Note: Descriptions are shown in the official language in which they were submitted.
1 335308
This application is a division of copending application
547,123, filed Septe~ber 17, 1987.
This invention relates to a speaker which is thin but
which can provide high-fidelity reproduction.
The present invention will be illustrated by way of the
accompanying drawings, in which:-
Fig. 1 is a schematic cross-sectional view of a rear-
opening speaker system mounted on a wall and a sound passage
formed therebetween;
O Fig. 2 is a graph of a sound pressure frequency
characteristics of the rear-opening speaker system mounted on
the wall with respect to a parameter which is the distance
between the speaker and the wall;
Fig. 3 is a schematic cross-sectional ~iew of a speaker
system and a sound passage, the system having a port formed
at the rear of an enclosure;
Fig. 4 is a graph of the relationship between the output
sound pressure levels and the phase difference between sounds
radiated from the front and rear of the speaker;
O Fig. 5 is an equivalent circuit diagram of a thin
speaker system having a phase difference;
Fig. 6 is a graph of the relationship between additional
mass and the area of a diaphragm;
Fig. 7 is a cross-sectional view of a speaker in which
the principle of the present invention is illustrated;
Fig. 8 is a graph of changes in the output sound
pressure in accordance with the ratio of a closed-system
portion and an opened-system portion of the diaphragm;
S ~
-- 1 --
1 3353~8
Fig. 9 is a cross-sectional view of a spea~er which
represents a first embodiment of the present invention;
Fig.s 10 and ll are cross-sectional views of essential
parts of second and third embodiments of the present lnvention;
Fig. 12 is a cross-sectional view of a fourth embodi-
ment of the present invention;
Fig. 1~ is a graph of sound pressure frequency charac-
teristics of the fourth embodiment;
Fig. 14 is a perspective view of a fifth embodiment of
the present invention;
Fig. 15A is a front view of a sixth embodiment of the
present invention;
Fig. 15B is a cross-sectional view of the sixth embodi-
ment;
Fig. 16 is a cross-sectional view of a seventh embodi-
ment of the present invention;
Fig. 17A is a front view of an eighth embodiment of the
present invention;
Fig. 17B is a cross-sectional view of the eighth embod-
iment of the present invention;
Fig. 18 is a front view of a ninth embodiment of the
present invention in which the positions at-which voice coils are
-fixed to the diaphragm are indicated;
.
Fig. 19 is a graph of the characteristics of the ninth
embodiment;
-- 2 --
`-- 1 335308
Fig. 20 is a schematic cross-sectional view of a tenth
embodiment of the present invention;
Fig.s 21A and 21B are enlarged cross-sectional views of
essential parts of the tenth ~ iment;
Fig.s 22A and 22B are graphs of the sound pressure fre-
quency characteristics and the distortion frequency characteris-
tics of the tenth embodiment;
Fig. 23 is a cross-sectional view of an essential part
of the speaker in accordance with the present invention, which
illustrates the state in which a voice coil is connected to a
diaphragm; and
Fig. 24 is a side view of the voice coil cap and voice
coil shown in Fig. 23.
There are great demands for speakers which are thin but
can provide high-fidelity reproduction because of their space-
saving characteristics. It is not very difficult to make a spea-
ker thinner while maint~;ning its performance in middle- and
high-ranges of frequencies above several hundred Hz. However, in
the reproduction of low frequencies below several hundred Hz, it
is not possible to ensure an adequate sound pressure level unless
the volume velocity of the diaphragm is increased. However, in a
speaker system using a closed type enclosure, the stiffness of
the enclosure SB (given by SB = ~OCO2S2/V, where ~ o: density
of air; CO: speed of sound; S: area of diaphragm; and V: volume
3~ of enclosure) is increased, so that the lowest resonance fre-
qeuncy fOB (given by foB = 2~ ~(Sg ~ SD)/M, where SD:
stiffness of vibration system; and M: mass of vibration system
including additional masses) of the system when the diaphragm is
attached to the enclosure is increased; and the sound pressure
level in the low-frequency range is thus reduced. To reduce the
value of fOB, M may be increased or (SB ~ SD) may be reduced.
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1 335308
However, if M is increased, the sound pressure level is reduced.
Therefore, there is no alternative but to reduce (SB + SD). If S
is made constant, fOB cannot be reduced because SB SD, since v
is small in the thin speaker system. If S is reduced, it is
neceC~ary to increase the amplitude of the diaphragm in order to
maintain a certain volume velocity. This may cause an increase
distortion.
For these reasons, conventional thin speaker systems
are usually provided with rear-opening type enclosures. A system
of this type will now be described with reference to Fig. 1. As
shown in Fig. 1, a diaphragm 1 is connected to an enclosure 3f by
an edge 2. The drive system, etc., are omitted in order to sim-
plify the description. Such a speaker system was mounted on a
rigid wall 4, and the axial ~n~ pressure frequency characteris-
tics obtained by varying the distance d ~e~een the system and
the rigid wall 4 were measured. The results of these measure-
ments are shown in Fig. 2. As is clear from Fig. 2, the sound
pressure level in the low-frequency range increases as the dis-
tance d increases. This is because a sound A which is radiatedfrom the front surface of the diaphragm and a sound B which is
radiated from the rear surface achieve opposite phases at a mea-
suring point P, and so cAncel each other, as the speaker system
is brought closer to the rigid wall. Therefore, this speaker
system cannot ensure a desired reproduction sound pressure level
unless it is spaced away from the wall by so to 60 cm. This
speaker system is thin but it cannot realize any space-saving
effect.
To overcome this problem of the rear-opening enclosure,
a type of system has been proposed in which an acoustic duct is
formed so as to improve the phase difference (at best, equalize
the phases) be~7een the sounds radiated from the front and rear
surfaces at the measuring point, even when the system is made in
close contact with a rigid wall. Fig. 3 shows this type of sys-
tem. An enclosure 3g has an opening 6 which ensures that a sound
1 335308
radiated from the rear surface of the diaphragm passes through
the duct 6 then through an acoustic passage 7 which is fonmed
be~.7een the enclosure 3g and a rigid wall 4, thereafter being
radiated toward the front. The sound pressure level is thereby
improved h~cAll~e the phase difference between the sounds-radiated
from the front and rear surfaces of the diaphragm is increased by
a phase corresponding to a distance ~, which should be compared
with that displayed in the above-described example.
If, in this method, the area of the diaphragm is
increased in order to reduce the amplitude of the diaphragm, a
mass Ma in the gap be~ee,l the rear side of the enclosure and the
rigid wall (Ma ( SD )2~ , where SD: area of diaphragm; Sp:
area of opening; and Mp: mass of air ln acoustic duct) is
increased, thereby reducing the output sound pressure level.
The present invention in consideration of the above-
described problems provides a speaker which is thin but which is
capable of providing high-fidelity reproduction and ensuring a
suitable level of sound pressure even when it is brought into
close contact with a wall.
To this end, present invention provides a speaker in
which a closed rhAmher is provided for a part of a diaphragm
mounted in an enclosure, and an open ch. ~her iS provided for the
other part of the diaphragm.
In this construction, the phase difference between
sounds radiated from the front and rear surfaces of the diaphragm
is increased as large as possible by using an acoustic duct for
the sound radiated through the open chamber, thereby minimizing
the cAncellation of colln~c and improving the sound pressure
level.
Thus, according to one aspect of the present invention
_ 5 _
1 335308
there is provided a speaker of the direct radiator-type compris-
ing an enclosure, a diaphragm connected to said enclosure, and at
least one voice coil unit connected to said diaphragm, wherein a
part of said diaphragm connected to said enclosure forms a
closed-type construction, and the other part of said diaphragm
forms an open-system having an opening. Suitably the speaker
further comprises an acoustic duct, wherein sound radiated from
said opening to the rear of said enclosure is radiated to the
front of said speaker through said acoustic duct.-
In one embodiment of the present invention said openingis formed in a rear plate of said enclosure, and a stepped por-
tion is formed on said rear plate, and wherein, said speaker is
mounted on a wall so that said rear plate is brought close to
said wall, an acoustic duct is formed by said wall and said
stepped portion on said real plate. Suitably the sectlonal area
of sald acoustic duct gradually increases as it approaches the
outlet of sound. Desirably a plate corresponding to said wall is
fixed to said enclosure at the rear of said rear plate.
In another embodiment of the present invention a plu-
rality of voice coil units for driving said diaphragm at a plu-
rality of points are disposed on said diaphragm, and wherein the
drivlng force of each of said voice coil units is weighted.
Suitably said diaphragm is separated into at least two indepen-
dent diaphragms which respectively form open and closed systems
and which are driven by voice coil units. Desirably said voice
coil unit is fixed to said diaphragm at a point or in the vicin-
ity thereof at which both vibrations of the primary free reso-
nance mode and those of the secondary free resonance mode arerestrained. Suitably an opening is formed in a rear plate of
said enclosure; sound is radiated to the rear of said enclosure;
and a gap between said rear plate of said enclosure and a wall is
utilized as an acoustic duct; and wherein the peripheral edge of
said opening which corresponds to an inlet of said acoustic duct
and the other peripheral edge of said rear plate which corre-
- 1 33530~
sponds to an outlet of said acoustic duct are curved smoothly or
tapered. Desirably said voice coil unit has a cylindrical ring
having a plurality of projections on its cylindrical wall, and a
flat end portion formed at its one end; and a voice-coil bobbin
having yLOu~ of slits of different depths formed at lts--one end
mated with the other end of said ring, said voice coil bobbin
being adapted for driving said diaphragm through said cylindrical
ring, wherein said pro~ections are inserted into and fixed to
said one of said groups of slits, and wherein said flat end por-
tion is fixed to one surface of said diaphragm.
In another aspect of the present lnvention there isprovided a speaker of the direct radiator-type type having a
diaphragm, a part of said diaphragm forming a closed system, and
the other part of said diaphragm forming an open system, said
speaker comprising a plurality of voice coils disposed on said
diaphragm and adapted for driving said diaphragm at a plurality
of points thereon. Suitably the driving force of each of said
voice coils for driving said diaphragm is weighted. Desirably
the driving force per unlt area Nc/Sc of said closed system
obtained by dividing the total driving force Nc of at least one
of said voice coils disposed on said closed-system portion of
said diaphragm ~y the area of said open-system diaphragm portion
SC is set to be larger than the driving force per unit area No/So
of said open system obtained by dividing the total driving force
No of at least one of said voice coils disposed on said open-sys-
tem portion of said diaphragm by the area of said open-system
diaphragm portion S0.
In a further aspect thereof the present invention pro-
vides a speaker having at least one closed-type speaker unit and
at least one open-type speaker unit each having an independent
diaphragm mounted in the same enclosure, said open-type speaker
unit having an opening formed in a rear plate of said enclosure,
said speaker comprising a duct through which sound radiated from
said opening is radiated to the front of said speaker. Suitably
- 1 335308
said open-type speaker unit is formed generally at the center of
said enclosure, and said closed-type speaker unit is formed at a
peripheral portion of-said enclosure, and wherein said opening is
fonmed in said rear plate of said enclosure generally at the cen-
ter thereof. Desirably said enclosure has a stepped portion onits rear surface so that, when said enclosure is mounted on a
wall in close contact therewith, an acoustic duct is formed by
said wall and by said stepped portion. Suitably said acoustic
duct is fonmed integrally with said enclosure.
In a still further aspect thereof the present invention
provldes a speaker havlng an opening formed in a rear plate of an
enclosure through which sound-is radiated to the rear of said
enclosure, and an acoustic duct formed of a gap between said rear
plate of said enclosure and a wall, wherein the peripheral edge
of-said opening which co.L~ponds to an inlet of said acoustic
duct and the outer peripheral edge of said rear plate which cor-
responds to an outlet of said acoustic duct are curved smoothly
or tapered.
In a further aspect thereof the present invention pro-
vides a speaker comprising a magnetic circuit having an annular
magnetic gap fonmed by an annular magnet and top and bottom
plates fixed to the upper and lower surfaces of said annular mag-
net; a voice coil suspended in said annular magnetic gap by adamper; and a flat rectangular diaphragm supported at its outer
periphery by an edge on a frame; wherein said voice coil is fixed
to said diaphragm at a point or in the vicinity thereof at which
both vibrations of the primary free resonance mode and those of
the secondary free resonance mode in the longitudinal direction
of said diaphragm are restrained.
Preferred embo~ nts of the present invention will be
described below with reference to the accompanying drawings.
The principle of the speaker in accordance with the
- 7a -
I 335308
present invention will first be described.
As mentioned above, the output sound pressure level is
reduced because the sound radiated from the rear side of the
speaker is reflected by the wall so as to c~ncel the sound radi-
ated from the front surface of the speaker. It is possible to
assume that, if the phase of the sound radiated from the rear
slde could be changed to a certain degree by some means, the out-
put sound pressure level could be improved in accordance with
this degree of change. It is possible to examine the change in
sound pressure level by assuming that a sound radiation front
having the opposite phase is formed around the speaker and the
phase of this sound is changed. The result of this is shown in
Fig. 4.
Fig. 4 is a graph of the relationship between the sound
pressure levels and the phase difference between sounds radiated
from the front and rear surfaces of the diaphragm. As shown in
Fig. 4, a slight change in the phase causes a large increase in
the output sound
- 7b -
1 335308
1 pressure level. If a speaker box is formed in the rear
of the diaphragm so as to cover the entire area of the
rear surface of the diaphragm, and if an opening is
formed in the rear plate of this spe~ker box, a phase ~
difference is created between sounds which are radiated
from the front and rear surfaces of the ~iAphragm
provided that the size of the spe~ker box is large,
even though the box has a small thickness. Therefore,
the provision of this speaker box enables the output
sound pressure level to be improved.
However, if, in this case, the area of the
~i~phragm is increased so as to limit the amplitude of
the diaphragm in order to reduce the thickness of the
speaker, the disadvantages which will be described below
are experienced. Fig. 5 shows an equivalent circuit
of such a speaker. As can be understood from Fig. 5,
the mass which is effective in the gap between the
rear of the speaker box and the wall is multiplied by a
transformation ratio n squared, and is added to the mass
of the diaphragm.
The present invention has been achieved by
further studying this speaker, as described below.
It is easy for the central portion of the
~iAphragm to increase the output sound pressure by
creating a certain phase difference, but it is easy for
the peripheral portion thereof to cancel the sounds
radiated from its front and rear surfaces since the
length of the passage for the sound radiated from the
- I 335308
1 rear is small. ~owever, it is necessary to reduce the
area of the diaphragm in order to el;~in~te this defect.
Fig. 6 shows the relationship between additional mass --
and the area of the diaphragm when the diaphragm is
S disposed at the center of a speaker box of 1 x 1 x 0.05 m.
As shown in Fig. 6, the additional mass is certainly
reduced if the area of the diaphragm is reduced. How-
ever, in this case, the diaphragm vibrates by a large
amplitude, resulting in a solution which is not practical
for the design of a thin speaker. For instance, if the
area S is 0.07 m , the amplitude must be about 10 mm
to achieve a practical maximum output sound pressure of
110 dB at 1 m.
If a portion about the center of a diaphragm
8, shown in Fig. 7, has a small area and a rear-opening
system so as to reduce the load while the r~m~ining
portion, including a peripheral portion of the diaphragm,
forms a closed system so as to prevent the cancellation
of sound, the entire area of a speaker box 9 can be
utilized for the area of the diaphragm so that the
thickness of the speaker is reduced while a large
output sound pressure at a low amplitude is ensured.
Fig. 8 shows changes in the output sound pressure when
the balance between the rear-opening-system portion
and the closed-system portion is varied. As shown in
Fig. 8, for a given size of box, there is a solution
which ensures suitable values of both fO and the output
sound pressure level.
g
1 335308
1 Fig. 9 shows a speaker which represents an
embodiment of the present invention in which a speaker
unit having a diaphragm 12 of 0.8 x 0.3 m is attached to
an enclosure of 1 x 1 x 0.05 m. Four magnetic circuit-s
which provide driving forces are attached to the
diaphragm 12. The magnetic circuits are constituted
by magnets 15, 15', 15", and 15-"'; lower platesl4,¦4',
14 n ~ and ! 4"'; and upper plates 10, 10', 10 n ~ and 10"'.
In the magnetic gaps of the magnetic circuit, driving
forces are generated by voice coils 6, 6', 6~, and
!6"' The interior of the enclosure is partitioned by a
woofer frame 20 and an internal edge 17, and is separated
into a closed enclosure portion 19 and a rear opening
13.
In the speaker constructed in this manner,
sound generated from the rear surface of a peripheral
portion of the diaphragm 12, which contributes most to
the cancellation of sound at the front side, is radiated
into the closed enclosure portion 19, and sound
generated from the rear surface of the central portion
of the diaphragm, which contributes only slightly to
the cancellation at the front side, is radiated through
the rear hole 13. It is therefore possible to realize
a speaker which can maintain its output sound pressure
- 25 level when it is mounted on a wall.
A stepped portion may be formed in the rear
plate of the enclosure in such a manner that an acoustic
duct is formed by this stepped portion and the wall when
1 3353~8
1 the speaker is mounted on the wall. This acoustic duct
may be formed in such a manner that the cross section
of the duct expands as it approaches the outlet opening
of the duct. Otherwise, a plate which acts as a wall-
may be previously fixed to the rear side of the speaker.
As described above, the present inventionrealizes a speaker which can maintain its output sound
pressure level when it is mounted on a wall and which
can be adapted for two kinds of use such as one in which
it is mounted on a wall and one in which it is free-
standing, since it can exhibit its basic performance
even when separated from the wall.
Fig. 10 shows a speaker which represents a
second embodiment of the present invention. As shown
in Fig. 10, the speaker has a diaphragm 21, an edge 22,
a speaker box 23, an intern~l edge 24, a partition
plate 25, a closed space 28, and an open space 29, and
the fundamental construction of this speaker is substan-
tially the same as that shown in Fig. 9. A portion 21c
of the diaphragm 21 forms a closed system of the
~ phragm 21, and a portion 21p form an open system of
the diaphragm 21. In this embodiment, the areas of the
portions 21c and 21p forming these two systems are
approximately equal to each other. The speaker is
further provided with a driver unit 26 which is attached
to the open-system portion 21p of the diaphragm 21,
and driver units 27a and 27b which are attached to the
closed-system portion 21c of the diaphragm 21. In
-- 11 --
1 335308 --
1 Fig. 10, the frame of the speaker is omitted in order
to avoid complications.
In this embodiment, the diaphragm 21 has one
drive point in the o~e., system portion 21p, and two ~
drive points in the closed-system portion 21c. Since
the areas of the closed-system portion 21c and the
open-system portion 21p of the diaphragm 21 are sub-
stantially the same, the driving force per unit area
of the ~iA~hragm 21 applied to the closed-system portion
21c is twice as large as that applied to the open-
system portion 21p. It is thereby possible to enable
suitable piston moitons of the diagram although, during
low-frequency reproduction, the vibration amplitude of
the closed-system portion 21c of the diaphragm is
basically less than that of the open-system portion 21p
because of the large stiffness of the closed system
due to the existence of the closed space 28.
The present invention will be further described
below with respect to other embodiments thereof in
conjunction with the correspon~ing drawings.
Fig. 11 shows a third embodiment of the
present invention which differs from that shown in Fig.
10 in that two driver units 26a and 26b are provided on
the open-system portion 21p of the diaphragm 21. In
this embodiment, the areas of the open-system portion
21p and the closed-system portion 21c are approximately
equal to each other, but the driving forces of the
driver units 26a and 26b are weighted so as to realize
1 335308
1 suitable piston motions of the diaphragm 21. That is,
the driving force of the driver units 27a and 27b
for driving the closed-system portion 21c of the
diaphragm 21 is set to be larger than that of the
S driver units 26a and 26b for driving the open-system
portion 21p so that the difference of the stiffnesses
for the open-system portion 21p and the closed-system
portion 21c at the time of low-frequency reproduction
is canceled; and the amplitudes of the portions 21c
and 21p of the diaphragm 21 are generally equalized,
thereby realizing piston motions of the diaphragm.
To provide a difference between the driving
forces per unit area of the closed-system portion and
the open-system portion of the diaphragm, the number of
driver units 26a and 26b are selected in the embodiment
shown in Fig. 10, and the driving forces of the driver
units are weighted. ~owever, the ratio of the areas
of the open- and closed-system portions may be
changed for this purpose within the design limitations.
2~ In the direct radiator-type speaker in
accordance with this embodiment in which a part of the
diaphragm forms a closed system and the other part forms
an open system, the diaphragm has a plurality of drive
points; and each of the driving forces is weighted or
the disposition of drive points is selected suitably,
so that the driving force per unit area of the closed-
system portion of the diaphragm is increased relative
to that of the open-system portion, thereby enabling
1 335308
1 suitable piston-motion vibrations of the diaphragm
at the tLme of low-frequency reproduction.
A fourth embodiment of the present invention
will now be described below with reference to Figs. 12-
and 13. As shown in Fig. 12, a speaker which representsthe fourth embodiment is provided with enclosures 30 of
1 x 1 x O.06 m, diaphragms 3la and 31b of 30 x 17 cm
and a diaphragm 31c of 30 x 46 cm. ~ach of the
diaphragms is formed of a member which is made of a
cellular material, which has a thickness of 8 mm and
which is sandwiched between aluminum surfacing members.
A closed enclosure is formed in the rear of the diaphragm
31a, and a rear-opening enclosure having an opening 36
is formed in the rear of the central diaphragm 31c.
The diaphragms are dirven by four voice coils 38.
This speaker also has edges 32, and magnetic circuits
39 for driving the voice coils 38 respectively.
Fig. 13 shows sound pressure frequency
characteristics of this embodiment. If the diaphragm
is formed in one united body as in the case of a tradi-
tional unit, the loads on the rear surfaces of the
closed-system portion and the open-system portion differ
from each other so that concentration of stress occurs
in the vicinity of the bonn~ry between the closed-
system portion and the open-system portion, resulting in
distortions and peaks and dips on the sound pressure
frequency characteristic curve which is exemplified by
the curve A in Fig. 13. The characteristics of the
- 14 -
1 3353~8
1 independently-driven diaphragm in accordance with the
present invention exhibit only small degrees of peaks
and dips, as exemplified by the curve B in Fig. 13,
compared with the characteristic indicated by the curve
S A in Fig. 13. In addition, the frequency response
range of the diaphragm in accordance with the present
invention is e~r~n~ed.
In this embodiment, the diaphragms are
rectangular, but the present invention is effective
irrespective of the shape of the diaphragm. It is
possible to realize the same effect by a speaker which
has, as shown in Fig. 14, coaxial ~;~p~ragms 33 and 34
connected to an enclosure 30a by an edge 32a and in which
an open chamber having an opening 36a is formed at
the rear of the central portion; and a closed chamber
is formed at the rear of the peripheral portion.
If the size of the enclosure is so small that
it is not possible to provide an adequate phase difference
between the sound radiated from the front surface and the
sound radiated through a port formed at the rear plate
of the speaker, the speaker may be designed to be
advantageous in such a manner that, as shown in Figs.
15A and lSB, an enclosure 33c is provided with an opening
36c; diaphragms 35 and 37 are supported by edges 32c
on the enclosure; and a crosspiece is attached to the
enclosure so as to form an acoustic duct 40 between
the rear surface of the enclosure and a rigid wall 4,
thereby radiating sound from the port to the front of
1 3353~8
l the speaker via the acoustic duct 40. It goes without
saying that, as shown in Fig. 16, the acoustic duct
40 may be formed so as to be integral with an enclosure
30b instead of utilizing the surface of the wall.
To limit the thickness of the enclosure, the speaker
may otherwise be constructed in such a manner that, as
shown in Figs. l?A and 17B, diaphragms 44 and 45 are
supported by edges 43 on the central portion of an
enclosure 42; and an acoustic duct 47 which communicates
with an opening 46 is formed in side portions of the
enclosure 42 such as to encircle this central portion.
These arrangements not only eliminate the need to closely
attach the enclosure to the rigid wall but also enable
the speaker to be mounted on the wall no matter how the
irregularity of the wall surface.
In accordance with the present invention,-as
described above, diaphragms are provided to form a rear-
opening system and a closed system independently, so
that the piston-vibration range of the diaphragm is
remarkably expanded; the frequency characteristic curve
is flattened; the ratio of distortion is reduced.
Moreover, the thickness and the weight of the diaphragm
assembly can be reduced since each diaphragm has a
reduced size and, hence, an improved flexural rigidity.
Therefore, the output sound pressure level is increased.
In addition, the degree of freedom in disposing the
~;~phragm is increased, thereby making the assembly work
easier.
- 16 -
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1 A ninth embodiment of the present invention
will be described below with reference to Figs. 18 and
19. The features of this e~o~iment reside in the
fixation of voice coils on the diaphragm. Voice -
S coils, each of which is susp~n~e~ by a damper in an
annular gap of a magnetic circuit formed between a top
plate and a bottom plate fixed to the upper and lower
surfaces of an annular magnet, are fixed to a flat
rectangular diaphragm at the points or in the vicinity
thereof at which-both vibrations of the primary free
resonance mode and those of the secondary free
resonance mode of the diaphragm in the longitll~in
direction thereof are restrained. The diaphragm is
supported at its outer periphery by an edge or the
like on a frame.
For the resonance in the longitll~;n~l direc-
tion alone in this construction, it is possible to
substitute the resonance form of an opposite-end-free
rod for that of the flat rectangular diaphragm. A
forced vibration displacement ~ by a concentrated driving
force Fxei~t is expressed by
PSI m ~2m - ~2 m m ~.... (1)
where p: density;
s: sectional area of rod;
Q: length of rod;
~m(x)~m(y): criterion function which represents
1 33530~
vibration form; and
~: angular velocity.
1 When the rod is driven at four points xl, x2,
X3 and X4, the forced vibration displacement ~ is
~sQ ~ 2 2 {FXl~m( 1) Fx3~m(X3)
m
x3 m(x3) + Fx4m(x4)}m(y)ei~t ,,,,, (2)
and thé driving method which is free of the occurrence
of vibrations of the primary and secondary modes (no
-5 asymmetrical mode vibration occurs since the rod is
driven symmetrically about the center thereof. Accord-
ingly, they are called the primary and seCo~ry
resonance modes in low-degree resonance modes other
than asymmetrical modes.) is to obtain the four points
xl, x2, X3 and X4 which satisfy the equation:
xl~m(Xl~ ~ Fx2m(X2) ~ Fx m(X3)
x4~m(X4) .................................... (3)
Since the rod is driven symmetrically about the center
thereof by the same magnitudes of forces,
F = F = F = F = F ..... (4)
xl x2 X3 X4 x
is formed.
- 18 -
1 335308
1 ( 1) ~1 (X2) ~ ~ X2) + ~l (l-xl) = o ,,, (5
2 1 ~2(X2) + ~3(1~X2) ~ ~3(1-xl) = O .. (6)
1 Driving points xl and x2 which satisfy both Equations
(5) and (6) are obtained as follows:
Xl = 0.1130
X2 = 0'37775 ~ ..... (7)
x3 (1 - x2) = 0.62225
x4 (1 - xl) = 0.8870
According to the present invention, the diaphragm
is driven at the points represented by Equations (7).
Therefore, there is no possibility of vibrations of
the primary or secondary resonance mode. The piston-
motion range of the diaphragm is thereby eYp~n~ed, and
the sound pressure frequency characteristic curve
is flattened.
The method in accordance with this embodiment
may be applied to each of the above-described embodiments.
This embodiment will be further described in detail with
reference to Figs. 18 and 19.
As shown in Fig. 18, four voice coils 51, 52,
53 and 54 are attached to a diaphragm 50 at the points
or in the vicinity thereof at which both free resonances
of the primary resonance mode and of the secondary
resonance mode of the diaphragm 50 in the longitudinal
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1 direction are restrained, that is, the points that
are located, if the width of the diaphragm 50 is W, at
distances of 0.113W, 0.37775W, 0.62225W, and 0.8870W
from the end of the diaphragm S0. --
These values represent the ratios of the
distances of points on the diaphragm to lengthwise
dimension of the ~iAr~agm. Since the voice coils are
fixed to the diaphragm at the positions defined by the
values shown in Equations (7), any one of the resonances
of the primary resonance mode and the seco~ry resonance
mode can be restrained.
Fig. 19 shows the results of calculations of
the vibration form on the basis of a finite element
method when the diaphragm is driven at the points which
satisfy Equations (7). The solid line indicates the
state before the occurrence of deformation, and the
broken line indicates the state after the occurrence of
deformation. As is also understood from Fig. 19, the
vibrations of the diaphragm of the primary and secondary
reson~nce modes are restrained so that the diaphragm
exhibits piston motions. Thus, the present invention can
provide a flat rectangular speaker which has flat and
smooth sound pressure frequency characteristics.
In the above-described embodiments, peripheral
edges of the opening formed in the rear plate the
enclosure are angular, but they may be surfaces which
are smoothly curved or tapered surfaces, such as those
shown in Fig. 20, which will be described below concretely.
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~ 33~30~
,
1 Fig. 20 is a cross-sectional view of a speaker
which represents a tenth embodiment of the present
invention and in which some parts are omitted. The
speaker shown in Fig. 20 has a diaphragm 61 of 30 x 80
cm formed of a member which is made of a cellular
material, which has a thickness of 8 mm and which is
sandwiched between aluminum-surfacing members.~ The
diaphragm 61 is supported by an edge 63 on a 1 m
square enclosure 62 having a thickness of 6 cm. A~ the
rear of a peripheral portion of the diaphragm 61 is
formed a closed-type construction which is defined by
this portion of the diaphragm, a rear plate 62a of the
enclosure, an intern~l plate 64 extending from the
enclosure rear plate 62a, and an internal edge 65 inter-
posed between the top of the internal plate 64 and adiametral intermediate portion of the diaphragm 61.
At the rear of a central portion 61b of the ~i~phragm
61 is formed an open-type construction which has a gap
67 between this central portion 61b and the internal
plate 64 and communicates with the space formed at
the rear of the enclosure rear plate 62a through a duct
66, which is encircled by the internal plate 64, and
which communicates with the gap 67 at its front end and
opens at its rear end in the rear plate 62a. The
duct 66 communicates with the outside through a gap 68
which is formed between the enclosure rear plate 62a
and a wall 69. An outer peripheral edge 70 of the
throat of the duct 66 and an outer peripheral edge 71
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1 of the ~outh of the duct 66 are curved smoothly or
tapered, and an outer peripheral edge 72 of the
enclosure rear plate 62 facing the wall 69 is also
curved smoothly or tapered. The diaphragm 61 is driven
by four voice coils ~not shown) at the positions that
correspond to the nodes of the primary and secondary
normal resonance modes in the longitudinal direction
of the diaphragm.
In this system, air in the gap 67 between the
central portion 61b of the diaphragm 61 and the internal
plate 64 is compressed and expanded by the vibration
of the diaphragm 61 so as to cause air flows A in the
direction of progress of sound waves, as indicated
by the arr~ws A in Fig. 2A. These air flows A pass
through the duct 66 and the gap 68 which serves as an
acoustic duct communicating with the outside. When the
air flows A pass over the peripheral edges 70 and-71 of
the throat and the mouth of the duct 66 and over the
outer peripheral edge 72 of the enclosure rear plate
62a, there is no possibility of occurrence of vortexes.
Therefore, the occurrence of distortion due to wind noise
can be limited, thereby enabling high-fidelity repro-
duction.
Figs. 22A and 22B show sound pressure frequency
characteristic and a distortion frequency characteristic
of this speaker in which the sectional area of the duct
66 is 230 cm ; the height of the duct is 4 cm; the
curvature of each of the opening peripheral edges 70 and
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1 71 and the outer peripheral edge 72 is Rzo; and the
gap 68 defined between the speaker and the wall 69 when
the speaker is mounted on the wall is 1 cm. The
distortion frequency characteristic of a speaker having
angular portions 73 which have not been rounded is
indicated by the broken line in Fig. 22B. As is clear
from Fig. 22B, the level of distortion of this-speaker
is higher than that of this embodiment of the present
invention.
In the above-described embodiments, the
diaphragm and the driving units may be connected to each
other by using a fixing method or fixing structure which
will be described below with reference to Figs. 23
and 24. Fig. 23 is a cross-sectional view of essential
parts of a diaphragm and a voice coil which are fixed
to each other, and Fig. 24 is a side view of the voice
coil cap and voice coil shcwn in Fig. 23.
The structure shown in Figs. 23 and 24
includes a flat diaphragm 81, a voice coil cap 82
which has a flat end surface of a flange portion con-
nected to the surface of the flat diaphragm 81 and which
has a plurality of projections 84 formed on the outer
periphery of a cylindrical portion and inserted into
~lOuyS of slits 86 formed at one end of a voice oil
bobbin 83, and the voice coil bobbin 83 which is
inserted into a magnetic gap (not sh~wn3 and around
which a voice coil 85 is wound. The groups of slits 86
comprise a plurality of opposed pairs of slits formed
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1 at the end of the voice coil bobbin 83 to different
depths. It is preferable for the projections 84 to be
disposed at regular intervais.
The voice coil cap 82 is inserted into and ~
fixed to the voice coil bobbin 83. At this time, one
of the groups of slits 86 at the end of the voice coil
bobbin 83 having a depth which minimizes the gap
between the voice coil cap and the diaphragm is selected,
and the pro3ections 84 of the voice coil gap 82 are
inserted into a group of slits selected to the ends
thereof. The voice coil cap 82 is thereafter fixed to
the voice coil bobbin 83. The number of slits in each
group is three to four which is preferred in terms of
- balance. Accordingly, the total number of slits is
obtained by multiplying this number by the number of
steps of adjusting the depth. That is, if the number
of projections 84 is four and if three different depths
are provided, the total number of slits is 4 x 3 = 12.
The speaker thus constructed operates as
described below. A driving force is generated in
accordance with an audio current which flows through
the voice coil 85, and it is transmitted to the flat
diaphragm 81 via the voice coil bobbin 83 and the voice
coil cap 82, thereby generating sound.
In accordance with the present invention, as
described above, when the plurality of projections 84,
which are formed at one end of the cylindrical portion
of the voice coil cap having the flat end surface at the
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-
1 335308
1 other end, are inserted into one of the groups of slits
86 formed at the corresponding end of the voice coil
bobbin 83 so as to fix the voice coil cap to the voice
coil bobbin, a group of slits having a depth which
minimize the gap between the surface of the diaphragm
and the flat end surface of the voice coil cap can be
selected from the groups of slits 86. It is therefore
possible to minimize a gap between the surface of the
diaphragm and the flat end surface of the voice coil
1~ cap due to a tolerance of the length of the voice coil
bobbin and a tolerance of the position at which the
damper is attached to the voice coil bobbin. It is
thereby possible to prevent any abnormal noise such as
buzzing caused by such a gap, thereby realizing a speaker
improved in reliability and having good acoustic charac-
teristics.
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