A double cone-type loudspeaker which comprises a subcone made of highly heat-conductive material, attached to the front of the main cone and to a voice coil bobbin made of highly heat-conductive material. Heat generated in the coil portion of the voice coil travels directly to the bobbin portion and subcone, and is released to the front of the speaker. Consequently, a double cone-type loudspeaker with high maximum input power and high heat radiating capability is achieved.

FIELD OF THE INVENTION 
The present invention relates to the field of sound-reproducing systems 
and, more particularly, to speakers with superior maximum input power 
capability resulting from improved heat dissipation characteristics. 
BACKGROUND OF THE INVENTION 
Recently, more audio equipment is being designed for higher power, 
resulting in a growing demand for speakers with higher maximum input 
power. The coil portion of the voice coil generates heat as a result of 
electrical resistance when the input signal is applied to the speaker and 
current flows in the coil portion. This heat is radiated through the voice 
coil bobbin, magnetic gap and other components. However, if excessive 
input power is applied to the speaker and a high current flows through the 
coil portion, insufficient radiation takes place and the coil portion will 
bum out due to the generation of excessive heat. 
Therefore, it is necessary to improve the heat radiating function to permit 
increased dissipation of the heat generated in the coil portion of the 
voice coil, to improve the maximum input power of the speaker. 
One known arrangement for a speaker with a better heat radiating function 
is a speaker unit disclosed in Japanese Utility Model Laid-open Patent No. 
S61-104698. More specifically, a through-hole is provided along the shaft 
direction of the center pole, and a heat sink made of non-magnetic 
material is disposed inside the hole. This speaker unit is installed in a 
cabinet and used as a speaker system. 
The heat transfer path of the above speaker unit is explained with 
reference to FIG. 16, which shows a section view of a speaker system 
employing the speaker unit disclosed in Japanese Utility Model Laid-open 
Patent No. S61-104698. When the input signal is applied to the speaker, 
heat is generated as a result of the electrical resistance of a coil 
portion 11 of the voice coil. This heat conducts through a magnetic gap 15 
to a top plate 13 and the yoke 14 thereof. The heat then reaches a heat 
sink 12 disposed inside a yoke 14, and is released to the inside of 
cabinet 16. Heat conductivity, which indicates how fast heat can travel, 
is approximately 0.02W/m.degree. K at the magnetic gap 15, 80 W/m.degree. 
K at the top plate 13 and the yoke 14, and 240 W/m.degree. K at the heat 
sink 12 (when it is made of aluminum). 
Since the magnetic gap 15, which has low heat conductivity, is included in 
the heat transfer path, sufficient radiating effect cannot be expected due 
to the heat transfer loss. 
In addition, heat released from the heat sink 12 will increase the 
temperature inside the cabinet 16. This results in a lower radiating 
effect due to the smaller temperature difference between the heat sink 12 
and the inside of cabinet 16. 
Furthermore, a die is needed for making the heat sink 12, resulting in 
higher cost. 
Furthermore, since air passes through the narrow space created by the heat 
sink 12, an abnormal noise is generated by the moving air. 
SUMMARY OF THE INVENTION 
A double cone-type loudspeaker is provided with high maximum input power as 
a result of sufficient heat radiation. 
A first exemplary embodiment of the present invention relates to a double 
cone-type loudspeaker comprising a subcone made of highly heat-conductive 
material. One part of the subcone is attached to the front of the main 
cone and another part is attached to the voice coil bobbin which is made 
of highly heat-conductive material. With this structure, heat generated in 
the coil portion of the voice coil travels directly through the voice coil 
bobbin and subcone, which have high heat conductivity, and is released 
through the front of the speaker. 
Thus, a high radiating effect can be easily achieved. Consequently, the 
present invention provides a double cone-type loudspeaker with higher 
maximum input power. 
A second exemplary embodiment of the present invention relates to a subcone 
with circumferential corrugations in addition to the structure of the 
first exemplary embodiment. The corrugations expand the surface area of 
the subcone. They also disperse the circumferential resonance of the 
subcone in a single frequency. 
Thus, increased radiating effect can be achieved easily. Therefore, the 
present invention provides a double cone-type loudspeaker with high 
maximum input power. In addition, the present invention enables the 
flattening of frequency characteristics by dispersing frequencies which 
cause resonance, so as to suppress the peak frequency characteristics 
caused by resonance. 
A third exemplary embodiment of the present invention, in addition to the 
structure of the first exemplary embodiment, relates to a subcone with a 
rib protruding toward the voice coil bobbin in an area bonded to the voice 
coil bobbin. The rib expands the area of the subcone contacting the voice 
coil bobbin, and also expands the bonding area of the subcone. 
Thus, increased radiating effect can be achieved easily, and consequently, 
the third exemplary embodiment provides a speaker with high maximum input 
power. At the same time, a larger bonding area improves the bonding 
strength. 
A fourth exemplary embodiment of the present invention, in addition to the 
structure of the first exemplary embodiment, relates to one or two holes, 
whose central angle is between 40.degree. and 120.degree., which are 
disposed on the main cone within the area of the subcone. 
In the case of two holes, they are disposed symmetrically about the center 
of the main cone. The outer periphery of the subcone is attached in an 
airtight fashion to the main cone. With this structure, heat in the 
subcone may also be released from the rear of the subcone. Furthermore, 
the hole(s) provided on the main cone convert high-frequency resonance 
above the second-harmonic to an axial asymmetry so as to suppress the peak 
frequency characteristics caused by resonance. 
Thus, increased radiating effect can be achieved easily, and consequently, 
the fourth exemplary embodiment provides the speaker with higher maximum 
input power. At the same time, the fourth exemplary embodiment enables the 
flattening of the frequency characteristic by suppressing the peak 
frequency characteristics caused by the resonance of the main cone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Exemplary embodiments of the present invention are explained below with 
reference to drawings. 
First Exemplary Embodiment 
FIG. 1 is a half section view of a .O slashed.30 cm double cone-type 
loudspeaker in a first exemplary embodiment of the present invention. A 
field system 1 comprises a top plate 1a, magnet 1b, and yoke 1c. The top 
plate 1a is made of iron. Its outer diameter is .phi.85 mm, inner diameter 
is .phi.42 mm, and thickness is 8 mm. The magnet 1b is made of ferrite. 
Its outer diameter is .phi.90 mm, inner diameter is .phi.40 mm, and 
thickness is 15 mm. The outer diameter of the yoke 1c is .phi.80 mm. At 
the center of the yoke 1c is a pole, whose outer diameter is .phi.38.2 mm 
and height is 29 mm. An iron frame 2 is 0.8 mm thick and damper 6 is made 
of cotton cloth. As shown in FIG. 1 a voice coil has an inner diameter of 
.phi.38.66 mm, and comprises a 0.05 mm thick aluminum bobbin portion 5 and 
a voice coil portion 5a where .phi.0.22 mm copper wire is coiled. The 
voice coil portion 5a is secured in the magnetic gap of the field system 1 
by the attachment of damper 6 to frame 2. The main cone 3 has an outer 
diameter of .phi.224 mm and is made of 0.5 mm thick paper which weighs 19 
g. The main cone 3 has an urethane edge with a roll outer diameter of 
.phi.253 mm, roll inner diameter of .phi.225 mm, and thickness of 0.8 mm 
at its outer periphery. The inner periphery of the main cone 3 is attached 
to the bobbin portion 5. The main cone 3 is fixed to the frame 2 with its 
edge on the outer periphery. 
A curved aluminum subcone 4 has a total weight of 3.5 g and is 0.11 mm 
thick. Its outermost diameter is .phi.120 mm, surface area is 160 
cm.sup.2, and radius of curvature is R100 mm. The outermost periphery of 
the subcone 4 is fixed to the front of the main cone 3 with rubber 
adhesive. The inner periphery of subcone 4 is tapered for 5 mm height with 
a taper angle of 5.5.degree.. This tapered portion is fitted to the inner 
periphery of the bobbin portion 5, and secured with rubber adhesive. The 
inside of subcone 4 is concaved to R55 mm with the center of the curve on 
the top. 
Operation of the double cone-type loudspeaker configured as above is 
explained below. 
When a signal is input to the speaker, the temperature of the voice coil 
portion 5a increases. The heat generated in voice coil portion 5a travels 
to the aluminum bobbin portion 5 which has heat conductivity of 240 
W/m.degree. K (about 12,000 times that of air), then to the aluminum 
subcone 4 attached to the voice coil bobbin, and released to the front of 
the speaker. 
Next, measured radiation characteristics of the prior art and the double 
cone-type loudspeaker in the first exemplary embodiment are compared. FIG. 
2 shows actual measurement results of the temperature increase which 
illustrate the relation of the input power to the speaker installed in a 
100 liter capacity cabinet and the temperature increase of the coil 
portion of the voice coil. The input power plotted along the abscissa 
indicates the power of the noise signal specified as DIN standard 45573 
Tei 112 (1979). The temperature increase plotted along the ordinate 
indicates the increase in temperature of the voice coil portion 5a when 
the noise power shown on the abscissa is applied. In general, the voice 
coil portion 5a will burn out and destroy the speaker if the temperature 
increase exceeds 200.degree. C. even if highly heat-resistant material is 
used for the voice coil. Line A shows the results of the prior art which 
employs a field system and voice coil identical to that of the first 
exemplary embodiment but comprises a heat sink at the center of the yoke. 
Line B shows the results of the first exemplary embodiment. Comparing the 
power at which the temperature reaches 200.degree. C. in the prior art and 
the present invention, power applied to the prior art is 135 W and that of 
the first exemplary embodiment is 163 W. The results show that the power 
handling capacity improves by approximately 21%. In other words, the 
exemplary embodiment increases the maximum input power of the speaker by 
21%. 
As explained above, the first exemplary embodiment of the present invention 
can i) reduce loss in heat transfer, ii) release heat to the cooler 
environment outside the cabinet, and iii) improve the radiating effect of 
the speaker by ensuring that heat generated in the voice coil portion 5a 
travels through only highly-conductive areas and releases the heat through 
the front of the speaker. Accordingly, the present invention offers a 
speaker with higher maximum input power. 
In the first exemplary embodiment, aluminum with heat conductivity of 240 
W/m.degree. K is used for the bobbin portion 5 and subcone 4. The 
equivalent effect is obtainable with titanium, copper or brass. Naturally, 
the same effect can be expected with the use of materials with about 80 
W/m.degree. K heat conductivity. 
Second Exemplary Embodiment 
FIG. 3 is a section view of a double cone-type loudspeaker in a second 
exemplary embodiment of the present invention. The only difference between 
the second exemplary embodiment and the first exemplary embodiment is the 
shape of the subcone. An aluminum subcone 7 has an outer diameter of 
.phi.120 mm and thickness of 0.11 mm. At the position of .phi.45 mm, 
.phi.60 mm, .phi.75 mm, .phi.90 mm, and .phi.105 mm on its oscillation 
portion, R1 mm corrugations protruding toward the front of the speaker are 
disposed in the circumferential direction. These corrugations enlarge the 
surface area of subcone 7 by about 15% compared to a subcone without 
corrugations. FIG. 4 shows the temperature increase curve of voice coil 
portion 5a. As in the first exemplary embodiment, the power at which the 
temperature reaches 200.degree. C. is compared between the prior art and 
the second exemplary embodiment. Power applied to the prior art is 135 W 
while the power applied to the second exemplary embodiment is 181 W. The 
results show that the power handling capability improves by approximately 
34%. Thus, the exemplary embodiment increases the maximum input power of 
the speaker by 34%. 
FIG. 5 shows simulation results of the sound pressure frequency 
characteristics of the first exemplary embodiment using finite element 
analysis. Line D shows the total frequency characteristics of the main 
cone including the edge and subcone. Line E is the frequency 
characteristics of only the subcone. As shown in the graph, a large peak 
is noticeable around 4.8 kHz. FIG. 6 is an original simulation model for 
one fourth of the oscillation system of the speaker. FIG. 7 shows a model 
distorted around 4.8 Hz. A large distortion is noticeable near the outer 
periphery of the subcone in the circumferential direction. 
Therefore, it is apparent that the peak in frequency characteristics around 
4.8 kHz is caused by distortion due to resonance of the subcone. FIG. 8 
shows simulation results of sound pressure frequency characteristics of 
the second exemplary embodiment. FIG. 9 is an original simulation model. 
Line F in FIG. 8 shows the total frequency characteristics of the speaker, 
and Line G shows the frequency characteristics of only the subcone. The 
simulation results indicate that circumferential distortion of the subcone 
due to resonance is dispersed by the provision of corrugations. The 
occurrence of a large in frequency characteristic peak caused by 
concentration of resonance at a single frequency is greatly suppressed 
Thus, the second exemplary embodiment improves the radiating effect by 
using a larger surface area for radiating heat and the provision of 
corrugations on the subcone. Moreover, the resonance frequency of the 
subcone is changed at each corrugation which enables the disbursement of 
peak characteristics caused by resonance distortion. Consequently, the 
second exemplary embodiment offers a speaker with higher maximum input 
power capability and flatter high-frequency characteristics. 
FIG. 10A is an enlarged section view of a bonding portion of the subcone 4 
and the bobbin portion 5 of the first exemplary embodiment. The bonding 
portion of the subcone is tapered for improving formability of the subcone 
4 and workability at inserting the bonding portion to the bobbin portion 
5. This provides linear contact between subcone 4 and bobbin portion 5 by 
filling a space created by the taper with a rubber adhesive 8. 
Third Exemplary Embodiment 
FIG. 10B is an enlarged section view of a bonding portion between the 
subcone and voice coil of the double cone-type loudspeaker of the third 
exemplary embodiment. 
A R1 mm rib, whose outer diameter is .phi.38.66 mm, protruding toward and 
contacting the bobbin portion 5 is provided on the taper of the bonding 
portion of a subcone 9 in the circumferential direction. The remainiing 
structure is the same as in the first exemplary embodiment. The structure 
of the third exemplary embodiment is enlarges the contact surface because 
subcone 9 and bobbin portion 5 contact each other at two points: a tip of 
the bobbin portion 5 and the rib. 
In the first exemplary embodiment, the subcone 4 and the tip of the bobbin 
portion 5 may come apart and the high-heat conductive portions may not 
remain in direct contact if the position of subcone 4 deviates upward or 
bobbin portion 5 deviates downward. In the third exemplary embodiment, 
however, the rib always remains in contact with the bobbin even if the tip 
of the bobbin portion 5 detaches from the subcone. In addition, the 
adhesion area will be enlarged because the surface area of the bonding 
portion of the subcone 9 becomes larger by provision of the rib. 
FIG. 11 shows the temperature increase curve of the voice coil portion 5a. 
As in the first exemplary embodiment, power applied at which the 
temperature reaches 200.degree. C. is compared between the prior art and 
the third exemplary embodiment. The prior art employs the field system and 
voice coil identical to that of the third exemplary embodiment but 
comprises a heat sink at the center of the yoke. Power applied to the 
prior art is 135 W and that to the third exemplary embodiment is 189 W. 
This shows that the power handling capability improves by approximately 
40%. Thus, the exemplary embodiment increases the maximum input power 
capability of the speaker by 40%. 
The third exemplary embodiment improves the radiating effect by expanding 
the contact area of the bonding portion between the subcone and bobbin 
portion 5. It also improves the adhesion strength of the bonding portion 
by providing a larger surface area for the bonding portion of the subcone. 
Consequently, the present invention offers a speaker with higher maximum 
input power and increased structural stability. 
FOURTH EXEMPLARY EMBODIMENT 
FIG. 12 is a front view of a double cone-type loudspeaker in a fourth 
exemplary embodiment of the present invention. The speaker has the same 
structure as the second exemplary embodiment except for main cone 10. A 
hole 10a whose inner diameter is .phi.46 mm, outer diameter is .phi.100 
mm, and central angle is 72.degree. is disposed on main cone 10. A second 
hole is disposed symmetrically with respect to the center of the main cone 
10. The outer periphery of the subcone 7 is attached in an airtight 
fashion to main cone 10. With this structure, the rear side of the subcone 
7 also contacts the air inside the cabinet, and helps improve the 
radiating effect of the speaker. 
FIG. 13 shows the temperature increase curve of the voice coil portion 5a. 
As in the first exemplary embodiment, the power at which the temperature 
reaches 200.degree. C. is compared between the prior art and the fourth 
exemplary embodiment. The prior art employs the same field system and 
voice coil as the exemplary embodiment but comprises a heat sink at the 
center of the yoke. Power applied to the prior art is 135 W and the power 
applied to the fourth exemplary embodiment is 192 W. The results show that 
the power handling capability improves by approximately 42%. Thus, the 
exemplary embodiment increases the maximum input power of the speaker by 
42%. 
FIG. 14 shows simulation results of the sound pressure frequency 
characteristics for the speaker of the second and fourth exemplary 
embodiments using finite element analysis. Line F shows the 
characteristics of the second exemplary embodiment and Line K shows the 
characteristics of the fourth exemplary embodiment. FIG. 15 is an original 
simulation model. Looking at FIG. 14, it is apparent that the fourth 
exemplary embodiment reduces the 2 kHz peak by about 4 db and the 2.5 kHz 
peak by about 5 dB over the second exemplary embodiment. This is due to 
the hole disposed on main cone 10. The hole converts high-frequency 
resonance above the second-harmonic to an axial asymmetry and suppresses 
the peak frequency characteristics caused by resonance. 
The hole also functions as a mechanical high frequency filter which 
attenuates sound emission in the high frequency band from the main cone, 
thereby suppressing interference with sound from the subcone. 
Thus, the fourth exemplary embodiment enables further improvement of the 
radiating effect by using the rear side of the subcone for radiation. 
Moreover, the hole on the main cone functions as a mechanical high 
frequency filter to reduce the peak caused by resonance of the main cone. 
Accordingly, the present invention provides a speaker with higher maximum 
input power and flatter high-frequency characteristics. 
In the exemplary embodiment, the hole disposed on the main cone 10 has a 
central angle of 72.degree.. It should be noted that, through computer 
simulation and actual measurement, the same effect is confirmed when the 
central angle is between 40.degree. and 120.degree.. 
Although the invention is illustrated and described herein with reference 
to specific embodiments, the invention is not intended to be limited to 
the details shown. Rather, various modifications may be made in the 
details within the scope and range of equivalents of the claims and 
without departing from the invention.