vibration generating element having a piezoelectric bimorph structure in which piezoelectric ceramic rectangular plates with metallized front and back surfaces are bonded to the front and back surfaces of an elastic plate, at least two or more piezoelectric ceramic rectangular plates at approximately equal positions on the front and back surfaces of the elastic plate An acoustic vibration generating element characterized in that the two are aligned and bonded in parallel.   2. The acoustic vibration generation according to claim 1, wherein the piezoelectric ceramic rectangular plate has a structure in which a plurality of internal electrode layers and piezoelectric ceramic layers are stacked in a thickness direction and connected to a common external electrode every other layer. element.   3. The acoustic vibration generating element according to claim 1, wherein the front and back surfaces or the entire surface is covered with a flexible material.

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