JPH0313200

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DESCRIPTION JPH0313200
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
magnetic circuit of an electroacoustic transducer such as a speaker and a microphone, and more
particularly to a magnetic circuit effective for reducing leakage flux. 2. Description of the Related
Art Conventionally, for example, a speaker used in a television receiver has a structure for
reducing the leakage flux in order to prevent magnetic disturbance to the cathode ray tube due
to the leakage flux of the magnetic circuit of the speaker. For example, the structure shown in
FIG. 6 is the structure disclosed in Japanese Patent Publication No. 37-10721, which is made of a
magnetic material surrounding a magnetic circuit consisting of the center pole 1, the first magnet
2, the front plate 3 and the back plate 4. The cover 5 is disposed, and in the cover 5, the second
magnet 6 whose magnetization direction is reversed from the first magnet 3 is disposed in
proximity to the back plate 4 of the magnetic circuit. According to the structure, it is possible to
cancel the magnetic flux to be leaked to the outside of the magnetic circuit by the second magnet
6 and to reduce the leakage flux. [Problems to be Solved by the Invention] However, it has been
found that the above-mentioned conventional magnetic shielding structure is particularly
inconvenient for sufficiently preventing the influence on the Braun tube, with the recent
improvement of the quality of television receivers. [Means for Solving the Problems] The present
invention is a magnetic circuit in which a magnetic body or a third magnet whose magnetization
direction is the same as that of the first magnet is disposed at the center of the second magnet in
the magnetic circuit structure. The present invention will be described based on the
embodiments shown in the drawings shown in FIG. 1 and the following. In FIG. 1, 1 is a center
pole, 2 is a ring-shaped first magnet disposed outward of the center pole 1, 3 is a front plate
disposed on the upper surface of the first magnet 2, and 4 is a center pole in general. A back
plate 4 integrally formed with 1 and disposed on the bottom surface of the first magnet 2, each
member being disposed concentrically about the central axis of the center pole 1, formed
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between the center pole 1 and the front plate 3 The gap formed acts as a magnetic gap. The ringshaped second magnet 6 is disposed on the back plate 4 so that the magnetization direction of
the magnet 6 is reversed. That is, the magnetic poles of the first magnet 2 and the second
magnet 6 are disposed to face each other with the back plate interposed therebetween. Reference
numeral 5 denotes a magnetic cover that encloses the side surface and the rear surface of the
magnetic circuit. Reference numeral 7 denotes a columnar magnetic body disposed at the center
of the second magnet 6 on the inner surface of the magnetic cover.
FIG. 2 shows another embodiment, in which the distance between the magnetic body 71 and the
back plate 4 can be adjusted by screwing the magnetic body 71 to the magnetic body cover 5.
FIG. 3 shows another embodiment, in which the central portion of the bottom of the magnetic
cover 5 is protruded inward to achieve the function of the magnetic body 7 of FIG. FIG. 4 shows
another embodiment, and 8 is a third magnet whose magnetization direction is the same as that
of the first magnet 2 at the central portion of the second magnet 6, that is, the first magnet 2 and
the magnetic pole direction are aligned. FIG. 5 shows another embodiment shown in FIG. 4 in
which magnetic members 81.82 are disposed on the upper and lower surfaces of the third
magnet. [Advantage of the Invention] FIG. 7 (a) shows the results of measurement of leakage flux
of the magnetic circuit of the embodiment of FIG. 1 and the magnetic circuit of the prior art of
FIG. The measurement was performed by moving the magnetic probe on a circle having a radius
of 6 cm around the pole top surface of the central axis of the center pole. As apparent from the
figure, according to the embodiment of the present invention, the leakage flux can be reduced,
and in particular, the reduction effect of the leakage flux behind the magnetic circuit (in the
direction of 90 °) is remarkable. 7 (b) shows the results obtained by measuring the leakage flux
of the magnetic circuit of this embodiment shown in FIG. 5 and the magnetic circuit of the prior
art shown in FIG. 6 in the same manner as above. The reduction effect of the leakage flux was
achieved. The magnetic cover may be integral with the speaker frame. The magnetic body 7 may
be disposed on the back plate 4 side, or may be formed integrally with the back plate.
[0002]
Brief description of the drawings
[0003]
1 to 5 are sectional views of the embodiment of the present invention, FIG. 6 is a sectional view
of a conventional example, and FIG. 7 is a distribution diagram of leakage flux.
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1: center pole 2: first magnet 3 front plate 4: back plate 5: magnetic cover 6 = second magnet 7:
magnetic 8: third magnet
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