Loudspeaker with burning resistant diaphragm

The invention discloses a loudspeaker with a diaphragm made of pulp fibers treated with a chemical condensation product of an ammonium salt of a polyphosphoric acid having a high degree of condensation with polyethyleneimine. The diaphragm has burning resistance, and the deterioration of acoustic characteristics may be prevented.

BACKGROUND OF THE INVENTION 
The present invention relates to a loudspeaker with a diaphragm made of 
pulp fibers combined with a burning resistant agent. 
The diaphragms of the conventional speakers are, in general, made of pulp 
fibers so that there is a fear that the voice coil and the diaphragm are 
burnt due to overheating of the voice coil when an excessive input is 
applied thereto. To overcome this problem, there has been devised and 
demonstrated a speaker diaphragm of the type in which a burning resistance 
layer is formed upon the surface of the diaphragm. However, the mass of 
the diaphragm is increased so that the satisfactory characteristics of the 
loudspeaker cannot be attained. Another attempt has been made to render 
the diaphragm burning resistance by impregnating the pulp fibers with a 
water soluble inorganic burning resisting agent such as ammonium secondary 
phosphate. However, the diaphragm exhibits a moisture absorbing property 
causing the corrosion of metal parts of the speaker so that such diaphragm 
is not adapted for use in the speaker. There has been a further attempt 
for rendering the pulp fibers burning resistant by treating them with an 
organic agent but the treatment cost is very expensive. Furthermore much 
smoke is produced when the diaphragm made of such pulp fibers should be 
burnt. 
The present invention was made to overcome the above problems.

DETAILED DESCRIPTION 
According to the present invention, pulp fibers are treated by the chemical 
reaction between water-insoluble highly condensed ammonium polyphosphate 
and highly active polyethyleneimine, and thereafter a neoprene series 
rubber latex is deposited or set upon the pulp fibers. The diaphragm of 
the loudspeaker of the present invention is made of these pulp fibers 
treated in the manner described above. 
The treated fibers in the diaphragm exhibit not only the improved physical 
strength properties such as swelling resistance, size degree, tearing 
strength and so on, but also are stable and have improved burning 
resistance. Furthermore since water-insoluble highly condensed ammonium 
polyphosphate is used, the poor moisture absorption property of and 
corrosion of metal parts of the loudspeakers by the conventional speaker 
diaphragms made of pulp impregnated with a burning resistance agent, are 
solved. 
When water-insoluble highly condensed ammonium polyphosphate is mixed with 
pulp fibers under a mechanical force, for example, by stirring the yield 
of the ammonium polyphosphate deposited on the pulp fibers may be lowered 
and the burning resistance properties of the paper prepared from the 
treated pulp fibers is also reduced due to the destruction of 
inter-molecular linkages of the particles of the ammonium polyphosphate 
and to the isolation of the particles of the ammonium polyphosphate from 
the pulp fibers. 
Furthermore, when the pH of the mixture of pulp fibers and the ammonium 
polyphosphate is not adjusted up to about pH 4 - 5, the ammonium 
polyphosphate quickly coagulates to form a heterogeneous gel in the form 
of flock which makes the mixture viscous and results in paper making 
operations difficult. 
In order to overcome such drawbacks, aluminum sulfate can be added to the 
mixture of pulp fibers and water-insoluble highly condensed ammonium 
polyphosphate for adjusting the pH of the mixture up to pH 4 - 5 to 
disperse the particles of the ammonium polyphosphate moderately and 
homogeneously and also the effect the fixation of particles of the 
ammonium polyphosphate on to the pulp fibers. 
Neoprene series rubber latex is further added to enforce the tight binding 
of the water-insoluble highly condensed ammonium polyphosphate and the 
pulp fiber. 
The water-insoluble highly condensed ammonium polyphosphate used in the 
present invention is a composition mainly consisting of an ammonium 
polyphosphate having the general formula (NH.sub.4).sub.n+2 P.sub.n 
O.sub.3n+1 (wherein n = 150 -200) represented by the following structural 
formula: 
##STR1## 
The phosphorus and nitrogen contents in the ammonium polyphosphate can be 
of P = 31 -32% by weight and N = 14 -16% by weight respectively. The 
water-insoluble highly condensed ammonium polyphosphate is added in the 
range of 30 - 40% by weight of the pulp fibers to give burning resistance 
property of the paper prepared therefrom. 
The neoprene series rubber latex is added in the range of 5 -6% by weight 
of the pulp fibers. 
The polyethyleneimine used in the present invention is an aqueous solution 
of polyethyleneimine having the general formula 
##STR2## 
and is added in the range of 2 - 3% by weight of the pulp fibers. 
Next referring to FIG. 1, one preferred embodiment of a loudspeaker in 
accordance with the present invention will be described. An external field 
system 1 comprises a plate 3 with a central pole piece 2, a ring-shaped 
magnet 4, and a ring-shaped upper plate 5. A frame 6 is attached to the 
field system 1, and a diaphragm 8 made of the material described above has 
its periphery attached to the frame 6 with a gasket 7. From the center 
aperture of the diaphragm 8 is suspended a coil bobbin 10 around which is 
wound a voice coil 9 and which is also supported by a damper 11. A dust 
cap 12 is attached to the diaphragm 8 to cover the center aperture 
thereof. 
Next some examples of the treatment of pulp fibers for loudspeaker 
diaphragms in accordance with the present invention will be described. 
EXAMPLE 1 
(Partial Treatment of Pulp Fibers) 
Pulp fibers were stirred in a beater and 15 - 35% by weight of Sumisafe PM* 
was added to the pulp fibers under stirring slowly. The pulp fibers were 
coagulated in the form of flock. 
FNT * Sumisafe PM -- a trademark for a water-insoluble highly condensed 
ammonium polyphosphate, manufactured by Sumitomo Chemical Company, Ltd., 
Osaka, Japan, having the following properties: Appearance: White powder: 
Phosphorus content: 20.42% wt. Nitrogen content: 32.21% wt. Solubility in 
water: Less than 5% wt. (Insoluble matters = more than 95% wt.) Paticle 
size: 98% by weight passed through 325 mesh sieve: Polymerization degree: 
150 - 200. Further, 2 - 3% by weight of Epomin P-1000** was added to the 
mixture. 
FNT ** Epomin P-1000 -- a trademark for polyethyleneimine manufactured by 
Nippon Shokubai Kagaku Kogyo Co., Ltd., Tokyo, Japan, having the following 
properties: Appearance: Transparent viscous aqueous solution: Ionic 
characteristics: Cathionic: Sp. gr.: 1.0 at 25.degree. C: Thermal 
decomposition temperature: 200.degree. - 250.degree. C: Solidification 
point: -3.degree. C. 
The yield of the water-insoluble highly condensed ammonium polyphosphate 
based on the pulp fibers was 70 - 80% by weight in this step. However, 
when the pulp fibers were uniformly dispersed by additional mixing, the 
yield was decreased to less than 50%. The results are shown in Table 1. As 
can be seen from the table, 25 - 30% by weight of highly condensed 
ammonium polyphosphate must be added for satisfying burning resistance and 
self-extinguishing properties of the paper. 
When the mixture was stirred until the pulp-fibers become uniformly 
dispersed, the burning resistance property of the paper was decreased, and 
the self-extinguishing property was lost even when 35% by weight of the 
water-insoluble highly condensed ammonium polyphosphate was added. Young's 
modulus of the paper was reduced by about 40% as compared with paper 
prepared from the pulp fibers without the treatment of the present 
invention, and the size degree of the paper was zero. A length of scorched 
area was measured by a method for blowing a paper by a gas burner of the 
type burning a gas mainly consisting of methane gas. 
Table 1 
__________________________________________________________________________ 
Ratio of addition in Yield of highly 
% of highly condensed condensed ammonium 
Length of 
ammonium polyphosphate 
Density 
Young's polyphosphate 
scorched 
(based on pulp 
g/cm.sup.3 
modulus (based on pulp 
area 
fibers = 100) 
.rho. 
E E/.rho. 
fibers = 100) 
cm 
__________________________________________________________________________ 
0% 0.383 
1.26 .times. 10.sup.10 
3.29 .times. 10.sup.7 
-- -- 
15% 0.390 
0.85 .times. 10.sup.10 
2.17 .times. 10.sup.7 
78% -- 
20% 0.354 
0.87 .times. 10.sup.10 
2.47 .times. 10.sup.7 
88% -- 
25% 0.352 
0.95 .times. 10.sup.10 
2.67 .times. 10.sup.7 
73% 10 
30% 0.349 
0.93 .times. 10.sup.10 
2.66 .times. 10.sup.7 
78% 6.5 
35% 0.369 
0.87 .times. 10.sup.10 
2.36 .times. 10.sup.7 
76% 4.5 
__________________________________________________________________________ 
EXAMPLE 2 
(Complete Treatment of Pulp Fibers) 
Pulp fibers were stirred in a beater and 20 - 30% by weight of Sumisafe PM 
was added to the pulp fibers under stirring slowly. Further, 2 - 3% by 
weight of Epomin P-1000 was added under stirring to the mixture. Then 
aluminum sulfate was added to the above mixture under stirring to adjust 
the pH of the mixture up to 4 - 5. Thereafter, 5 - 6% by weight of 
Neoprene Latex 736*** was added under stirring and the whole mixture was 
sufficiently mixed until the pulp fibers were uniformly dispersed. 
FNT *** Neoprene Latex 736 -- a trademark for a neoprene series rubber latex 
manufactured by Showa Neoprene Co., Ltd., Tokyo, Japan, having the 
following properties: Content of solid matter: 34.5% by weight: pH: 11.5: 
Sp. gr.: 1.06. 
The yield of the water-insoluble highly condensed ammonium polyphosphate 
based on the pulp fibers and this physical properties of the papers thus 
prepared are shown in Table 2. 
Table 2 
__________________________________________________________________________ 
Ratio of addition in Yield of highly 
% of highly condensed condensed ammonium 
Length of 
ammonium polyphosphate 
Density 
Young' polyphosphate 
scorched 
(based on pulp 
g/cm.sup.3 
modulus (based on pulp 
area 
fibers = 100) 
.rho. 
E E/.rho. 
fibers = 100) 
cm 
__________________________________________________________________________ 
0% 0.343 
1.03 .times. 10.sup.10 
3.0 .times. 10.sup.7 
-- -- 
20% 0.357 
1.07 .times. 10.sup.10 
3.0 .times. 10.sup.7 
81% -- 
26% 0.326 
0.98 .times. 10.sup.10 
3.0 .times. 10.sup.7 
75% 7.5 
30% 0.333 
1.10 .times. 10.sup.10 
3.3 .times. 10.sup.7 
73% 5.0 
__________________________________________________________________________ 
The stability of burning resistance as well as Young's modulus may be 
improved by the addition of 5 to 6% of neoprene series rubber latex. 
Furthermore, swelling resistance, water resistance and tearing strength 
may also be improved. Therefore the diaphragms made of the materials 
prepared in accordance with the present invention are highly reliable. The 
results are shown in Table 3. 
Table 3 
______________________________________ 
Ratio of addition 
of neoprene 
(based on pulp Swelling Tearing 
fibers = 100) resistance Strength 
______________________________________ 
0% 100 100 
2% 122 110 
5% 150 123 
10% 175 145 
______________________________________ 
The pulp fibers which are treated in the manner described above may be made 
into the diaphragms by the conventional paper making method. 
Alternatively, the sheet of paper made of these pulp fibers may be pressed 
into a diaphragm. 
The frequency characteristic curve of the speaker with the diaphragm made 
of the pulp fibers treated in the manner described in Example 1 is 
indicated by the solid line curve in FIG. 2. Since the Young's modulus is 
smaller than that of the diaphragm made of the pulp fibers not treated, 
the response at higher frequencies is not satisfactory even though the 
response at lower frequencies is satisfactory. The frequency response 
curve of the loudspeaker with the diaphragm made of the fibers not treated 
is shown by the dotted curve in FIG. 2. 
The frequency response curves of the loudspeakers with the diaphragms made 
of the pulp fibers treated in the manner described in Example 2 and the 
pulp fibers not treated, respectively, are indicated by the solid and 
dotted curves, respectively, in FIG. 3. Young's modulus of the former is 
substantially equal to that of the latter so that their frequency 
responses are substantially equal in both the low and high frequency 
ranges. 
The burning resistance may be further improved when glass fibers, asbestos 
fibers, Polyclar and Modaacrylic fibers are added. The mixture may be made 
into the diaphragms with the burning resistance property by the 
conventional paper making method. 
As described above, according to the present invention, the physical 
strength properties such as swelling resistance, tearing strength and so 
on of the diaphragms for the loudspeakers may be considerably improved, 
and the diaphragms are made burning resistant. Furthermore the corrosion 
of the metal parts of the speakers may be prevented, and the deterioration 
of the acoustic characteristics may be prevented.