Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
203~185
The present invention relates to a magnetic circuit
for a speaker used in an audio system.
An object of the present invention is to provide a
magnetic circuit for a speaker which may increase magnetic
efficiency, thereby enabling improvement of the tonal
quality of the speaker.
Another object of the present invention is to
provide a magnetic circuit which can be light in weight and
may be manufactured at a low cost.
According to the present invention, there is
provided a magnetic circuit for a speaker having a yoke
base, a pole formed on the yoke base, an annular magnet
mounted on the yoke base, and a top plate mounted on the
magnet so as to form a gap between an inside wall thereof
and an outer wall of the pole. At least one of the yoke
base, pole and top plate has a periphery having a non-
linear sectional shape.
The non-linear sectional shape is an outwardly
curved sectional shape or an inwardly curved sectional
shape.
In one aspect of the invention, the magnet has a
periphery having an outwardly curved sectional shape.
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The magnet, yoke base and top plate may have
peripheries having respectively outwardly curved sectional
shapes.
In another aspect, at least one of the yoke base and
the top plate has an inwardly curved sectional shape.
The magnet may have a periphery having a sectional
shape combining straight lines.
The other objects and features of this invention
will become understood from the following description with
reference to the accompanying drawings, in which
~ Fig. 1 is a sectional view showing a magnetic
;~ circuit according to a first embodiment of the present
invention;
Fig. 2 is a sectional view of a second embodiment;
15Figs. 3 and 4 are sectional views showing third and
fourth embodiments;
Fig. 5a is an illustration showing lines of magnetic
force generated in the first embodiment;
Fig. 5b is an illustration showing lines of magnetic
force generated in the conventional magnetic circuit;
Figs. 6, 7a and 7b are sectional views showing
modifications of the sectional shape;
Fig. 8 is a sectional view of a fifth embodiment;
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Fig. 9 is a sectional view showing a modification of
the fifth embodiment;
Fig. 10 is a sectional view explaining the effects
of tapered portions;
Fig. 11 is an illustration showing lines of magnetic
force generated in the magnetic circuit of Fig. 9;
Fig. 12 is a table comparing performance and weight
of the magnetic circuit of Fig. 8 with conventional
magnetic circuits;
Figs. 13 to 19 are sectional views showing sixth to
twelfth embodiments of the present invention;
Fig. 20 is a sectional view showing a conventional
cone speaker; and
Figs. 21 and 22 are sectional views showing
lS sectional shapes of prior art magnetic circuits.
Reference will first be made to Figures 20 - 22,
illustrating prior art speakers.
Fig. 20 shows a widely used cone speaker. The
speaker comprises a yoke 51 having a yoke base 53 and a
cylindrical pole 52 formed on the yoke base 53, an annular
top plate 54, and a magnet 55 disposed between the yoke
base 53 and the top plate 54. A voice coil 56 is supported
by a damper 60 and disposed in a magnetic gap G between the
pole 52 and the top plate 54 so as to be moved in the
direction shown by arrows a and b. A frame 57 is secured
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to the plate 54, and a diaphragm 59 is provided between an
edge 58 of the frame 57 and the voice coil 56. Reference
numeral 61 designates a terminal and 62 is a lead.
Referring to Fig. 21, the magnetic circuit is formed
by the yoke 51, magnet 55 and plate 54, each of which has
a rectangular cross section. Consequently, the magnetic
flux ~D in the gap G is reduced by magnetic flux leakages
~A, ~B and ~C.
Furthermore, no effective measures are taken to
prevent the leakage of magnetic flux. Although the
periphery of the magnet 55 serves as a magnetic guard
against the magnetic flux leakage ~A, the upper and lower
corners of the magnet have but a small guarding effect.
Japanese Utility Model Publication 46-8272 discloses
a magnetic circuit for a speaker intended to prevent the
reduction of the magnetic efficiency. Fig. 22 shows the
magnetic circuit disclosed in that publication. The yoke
base 53 has a tapered underside 64 which serves to reduce
the leakage of the magnetic flux ~A and ~B. However, the
tapered yoke base has little effect in increasing the
magnetic efficiency at the gap G.
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Referring to Fig. 1, the magnetic circuit
according to the present invention comprises a yoke 1
having a yoke base 2 and a cylindrical pole 3 having an
aperture 3a, an annular magnet 4 mounted on the yoke
base 2, and an annular top plate 5. The magnet 4 is
secured to the yoke base 2 with adhesive, and the top
plate 5 is also secured to the magnet with adhesive.
The magnetic circuit has a compressed spherical
periphery. Namely, the yoke base 2 has a convex
periphery 2a and the top plate 5 also has a convex
periphery 5a, and the periphery 4a of the magnet 4 has
an outwardly curved sectional shape.
In the magnetic circuit shown is Fig. 2, a solid
pole 6 is formed on the yoke base 2. The yoke base 2
of the third embodiment shown in Fig. 3 has an annular
recess 7 on the inside wall thereof, for allowing large
axial movement of the voice coil, which is designed to
be used for the woofer. A pole piece 8 is mounted on
the pole 3.
As shown in Fig. 4, the periphery 4a of the magnet
4 may have a sectional shape defined by straight lines.
Namely, a periphery lla has a triangle sectional shape,
and peripheries llb and llc have trapezoidal sectional
shapes, respectively. Although the yoke base 2 and the
top plate 5 have rectangular sectional shapes, they may
have curved sectional shapes as shown in Fig. 1.
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6 20381~5
Referring to Fig. 5a showing lines of magnetic
force generated in the magnetic cir~uit of Fig. 1, most
of magnetic lines gl of force generated from the north
pole of the magnet 4 enter in the plate 5 and are led
to the gap G as magnetic flux g2. Magnetic flux g3 in
the gap G is induced in the pole 3 and the yoke base 2
as magnetic flux g4 and led to the south pole of the
magnet 4.
Since the periphery of the magnet has an outwardly
curved sectional shape, the magnetic flux gl in the
magnet 4 is led to the plate 5 along the periphery.
Consequently, magnetic flux density is high in the gap
G as indicated by the equal magnetic flux density
contours Ll~ L4. On the other hand, leakages ~1 and ~2
of magnetic flux outside the magnet 4, plate 5 and yoke
2 is in low density.
To the contrary, the conventional magnetic circuit
has a low magnetic flux density in the gap G and a
large leakage ~1 and ~2 of magnetic flux as shown in
Fig. 5b.
In a magnetic circuit according to the first
embodiment having a magnet radius of 65 mm, a maximum
magnetic flux density in the gap G is 1.312 tesla. In
the conventional magnetic circuit of Fig. 21 having the
same magnetic radius as the first embodiment, a maximum
magnetic flux density in the gap G is 1.309 tesla,
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because of a large amount of the leakage ~A and the
leakage ~C of the magnetic flux from the plate 54 to
the pole 52.
In addition, the weight of the magnetic circuit of
the present invention is reduced by 20 ~ of the weight
of the conventional magnetic circuit, since edges of
the yoke base 2, magnet 4 and plate 5 are rounded.
Accordingly, expensive magnetic material is reduced in
quantity to lower the manufacturing cost thereof.
Furthermore, the magnetic f lux density in the magnet 4
is more equalized, so that reduction of magnetization
at low temperature is prevented.
Although each of the periphery of the magnetic
circuits of Figs. 1 to 3 has a continuously curved
sectional shape, a discontinuous periphery may be
employed as shown in Figs. 6, 7a and 7b.
Each of the magnetic circuit of Figs. 1 to 4 is a
solid of revolution about a center line ~. However,
another magnetic circuit having a square shape or
ellipse shape in plan view may be used in embodying the
present invention.
Referring to Fig. 8 showing the fifth embodiment,
the periphery of a yoke base 17 of yoke 15 has a
tapered surface 17a, and a top plate 18 has a tapered
surface 18a. A solid pole 16 is formed on the yoke 15.
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8 203~85
Each of the tapered surfaces 17a and 18a is inwardly
curved.
Since the outer periphery of the tapered surface
17a (18a), which has a circular shape in section, has a
thin thickness, leakage ~B from the circular periphery
can be prevented or reduced to a very small amount.
Fig. 9 shows a most preferable tapered surface.
The periphery of each of the yoke base 17 and the top
plate has no circular sectional shape. Namely, the
inwardly curved surface converges on the surface of a
magnet 19.
The magnetic f lux and dimension of the magnetic
circuit of Fig. 9 will be described with reference to
Fig. 10. In the figure, X1 designates the outer radius
of the magnet 19, X2 represents the inner radius, X3
the radius of the pole 16. Thickness t of the yoke
base 17 at which inner magnetic flux density Bi becomes
constant will be obtained as described hereinafter.
Here it is assumed that all of the magnetic flux in the
magnet 19 flows in the yoke 15, hence there is no
leakages ~A and ~B.
1. When X2_X_X1, magnetic flux between X and Xl is
~m(X)=ll(X12-X2)Bm
Magnetic flux in the tapered portion of the yoke
base 17 is
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~i(X)=2lt~X.t.Bi
Since ~m=~i,
X(X12-X2)Bm=2~-X-t~Bi
Therefore
t~X)=(X12-X2)Bm/(2X-Bi)
When X3<X_X2, magnetic flux between X and Xl is
~m(X)=T~(X12-X22)Bm
Magnetic flux in the tapered portion of the yoke
base 17 is
~i(X)=2~X.t~Bi
Therefore
t(X)=(X12-X22)Bm/(2X-Bi)
When X=X1,
dt(X)/dX=Bm/(2Bi)-(-xl2/X2-1)
dt(X)/dt(X=X1)=-Bm/Bi
These are the conditions which define the
dimensions of the yoke base.
The following is the analysis of these conditions.
When X2~X<X1,
t(X)=(cl/X)-c2~X
(cl and c2 are constants)
Consequently, the tapered portion of the yoke base
becomes an inwardly curved sectional shape.
; When X3SX_X2,
t(X)=c3/X (c3 is a constant)
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Therefore, in this case also, the tapered portion
becomes inwardly curved.
The above values are obtained on the assumption
that the leakage of the magnetic flux ~A does not
exist. Actually a part of the flux does not pass
through the pole 16 because of the leakage ~A.
Therefore, the value of the increasing rate of the
thickness t is smaller than the above equations.
However, the conditions for forming an inwardly curved
sectional shape do not change. This is confirmed by
numerical calculation, for example by the finite
element method.
The above theory is also applied to the tapered
portion of the plate 18.
Fig. 11 shows a distribution of the magnetic flux
in the magnetic circuit of Fig. 9. The same references
as Fig. 5a are used. From the figure, it will be
understood that a high density of magnetic flux is
obtained in the gap G.
Fig. 12 shows magnetic flux densities and weight
of prior arts 1 and 2 and the present invention. In
the table, Bg represents the averaged magnetic flux
density in the gap G, ~g is the magnetic flux in the
gap, and ~m is a total magnetic flux in the magnet. It
will be seen that the magnetic flux density of the
present invention is higher than the prior arts 1 and 2
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11 2~3~185
and the magnetic circuit of the present invention islighter than the prior arts in weight.
The magnetic circuit of Fig. 13 has a recess 16a
on the underside of the pole 16 in order to reduce the
weight thereof.
In the seventh embodiment shown in Fig. 14, an
additional recess 16b is formed on the upperside of the
pole 16 so as to further reduce the weight. Since each
the recesses 16a and 16b has a curved inner surface,
leakage of magnetic flux therefrom can be reduced.
The magnetic circuit of Fig. 15 has a perforation
16c in the pole 16.
The magnet 19 of the magnetic circuit shown in
Fig. 16 has an outwardly curved sectional shape similar
to the first embodiment of Fig. l.
Figs. 17 to 19 show various sectional shapes of
the magnet circuit.
While the presently preferred embodiments of the
present invention have been shown and described, it is
to be understood that this disclosure is for the
purpose of illustration and that various changes and
modifications may be made without departing from the
scope of the invention as set forth in the appended
claims.
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