JPS6051099A - Acoustic transducer - Google Patents

Abstract

PURPOSE:To make stable acoustic transducing at high sound pressure irrespective of the fluctuation of an external load by selecting the resonance frequency of a hollow formed in front or back side of a diaphragm lower than the resonance frequency of the diaphragm. CONSTITUTION:A yoke 4 having an iron core 3 wound with a coil 2 is attached to the bottom of cylindrical case 1 of an acoustic transducer. A cylindrical permanent magnet 5 surrounding an electric magnet made of the coil 2 and iron core 3 is provided in the yoke 4. A discoid diaphragm 6 is mounted at the upper opening of the case 1 via a cavity through the tip 3a of the iron core 3 and tip face 5a of the magnet 5. A hollow 83 is formed in front or back of the diaphragm 6. The inherent resonance frequency f1 of the diaphragm 6 and the hollow resonance frequency f2 of the hollow 83 are selected to form the frequency relation of f1>f2. Thus, the stable acoustic transducing at high sound pressure is performed regardless of the fluctuation of the external load etc.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an acoustic transducer used for making a sound in, for example, various watches, cameras, microwave ovens, portables, video tapes, recorders, pocket bells, etc. Concerning vessels.

PRIOR ART AND THEIR PROBLEMS Although piezoelectric type acoustic transducers and the like are known as this type of acoustic transducer, recently, electromagnetic type acoustic transducers having a resonance point in a low frequency region of around 2 kHz have been attracting attention. This electromagnetic acoustic transducer excites a diaphragm through magnetic interaction between a direct current magnetic field and an alternating current magnetic field to obtain vibrating sound.

FIG. 1 is a sectional view showing a conventional example of the electromagnetic acoustic transducer, in which a yoke 4 having an iron core 3 around which a coil 2 is wound is attached to the bottom of a cylindrical case l made of a non-magnetic metal material or the like. At the same time, a cylindrical permanent magnet 5 is placed on the yoke 4 so as to surround the electromagnet made up of the coil 2 and the iron core 3.
is arranged, and a disc-shaped diaphragm 6 is mounted in the upper opening of the case l, which faces the tip 3a of the iron core 3 and the tip surface 5a of the permanent magnet 5, respectively, with a gap therebetween. It has become. A ballast door made of a magnetic material or the like is fixed to approximately the center of the diaphragm 6.

In addition, in order to improve the sound pressure level and protect the acoustic converter at the same time, a sound emitting cylinder 8 is attached to the sound emitting side of the case l, and a sound emitting cylinder 8 is provided with a hole in the front plate 81 of the sound emitting cylinder 8. Sound hole 82
It emits a vibrating sound.

To drive the acoustic transducer having the above structure, an oscillation circuit 9 supplies a periodic current of an appropriate frequency, for example a rectangular wave current, to the coil 2 forming an electromagnet. Then, the alternating current magnetic field generated in the electromagnet 2.3 and the permanent magnet 5
Due to the magnetic interaction with the unidirectional magnetic bias, the diaphragm 6 vibrates depending on the frequency of the drive current, and the vibration sound is generated inside the sound emitting cylinder 8 by a cavity resonance effect of the cavity 83. In response to this, the sound is emitted from the sound emitting hole 82 to the outside.

The oscillation circuit 9 is generally a small automatic oscillation circuit with a simple circuit configuration, such as a blocking oscillation circuit,
Usually, it is attached integrally with the case 1. FIG. 2 shows a specific example of the oscillation circuit 9, in which an inductor L1 connected to the collector side of a transistor Q+ and an inductor L2 connected to the base side are inductively coupled to form a transistor. The circuit configuration is such that positive feedback is applied from the output side of l to its input side to perform blocking oscillation operation. The inductors L1 and L2 are constructed using a coil 2 that constitutes an electromagnet. R1 is a base bias resistor, and Dl is a diode.

In the above acoustic transducer, in order to improve the sound pressure level, conventionally, the frequency fo of the drive current is made approximately equal to the natural resonant frequency f of the diaphragm 6, and at the same time, the amplitude of the diaphragm 6 is increased. Natural resonance frequency fl of 6 and cavity 8
The cavity resonance frequencies f2 and No. 3 were located close to each other under the condition of f2 > fl to improve the cavity resonance effect. In this case, the stable point of the mechanical vibration of the diaphragm 6 is near the peak of the sound pressure characteristics, that is, the resonance frequency f,
, around f2. Furthermore, the electrical stability points are also near the resonance frequencies f, , f2.

When a device with such characteristics is automatically oscillated, the stable point is usually at a frequency f11 or f22 (see Figure 3), which is slightly lower than the resonance frequency f, f2. Electrical impedance 2. is the electrical impedance Z for the resonance frequency f2 of the cavity 83
2 (see FIG. 4), the self-excited vibration operation is normally performed at a frequency fl1 slightly lower than the resonance frequency f1 of the diaphragm 6. However, if the long voltage rises suddenly, or if the external load conditions change due to the sound emission hole 82 being blocked, etc., a stable point will temporarily appear near the frequency f22, which is higher than the frequency fl1. shifts, and even after the external conditions are removed, self-excited oscillation operation may occur around this frequency f22. In other words, there is a problem in that the stable oscillation frequency moves to either frequency fll or f22 depending on external conditions, making the vibration sound frequency unstable.

As a means to solve this drawback, as shown in Figure 5,
A method has been tried in which the natural resonance frequency f1 of the diaphragm 6 and the cavity resonance frequency f2 are selected apart from each other. In this case, the electrical impedance characteristic becomes a single peak characteristic as shown in Figure 6, so even when driven by a self-supporting oscillation circuit, stable oscillation operation is achieved, but the cavity resonance effect is impaired. No increase in sound pressure level can be expected.

Purpose of the Invention The present invention solves the above-mentioned conventional problems, and
The purpose of the present invention is to provide an acoustic transducer that performs stable oscillation operation at high sound pressure.

Configuration of the Present Invention In order to achieve the above object, the present invention provides an acoustic transducer having a cavity on the front or rear side of a diaphragm constituting an acoustic transducer, in which the resonant frequency of the cavity is lower than the resonant frequency of the diaphragm. It is characterized by having been selected to have a low value.

That is, the natural resonant frequency fl of the diaphragm 6 and the cavity resonant frequency f2 of the cavity 82, which were conventionally set in the relationship f<f2, are selected so that fl>f2 as shown in FIG. That's what I do.

In this case, the electrical impedance characteristics become as shown in FIG. 8, and the electrical impedance Zl corresponding to the resonant frequency f of the diaphragm 6 becomes larger than the electrical impedance Z2 corresponding to the resonant frequency f2 of the cavity 83. When oscillation is performed, stable automatic oscillation is performed at a frequency fi1 slightly lower than the resonance frequency f of the diaphragm 6. Furthermore, since the frequency fll is between the peaks of the bimodal characteristics corresponding to the resonant frequencies fl and f2, a high sound pressure vibration sound with little variation can be obtained.

Furthermore, even if the opening degree of the sound emitting hole 82 changes temporarily, such as when the sound emitting hole 82 is blocked, a stable oscillation operation is performed at the frequency f. FIG. 9 is a characteristic diagram showing changes in the resonance frequency f and f2 with respect to the diameter of the sound emission hole 82. As shown in FIG. 9, when the opening degree of the sound emission hole 82 becomes smaller, f
, > f2 (A). Therefore, even if the opening degree of the sound emitting hole 82 changes, the frequency remains stable at the frequency fl side without shifting to the frequency f2 side.

Effects of the Invention As described above, the present invention provides an acoustic transducer having a cavity on the front or back side of a diaphragm constituting an acoustic transducer, in which the resonance frequency of the cavity is lower than the resonance frequency of the diaphragm. This feature makes it possible to provide an acoustic transducer that performs stable oscillation operation at high sound pressure despite external load fluctuations, etc.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a conventional example of the electromagnetic acoustic transducer;
Figure 2 is an electric circuit diagram of the oscillation circuit, Figure 3 is a sound pressure characteristic diagram of a conventional acoustic transducer, Figure 4 is its electrical impedance characteristic diagram, and Figure 5 is a sound pressure characteristic diagram of a conventional acoustic transducer. figure,
FIG. 6 is an electric impedance characteristic diagram, FIG. 7 is a sound pressure characteristic diagram of an embodiment of the acoustic transducer according to the present invention, FIG. 8 is an electric impedance characteristic diagram, and FIG. 9 is a sound emission hole diagram. FIG. 2 is a diagram showing the relationship between pore diameter and frequency. l-・Case 2・Fist・Coil 3φ・φ Iron core No. 5・Permanent magnet 6φ・・Diaphragm 8・φ・Sound emitting cylinder 9−・φ Oscillation circuit 83＠・Cavity patent applicant TDC Co., Ltd. *2 Figure 3 Figure 4 Figure 5 Figure 6 Jun Q -1 □

Claims (5)

[Claims] (1) An acoustic transducer having a cavity on the front or rear side of a diaphragm constituting an acoustic transducer, characterized in that the resonance frequency of the cavity is selected to be lower than the resonance frequency of the diaphragm. vessel. (2) The acoustic transducer is arranged to face the electromagnet through a gap due to magnetic interaction between an electromagnet driven by a periodic current and a permanent magnet arranged to surround the electromagnet. The acoustic transducer according to claim 1, wherein the diaphragm is vibrated. (3) The acoustic transducer according to claim 2, wherein the electromagnet is driven by an automatic oscillation circuit. (4) The acoustic transducer according to claim 3, wherein the self-excited sales circuit is a blocking oscillation circuit in which the coil of the electromagnet is used as a feedback winding. (5) The acoustic transducer according to claim 2, 3, or 4, wherein the electromagnet is driven by a periodic current having a frequency substantially equal to the resonant frequency of the diaphragm. .