JPS602717Y2 - electroacoustic transducer - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an electroacoustic transducer using a piezoelectric element, and particularly provides an electroacoustic transducer having high conversion efficiency and optimal for use in the audio band.

Generally, a piezoelectric speaker using a piezoelectric element uses a bimorph element as a driving source and is realized as a high-pitched speaker.

This is due to the relationship between the reproduction frequency band and the output sound pressure level.

Bimorph elements are used in vibrating bodies such as microphones, pickups, buzzers, and speakers, and use of this element allows for power saving and thinning.

Bimorph elements are made by pasting together two piezoelectric elements that expand and contract in the length direction, and when one expands, the other contracts, resulting in a bending motion as a whole.It is designed to reduce mechanical impedance and produce a large output voltage. This is what I did.

Usually, a piezoelectric element used in a bimorph element has a thin disk shape, and the resonance frequency of a disk-shaped piezoelectric element that is allowed to freely vibrate is generally expressed by the following equation.

(Here, r is radius, t is thickness, E is Young's modulus, ρ is density, σ is Poisson's ratio, and a is a constant.

) As is clear from this equation, E, ρ, and σ are unique characteristics determined by the material, and bimorph elements require a high dielectric constant and a high electromechanical coupling coefficient, so they are mainly used as materials for piezoelectric elements. Piezoelectric ceramics are used, but there are limits to changing the material and composition of piezoelectric ceramics.

Therefore, the fundamental resonant frequency of a bimorph element is approximately determined by its size and shape.

Fig. 1 shows a conventional bimorph element having the above-described structure, and Fig. 1A shows a partially cutaway plan view, and Fig. 1B shows a c/cross-sectional view of Fig. 1A. .

A polarized piezoelectric ceramic 1 having electrodes 3 on both sides and having a predetermined diameter and thickness is mechanically and electrically connected to an intermediate electrode plate 2 via the electrodes 3.

In order to use a bimorph element with such a structure as a bimorph element for the audio band, a fundamental resonance frequency lower than the reproduction frequency band is required. One possible method is to increase the diameter.

As mentioned above, ceramics are mainly used for piezoelectric elements due to their characteristics, but since ceramics need to be sintered at high temperatures, manufacturing becomes more difficult as they become thinner.

Furthermore, it is also difficult to manufacture a larger diameter with the same thickness.

Japanese Patent Publication No. 46-2716 states that the fundamental resonance frequency can be lowered by providing 8-digit slits in the radial direction. Although this method lowers the fundamental resonance frequency, it also lowers the electro-mechanical coupling coefficient, which indicates conversion efficiency.

It will drop.

Note that the electro-mechanical coupling coefficient is the fundamental resonance frequency fr,
When the antiresonance frequency is fa, it is given by the following equation.

On the other hand, it is conceivable to leave the dimensions of the piezoelectric element the same and increase the dimensions of the intermediate electrode plate, but this would also lower the electro-mechanical coupling coefficient.

When used as a drive source for acoustic conversion elements such as speakers, high conversion efficiency and high force coefficient are required.

All of the conventional examples described so far are not ideal as drive elements in some respect.

The present invention solves these problems and provides an electroacoustic transducer that lowers the fundamental resonance frequency while maintaining high conversion efficiency.

As shown in FIG. 2, when the bimorph element shown in FIG. 1 is allowed to freely vibrate, it vibrates concentrically with reference to the fundamental vibration node b'.

This node exists in the vicinity of about 0.6 to 0.7 of the diameter a of the bimorph element as a flexural vibration of the disk.

In the present invention, by partially providing a gap at the intermediate electrode plate position corresponding to this basic vibration node, the elastic compliance of the vibration node can be locally increased without reducing the force coefficient, and the fundamental resonance frequency can be increased. is decreasing.

Fig. 3 shows the structure of the electroacoustic transducer of the present invention, and Fig. 3A shows its partially cutaway plan view, and Fig. 3B
3A shows a sectional view taken along line d-d' in FIG. 3A.

In the figure, parts having the same functions as those in FIG. 1 are given the same numbers.

Here, the difference from Fig. 1 is that the intermediate electrode plate 2 has a fundamental vibration node 5.
There are intermittent voids 4 in the radial direction.

This cavity 4 is provided in the radial direction, and it is necessary that it does not reach the outer periphery.

The void 4 provided at the vibration node is not limited to a circular shape as shown in the drawings, but may have other shapes with the same effect, and if it is arranged concentrically and symmetrically, it is designed to have the optimum dimensions in accordance with the characteristics.

In addition to shaping the intermediate electrode plate, it is also possible to lower the resonant frequency by creating a void that penetrates the piezoelectric ceramic sheet at the same location, but this makes machining the ceramic sheet difficult and reduces the impedance in terms of electrical characteristics. There is a drawback that appears as an increase in

The present invention will be described in detail below based on Examples.

Example 1 A piezoelectric ceramic sheet having a thickness of 100 μm and a diameter of 38 mm was adhered to an intermediate electrode plate with the polarization directions aligned.

Holes with a diameter of 4 mm were provided in the intermediate electrode plate symmetrically at a distance of 14 mm from the center point corresponding to the fundamental vibration node under various conditions shown in Table 1.

Table 1 shows the piezoelectric characteristics of the bimorph element under these conditions.

From this result, acoustic conversion elements such as speakers are desired to have high characteristics of capacitance, coupling coefficient, and admittance, and a low resonant frequency.

Taking these into consideration, it was found that when 8-digit holes are provided, the resonance frequency is the lowest and other characteristics are not degraded.

When 12 digits are provided, the admittance characteristic deteriorates due to divided vibration.

Example 2 A piezoelectric ceramic sheet similar to that used in Example 1 was bonded to a bimorph element using brass with a thickness of 1100 mm and a diameter of 40 mm as an intermediate electrode plate, and bonded to both sides of the piezoelectric ceramic sheet with the polarization directions aligned. did.

Eight-digit holes, which were the most effective in Example 1, were provided in the intermediate electrode plate under various conditions as shown in Table 2.

The piezoelectric characteristics under each condition are shown in Table 2.

As is clear from Table 2, if the device of the present invention is used, the piezoelectric properties will be reduced in all cases.

It is possible to lower the resonance frequency by about 20% without any problem.

Table 2 shows an example in which a slit is made to the outer periphery and an example in which a hole is also provided in the piezoelectric element as a comparative example.When the slit is made to the outer periphery, the resonance frequency can be lowered, but the coupling coefficient Both the converter and admittance characteristics become low, making it impossible to obtain sufficient conversion efficiency.

Furthermore, when holes are provided in the piezoelectric element, the capacitance decreases and the admittance characteristic also decreases.

In addition, in the above two embodiments, a disc-shaped piezoelectric element was used, but the present invention is not limited to this shape, and the same effect can be obtained even if other shapes such as an ellipse are used. Needless to say, you can get it.

As is clear from the above description, the electroacoustic transducer of the present invention has a bimorph element in which two piezoelectric elements are sandwiched between one electrode plate, and a gap is formed at the electrode plate position corresponding to the fundamental vibration node of the bimorph element. It is possible to obtain an electroacoustic transducer having a low resonant frequency without degrading other piezoelectric properties, and therefore its practical effects are great.

[Brief explanation of the drawing]

1A and 1B are a partially cutaway plan view and a side sectional view of a conventional electroacoustic transducer element, FIG. 2 is a principle diagram showing the vibration mode of the element in FIG. 1, and FIG. 3A. B is a partially cutaway plan view and side sectional view of the electroacoustic transducer of the present invention. 1... Piezoelectric element, 2... Electrode plate, 3.
...Electrode, 4...Gap, 5...
・Basic vibration node.

Claims (2)

[Scope of utility model registration request] (1) In an electroacoustic transducer in which two piezoelectric elements with electrodes are provided on both sides of an electrode plate, a gap is partially formed in the electrode plate at a position corresponding to the fundamental vibration node of the electroacoustic transducer. An electroacoustic transducer characterized by being provided. (2) In the statement in paragraph 1 of the claims for utility model registration,
An electroacoustic transducer element characterized in that a disk-shaped piezoelectric element is used, and a gap in an electrode plate is provided on a concentric basic vibration node.