Loudspeaker and a method for producing the same

Water-proofed natural pulp or organic synthetic fibers are with polyester-type fibers having a low melting point, and subjected to a paper fabrication process. The fabricated product is dried with hot air at a temperature higher than the melting point of the polyester-type fibers, thereby melt-bonding only intersections of the fibers without completely fusing the polyester-type fibers. The pressure of the hot air contributes to the formation of a predetermined shape. Thus, a water-proof diaphragm for a loudspeaker, having a large thickness, a low density, a high internal loss and a high stiffness, is obtained. By incorporating the thus formed diaphragm, a high-performance loudspeaker having a low distortion and a broad reproducing range is obtained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a loud speaker to be used for various 
acoustic apparatuses, and a method for producing the same. 
2. Description of the Related Art 
FIG. 1 is a half cross-sectional view showing a configuration for a typical 
loud speaker 20. FIG. 2 is an exploded perspective view showing details of 
the loud speaker 20. The same constituent elements are indicated by the 
same reference numerals in FIGS. 1 and 2. 
As shown in FIGS. 1 and 2, the loud speaker 20 includes a lower plate 3 
integral with a center pole 2, a magnet ring 4 provided on a bottom 
portion of the lower plate 3 so as to surround the center pole 2, and an 
upper plate 5 provided on an upper face of the magnet ring 4. The lower 
plate 3, the magnet ring 4, and the upper plate 5 are coupled to one 
another to constitute a magnet circuit 1. 
On an upper face of the upper plate 5, an inner periphery of the frame 6 is 
coupled. A gasket 7 and an outer periphery of a diaphragm 8 are attached 
to an outer periphery of the frame 6 using an adhesive. A voice coil 9 is 
coupled to an inner periphery of the diaphragm 8. 
A middle portion of the voice coil 9 is supported by an inner periphery of 
the damper 10, an outer periphery of the damper 10 being supported by the 
frame 6. A lower portion of the voice coil 9 is inserted into a magnetic 
gap 11 formed between the center pole 2 of the lower frame 3 and the upper 
frame 5 (which are included in the magnetic circuit 1) without being 
eccentric. Moreover, a dust cap 12 for preventing dust from entering the 
magnetic circuit 1 is provided on the upper side of a central portion of 
the diaphragm 8. 
It is preferable that the material constituting the diaphragm 8 has such 
properties as high elasticity, low density, and high internal loss for the 
following reasons. 
The high-frequency range resonance frequency of the diaphragm 8 increases 
as a specific elasticity E/.rho. (where E represents the elasticity 
modulus and .rho. represents the density) of the material constituting the 
diaphragm 8 increases, that is, as the elasticity modulus E increases and 
as the density .rho. decreases. Such a loud speaker is capable of 
reproducing sounds in a higher frequency range and therefore realizing a 
broader reproduction range. 
Moreover, the diaphragm 8 achieves a flatter frequency characteristic curve 
and a lower distortion rate as the internal loss of its material 
increases. 
In view of the above, a principal material used for the diaphragm 8 of the 
conventional loud speaker 20 is paper which is composed mainly of natural 
pulp such as wood pulp. This is because paper has an appropriate 
elasticity modulus and internal loss as well as low density, and therefore 
provides advantages that a diaphragm composed of a synthetic resin or a 
complex thereof cannot attain. 
On the other hand, the voice coil 9 is required to withstand a large input 
signal applied thereto. In order for a loud speaker to have good 
resistance for such a large input, the voice coil 9 is required to have an 
increased inflammability and heat resistance for the following reasons. 
When an input signal is applied to the voice coil 9, an electric current 
flows in a coil (not shown in FIG. 1 or 2) of the voice coil 9 so as to 
generate Joule's heat. The Joule's heat increases as the level of the 
input signal increases, thereby drastically raising the temperature of the 
voice coil 9. As a result, a bobbin (not shown in FIG. 1 or 2) around 
which the coil is wound may be burnt, or varnish which is used to couple 
the coil to the bobbin may deteriorate through softening, causing the coil 
to fall off the bobbin. 
FIG. 3 shows an exemplary configuration for a conventional voice coil 9 
designed so as to overcome the above-mentioned problem. The voice coil 9 
includes a bobbin 13 composed of a strip of a metal foil, e.g., aluminum, 
bent into a cylindrical shape. Kraft paper 14 is wound, for reinforcement 
and insulation, around an outer periphery of the voice coil 9 where a coil 
15 is not wound. The bobbin 13 is obtained by winding the voice coil 9 on 
a portion of the bobbin 13 where the kraft paper 14 is not wound. In this 
configuration, the coil 15 is directly wound on the metal foil 
constituting the bobbin 13, so that the metal foil functions to radiate 
the heat generated in the coil 15, thereby preventing elevation of 
temperature. 
Recently, there has been a trend for using metals such as aluminum or 
organic foams for the material of the diaphragm 8, instead of the 
above-mentioned paper. However, organic foams have low elasticity and 
cannot attain sufficient characteristics. On the other hand, a metal 
diaphragm has only a small internal loss and the weight thereof is large. 
Therefore, these substitute materials for paper are not optimum materials 
for diaphragms of loud speakers for use in acoustic apparatuses. 
There have been developed diaphragms for loud speakers made of materials 
consisting of inorganic fibers and/or organic synthetic fibers mixed with 
paper so as to improve the elasticity of the paper. However, the expected 
effect of improving the elasticity has not been attained. 
Furthermore, paper diaphragms tend to absorb, and therefore are generally 
susceptible to, moisture. For example, paper diaphragms are not 
appropriate for such applications as loud speakers to be attached on the 
doors of automobiles, which require a particularly good water-proofness. 
In order to solve this problem, diaphragms for loud speakers requiring a 
high degree of water-proofness have typically been produced by adhering 
water repellent on pulp fibers during fabrication, or impregnating the 
fabricated paper diaphragm with a synthetic resin solution so as to 
provide the paper with water-proof properties. 
Very recently, however, the loud speakers to be attached on the doors of 
automobiles have particularly been required to be sufficiently resistant 
against surfactants included in detergents for washing automobiles, e.g., 
car shampoos. The above-mentioned method of adhering water repellent on 
pulp fibers or impregnating the fabricated paper diaphragm with a 
synthetic resin solution cannot attain sufficient resistance against such 
surfactants. 
One solution to this problem has been proposed, according to which a 
water-proof synthetic resin film is laminated onto a surface of a paper 
diaphragm after the fabrication thereof. However, this creates a new 
problem of the need for specific jigs and equipment for attaching the 
synthetic resin film onto the paper diaphragm. 
In order to overcome the above-mentioned problems, Japanese Patent 
Publication No. 57-40718 describes a diaphragm produced by using a 
material including a principal material of short fibers, such as 
polyethylene, polypropylene, nylon, and polyacrylonitrile, or synthetic 
pulp obtained by fibrillating these fibers, and a subordinate material of 
fibers such as inorganic fibers, organic synthetic fibers, or natural 
fibers mixed in the principal material, subjecting the material to a 
paper-fabrication process, and melting the resultant complex synthetic 
pulp so as to mold it into a desired shape. This diaphragm has excellent 
environmental characteristics such as water-proofness. However, the 
diaphragm also has the three following problems. 
First, it is difficult to reduce the density of the obtained diaphragm 
because high-density inorganic fibers, e.g., carbon fibers, alumina 
fibers, and glass fibers, are mixed into the principal material in order 
to improve the elasticity of the molded product. 
Second, the synthetic pulp used for the above-mentioned diaphragm has 
relatively short fiber lengths and therefore has low freeness. As a 
result, the fabrication process takes a long time. 
Third, the synthetic pulp used for the above-mentioned diaphragm has a 
relatively high beating degree, and has relatively short fiber lengths, so 
that it is difficult to obtain a bulky product after a percolation 
process. Moreover, since the synthetic pulp is melted during the 
drying-molding process, the obtained molded product has a film-like shape, 
so that it is difficult to increase the thickness of the molded product 
and to adequately reduce the density and increase the internal loss 
thereof. 
On the other hand, in the conventional voice coil 9 shown in FIG. 3, the 
metal foil used for the bobbin 13, which is incorporated with a view to 
improving the heat resistance of the voice coil 9, has a large weight, 
thereby deteriorating the performance of the loud speaker. Moreover, since 
metals are good electrical conductors, the use of a metal foil for the 
bobbin 13 may cause a short-circuiting of the coil 15. 
Alternatively, a sheet composed of heat-resistant chemical fibers, such as 
paper composed of aromatic polyamide fibers, e.g., aramid paper or NOMEX 
paper (manufactured by Du Pont Ltd.) is occasionally used for the bobbin 
13 of the voice coil 9. However, such paper slightly absorbs moisture. As 
a result, when the temperature of the voice coil 9 rapidly increases, the 
moisture absorbed in the paper is gasified so that swelling may occur in a 
portion of the bobbin 13 where the coil 15 is wound. Furthermore, it is 
difficult for the bobbin 13 as described above to be completely severed. 
Thus, a portion of one or more of the aromatic polyamide fibers may be 
left at the severed surface. These fibers may also remain in a plumous 
state on the surface of the sheet. In either case, such portions of the 
aromatic polyamide fibers can cause extraordinary noises during the 
operation of the loud speaker, thus deteriorating the quality of the loud 
speaker. 
Furthermore, as described above in connection with the diaphragm 8, loud 
speakers to be attached on the doors of automobiles are required to be 
particularly water-proof, so that the voice coil 9 is also required to 
have an improved water-proofness as well as the diaphragm 8. 
SUMMARY OF THE INVENTION 
A loudspeaker of the invention includes: a magnetic circuit portion 
including a magnetic gap; a frame coupled to an upper face of the magnetic 
circuit portion; a diaphragm, an outer periphery thereof being attached to 
an outer periphery of the frame; and a voice coil coupled to an inner 
periphery of the diaphragm, the voice coil being inserted into the 
magnetic gap, wherein the diaphragm is formed by mixing water-proofed 
natural pulp or organic synthetic fibers as a principal material with 
polyester-type fibers having a low melting point as a subordinate 
material, only intersections among the fibers being melt-bonded. 
A method for producing a loudspeaker of the invention includes: a beating 
step for obtaining a principal material of natural pulp or organic 
synthetic fibers; a mixing step for mixing the principal material with 
polyester-type fibers having a low melting point, and further with a water 
repellent to be affixed thereto; a paper fabrication step for subjecting a 
slurry obtained in the mixing step to a paper fabrication process; a 
forming step for drying the fabricated slurry by being heated with hot air 
at a predetermined temperature higher than the melting point of the 
polyester-type fibers, and forming the fabricated slurry into a 
predetermined shape; a trimming step for conducting a trimming process so 
as to obtain the diaphragm; and a fabricating step for forming a 
loudspeaker using the obtained diaphragm. 
In one embodiment, the melting point of the polyester-type fibers is in the 
range from about 120.degree. to about 180.degree. C. 
In another embodiment, a thickness of the polyester-type fibers is in the 
range of about 0.5 to about 5 deniers. 
In still another embodiment, a fiber length of the polyester-type fibers is 
in the range of about 1 to about 15 mm. 
In still another embodiment, the polyester-type fibers are mixed in an 
amount of about 1 to about 50% by weight based on the principal material. 
According to another aspect of the invention, a loudspeaker includes: a 
magnetic circuit portion including a magnetic gap; a frame coupled to an 
upper face of the magnetic circuit portion; a diaphragm, an outer 
periphery thereof being attached to an outer periphery of the frame; and a 
voice coil coupled to an inner periphery of the diaphragm, the voice coil 
being inserted into the magnetic gap, wherein the diaphragm is formed of a 
molded product obtained by dry-molding a slurry of a principal material of 
water-repellentized natural pulp mixed with water-proof synthetic pulp 
having a minute film-like shape, a water-repellent synthetic resin film 
being disposed on a surface of the molded product. 
A method for producing a loudspeaker of the invention includes: a beating 
step for obtaining a principal material of natural pulp or organic 
synthetic fibers; a mixing step for mixing the principal material with 
water-proof synthetic pulp having a minute film-like shape, and further 
with a water repellent to be affixed thereto; a paper fabrication step for 
subjecting a slurry obtained in the mixing step to a paper fabrication 
process; a forming step for drying the fabricated slurry by being heated, 
thereby obtaining a molded product having a predetermined shape; an 
immersion step for impregnating the molded product with an organic resin 
solution mixed with a water repellent, and drying, thereby forming a 
water-repellent synthetic resin film on a surface of the molded product; a 
trimming step for conducting a trimming process so as to obtain the 
diaphragm; and a fabricating step for forming a loudspeaker using the 
obtained diaphragm. 
In one embodiment, the synthetic pulp is formed of a meta-type aramid 
resin. 
In another embodiment, the diaphragm is further mixed with water-proof 
fibers. The water-proof fibers are mixed into the principal material in 
the mixing step. Preferably, the water-proof fibers are polyester-type 
fibers. 
According to still another aspect of the invention, a loudspeaker includes: 
a magnetic circuit portion including a magnetic gap; a frame coupled to an 
upper face of the magnetic circuit portion; a diaphragm, an outer 
periphery thereof being attached to an outer periphery of the frame; and a 
voice coil coupled to an inner periphery of the diaphragm, the voice coil 
being inserted into the magnetic gap, wherein the voice coil further 
includes: a cylindrical bobbin formed of a sheet obtained by mixing 
water-proof heat-resistant synthetic pulp having a minute film-like shape 
with inorganic fillers and water-proof heat-resistant synthetic fibers and 
then subjecting to a paper fabrication process and to a pressure-heating 
process using a calender; and a coil wound on an outer surface of at least 
a portion of the bobbin. 
A method for producing a loudspeaker of the invention includes the steps 
of: mixing water-proof heat-resistant synthetic pulp having a minute 
film-like shape with inorganic filler and water-proof heat-resistant 
synthetic fibers; subjecting the mixed synthetic pulp to a paper 
fabrication process; forming a sheet by subjecting the fabricated 
synthetic pulp to a pressure heating process using a calender; forming a 
cylindrical bobbin using the sheet; forming a voice coil using the bobbin; 
and fabricating a loudspeaker using the obtained voice coil. 
In one embodiment, the synthetic pulp is formed of a meta-type aromatic 
polyamide. 
In another embodiment, the synthetic fibers are short fibers formed of a 
para-type aromatic polyamide. 
In still another embodiment, a thickness of the sheet after being processed 
by the calender is in the range from about 30 to about 500 .mu.m. 
In still another embodiment, a bulk density of the sheet after being 
processed by the calender is in the range from about 0.6 to about 1.5 
g/cm.sup.3. 
Thus, the invention described herein makes possible the advantages of (1) 
providing a loud speaker including a diaphragm having a high elasticity 
modulus and excellent water-proofness and/or a light-weight voice coil 
having excellent inflammability, water-proofness, and adhesion, the loud 
speaker therefore being capable of withstanding a large input; and (2) a 
method for producing the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described by way of examples, 
with reference to the accompanying figures. 
(EXAMPLE 1) 
FIG. 4 is a half cross-sectional view showing a configuration for a loud 
speaker 120 produced according to a first example of the present 
invention. 
As shown in FIG. 4, the loud speaker 120 includes a lower plate 103 
integral with a center pole 102, a magnet ring 104 provided on a bottom 
portion of the lower plate 103 so as to surround the center pole 102, and 
an upper plate 105 provided on an upper face of the magnet ring 104. The 
lower plate 103, the magnet ring 104, and the upper plate 105 are coupled 
to one another to constitute a magnet circuit 101. 
On an upper face of the upper plate 105, an inner periphery of the frame 
106 is coupled. A gasket 107 and an outer periphery of a diaphragm 108 are 
attached to an outer periphery of the frame 106 by using an adhesive. A 
voice coil 109 is coupled to an inner periphery of the diaphragm 108. 
A middle portion of the voice coil 109 is supported by an inner periphery 
of the damper 110, an outer periphery of the damper 110 being supported by 
the frame 106. A lower portion of the voice coil 109 is inserted into a 
magnetic gap 111 formed between the center pole 102 of the lower frame 103 
and the upper frame 105 (which are included in the magnetic circuit 101) 
without being eccentric. Moreover, a dust cap 112 for preventing dust from 
entering the magnetic circuit 101 is provided on the upper side of a 
central portion of the diaphragm 108. 
FIG. 5 is a half cross-sectional view showing the diaphragm 108. 
Hereinafter, a method for producing the diaphragm 108 will be described 
with reference to a flow chart shown in FIG. 6. 
First, in a beating step 610, an un-bleached kraft pulp (hereinafter 
referred to as "UKP") having a freeness (Canadian Freeness: as measured by 
the Canadian standard freeness measuring apparatus) of 550 cc is beaten so 
as to give a slurry of UKP. The UKP functions as a principal material. 
Next, in a mixing step 620, predetermined additives such as a reinforcement 
material, a dye, and a binder are mixed with the UKP slurry thus obtained. 
Specifically, in the present example, modified polyester fibers (melting 
point: about 120.degree. C. to about 180.degree. C.; fiber length: about 1 
to about 15 mm; thickness: about 0.5 to about 5 deniers) are added in an 
amount of about 1% to about 50% by weight based on the absolute dry weight 
of the UKP. Typically, modified polyester fibers having a melting point of 
130.degree. C., a fiber length of 5 mm, and a thickness of 2 deniers are 
added in an amount of 10% by weight based on the absolute dry weight of 
the UKP. Furthermore, a fluorine-type water repellent is added in an 
amount of about 0.05% to about 0.5%, e.g., 0.1%, by weight based on the 
absolute dry weight of the UKP so as to obtain a mixture to be subjected 
to a paper-fabrication process. The modified polyester fibers function as 
a subordinate material. As the fluorine-type water repellent, a product 
designated as "DICGUARD F-400" (manufactured by Dainippon Ink and 
Chemicals, Inc.) can be used, for example. 
Furthermore, in a molding step 630, that is, in a paper-fabrication step, 
the above-mentioned mixture (slurry) is subjected to a paper-fabrication 
process by using a screen formed into a desired shape of the diaphragm 
108, e.g., a conical shape, and is dehydrated. 
Then, in a forming step 640, the fabricated product is dried by being 
heated with pressurized hot air at a temperature higher than the melting 
point of the polyester-type fibers, e.g., 220.degree. C., for about 40 
seconds. Thus, only intersections of the fibers are fuse-bonded without 
completely fusing the polyester-type fibers. The pressure of the hot air 
contributes to the formation of a predetermined shape. 
Finally, the molded diaphragm is subjected to a trimming process in a 
trimming step 650 so as to have predetermined inner and outer shapes. 
Thus, the diaphragm 108 having a predetermined shape, for example, a 
conical shape with a diameter of 120 mm and a weight of 2.2 g is obtained. 
The diaphragm 108 obtained in the above-mentioned manner has a freeness of 
670 cc, an elasticity modulus of 4.times.10.sup.9 N/cm.sup.2, an internal 
loss (tan.delta.) of 0.067, a thickness of 0.73 mm, and a density of 0.052 
g/cm.sup.3. 
For comparison, a conventional paper diaphragm was produced by subjecting 
the above-mentioned UKP having a freeness of 550 cc to a paper-fabrication 
process and a heat-press molding by using a mold maintained at 180.degree. 
C. with a pressure of 2 kg/cm.sup.2 applied, the diaphragm having the same 
shape and diameter as the diaphragm 108 of the present example. 
Measurement of the characteristics of this conventional paper diaphragm 
revealed an elasticity modulus of 1.4.times.10.sup.9 N/cm.sup.3, an 
internal loss (tan.delta.) of 0.035, a thickness of 0.36 mm, and a density 
of 0.067 g/cm.sup.3. 
A diaphragm, as a constituent element of a loud speaker, is subjected to 
long-term use, and is preferably required to have a large strength 
(stiffness). In general, in order to improve the stiffness of a diaphragm, 
the molded diaphragm is required to be sufficiently thick. The diaphragm 
108 produced according to the present example has a thickness of 0.73 mm, 
thereby achieving a thick diaphragm 108 with a large stiffness in spite of 
its small weight. 
On the other hand, the above-mentioned conventional diaphragm has only a 
thickness of 0.36 mm, that is, it is difficult to increase the stiffness 
of the conventional diaphragm by increasing the thickness thereof. 
Although the stiffness of such a diaphragm may be increased by increasing 
the density thereof, there is an adverse effect in that the high-frequency 
range resonance frequency of a loud speaker incorporating the diaphragm 
lowers as the density of the diaphragm increases, thereby narrowing the 
range of frequencies reproducible by the loud speaker. 
An observation of the surface and the interior of the diaphragm 108 
obtained in the present example with a scanning electron microscope has 
revealed that the modified polyester fibers are present as if stitching 
through the UKP fibers. The UKP fibers and the modified polyester fibers 
are integrated with each other by being completely fused at the 
intersections thereof. Moreover, the intersections of the modified 
polyester fibers themselves are also fuse-bonded. As a result, the 
modified polyester fibers constitute a three-dimensional net-like 
structure present in the interspaces between the UKP fibers. The diaphragm 
108 of the present example retains its shape owing to such interfusion 
between fibers. 
FIG. 7 is a graph showing the sound pressure level (S.P.L.)-frequency 
characteristics (solid line a) of a loud speaker incorporating the 
diaphragm 108 of the present example and the S.P.L.-frequency 
characteristics (broken line b) of a loud speaker incorporating the 
conventional diaphragm 8 (shown in FIG. 1). The S.P.L. was measured with a 
microphone placed apart from the tested loudspeaker by 1 m. As seen from 
FIG. 7, the loud speaker incorporating the diaphragm 108 of the present 
example is capable of reproducing a broader range of frequencies than the 
loud speaker incorporating the conventional diaphragm 8. 
Although UKP is used as the principal material in the above description, 
the principal material for the diaphragm is not limited thereto. For 
example, natural pulp such as wood, cotton, and linen, or organic 
synthetic fibers having a high elasticity modulus and a high melting 
point, e.g., an aromatic polyamide and highly crystalline vinylon. 
Regardless of the material to be used, the principal material is subjected 
to a water-proofing process by affixing a water repellent thereto. 
The low melting point polyester-type fibers used as the subordinate 
material preferably have a thickness of about 0.5 to about 5 deniers and a 
melting point of about 120.degree. C. to about 180.degree. C. This is 
because fibers having characteristics in the above-mentioned ranges do not 
completely fuse during the drying process and therefore are appropriate 
for the purpose of obtaining the above-mentioned structure where only the 
intersections are melt-bonded. 
In order to ensure that the subordinate material of modified polyester 
fibers constitute a three-dimensional net-like structure in the 
interspaces between the UKP fibers while maintaining a high elasticity, it 
is preferable to prescribe the fiber length of the polyester-type fibers 
to be about 1 to about 15 mm, the polyester-type fibers being mixed in an 
amount of about 1% to about 50% by weight based on the principal material. 
The freeness of the diaphragm increases as the content ratio of the 
subordinate material increases. 
As described above, the diaphragm of the present example is produced by 
mixing low-melting point polyester-type fibers having a relatively large 
fiber diameter and a long fiber length with natural pulp or organic 
synthetic fibers having a small density and then subjecting the mixture to 
a paper-fabrication process. Thus, a diaphragm having a high freeness is 
obtained by a relatively short fabrication process, whereby a bulky 
product is easily obtained. Furthermore, during the dry-heating step, hot 
air at a temperature higher than the melting point of the polyester type 
fibers is used, so as to fuse only the intersections of the fibers without 
completely fusing the polyester-type fibers. The pressure of the hot air 
contributes to the formation of a predetermined shape. Since the method 
for producing the diaphragm according to the present invention performs no 
pressure-drying using a heated mold, which would be performed in the case 
of producing a diaphragm through a common paper-fabrication process, a 
diaphragm having a large thickness, a small density, a high internal loss, 
and a high stiffness can be obtained. 
Thus, according to the present example, a diaphragm having a high 
elasticity modulus, a high internal loss, and a large thickness can be 
obtained. By employing this diaphragm, a loud speaker having small 
distortion and a broad reproducible frequency range can be obtained. 
Moreover, the polyester-type fibers themselves have an extremely low 
moisture absorption, so that a diaphragm with a sufficient mechanical 
strength can be obtained even if a water-immersion process is conducted, 
which is conducted for mixing the polyester-type fibers with natural pulp 
or organic synthetic fibers and for the paper-fabrication of the mixture. 
(EXAMPLE 2) 
FIG. 8 is a half cross-sectional view showing a configuration for a loud 
speaker 220 according to a second example of the present invention. FIG. 9 
is a half cross-sectional view showing a configuration for a diaphragm 
108A incorporated in the loud speaker 220. 
Since the configuration for the loud speaker 220 is basically the same as 
that of the loud speaker 120 of Example 1, which was described with 
reference to FIGS. 4 and 5, constituent elements in FIGS. 8 and 9 which 
also appear in FIGS. 4 and 5 are indicated by the same reference numerals. 
Consequently, the description thereof is omitted here. 
The loud speaker 220 differs from the loud speaker 120 with respect to the 
diaphragm 108A. Hereinafter, a method for producing the diaphragm 108A 
will be described with reference to a flow chart shown in FIG. 10. 
First, in a beating step 610, UKP having a freeness (Canadian Freeness) of 
550 cc is beaten so as to give a slurry of UKP. The UKP functions as a 
principal material. 
Next, in a mixing step 620, predetermined additives such as a reinforcement 
material, a dye, and a binder are mixed with the UKP slurry thus obtained. 
Specifically, in the present example, meta-type aramid resin pulp is first 
added in an amount of about 5% to about 20% by weight based on the 
absolute dry weight of the UKP. Typically, a product designated as "CONEX 
pulp" (manufactured by Teijin Ltd.) is added in an amount of 10% by weight 
based on the absolute dry weight of the UKP. Furthermore, a fluorine-type 
water repellent is added in an amount of about 2 to about 20 cc to an 
absolute dry weight of 100 g of the UKP. For example, 10 cc of "DICGUARD 
F-400" (manufactured by Dainippon Ink and Chemicals, Inc.) may be added to 
an absolute dry weight of 100 g of the UKP. After adding a dye to the 
resultant mixture and stirring the mixture, an aluminium sulfate is 
employed to adjust the pH of the slurry to be in the range of about 4.5 to 
about 5.0. Thus, the fluorine-type water repellent is affixed to the UKP. 
Furthermore, in a molding step 630, that is, in a paper-fabrication step, 
the above-mentioned mixture (slurry) is subjected to a paper-fabrication 
process by using a screen formed into a desired shape of the diaphragm 
108A, e.g., a conical shape, and is dehydrated. 
Then, in a forming step 640, the fabricated product is subjected to a 
heat-pressure-drying process by setting the product in a mold having the 
shape of the diaphragm 108A and preheated at about 160.degree. C. to about 
220.degree. C., e.g., 200.degree. C. Thus, the diaphragm 108A is obtained 
as a molded product having a predetermined shape. 
Next, in an immersion step 645, the molded product obtained in the forming 
step 640 is immersed in a pre-formulated immersion solution, so as to 
impregnate the product with the solution. Thereafter, the product 
impregnated with the solution is dried against wind at room temperature 
for about 10 minutes, and is further dried for about 10 minutes in an oven 
set at an appropriate temperature, e.g., 120.degree. C. Thus, a 
water-repellent synthetic resin film is formed on the surface of the 
molded product. 
The immersion solution is prepared by diluting 50 g of a saturated 
copolymer polyester resin solution, e.g., a product designated as 
"Polyester LP-011S50TO" (manufactured by Nippon Synthetic Chemical 
Industry, Co., Ltd.) with 200 cc of methyl ethyl ketone, adding 10 cc of a 
fluorine-type water repellent, e.g., a product designated as "SURFRON 
SR-137AR" (manufactured by SEIMI Chemical Co., Ltd.) to the resultant 
mixture, and then stirring the resultant mixture. 
Finally, the molded diaphragm is subjected to a trimming process in a 
trimming step 650 so as to have predetermined inner and outer shapes. 
Thus, the diaphragm 108A having a predetermined shape, for example, a 
conical shape with a diameter of 160 mm is obtained. 
In the above description, a meta-type aramid resin is mixed in a pulp 
material which includes natural pulp as a principal material and is 
water-proofed with a water-repellent. However, any other material which is 
water-proof synthetic pulp having a minute film-like shape may be mixed in 
the pulp material in the place of a meta-type aramid resin. 
Although all the water-repellents used for the purposes of pulp affixation, 
immersion, and addition of a synthetic resin in the above description are 
fluorine type, it is also applicable to use water-repellents of other 
kinds. 
Although a saturated modified polyester resin is used as the synthetic 
resin in the immersion step in the above description, any other material 
can be employed as long as the material sufficiently forms a film after 
being dried and does not degrade the paper diaphragm in terms of either 
the characteristics or the sound quality thereof. For example, an 
acryl-type resin may be employed. 
Although a water-repellent synthetic resin film is formed on the surface of 
the molded product (diaphragm) by immersion-based impregnation in the 
above description, the immersion step may be replaced by any other method 
as long as the molded product (diaphragm) is appropriately impregnated 
with the synthetic region so that a water-repellent synthetic resin film 
with an appropriate thickness is formed. In this respect, the 
above-mentioned immersion step 645 may be interpreted as an impregnation 
step. 
(EXAMPLE 3) 
A diaphragm according to a third example of the present invention is 
produced as follows. Since the configuration of the loud speaker of the 
present example is basically the same as those of the loud speakers 120 
(Example 1; FIG. 4) and 220 (Example 2; FIG. 8), the description thereof 
is omitted. Since the same flow chart described in Example 2 (FIG. 10) 
applies to the production process of the loud speaker of the present 
example, the description thereof is also omitted. 
First, in a beating step, UKP having a freeness (Canadian Freeness) of 550 
cc is beaten so as to give a slurry of UKP. The UKP functions as a 
principal material. 
Next, in a mixing step, predetermined additives such as a reinforcement 
material, a dye, and a binder are mixed with the UKP slurry thus obtained. 
Specifically, in the present example, modified polyester fibers (melting 
point: about 120.degree. C. to about 180.degree. C.; fiber length: about 1 
to about 15 mm; thickness: about 0.5 to about 5 deniers) are first added 
in an amount of about 1% to about 50% by weight based on the absolute dry 
weight of the UKP. Typically, modified polyester fibers having a melting 
point of 130.degree. C., a fiber length of 5 mm, and a thickness of 2 
deniers are added in an amount of 10% by weight based on the absolute dry 
weight of the UKP. Furthermore, meta-type aramid resin pulp is added in an 
amount of 5% to 20% by weight based on the absolute dry weight of the UKP. 
Typically, "CONEX pulp" (manufactured by Teijin Ltd.) is added in an 
amount of 10% by weight based on the absolute dry weight of the UKP. 
Furthermore, a fluorine-type water repellent is added in an amount of 
about 2 to about 20 cc to an absolute dry weight of 100 g of the UKP. For 
example, 10 cc of "DICGUARD F-400" (manufactured by Dainippon Ink and 
Chemicals, Inc.) may be added to an absolute dry weight of 100 g of the 
UKP. After adding a dye to the resultant mixture and stirring the mixture, 
an aluminium sulfate is employed to adjust the pH of the slurry to be in 
the range of about 4.5 to about 5.0. Thus, the fluorine type water 
repellent is affixed to the UKP. 
Thereafter, a molding step (a paper-fabrication step), a forming step, an 
immersion step, and a trimming step are conducted in the same manner as in 
Example 2, the descriptions thereof being omitted. Thus, a diaphragm, for 
example, having a conical shape with a diameter of 160 nm, is obtained. 
In the above description, low-melting point polyester fibers and a 
meta-type aramid resin are mixed in a pulp material which includes natural 
pulp as a principal material and is water-proofed with a water-repellent. 
However, any other material which is water-proof synthetic pulp having a 
minute film-like shape may be mixed in the pulp material in the place of a 
meta-type aramid resin. It is also applicable to use, in the place of 
polyester fibers, any other material which has good comformability with 
pulp and appropriate water-proofness, e.g., aramid fibers. There is no 
limitation to the shape of the fibers to be mixed, either. 
Although all the water-repellents used for the purposes of pulp affixation, 
immersion, and addition of a synthetic resin in the above description are 
fluorine type, it is also applicable to use water-repellents of other 
kinds. 
Although a saturated modified polyester resin is used as the synthetic 
resin in the immersion step in the above description, any other material 
can be employed as long as the material sufficiently forms a film after 
being dried and does not degrade the paper diaphragm in terms of either 
the characteristics or the sound quality thereof. For example, an 
acryl-type resin may be employed. 
The following examination was conducted in order to examine the 
water-proofness of the diaphragms of Examples 2 and 3. A cylindrical water 
tank was placed behind each of loud speakers incorporating diaphragms 
produced according to Examples 2 and 3. The loudspeaker corresponds to a 
bottom face of the tank. Tap water or an aqueous solution of a 
commercially available detergent for washing automobiles, e.g., a car 
shampoo, diluted so as to have a concentration of 5% was poured into each 
tank so as to be 30 mm deep. The infiltration of the respective solutions 
toward a front face of each loud speaker, i.e., a front face of each 
diaphragm, was observed. 
In this water-proofness examination, neither the tap water or the car 
shampoo solution infiltrated through the diaphragms produced according to 
Examples 2 and 3 after a lapse of 96 hours, either to the surfaces or the 
side faces thereof. 
For comparison, two conventional diaphragms A and B were subjected to the 
same water-proofness examination, the conventional loud speakers being 
produced as follows: 
Conventional diaphragm A was produced by using a UKP slurry having a 
freeness of 550 cc, adding a fluorine-type water-repellent and a dye so as 
to be affixed thereto (in the same manner as in Example 2), subjecting the 
slurry to a paper-fabrication process by using a screen formed into the 
shape of a diaphragm, dehydrating the slurry, subjecting the slurry to a 
heat-pressure-drying process in a mold having the shape of the diaphragm 
and preheated at 200.degree. C. Thus, the diaphragm was obtained as a 
molded product having a conical shape with a diameter of 16 cm. 
Conventional diaphragm B was produced by using a UKP slurry having a 
freeness of 550 c, subjecting the slurry to a paper-fabrication process by 
using a screen formed into the shape of a diaphragm, dehydrating the 
slurry, subjecting the slurry to a heat-pressure-drying process in a mold 
having the shape of the diaphragm and preheated at 200.degree. C. The 
molded material with a diaphragm shape thus obtained was subjected to an 
immersion process as in Example 2, whereby the diaphragm was obtained as a 
molded product having a conical shape with a diameter of 16 cm. 
On conducting the same water-proofness examination for conventional 
diaphragms A and B thus obtained, it was observed that tap water 
infiltrated through conventional diaphragms A and B after a lapse of 24 to 
48 hours, both to the surfaces or the side faces thereof. Car shampoo was 
recognized to have infiltrated to the surfaces or the side faces thereof 
after a lapse of 1 hour. 
Moreover, the buckling strengths of the diaphragms produced according to 
Examples 1 and 2 and conventional diaphragms A and B were measured as 
follows. Each diaphragm was immersed in the above-mentioned car shampoo 
solution for 24 hours. Thereafter, each conical-shaped diaphragm was 
placed on a surface plate face down, the diaphragm being in a moistened 
state. A disk was placed on a neck portion of each diaphragm maintained in 
this state. Thus, a load was applied onto the disk in such a manner that 
the disk and the surface plate were kept parallel to each other. The load 
was gradually increased until reaching a value at which each diaphragm was 
destroyed, which value was defined as a buckling destruction strength. The 
same measurement was conducted for diaphragms, both conventional and 
according to the present invention, that were not immersed in car shampoo 
(hereinafter referred to as "non-immersed diaphragms"). 
Table 1 shows the measured buckling destruction strength values. Each of 
the reduction rates shown in Table 1 represents a rate by which the 
buckling destruction strength of each diaphragm decreased after immersion, 
with respect to the buckling destruction strength of the non-immersed 
diaphragm. 
TABLE 1 
______________________________________ 
Buckling strength 
Before After Reduction 
examination 
examination rate 
(kg) (kg) (%) 
______________________________________ 
Invention 3.44 1.22 64.5 
Example 2 
Invention 3.88 1.54 60.3 
Example 3 
Conventional 
3.29 0.76 76.9 
Example A 
Conventional 
3.38 0.81 76.3 
Example B 
______________________________________ 
As shown in Table 1, the diaphragms produced according to Examples 2 and 3 
of the present invention have smaller reduction rates of buckling 
destruction strength than the conventional diaphragms. Thus, the 
diaphragms according to the present invention have excellent 
water-proofness and maintain high buckling strength. As for Examples 2 and 
3, in comparison, the reduction rate of the diaphragm of Example 3 is 
smaller than that of the diaphragm of Example 2, indicating the superior 
water-proofness and high buckling strength of the diaphragm of Example 3. 
These are the advantages which result from the low-melting point and 
strong polyester fibers mixed in the material being fuse-bonded at 
intersections thereof during the heat-pressure-drying molding step, the 
fibers thus constituting a three-dimensional net-like structure. 
As described above, in accordance with the diaphragms produced according to 
Examples 2 and 3, a molded product is obtained by dry-molding a principal 
material of water-repellentized natural pulp, which is mixed with 
water-proof synthetic pulp having a minute film-like shape. Furthermore, 
the molded product is impregnated with a synthetic resin solution mixed 
with a water-repellent and is dried, so as to obtain a water-repellent 
synthetic resin film on the surface of the molded product. As a result, 
water is prevented from entering the diaphragm, so that the water 
absorption of the diaphragm is reduced without ruining the advantages of 
the paper diaphragm and without requiring specific jigs and equipment. In 
particular, the diaphragm has a sufficient resistance against surfactants. 
Moreover, since the water-proof synthetic pulp having a minute film-like 
shape adheres to the surface of the natural pulp fibers, such as wood 
pulp, so as to form a film thereon strongly entangled with the natural 
pulp, the diaphragm maintains a strong buckling strength even if water 
enters the inside of the diaphragm so as to moisten it. In addition, the 
diaphragm, although water-repellent and water-proofed, can be produced at 
a relatively low cost. By using the above-mentioned diaphragm, a high 
performance loud speaker having an excellent water-proofness can be 
obtained. 
(EXAMPLE 4) 
FIG. 11 is a half cross-sectional view showing a configuration for a loud 
speaker 420 according to a fourth example of the present invention. FIG. 
12 is a half cross-sectional view showing a configuration for a voice coil 
409 incorporated in the loud speaker 420. 
Since the configuration for the loud speaker 420 is basically the same as 
that of the loud speaker 120 of Example 1, which was described with 
reference to FIGS. 4 and 5, constituent elements in FIGS. 11 and 12 which 
also appear in FIGS. 4 and 5 are indicated by the same reference numerals. 
Consequently, the description thereof is omitted here. 
The loud speaker 420 differs from the loud speaker 120 with respect to the 
voice coil 409. Hereinafter, a configuration for the voice coil 409 will 
be described with reference to FIG. 12. 
A bobbin 413 included in the voice coil 409 is formed by using a sheet 
which includes water-proofed and heat-resistant synthetic pulp having a 
minute film-like shape as a principal material, the synthetic pulp 
exhibiting auto-fusion properties by pressure-heating. As the film-like 
synthetic pulp, pulp composed of a meta-type aromatic polyamide (aramid) 
may be used. 
An inorganic filler, such as mica, is mixed in the film-like synthetic pulp 
in an amount of about 20% to about 50%, and preferably about 35% to about 
50%, by weight. Furthermore, water-proof and heat-resistant synthetic 
fibers are mixed in the film-like synthetic pulp in an amount of about 5% 
to about 30%, and preferably about 15% to about 25%, by weight. As the 
synthetic fibers, short fibers composed of para-type aromatic polyamide 
may be used. 
The film-like synthetic pulp, in which the inorganic filler and the 
synthetic fibers are mixed in the above-mentioned manner, is subjected to 
a paper-fabrication process and a heat-pressure process by means of a 
calender so as to form a sheet to be used as the bobbin 413. The thickness 
of the sheet after the calender process is typically about 30 to about 500 
.mu.m. The bulk density is typically about 0.6 to about 1.5 g/cm.sup.3. 
Since the pulp composed of meta-type aromatic polyamide (aramid) exhibits 
auto-fusion properties by pressure-heating, a flame-resistant sheet with 
excellent thermal stability can be obtained by the inclusion of the 
inorganic filler, such as mica, in an amount of about 35% to about 50% by 
weight. 
By impregnating this sheet with about 15% to about 25% by weight of a 
thermosetting resin such as epoxy resin or phenol resin, the stiffness 
thereof can be improved. Thus, a light-weight bobbin 413 having excellent 
flame resistance, stiffness, and thermal stability can be obtained. 
By the above impregnation process, it becomes easy to cut the sheet 
constituting the bobbin 413. As a result, cross sections resulting from 
cutting the sheet are prevented from having burrs, and the aromatic 
polyamide fibers are prevented from remaining without being completely 
severed. Thus, fibers are prevented from projecting from the surface of 
the sheet, which may cause extraordinary noises during the operation of a 
loud speaker. 
If the impregnated amount of the thermosetting resin is less than about 15% 
by weight, the water-proofness and the stiffness of the sheet are 
deteriorated. If the impregnated amount of the thermosetting resin is more 
than about 30% by weight, the sheet becomes fragile. 
It is preferable that the meta-type aromatic polyamide pulp exists in an 
amount of about 10% to about 80% by weight. If the meta-type aromatic 
polyamide pulp content is less than about 10% by weight, the sheet does 
not attain sufficient strength. If the meta-type aromatic polyamide pulp 
content is more than about 80% by weight, the specific elasticity of the 
sheet becomes insufficient. 
It is preferable that the para-type aromatic polyamide short fibers exist 
in an amount of about 5% to about 30% by weight in the sheet. 
Mica is most preferable as the inorganic filler to be mixed in the sheet. 
Since the sheet is subjected to a paper-fabrication process, in 
particular, it is preferable to use mica grains having diameters smaller 
than about 16 mesh and larger than about 200 mesh as the principal 
material. If the inorganic filler content is less than about 35% by 
weight, the heat-resistance and stiffness of the sheet become slightly 
insufficient. If the inorganic filler content is more than about 50% by 
weight, the sheet surface has too large bumps and dents, which results in 
fragility in terms of the physical characteristics of the material. 
The paper to be used in the paper-fabrication process may be the usual 
round net type or long net type. 
Moreover, by performing a heat-press process with a calender after the 
paper-fabrication process, the specific elasticity and the water-proofness 
of the resultant sheet can be further improved. By the calender process, 
the auto-fusion properties of the meta-type aromatic polyamide (aramid) 
pulp are exhibited, and the adhesion of the mica is improved. Moreover, 
since the water-proofness of the sheet is also improved so that moisture 
is prevented from being absorbed into the voice coil 409 (bobbin 413), the 
generation of gas and/or voids due to an increase in the temperature of 
the voice coil 409 is reduced, thereby further improving the 
heat-resistance of the voice coil 409. 
The above-mentioned sheet has excellent water-proofness. Since the sheet 
has moderate bumps and dents on the surface thereof, it exhibits excellent 
adhesion. By cutting this sheet into strips and forming the strips into 
cylinders, the light-weight bobbin 413 having excellent flame resistance, 
stiffness, and thermal stability can be obtained. A coil portion 415 is 
formed by winding a heat-resistant magnet wire around the outer periphery 
of the bobbin 413. Reinforcement paper 414 is wound around the outer 
periphery of the bobbin 413 excluding the coil portion 415 for 
reinforcement and insulation. Thus, the voice coil 409 shown in FIG. 12 is 
obtained. 
By forming the reinforcement paper 414 of the same material as that of the 
bobbin 413 instead of kraft paper, the above-mentioned advantages of 
light-weight, flame resistance, stiffness, and thermal stability of the 
voice coil 409 (bobbin 413) can be further improved. 
The voice coil 409 thus produced has excellent heat-resistance and 
stiffness, and yet has a small weight. By incorporating the voice coil 409 
into a loud speaker, the bobbin 413 is prevented from being burnt and the 
coil 415 is prevented from falling off the bobbing 413, thereby providing 
a loud speaker having a stably excellent performance. 
Tables 2 and 3 shown below indicate typical measurement values of the 
physical characteristics, e.g., thermal shrinkage, of three sheets to be 
used for the bobbin according to the present invention and a conventional 
material for a bobbin composed only of aromatic polyamide fibers. The 
three sheets to be used for the bobbin according to the present invention 
are all obtained by using aromatic polyamide fibers, mica powder, and 
phenol resin in three different content ratios as shown in Tables 2 and 3. 
These data were measured by a common method and the detailed description 
of the measurement method itself is omitted here. 
TABLE 2 
______________________________________ 
(The following thermal shrinkage data are measured by 
leaving the sheet in atmospheres at the respective 
temperatures for 30 minutes.) 
Invention 
Invention 
Invention 
Conven- 
1 2 3 tional 
______________________________________ 
Composition 
aromatic polyamide 
70 60 50 100 
fibers (wt %) 
mica powder 30 40 50 -- 
(wt %) 
phenol resin* 
20 20 20 -- 
(wt %) 
Thermal 
Shrinkage 
200.degree. C. 
0.1% 0.1% 0.1% 0.5% 
or less or less or less 
250.degree. C. 
0.2% 0.1% 0.1% 1.0% 
or less or less 
300.degree. C. 
0.2% 0.2% 0.1% 1.8% 
or less 
350.degree. C. 
0.5% 0.4% 0.2% 5.0% 
400.degree. C. 
2.0% 1.5% 1.0% -- 
______________________________________ 
TABLE 3 
______________________________________ 
Invention 
Invention 
Invention 
Conven- 
1 2 3 tional 
______________________________________ 
Composition 
aromatic polyamide 
70 60 50 100 
fibers (wt %) 
mica powder 30 40 50 -- 
(wt %) 
phenol 20 20 20 -- 
resin (wt %)* 
Physical 
Characteristics 
thickness (mm) 
0.134 0.132 0.130 0.131 
density (g/cm.sup.2) 
0.88 0.95 1.02 0.86 
elastic modulus 
8.50 8.90 9.15 6.50 
(.times. 10.sup.10 dyn/cm.sup.2) 
specific elastic 
3.31 3.14 2.97 2.96 
modulus 
(.times. 10.sup.5 cm/sec) 
internal 3.05 3.50 3.45 2.60 
loss (.times. 10.sup.-2) 
______________________________________ 
In Tables 2 and 3, the content of the phenol resin (*) is indicated as 
percent by weight when the entire sheet is defined as 100. 
As seen from Table 2, the sheets according to the present invention have 
excellent heat resistance and good dimensional stability. Therefore, by 
using any of these sheets for a voice coil incorporated in a loud speaker 
for receiving a large input, the voice coil can achieve sufficient 
characteristics. In addition, as seen from Table 3, the sheets of the 
present invention all have light weight and excellent stiffness, as well 
as good water-proofness. 
It is also applicable to form a metal powder layer on the surface of the 
outer surface of the bobbin 413 of the voice coil 409 shown in FIGS. 11 
and 12 by vapor-depositing metal powder composed of light-weight 
non-magnetic material, e.g., aluminum, on the surface and thereafter 
coating resin on the surface. Alternatively, a metal powder layer may be 
formed by coating resin mixed with the above-mentioned metal powder on the 
surface. By adopting such a configuration including a metal powder layer, 
the heat radiation properties of the voice coil can be further improved, 
thereby preventing the temperature increase more effectively. As a result, 
a loud speaker incorporating the voice coil can have its resistance 
against large input signals improved by about 10% to about 15% as compared 
with the case where no such metal powder layer is included. 
Thus, the voice coil for a loud speaker according to the present example is 
formed by using as a bobbin a sheet which is obtained by subjecting 
water-proof and heat-resistant synthetic pulp having a minute film-like 
shape and exhibiting auto-fusion properties by a pressure-heating or a 
calender process, e.g., aromatic polyamide pulp, mixed with inorganic 
fillers and water-proof heat-resistant synthetic fibers, to a 
paper-fabrication process and subjecting the synthetic pulp to a 
heat-press process by means of a calender. As a result, a light-weight, 
water-proof and heat-resistant voice coil is obtained which is capable of 
sufficiently withstanding a large input signal applied thereto. 
Furthermore, by using the thus fabricated voice coil having 
flame-resistance, the risk of ignition and possible combustion is reduced, 
thereby providing for safety. Moreover, the voice coil has improved 
water-proofness, so that it will exhibit stably high performance even when 
applied to uses such as loud speakers for the doors of automobiles, where 
water-proofness is a strong requirement. 
The above described loud speaker according to the present invention 
incorporates a diaphragm having high internal loss and large stiffness, 
and therefore is capable of sound reproduction with little distortion in a 
broad range of frequencies as well as having improved water-proofness. By 
incorporating the heat-resistant voice coil of the present invention, the 
loud speaker is made capable of withstanding a large input signal applied 
thereto. Thus, a high-performance loud speaker having excellent 
flame-resistance and water-proofness can be provided. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.