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JPS61169098

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DESCRIPTION JPS61169098
[0001]
The present invention relates to an electromagnetic acoustic transducer in which a diaphragm
opposed to the electromagnet via a gap is vibrated by an electromagnet driven by an alternating
current and a permanent magnet provided to surround the electromagnet. FIG. 1 shows a
conventional example of an electromagnetic acoustic transducer of this type, wherein an annular
step 1a is formed by rolling the inner circumferential surface at the bottom of a cylindrical case 1
made of nonmagnetic metal material. While fitting and adhering the washer 5 having the printed
circuit board 2 and the iron core 4 with the coil 3 in a stacked state in the step portion la, the
electromagnet consisting of the coil 3 and the iron core 4 is surrounded on the washer 5 A
cylindrical permanent magnet 6 is disposed, and at the upper opening of the case l, the tip face
4a of the iron core 4 and the upper end face 6a of the permanent magnet 6 are respectively '! ! ! :
WA. The diaphragm 7 is mounted so as to face each other via Gl and G2. In the electromagnetic
acoustic transducer of the above structure, when an alternating current of an appropriate
frequency is supplied to the coil 3 via the terminal 8.9, an alternating magnetic field generated at
this time. The diaphragm 7 vibrates in the air gap Gl depending on the frequency of the coil
current due to the magnetic interaction between the permanent magnet 6 with the bias DC
magnetic field and the air interaction with the diaphragm 7 depending on the frequency of the
coil current. In this case, in order to vibrate the diaphragm 7 efficiently and raise the sound
pressure level, the frequency of the coil current is conventionally set to a value matching the
natural resonance frequency of the diaphragm 7. FIG. 2 is a characteristic diagram showing the
relationship between the coil current frequency and the sound pressure, and the horizontal axis
represents the coil current frequency (H2) and the vertical axis represents the sound pressure
(dB). A characteristic curve 1 curve A2 when the distance of G1 is gl 1 shows a characteristic
curve when the distance of the air gap G1 is a value g2 larger than the distance g1. As shown in
the figure, the electromagnetic acoustic transducer shown in FIG. 1 is configured as a vibration
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system having no resonance point on the lower frequency side than the natural resonance
frequency fo of the diaphragm 7, and the sound pressure level is It changes in accordance with
the frequency, and when the frequency of the coil current coincides with the natural resonance
frequency fo of the diaphragm 7, it becomes maximum. The natural resonance frequency fo is
determined by the mechanical structure, physical conditions, and the like of the electromagnetic
acoustic transducer, and when the design conditions are specified, it is a known amount
determined by itself. Therefore, conventionally, by matching the frequency of the coil current to
the natural resonance frequency fo, the diaphragm 7 is vibrated at a position where the sound
pressure level is maximized. However, when the distance of the air gap G1 becomes a distance g2
larger than the distance g1, the characteristic curve AI becomes the characteristic curve A2 of
FIG. 2, and the sound pressure level decreases for the same coil current frequency, The natural
resonance frequency fo at which the level is maximized moves to the low frequency side.
For this reason, as in the conventional case, when the frequency of the coil current is matched to
the natural resonance frequency fo, the fluctuation range of the sound pressure level when the
distance of the air gap G1 fluctuates is large, and the electromagnetic acoustic conversion of
constant quality The disadvantage is that it becomes difficult to provide the For example, when
the distance of the air gap G is gl. If the frequency of the coil current is set to the natural
resonance frequency fo, the sound pressure level becomes the maximum value al, but if the
distance of the air gap Gl becomes gzcgz) g +), the sound pressure level is on the characteristic
curve A2 It becomes az, and the fluctuation range (al-az) becomes very large. Moreover, in this
type of electromagnetic acoustic transducer, the dimension of the air gap G1 is practically a
minute value of at most about 1004 m, and the processing error or assembling error of the case
1, the iron core 4 or the washer 5 is very small. Even if there is, the fluctuation rate of the air gap
Gl becomes very large, and it is a structure that easily causes the fluctuation of the sound
pressure level accompanying the above-mentioned fluctuation of the air gap Gl. An object of the
present invention is to provide an inexpensive electromagnetic acoustic transducer in which the
width of the sound pressure fluctuation is small and the quality is constant even if the distance of
the air gap changes. In order to achieve the above object, according to one aspect of the present
invention, there is provided an electromagnetic type acoustic conversion in which an oscillating
plate opposed to the electromagnet via a gap is vibrated by an electromagnet driven by an
alternating current and a permanent magnet provided to surround the electromagnet. A vibration
system having no resonance point on the lower frequency side than the natural resonance
frequency of the diaphragm, and driving the electromagnet with an alternating current having a
frequency lower than the natural resonance frequency of the diaphragm. Do. That is, in the
electromagnetic acoustic transducer as shown in FIG. 1, when the distance of the air gap G1 is set
to the value gl which can obtain the characteristic curve AI of FIG. An alternating current of lower
frequency fb is supplied to drive the electromagnets 3 and 4. Then, when the distance of the air
gap G1 changes from gl to g2, as shown in FIG. 2, the decrease width (bI-bz) of the sound
pressure drops from the level b1 to the level b2 as in the prior art. Because the width is much
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smaller than the width (at-az), it is possible to obtain an electromagnetic acoustic transducer
having a small sound pressure fluctuation width and a uniform quality even if the distance of the
air gap G1 changes. Moreover, as shown in FIG. 2, there is also an advantage that the sound
pressure level b2 after change is higher than the conventional level a2. In addition, when the
sound pressure fluctuation range with respect to the change of the air gap distance becomes
smaller, the tolerance for the processing error and the assembly error of each component can be
wider at least than in the conventional case, thereby reducing the processing cost and facilitating
the assembly.
Furthermore, since the air gap adjustment mechanism and the like are not required, the cost can
be further reduced. In the above description, an example in which the present invention is
applied to an electromagnetic acoustic transducer not having a resonance box is shown.
However, when the resonance box is provided, as shown in FIG. In order not to have a resonance
point on the lower frequency side than fo, the natural resonance frequency fo of the diaphragm 7
is the resonance frequency of the resonance box! Make it lower than k f +. Then, by selecting the
drive frequency fb to a value lower than the natural resonance frequency fO of the diaphragm 7,
the same function and effect can be obtained. Curves A, B and C in FIG. 3 respectively indicate
the sound pressure frequency characteristics of the diaphragm 7, the resonance box and their
combination. When a resonance box is provided, as shown in FIG. 4, the natural resonance
frequency fo of the diaphragm 7 may be set higher than the resonance frequency f1 of the
resonance box, but in this case, the natural resonance Since relatively flat synthetic sound
pressure characteristics C can be obtained from the frequency fo to the resonance frequency f1,
there is little merit by selecting the drive frequency fb to be lower than the natural resonance
frequency fo, that is, the present invention As shown in FIGS. 1 to 3, the present invention is
effective for an electromagnetic acoustic transducer comprising a vibration system having no
resonance point on the lower frequency side than the natural resonance frequency fo of the
diaphragm 7. As described above, according to the second aspect of the present invention, an
electromagnetic type acoustic transducer which vibrates a diaphragm opposed to the
electromagnet via a gap by an electromagnet driven by an alternating current and a permanent
magnet provided to surround the electromagnet. In the vibration system, the vibration system
does not have a resonance point on the lower frequency side than the natural resonance
frequency of the vibration plate, and the electromagnet is driven by an alternating current having
a frequency lower than the natural resonance frequency of the vibration plate. Thus, it is possible
to provide an inexpensive electromagnetic acoustic transducer that has a small sound pressure
fluctuation range with respect to air gap changes and is constant in quality.
[0002]
Brief description of the drawings
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[0003]
1 is a cross-sectional view of an example of an electromagnetic acoustic transducer, FIG. 2 is a
characteristic diagram showing the relationship between the coil current frequency and the
sound pressure, and FIGS. 3 and 4 are sounds when a resonance box is provided. It is a pressure
frequency characteristic view.
1 и и и Case 3 и и ? ? ? coil 4 и battle и iron core 5 иии Washer 6 fist 0 и permanent magnet fo, f'o и
?? и natural resonance frequency Fig. 1 Fig. 2 coil J with a number of points J
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