JPH1013989

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DESCRIPTION JPH1013989
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
magnetic circuit and a speaker, and more particularly to a speaker which is small in volume, light
in weight and low in magnetic leakage.
[0002]
2. Description of the Related Art Most conventional speakers use permanent magnets and voice
coils to activate transducers such as diaphragms to produce sounds of various heights. This type
of loudspeaker uses a permanent magnet and a bulk of magnetically conductive material and
forms a magnetic circuit by maintaining an air gap in the magnetic circuit. A voice coil for
conducting current to the air gap is accommodated. Permanent magnets, usually of ferrite
material, surround the bulk of the magnetically conductive material. The driving force generated
by the interaction of the magnetic field and the current conducting the voice coil according to the
Lorentz equation is applied to the voice coil and activates the diaphragm coupled with the voice
coil. With such a configuration, in addition to the increase in volume and weight of the speaker, a
large amount of magnetic leakage occurs on the top and side surfaces of the speaker, so there is
the problem that the efficiency is extremely low if permanent magnets are used.
[0003]
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With the development of the information industry, personal computers are entering into
multimedia applications. Thus, speakers are introduced into personal computers and serve as
important peripheral components of multimedia systems. By the way, it appears that
conventional coil activated speakers can not meet the requirements in terms of size and weight in
such applications. Furthermore, cathode ray tubes (CRTs) often used in personal computer
display monitors are sensitive to external magnetic fields. As such, conventional loudspeakers
with high magnetic leakage may result in non-uniform saturation, image twisting and flashing,
which are unsuitable for combination with a monitor.
[0004]
SUMMARY OF THE INVENTION Several improved techniques have been developed to overcome
the above disadvantages. Japanese Mitsubishi Publication "Electric Technology" (Vol. 64, No. 8,
1990) discloses a novel coil start-up speaker that retains the use of the external magnet structure
and the ferrite magnet. The improvement is to mount on the bottom a counter magnet which
repels the primary magnet, and to seal the whole structure with a magnetically conductive
material enclosure. Although such a method can actually reduce the magnetic leakage on the side
of the magnetic circuit, it has little or no effect on the magnetic leakage above the air gap, in
addition to which an auxiliary magnet is added. There is a problem that the capacity and weight
of the magnetic circuit increase.
[0005]
U.S. Pat. No. 5,321,762 discloses an internal magnetic circuit comparable to the external
magnetic circuit described above, which comprises a radially polarized magnet and a core
surrounding the radially polarized magnet. Have. Although such an approach can minimize field
inhomogeneities and violations, it can not solve the problem of magnetic leakage above the air
gap. Furthermore, radially polarized magnets which can be composed of a plurality of arc-shaped
segments of radially polarized magnetic material are difficult to assemble and have the problem
of increased manufacturing costs.
[0006]
U.S. Pat. No. 5,390,257 discloses another internal magnetic circuit. The characteristic is that the
magnet has the same volume as the main magnet, and a magnet that repels the main magnet is
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mounted above the main magnet. The main magnet and the repelling magnet can be made of
neodymium. Thus, this type of loudspeaker is light in weight, small in volume and extremely easy
to assemble. Furthermore, by configuring the main magnet and the repelling magnet in this way,
it is possible to reduce distortion of the speaker and to increase its sensitivity. However, due to
the configuration of the magnet, there is a disadvantage that the magnetic field in the vicinity of
the air gap becomes nonuniform, and in addition there is a problem that the problem of magnetic
leakage above the air gap is still not solved.
[0007]
Therefore, a first object of the present invention is to provide a magnetic circuit that has less
magnetic leakage that interferes with the nearby CRT monitor than conventional magnetic
circuits, and such a magnetic circuit is a magnet of the internal primary magnet. It also enhances
energy efficiency.
[0008]
A second object of the present invention is to provide a lightweight speaker with low magnetic
leakage, such a speaker can be incorporated more easily into a multimedia computer system than
conventional ones.
[0009]
SUMMARY OF THE INVENTION In order to achieve the above object, the present invention
provides a magnetic circuit having a primary magnet, a yoke, a first upper center plate, a
peripheral upper plate, and a magnetic thin film. It is
The yoke can then be divided into a bottom and an annulus attached to the periphery of the
bottom.
A permanent magnet having a first surface, a second opposing surface, and a third surface is
disposed at a position in contact with the bottom of the yoke. The first central top plate is
disposed on the second opposing surface of the primary magnet, and the peripheral top plate is
disposed on the annular portion of the yoke. An air gap is formed between the first central upper
plate and the peripheral upper plate. Therefore, a closed loop magnetic circuit is configured by
the primary magnet, the first center top plate, the peripheral top plate, and the yoke. A magnetic
thin film that is repulsive to the primary magnet is disposed on the first central upper plate to
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reduce magnetic leakage from the air gap. Furthermore, a second center top plate can be
mounted on the magnetic thin film to further reduce the magnetic leakage above the magnetic
thin film. Furthermore, the annular portion of the yoke is provided with a groove at a position in
contact with the surrounding upper plate in order to enclose the magnetic energy contained in
the air gap.
[0010]
By using the above-mentioned magnetic circuit, the present invention can provide a novel
speaker. The speaker further includes a bobbin, a voice coil wound around the bobbin and
disposed in the air gap, and a diaphragm connected to the bobbin for emitting sound, in addition
to the magnetic circuit described above. The voice coil can interact with the closed loop magnetic
circuit to generate a driving force perpendicular to the tangential direction of the voice coil and
to the direction of the closed loop magnetic circuit. Then, the diaphragm vibrates according to
this driving force to emit sound. Furthermore, support means can be used to support the
diaphragm and to prevent lateral movement of the diaphragm. The support means comprises a
frame, an edge and a damper. The frame has a first rim attached to the peripheral top plate and a
second opposing rim connected to the diaphragm. The damper is coupled between the frame and
the diaphragm. Furthermore, a cap is used to protect the closed loop magnetic circuit from
contamination by dust.
[0011]
DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention
will be described in detail with reference to the drawings. The features of the magnetic circuit
utilized in the speaker of the present invention are: (a) a primary permanent magnet functioning
as a magnetic field source surrounded by a cup-shaped yoke to form a magnetically conductive
closed loop; and (b) magnetic leakage In order to suppress, the magnetic thin film which is
repelled to the primary permanent magnet is attached on the primary permanent magnet. Such
an arrangement can increase the efficiency of magnetic energy.
[0012]
FIG. 1 is a cross-sectional view of a magnetic circuit in an embodiment of the present invention.
The main magnetic loop starts from the north pole (north pole) of the primary magnet 60, passes
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through the yoke 50, the peripheral top plate 52, and the first center top plate 54, and then
terminates at the south pole (south pole) of the primary magnet 60. In order to reduce the weight
of the assembly, the primary magnet 60 can be made of rare earth magnetic materials such as
SmCo5, Sm2 Co17 and NdFeB systems, which have high magnetic energy density. In addition, the
yoke 50, the peripheral top plate 52 and the first center top plate 54 are made of a material
having a high magnetic flux conductivity such as low carbon steel in order to enclose the
magnetic flux and to prevent the magnetic leakage to the side. Can.
[0013]
Next, the structure of the magnetic circuit will be described in detail. The cup-shaped yoke 50
includes a bottom 50a and an annular portion 50b mounted around the bottom 50a. Preferably,
the bottom portion 50a and the annular portion 50b can not be separated. The primary magnet
60, which is cylindrical in shape, is bonded to the bottom 50a by an adhesive 60a and serves as a
magnetic field source. In this embodiment, the upper surface of the primary magnet 60 is a south
pole, and the lower surface of the primary magnet is a north pole. The first central upper plate
54 and the peripheral upper plate 52 are attached to the primary magnet 60 and the annular
portion 50 b of the yoke 50, respectively. Furthermore, the peripheral top plate 52 and the first
central top plate 54 are horizontally spaced apart to form an annular gap 62 for receiving the
voice coil therebetween. However, the width of the air gap 62 is preferably as narrow as possible
to avoid the divergence of the magnetic field in the vicinity of this region.
[0014]
In order to enclose the magnetic energy contained in the air gap 62, a circular groove 51 formed
on the upper inside of the annular portion 50b is utilized. In addition, an annular magnetic thin
film 56 that is repelled by the primary magnet 60 is mounted on the first central upper plate 54
to further enclose magnetic energy in the upper space of the air gap. It is a well known fact that
the magnetic flux resulting from two adjacent poles of opposite polarity are compressed and
extend along a vertical line between them. Thus, the original magnetic leakage above the air gap
62 is guided into the air gap 62 and minimized. Furthermore, the magnetic thin film 56 can also
increase the magnetic flux density in the air gap 62.
[0015]
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It should be noted that the magnetic thin film 56 itself also generates a magnetic field in the
upper region (above the N pole of the magnetic thin film 56 shown in FIG. 1), which also causes
undesirable magnetic leakage. The first measure is to attach a second central upper plate serving
as a magnetic shield on the magnetic thin film 56. The second central top plate 58 may be made
of a material having high magnetic flux conductivity, such as low carbon steel. The second
measure is to reduce the magnetic field strength of the magnetic thin film 56.
[0016]
A thickness of 1.5 mm or less of the magnetic thin film 56 is sufficient to achieve the required
magnetic field encapsulation. Furthermore, the magnetic field emitted from the peripheral
portion of the magnetic thin film 56 is effective in reducing the magnetic leakage above the air
gap 62, while the magnetic field from the central portion of the magnetic thin film 56 is
undesirable. You may As a result of calculation and confirmation, it has been found that the ratio
of the radius of the inner circle of the magnetic thin film 56 to the radius of the primary magnet
60 should be 0.37 to 0.43 in order to obtain the best magnetic leakage characteristics.
[0017]
2 and 3 show graphs of the driving force in the vicinity of the regions I and II in FIG. 1,
respectively, where • indicates a current flowing outward from the paper and x indicates
a current flowing to the paper. There is. In FIG. 2, the resultant force F acting on the conductor
positioned vertically in the magnetic field B is upward according to Lorentz's law, the direction of
B is from left to right, and the direction of current is outward from the paper. On the other hand,
the resultant force F in FIG. 3 is also upward, the direction of the magnetic field B is from right to
left, and the direction of the current is on the paper. Therefore, an object received in the air gap
62 like a voice coil receives an upward driving force. Furthermore, the magnitude of the driving
force acting on the object is proportional to the total current flowing across the air gap 62, which
in the case of a voice coil also depends on the number of turns and the magnitude of the
magnetic field B. If the current flowing through the voice coil includes a time-varying element, a
time-varying driving force is also generated.
[0018]
FIG. 4 is a graph showing the magnetic flux density of the magnetic leakage calculated along the
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test line III of FIG. The distance between the test line III and the outer surface of the magnetic
circuit 10 is 10 mm, and the total length of the test line III is 80 mm. The relative positions on
the test line III are each described as the corresponding length (0 mm-80 mm). In the absence of
the magnetic thin film 56 (in the case of a solid line), a large amount of magnetic leakage is
observed along the test line III, particularly near the upper space of the air gap 62. However,
when the magnetic thin film 56 is present (in the case of the dotted line), the magnetic flux
density of the magnetic leakage calculated along the test line III is greatly reduced.
[0019]
It should be noted that the magnetic leakage of the magnetic circuit 10 is not a complete source
of the dispersion of the magnetic field in the vicinity of the air gap. According to Maxwell's
electromagnetic equation, the loop circuit conducting the current generates a magnetic field
around its vicinity. Although the voice coil is housed in the magnetic circuit, it is inevitable that
part of the induced magnetic field emanating from the voice coil situation is dispersed, causing
magnetic leakage. However, in this embodiment of the invention, the magnetic film 56 can be
utilized to encapsulate the induced magnetic field and guide it to the normal magnetic circuit.
[0020]
By using the magnetic circuit 10 shown in FIG. 1, it is possible to easily configure a light-weight,
small-volume, low-leakage, efficient speaker. FIG. 5 is a cross-sectional view of the speaker of this
embodiment. Hereinafter, all components other than the components in the magnetic circuit 10
will be described. The voice coil 70 fixedly wound around the bobbin 72 is disposed in the air
gap 62 and interacts with the magnetic field present in the air gap 62 to generate a driving force.
The direction of this driving force is perpendicular to the conduction direction of the current
flowing through the voice coil 70 and the direction of the magnetic field. A conical diaphragm 80
connected to the bobbin 72 receives this driving force. The driving force changes with a change
in current flowing through the voice coil 70, where the diaphragm 80 emits sounds of various
heights according to the driving force. A frame 82, an edge 84 and a damper 86 are used to
support the diaphragm 80. The frame 82 is in the form of a truncated cone in this embodiment
and can be made of aluminum, magnesium, an alloy of aluminum and magnesium, plastic, or
reinforced plastic for weight reduction.
[0021]
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The lower rim portion of the frame 82 is connected to the peripheral top plate 52.
Correspondingly, the upper rim of the frame 82 is connected to an edge 84 which is further
connected to the diaphragm 80. The damper 86 is wider than the edge 84 and is also connected
between the frame 82 and the diaphragm 80. The edge 84 and the damper 86 are used not only
to support the diaphragm 80 but also to prevent lateral movement of the diaphragm 80. In
addition, a cap 75 can be used to protect the magnetic circuit 10, in particular in the area near
the primary magnet 60, in order to prevent contamination by dust.
[0022]
The speaker of this embodiment can also be used as a tweeter by adjusting the height of the
bobbin 72. Furthermore, although the diaphragm 80 vibrates vertically, the cap 75 does not
contact the magnetic thin film 56 or the second center top plate 58. The reason is that the
thickness of the magnetic thin film 56 is 1.5 mm or less. In addition, since the driving force
acting on the voice coil 70 having such a configuration is extremely uniform, the speaker can
emit sound with less distortion.
[0023]
The foregoing description of the embodiments of the present invention has been made for the
purpose of describing the illustrated ones. This is not a complete description of the present
invention, and is not intended to limit the present invention strictly by this embodiment. It will be
apparent to the expert that many modifications and variations are possible. This embodiment has
been chosen and described to most clearly illustrate the principles and practical applications of
the present invention, and thus, even if it is not an expert, the present invention is suitable for a
particular application. It will be appreciated that various modifications can be made and
implemented. The scope of the invention is defined by what is stated in the claims.
[0024]
As described above, according to the present invention, the magnetic leakage of the magnetic
circuit is reduced and the magnetic energy efficiency of the primary magnet is also enhanced by
attaching the magnetic thin film which is repelled to the primary magnet to the upper central
plate. It has the effect of
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[0025]
Brief description of the drawings
[0026]
1 is a cross-sectional view of the magnetic circuit utilized to drive the voice coil of the
embodiment of the present invention.
[0027]
FIG. 2 FIG. 2 is a graph relating to the area I of the driving force of the voice coil disposed
horizontally in the air gap of the above magnetic circuit.
[0028]
FIG. 3 FIG. 3 is a graph relating to the region II of FIG.
[0029]
4 is a graph showing the calculated magnetic leakage along the test line III of FIG.
[0030]
5 is a cross-sectional view of the speaker of the embodiment of the present invention.
[0031]
Explanation of sign
[0032]
10 Magnetic circuit 50 Yoke 50a Yoke bottom 50b Yoke ring 52 peripheral upper plate 54 first
center upper plate 56 magnetic thin film 58 second center upper plate 60 primary magnet 60a
adhesive 62 air gap 70 voice coil 72 bobbin 75 cap 80 diaphragm 82 frame 84 Edge 86 Damper
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