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JPS6051099

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DESCRIPTION JPS6051099
[0001]
The invention relates to acoustic transducers used for sounding, for example in various watches,
cameras, microwave ovens, portables, video, tapes, recorders or pages 7) bells and the like. The
prior art and its problems As a sound transducer of this type, a piezoelectric type or the like is
also known, but recently, an electromagnetic type having a resonance point in a low frequency
region of around 2 KHz has attracted attention. This electromagnetic acoustic transducer excites
the diaphragm by the magnetic interaction between a direct current magnetic field and an
alternating current magnetic field to obtain vibration sound. FIG. 1 is a cross-sectional view
showing a conventional example of the electromagnetic acoustic transducer, in which a yoke 4
having an iron core 3 having a coil 2 wound thereon is attached to the bottom of a cylindrical
case l made of nonmagnetic metal material or the like. In addition, a cylindrical permanent
magnet 5 is disposed on the yoke 4 so as to surround the electromagnet composed of the coil 2
and the iron core 3, and the tip 3 a of the iron core 3 and the permanent magnet are further
provided in the upper opening of the case l. A disc-shaped diaphragm 6 is mounted so as to face
each of the front end faces 5a of the fifth example via a gap. A ballast door made of a magnetic
material or the like is fixed to a substantially central portion of the diaphragm 6. Also, in order to
improve the sound pressure level and at the same time to protect the acoustic conversion unit,
the sound emitting cylindrical body 8 is mounted on the sound emitting side of the case l, and
the sound emitting cylindrical body 8 is perforated. Vibration sound is emitted from the sound
hole 82. In order to drive the acoustic transducer having the above structure, a periodic current
of an appropriate frequency, for example, a rectangular wave current is supplied from the
oscillation circuit 9 to the coil 2 forming the electromagnet. Then, due to the magnetic
interaction between the alternating magnetic field generated in the electromagnet 2.3 and the
unidirectional magnetic bias of the permanent magnet 5, the diaphragm 6 vibrates depending on
the frequency of the drive current, and the vibration thereof. The sound is emitted to the outside
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through the sound emission hole 82 under the cavity resonance action of the cavity 83 formed
inside the sound emission cylinder 8. The oscillation circuit 9 generally uses a small-sized
automatic oscillation circuit having a simple circuit configuration, such as a blocking oscillation
circuit, and is usually mounted 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 of the transistor Q + is
inductively coupled to an inductor L2 connected to the base. A positive feedback is applied from
the output side of l to its input side to cause a blocking oscillation operation. The inductors L1
and L2 are configured using a coil 2 that constitutes an electromagnet. R1 is a base bias resistor,
and D1 is a diode.
In the above acoustic converter, in order to improve the sound pressure level, conventionally, the
frequency fo of the drive current is made substantially equal to the natural resonance frequency f
of the diaphragm 6, and the amplitude of the diaphragm 6 is increased. The natural resonance
frequency f1 of 6 and the cavity resonance frequency f2 of the cavity 83 are positioned close to
each other under the condition of f2> fl to improve the cavity resonance action. The stable point
of the mechanical vibration of the diaphragm 6 in this case is in the vicinity of the peak of the
sound pressure characteristic, that is, in the vicinity of the resonance frequencies f, f2 and f2. In
addition, the electrically stable point is also in the vicinity of the resonance frequencies f, f, f2. In
the case where such a characteristic is automatically oscillated, the resonance frequency f and a
frequency f11 or f22 (see FIG. 3) slightly lower than f 2 (see FIG. 3) are usually stable points.
Electrical impedance However, since the electrical impedance Z2 of the cavity 83 for the
resonance frequency f2 is larger (see FIG. 4 @), the self-oscillation operation is normally
performed at a frequency fl1 slightly lower than the resonance frequency f1 of the diaphragm 6.
However, when the long-term voltage suddenly rises E, or when the external load conditions
change due to the sound release hole 82 being blocked, etc., a stable point is temporarily
established near the frequency f22 higher than the frequency fll. After the external conditions
are removed, the self-oscillation operation may be performed around this frequency f22. That is,
there is a disadvantage that the stable oscillation frequency moves to either the frequency fll or
f22 depending on the external conditions, and the vibration sound frequency becomes unstable.
As means for solving this defect, as shown in FIG. 5, a method of selecting the natural resonance
frequency f1 of the diaphragm 6 and the cavity resonance frequency f2 apart is tried. In this
case, since the electrical impedance characteristic is a single peak characteristic as shown in FIG.
6, stable oscillation operation is achieved even when driven by the self-helping oscillation circuit,
but the cavity resonance effect is impaired. Sound pressure level can not be expected to rise.
SUMMARY OF THE INVENTION An object of the present invention is to solve the abovementioned conventional problems and to provide an acoustic transducer which performs stable
oscillation operation with 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 back side of a diaphragm constituting an acoustic transducer, wherein the
resonant frequency of the cavity is greater than the resonant frequency of the diaphragm. It is
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characterized in that it is selected to a low value. That is, as shown in FIG. 7, the natural
resonance frequency fl of the diaphragm 6 and the cavity resonance frequency f2 of the cavity
82, which were conventionally set to the relationship of f, <f2, are selected so as to be fl> f2. It is
to do.
In this case, the electric impedance characteristic is as shown in FIG. 8, and the electric
impedance Zl corresponding to the resonance frequency f of the diaphragm 6 is larger than the
electric impedance Z2 corresponding to the resonance frequency f2 of the cavity 83. When the
oscillation operation is performed, a stable automatic oscillation operation is performed at a
frequency f i1 slightly lower than the resonance frequency f of the diaphragm 6. Moreover, since
the frequency fll is between the peaks of the bimodal characteristics respectively corresponding
to the resonance frequencies fl and f2, a high sound pressure vibration sound with small
variation can be obtained. In addition, even when the opening degree is temporarily changed, for
example, when the sound release hole 82 is closed, a stable oscillation operation is performed on
the frequency f side. FIG. 9 is a characteristic diagram showing changes in the resonance
frequency f and f2 with respect to the diameter of the sound output hole 82. As shown in FIG. 9,
when the opening degree of the sound release hole 82 becomes smaller, oscillation occurs in a
region (f) where f,> f 2. Therefore, even when the opening degree of the sound release hole 82
changes, the frequency fl side is stabilized without shifting to the frequency f2 side. As described
above, according to the present invention, in the acoustic transducer having a cavity on the front
or back side of the diaphragm constituting the acoustic transducer, the resonant frequency of the
cavity is lower than the resonant frequency of the diaphragm. It is possible to provide an acoustic
transducer that performs stable oscillation operation with high sound pressure regardless of
external load fluctuation and the like because it is characterized in that the value is selected.
[0002]
Brief description of the drawings
[0003]
1 is a cross-sectional view showing a conventional example of the electromagnetic acoustic
transducer, FIG. 2 is an electric circuit diagram of an oscillator circuit, FIG. 3 is a sound pressure
characteristic diagram of the conventional acoustic transducer, and FIG. Electrical impedance
characteristic diagram, FIG. 5 is a sound pressure characteristic diagram of a conventional
acoustic transducer, FIG. 6 is the same electrical impedance characteristic diagram, and FIG. FIG.
8 is an electric impedance characteristic diagram of the same, and FIG. 9 is a diagram showing
the relationship between the diameter of the sound emission hole and the frequency.
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· · · Case 2 · fist · coil 3 φ · φ iron core 5 · · · permanent magnet 6 φ · · diaphragm 8 · φ · sound
emitting cylinder 9-· 発 振 oscillation circuit 83 @ · · Patent applicant Tidy K Co., Ltd. * 2 Fig. 3
Fig. 4 Fig. 5 Fig. 6 Jun Q -1 衷 □
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