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JP2000217192

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DESCRIPTION JP2000217192
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
speaker that is suitable for reproducing a large input sound signal and has an edge feature.
[0002]
2. Description of the Related Art In recent years, compact and space-saving sound reproducing
apparatuses have been provided for stereo and personal computers used at home. Speakers used
in such sound reproducing apparatuses are often small in diameter.
[0003]
FIG. 9A is a cross-sectional view showing a structural example of a conventional small-aperture
speaker. FIG.9 (b) is an expanded sectional view which shows the vicinity of the edge 9 of the
speaker of Fig.9 (a). In FIG. 9A, an annular magnetic circuit including a center pole 2, a magnet 3,
and a top plate 4 is formed at the lower end portion of the annular frame 1.
[0004]
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1
A magnetic flux of high density is generated in an annular gap 5 formed between the upper outer
periphery of the center pole 2 and the inner periphery of the top plate 4, and the voice coil
bobbin 6 is vertically and vibratably held in the gap 5. There is. The voice coil 10 is wound
around the lower outer periphery of the voice coil bobbin 6. When a drive current corresponding
to the voice signal is applied to the voice coil 10, an electromagnetic force is generated in the
voice coil 10, and the voice coil bobbin 6 vibrates in a piston in the magnetic field of the air gap
5.
[0005]
A diaphragm 8 is fixed to the upper end of the voice coil bobbin 6, and a damper (also referred to
as a suspension) 7 is connected near the upper end of the voice coil bobbin 6. The diaphragm 8
is attached to the frame 1 via the edge 9 and the damper 7 is attached directly to the frame 1.
The diaphragm 8 is vibratably held by the edge 9 and the damper 7.
[0006]
In the speaker thus configured, when a driving current proportional to the voice signal is applied
to the voice coil 10, a driving force is generated in the voice coil 10 due to the interaction
between the electromagnetic force of the voice coil 10 and the magnetic flux of the air gap 5. As
a result, the voice coil 10 vibrates, the diaphragm 8 held by the damper 7 and the edge 9
vibrates up and down, and a sound is output from this speaker.
[0007]
As the edge 9 of the speaker, those shown in FIGS. 9A and 9B are most generally used.
The edge 9 has a roll shape whose cross-sectional shape is semicircular and is called a roll edge,
and is formed on the outer periphery of the diaphragm 8 so as to be concentric with the
diaphragm 8. As a material of the edge 9, a cloth impregnated with a resin, a urethane foam
sheet, a rubber sheet or the like is applied. The roll-shaped edge 9 and the wave-shaped damper
7 constitute a support system for the diaphragm 8 of the speaker, and a large vibration
amplitude of the diaphragm 8 is secured.
[0008]
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However, in the conventional small-sized speaker shown in FIG. 9 (a), since the diameter of the
diaphragm 8 is small, it is difficult to obtain a large sound pressure. That is, to obtain a
predetermined sound pressure, the vibration amplitude of the diaphragm 8 must be increased in
inverse proportion to the area of the diaphragm, and the vibration amplitude is increased in
inverse proportion to the square of the frequency of the audio signal. There must be.
[0009]
The maximum amplitude of the diaphragm is proportional to the size of the roll of the edge 9.
The size of the roll does not differ greatly between the small aperture speaker and the large
aperture speaker. For this reason, with a small-aperture speaker, it has been difficult to secure a
sufficiently large amplitude, and in particular there has been a problem that the reproduced
sound in the low frequency band becomes worse.
[0010]
FIG. 10 is a schematic cross-sectional view showing a displacement state of the roll-shaped edge
9. In FIG. 10, the broken line shows the state of the edge 9 when the drive current is not applied
to the voice coil 10 and the diaphragm 8 is at the neutral point. The solid line shows the state of
the edge 9 when a large drive current of low frequency is applied to the voice coil 10 and the
diaphragm 8 is displaced in the + direction along the Z axis. As is apparent from FIG. 10, when a
large low-frequency drive current is applied to the voice coil 10, the edge 9 is linearly extended
and extended.
[0011]
FIG. 11 shows the displacement of the edge 9 when the diaphragm 8 is vibrated with an
amplitude of up to ± 10 mm along the Z axis by applying a driving current corresponding to the
attack sound of the drum to the voice coil 10 by laser Doppler displacement. It is measured with
a meter and shows one cycle of displacement of the edge 9. As is clear from FIG. 11, when the
diaphragm 8 is at the neutral point, the edge 9 is in a roll shape, but when the amplitude of the
diaphragm 8 becomes ± 10 mm, the edge 9 changes linearly and the edge 9 becomes tense. ing.
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[0012]
As described above, when a large low-frequency drive current is applied to the voice coil 10 and
the edge 9 is extended and squeezed, an abnormal sound (slump sound) is generated from the
edge 9 and the quality of the reproduced voice is significantly deteriorated. . This abnormal
sound is completely the same as the sound generated when the thin sheet-like rubber, cloth,
paper, etc. is bent suddenly after being bent.
[0013]
Therefore, the present invention has been made in view of such conventional problems, and it is
desirable to make the diaphragm large without deteriorating the linearity of the displacement of
the diaphragm with respect to the drive current applied to the voice coil. It is an object of the
present invention to provide a speaker which does not generate an abnormal sound (tension
noise) due to the edge tension at the time of amplitude.
[0014]
SUMMARY OF THE INVENTION In order to solve the above-mentioned conventional problems,
the loudspeaker according to the present invention comprises a diaphragm, a frame surrounding
the diaphragm, and a frame which is joined to the outer periphery of the diaphragm. And an edge
for supporting the diaphragm on the inner side of the frame, the edge being thinned near the
approximate center between the diaphragm and the frame and approaching the diaphragm and
the frame It is thickened according to.
[0015]
According to the present invention, the edge is thinned near its approximate center, and
thickened as it approaches the diaphragm and the frame, that is, as it approaches the inner and
outer peripheries.
Therefore, the rigidity is low near the center of the edge and high near the inner and outer
circumferences.
Because the rigidity is low near the center of the edge, if the vibration amplitude of the
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diaphragm is within a certain range, the vicinity of the center of the edge is easily deformed and
the linearity of displacement of the diaphragm with respect to the drive current applied to the
voice coil is maintained. Get down. Further, when a large tensile stress is applied to the edge at
the time of large amplitude of the diaphragm, the tensile stress is dispersed by the inner and
outer peripheral portions of the high rigidity edge, so the edge does not extend rapidly and is not
stretched. It is not necessary to generate an abnormal sound (stag noise) caused by the stab.
[0016]
In one embodiment, the cross-sectional shape of the edge is a roll shape. That is, the present
invention can be applied to the edge where the most common cross section is a roll shape.
[0017]
In one embodiment, the edge is dense in the surface layer and is made of a porous foam inside. In
this case, since the edge is dense and hard in the surface layer and has a rough and light
sandwich structure in the inside, the edge can be thickened while being light in weight, and the
appropriate rigidity as a support of the diaphragm Viscoelasticity is obtained. And, compared to
the foam which is homogeneous from the surface layer to the inside, the rigidity is unlikely to
change even under high temperature and high humidity without being affected by ultraviolet
rays and humidity, and the bass characteristic of the speaker is stably maintained.
[0018]
Further, the speaker of the present invention is joined to the diaphragm, a frame surrounding the
diaphragm, and the outer periphery of the diaphragm, and is joined to the inner periphery of the
frame, and the diaphragm is supported inside the frame And the cross-sectional shape of the
edge is composed of three or more continuous roll shapes, and the thickness of both outer rollshaped portions is greater than the thickness of the central roll-shaped portion.
[0019]
Furthermore, the speaker according to the present invention is joined to the diaphragm, the
frame surrounding the diaphragm, and the outer periphery of the diaphragm, and is joined to the
inner periphery of the frame, and the diaphragm is supported inside the frame The crosssectional shape of the edge is composed of three or more continuous roll shapes, and the radius
of both outer roll shapes is different from the radius of the central roll shape.
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[0020]
Even in such a configuration, the rigidity is low near the center of the edge, the rigidity is high
near the inner and outer peripheries, and if the vibration amplitude of the diaphragm is within a
certain range, the vicinity of the center of the edge is easily deformed and a voice coil is formed.
The linearity of the displacement of the diaphragm with respect to the applied drive current is
maintained, and when a large tensile stress is applied to the edge at the time of large amplitude
of the diaphragm, the tensile stress is dispersed by the inner and outer peripheral portions of the
high rigidity edge. The edge does not extend and squeeze sharply, and no abnormal sound is
generated.
[0021]
In one embodiment, the radiuses of the two outer roll shapes are smaller than the radius of the
central roll shape.
[0022]
Further, the speaker of the present invention is joined to the diaphragm, a frame surrounding the
diaphragm, and the outer periphery of the diaphragm, and is joined to the inner periphery of the
frame, and the diaphragm is supported inside the frame And the cross-sectional shape of the
edge is composed of three or more continuous roll shapes, and the thickness of both outer rollshaped portions is different from the thickness of the central roll-shaped portion, and both outer
rolls The radius of the shape is different from the radius of the central roll shape.
[0023]
Even in such a configuration, the rigidity is low near the center of the edge and the rigidity is
high near the inner and outer peripheries. Therefore, if the vibration amplitude of the diaphragm
is within a predetermined range, the diaphragm for the drive current applied to the voice coil In
addition, the edge does not rapidly extend and stretch when a large tensile stress is applied to the
edge at the time of large amplitude of the diaphragm, and no abnormal sound is generated.
[0024]
In one embodiment, the thickness of the both outer roll-shaped portions is greater than the
thickness of the central roll-shaped portion, and the radius of the both outer roll shapes is
smaller than the radius of the central roll shape. It is done.
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[0025]
Further, the speaker of the present invention is joined to the diaphragm, a frame surrounding the
diaphragm, and the outer periphery of the diaphragm, and is joined to the inner periphery of the
frame, and the diaphragm is supported inside the frame The rigidity of the edge is small near the
approximate center between the diaphragm and the frame and is increased as the diaphragm and
the frame are approached.
[0026]
Even in such a configuration, the rigidity is low near the center of the edge and the rigidity is
high near the inner and outer peripheries. Therefore, if the vibration amplitude of the diaphragm
is within a predetermined range, the diaphragm for the drive current applied to the voice coil In
addition, the edge does not rapidly extend and stretch when a large tensile stress is applied to the
edge at the time of large amplitude of the diaphragm, and no abnormal sound is generated.
[0027]
DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIG. 1 (a) is a crosssectional view showing a first embodiment of a loudspeaker according to the present invention.
Moreover, FIG.1 (b) is an expanded sectional view which shows the edge vicinity of the speaker
of Fig.1 (a).
[0028]
In FIGS. 1A and 1B, an annular magnetic circuit including a center pole 2, a magnet 3 and a top
plate 4 is formed at the lower end portion of the annular frame 1.
[0029]
A voice coil bobbin 6 is vertically and vibratably held in an annular space 5 formed between the
upper outer periphery of the center pole 2 and the inner periphery of the top plate 4.
The voice coil 10 is wound around the lower outer periphery of the voice coil bobbin 6.
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A diaphragm 8 is fixed to the upper end of the voice coil bobbin 6, and a damper (also referred to
as a suspension) 7 is connected near the upper end of the voice coil bobbin 6.
[0030]
The edge 11 is joined to the outer periphery of the diaphragm 8 and is joined to the inner
periphery of the frame 1 to support the diaphragm 8 inside the frame 1.
The damper 7 is directly attached to the frame 1.
The diaphragm 8 is vibratably held by the edge 11 and the damper 7.
[0031]
The edge 11 is formed by molding a rubber material, and its cross section has a convex roll
shape.
The thickness of the edge 11 is the thinnest at the top 14 and gradually increases from the top
14 toward the inner periphery 12 and the outer periphery 13 thereof.
The outer peripheral portion 13 is bonded to the frame 1, and the inner peripheral portion 12 is
fixed to the diaphragm 8.
[0032]
Here, the roll shape of the edge 11 draws a semicircle, but even if drawing an arc, an oval, etc.
combining other arcs, arcs and straight lines, or combining a convex arc and a concave arc
Furthermore, it may be a waveform in which a plurality of semicircles or arcs are combined, a
trapezoidal shape, a flat shape or the like.
[0033]
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In such a configuration, when a driving current proportional to the voice signal is applied to the
voice coil 10, a driving force is generated in the voice coil 10 due to the interaction between the
electromagnetic force of the voice coil 10 and the magnetic flux of the air gap 5. The voice coil
10 vibrates, the diaphragm 8 vibrates up and down along with this, and a sound is output from
this speaker.
[0034]
FIG. 2 is a schematic cross-sectional view showing the displacement state of the edge 11.
In FIG. 2, the vibration direction of the voice coil bobbin 6 is taken as a Z axis, and the front
(direction of sound output) of the speaker is taken + and the back is taken −.
The broken line indicates the state of the edge 11 when the drive current is not applied to the
voice coil 10 and the diaphragm 8 is at the neutral point.
The solid line shows the state of the edge 11 when a large drive current with low frequency is
applied to the voice coil 10 and the diaphragm 8 is displaced in the + direction along the Z axis.
[0035]
The inner peripheral portion 12 of the edge 11 is bonded to the diaphragm 8 and vibrates
integrally with the diaphragm 8.
The other outer peripheral portion 13 is fixed to the frame 1 and is not displaced.
The top portion 14 of the edge 11 has small rigidity and extends linearly because of its small
thickness.
[0036]
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9
Since the inner and outer peripheral portions 12 and 13 of the edge 11 are thick and high in
rigidity, the inner and outer peripheral portions 12 and 13 for the load (corresponding to the
displacement amount) of the diaphragm 8 as compared with the conventional edge of uniform
thickness in FIG. The edge 11 does not extend like the conventional edge of FIG. When an
excessive drive current is applied to the voice coil 10 and an excessive load acts on the
diaphragm 8, the inner and outer peripheral portions 12 and 13 of the edge 11 gently extend,
and the diaphragm 8 is gently braked.
[0037]
FIG. 3 shows the amount of displacement of the inner circumferential portion with respect to the
force (N) applied to the inner circumferential portion 12 of the edge 11. In FIG. 3, the broken line
(1) is a characteristic curve measured for a conventional rubber edge having a roll shape in cross
section and a uniform thickness of 0.5 mm, and the dashed line (2) is a cross section FIG. 7 is a
characteristic curve measured for a conventional rubber edge of roll shape and uniform
thickness 1.0 mm, and the solid line (3) shows that the top portion 14 is 0.5 mm thick and the
inner circumferential portion It is a characteristic curve measured about edge 11 made of rubber
which becomes thick gradually as it approaches 12 and perimeter part 13 and inner perimeter
part 12 and perimeter part 13 become thickness 1.0 mm.
[0038]
In the case of the conventional edge having a uniform thickness of 0.5 mm, as is apparent from
the characteristic curve of the broken line (1), the amount of displacement rapidly changes at a
force (load) of 10 (N). N) In the above, the displacement amount becomes almost constant, and
the edge is in a stretched state. Further, in the case of the conventional edge having a uniform
thickness of 1.0 mm, as apparent from the characteristic curve of the alternate long and short
dash line (2), the displacement becomes approximately constant at a force of about 20 (N) or
more. The slopes of these characteristic curves represent the stiffness of the edges. The
characteristic curve of the broken line (1) has a large inclination, so that the stiffness of the edge
is small, which is an appropriate value as a support system of the diaphragm. On the other hand,
the characteristic curve of the alternate long and short dash line (2) has a small slope, and the
edge has a large stiffness, which is too hard for the support system of the diaphragm to impede
the reproduction of the low frequency audio signal.
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[0039]
On the other hand, in the case of the rubber edge 11 of the present embodiment in which the top
portion 14 has a thickness of 0.5 mm and the inner circumferential portion 12 and the outer
circumferential portion 13 have a thickness of 1.0 mm, the characteristic curve of the solid line
(3) As apparent from the above, when the force (N) applied to the inner circumferential portion
12 of the edge 11 is small, the thinned top portion 14 is deformed, and the displacement amount
linearly changes corresponding to the force (N). Further, as the force (N) increases, the thickened
inner circumferential portion 12 and the portion in the vicinity of the outer circumferential
portion 13 gradually deform, so that the change of the displacement amount with respect to the
force (N) gradually becomes slow. That is, when the force (N) is small, the characteristic curve of
the solid line (3) substantially matches the characteristic curve of the broken line (1). Also, in the
range of 8 (N) to 15 (N) of the force (N), the characteristic curve of the solid line (3) shows the
middle of the characteristic curves of the broken line (1) and the dashed dotted line (2). The
change in displacement with respect to force (N) becomes gentler as the
[0040]
Therefore, when the force (N) becomes large, the displacement of the inner peripheral portion 12
of the edge 11 becomes gentle, and the edge 11 does not become suddenly stretched. In terms of
stiffness, when the force (N) is relatively small, the thinned top portion 14 is mainly deformed, so
the stiffness is small and becomes an appropriate value, and when the force (N) becomes large,
the thickness is increased. Since the portions in the vicinity of the inner peripheral portion 12
and the outer peripheral portion 13 are mainly deformed, the stiffness is increased, and the
support system of the diaphragm 8 is hard against excessive input and the diaphragm 8 becomes
difficult to move.
[0041]
FIG. 4 shows an edge 11 when the diaphragm 8 is vibrated with an amplitude of up to ± 10 mm
along the Z-axis by applying a drive current corresponding to the attack sound of the drum to the
voice coil 10 of the speaker of this embodiment. The displacement of the edge 11 is measured by
a laser Doppler displacement meter, and one cycle of the displacement of the edge 11 is shown.
As apparent from FIG. 4, even if the diaphragm 8 vibrates with an amplitude of ± 10 mm, the
edge 11 is not stiff and there is room in the shape of the edge 11. For this reason, the edge 11
does not extend completely, and the abnormal sound caused by the tension of the edge 11 does
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11
not occur.
[0042]
As described above, the edge 11 of the speaker according to the present embodiment has a
biased thickness in which the central portion is thin and thick in the vicinity of the inner and
outer peripheral portions. It has the advantage of both uniform thick edges.
[0043]
Second Embodiment FIG. 5 is an enlarged sectional view showing an edge in a speaker according
to a second embodiment of the present invention.
Although the edge 21 of this embodiment is different in material from the edge 11 of the first
embodiment, it has the same shape as the edge 11 of the first embodiment, and is applied to the
speaker shown in FIG.
[0044]
The edge 21 of the present embodiment is a material to which porous foam rubber is applied,
and is formed by molding.
[0045]
The hollow portion in the mold in which the edge 21 is formed has the same shape as the outer
shape of the edge 21, the cross section is a roll shape, and the portion corresponding to the top
24 of the edge 21 is the thinnest The respective portions corresponding to the inner
circumferential portion 12 and the outer circumferential portion 13 are thicker.
The edge 21 is formed by foaming a rubber material in the hollow portion. At the time of this
foam molding, the expansion ratio of the rubber material is set so that the expansion ratio
becomes higher as the surface layer becomes denser and goes to the inside.
[0046]
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12
As a result, the edge 21 has a sandwich structure in which the surface layer is dense and hard,
and the inside is rough and light, so that the edge 21 can be thicker and rigidity can be increased
as compared with a rubber sheet of the same weight. Furthermore, since the expansion ratio is
changed, the weight and rigidity of the edge 21 can be easily adjusted, and appropriate rigidity
and viscoelasticity can be set. By this, as in the first embodiment, the advantage that the vibration
amplitude of the diaphragm 8 is soft and easy to be deformed within a certain range, and even if
the vibration amplitude becomes large, it is difficult to hold and abnormal noise (shearing noise)
does not occur. It has the advantage of Further, compared with the edge made of urethane foam
having a uniform expansion ratio, the surface layer of the edge 21 is dense and hard, so the edge
21 is excellent in weather resistance and moisture resistance, and the rigidity hardly changes
even under high temperature and high humidity. The bass reproduction characteristic of the
speaker is stably maintained.
[0047]
Third Embodiment FIG. 6A is a cross-sectional view showing a third embodiment of the speaker
of the present invention. 6 (b) is an enlarged sectional view showing the vicinity of an edge of the
speaker of FIG. 6 (a). In FIGS. 6 (a) and 6 (b), the same reference numerals are given to the
portions that perform the same function as in FIG.
[0048]
In the present embodiment, the edge 41 has three continuous roll portions 42, 43 and 44. The
edge 41 is thin at the central portion and gradually thickens toward the inner circumferential
portion 46 and the outer circumferential portion 45. The outer peripheral portion 45 is bonded
to the frame 1, and the inner peripheral portion 46 is bonded to the diaphragm 8.
[0049]
Similar to the first and second embodiments, the edge 41 of the present embodiment also has an
advantage that the vibration amplitude of the diaphragm 8 is soft and easy to be deformed within
a certain range, and it is difficult to hold even if the vibration amplitude becomes large. It also
has the advantage that no sound (crying noise) occurs.
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[0050]
In the present embodiment, the thickness of the edge 41 is gradually changed, but instead, only
the central roll portion 43 may be thinned and the outer side roll portions 42 and 44 may be
thickened. .
In addition, the number of continuous roll units may be four or more.
[0051]
Fourth Embodiment FIG. 7A is a cross-sectional view showing a fourth embodiment of the
speaker of the present invention. FIG. 7B is an enlarged cross-sectional view showing the vicinity
of an edge of the speaker of FIG. 7A. In FIGS. 7 (a) and 7 (b), the same reference numerals are
given to the portions that perform the same function as in FIG.
[0052]
The edge 51 of the present embodiment is composed of three continuous inner circumferential
roll portions 52, a central roll portion 53, and an outermost circumferential roll portion 54. The
radius of the inner and outer peripheral roll portions 52 and 54 is smaller than the radius of the
central roll 53. When the radius of the roll portion decreases, the stiffness for supporting the
diaphragm 8 increases, so the amount of displacement of the roll portion with respect to the
force (N) acting on the inner peripheral portion of the edge 51 decreases. This corresponds to an
increase in edge thickness. Therefore, in the edge 51 of the present embodiment, the central roll
53 is soft and easily deformed, and the inner and outer rolls 52 and 54 are hard and difficult to
be stretched, as in the first to third embodiments. It is possible to realize an advantage that the
vibration amplitude of 8 is soft and easy to be deformed within a certain range, and an advantage
that it is difficult to hold even if the vibration amplitude becomes large and abnormal noise
(shear sound) is not generated.
[0053]
Incidentally, by changing the thickness of the edge 51 in the present embodiment, it is possible
to adjust the linearity of the braking effect of the edge 51 and the restoring force characteristic.
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14
[0054]
Further, as in the third embodiment, the center of the edge 51 may be thinned and the edge 51
may be gradually thickened toward the inner and outer peripheral portions, whereby the braking
effect can be further increased. .
[0055]
The present invention is not limited to the above embodiments, and can be variously modified.
FIG. 8 shows various modifications of the cross-sectional shape of the edge.
The edge 81 in FIG. 8 (a) is generally flat, the edge 82 in FIG. 8 (b) is mountain-shaped, and the
edge 83 in FIG. 8 (c) is a continuous uneven two roll portion 83a, 83b. The edge 85 of FIG. 8 (d)
is trapezoidal, the edge 86 of FIG. 8 (e) has two continuous wave portions 86a and 86b, and the
edge 87 of FIG. 8 (f) is continuous. The edge 88 of FIG. 8 (g) has three concave portions 88a at
the center and the roll portions 88b and 88c of small radius at the inner and outer
circumferences, as shown in FIG. 8 (h). The edge 89 has a central recess 89a and inner and outer
peripheral roll portions 89b and 89c, and the edge 90 in FIG. 8I has a central recess 90a and
inner and outer peripheral corrugated portions 90b and 90c. In each of the edges 81 to 90, the
central portion is thinner and thicker toward the inner and outer peripheries, and the vibration
amplitude of the diaphragm is soft and easily deformed within a certain range, and the vibration
amplitude becomes large. It has the advantage that it is hard to stand and that no abnormal
sound is generated.
[0056]
Considering the edges of each of the above-described embodiments and the modifications, the
rigidity at the center of the edge is small at any of the edges, and the rigidity increases as it
approaches the inner and outer periphery of the edge. Thus, the edges to which the rigidity is
distributed in this way fall within the scope of the present invention.
[0057]
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15
As described above, according to the present invention, the rigidity is low near the center of the
edge, and the rigidity is high near the inner and outer peripheries. Because the rigidity is low
near the center of the edge, if the vibration amplitude of the diaphragm is within a certain range,
the vicinity of the center of the edge is easily deformed and the linearity of displacement of the
diaphragm with respect to the drive current applied to the voice coil is maintained. Get down.
Further, when a large tensile stress is applied to the edge at the time of large amplitude of the
diaphragm, the tensile stress is dispersed by the inner and outer peripheral portions of the high
rigidity edge, so the edge does not extend rapidly and is not stretched. It is not necessary to
generate an abnormal sound (stag noise) caused by the stab.
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