U.S. patent number 3,862,376 [Application Number 05/325,162] was granted by the patent office on 1975-01-21 for cone construction for loudspeaker.
Invention is credited to Stanley F. White.
United States Patent |
3,862,376 |
White |
January 21, 1975 |
CONE CONSTRUCTION FOR LOUDSPEAKER
Abstract
A cone for a loudspeaker in which the shear function is more
nearly the same at all radii, the shear function being the product
of the effective axial thickness at a given radius multiplied by
the circumferential length at such radius. The cone is preferably
of curved or "exponential" cross section, deeply angled at a
central throat but flattening out to approach a plane at the
periphery. The cone is made of light, dimensionally stable material
characterized by a high velocity of sonic conduction and is
preferably in the form of hollow spheres of glass of small
dimension in a binder of epoxy or the like. In an alternate
embodiment the cone is made of a hard solid acrylic plastic in thin
section and with supporting ribs to provide both rigidity and a
substantially constant shear function. Encircling the cone is a
flexible hinge or "surround." Interposed between the hinge and the
edge of the cone is a light collar having axial rigidity to ensure
that the edge of the cone remains in a planar locus. The cone,
collar and surround may be made integral with one another in a
production line setup. At the center of the cone a stiff cap is
provided for closing the central opening and for imparting circular
and axial rigidity in the region of attachment of the voice
coil.
Inventors: |
White; Stanley F. (Elmhurst,
IL) |
Family
ID: |
23266696 |
Appl.
No.: |
05/325,162 |
Filed: |
January 19, 1973 |
Current U.S.
Class: |
381/404; 181/167;
381/423 |
Current CPC
Class: |
H04R
7/20 (20130101); H04R 3/002 (20130101); H04R
7/12 (20130101); H04R 31/006 (20130101); H04R
31/003 (20130101); H04R 2307/207 (20130101); H04R
2231/003 (20130101); H04R 2307/204 (20130101); H04R
9/025 (20130101) |
Current International
Class: |
H04R
3/00 (20060101); H04R 7/12 (20060101); H04R
7/00 (20060101); H04R 31/00 (20060101); H04R
7/20 (20060101); H04R 9/02 (20060101); H04R
9/00 (20060101); H04r 007/12 (); G10k 013/00 () |
Field of
Search: |
;179/115R,115.5R,115.5H,116,180,181R,181F,115DV,115ES ;181/31R,32R
;116/142R ;340/388 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IBM Tech. Disclosure Bulletin, Vol. 15, No. 2, July, 1972..
|
Primary Examiner: Robinson; Thomas A.
Attorney, Agent or Firm: Wolfe, Hubbard, Leydig, Voit &
Osann, Ltd.
Claims
What I claim is:
1. In a loudspeaker having a frame and a magnetic structure
providing an annular magnetic air gap, a cone assembly which
comprises a cone body forming a convexly curved front surface and
having a periphery and a central throat with a voice coil at the
throat extending into the air gap, a flexible surround and a
flexible spider for mounting the periphery and voice coil for free
axial movement, the cone body having an effective axial thickness
which varies generally inversely with the radius so that the
product of the effective axial thickness at a given radius
multiplied by the circumferential length at that radius is
substantially constant across the width of the cone body.
2. A loudspeaker as set forth in claim 1 wherein said cone body has
an exponentially shaped cross section which is deeply angled at the
throat and which approaches a plane at the periphery.
3. The combination as claimed in claim 2 in which the exponent
which characterizes the exponential function lies within a range of
2 and 4.
4. The combination as claimed in claim 1 in which the cone is
formed by a layer of glass cloth embedded in epoxy plastic.
5. The combination as claimed in claim 4 in which the glass cloth
has a thickness on the order of 3 to 4 mils while the cone has a
thickness on the order of 5 mils.
6. The combination as claimed in claim 1 in which the surround is
of curved cross section having an edge which meets the periphery of
the cone body at right angles for axial reinforcement of the
periphery.
7. The combination as claimed in claim 1 in which the radial width
of the surround varies from point to point about the periphery of
the speaker body.
8. The combination as claimed in claim 1 in which the surround
extends between the periphery of the cone body and the frame and in
which the frame is of rectangular configuration while the periphery
of the cone body is of circular configuration thereby producing a
surround having a variable radial width.
9. The combination as claimed in claim 1 in which both the surround
and the spider have widths which vary as the function of polar
angle.
10. In a loudspeaker having a frame and a magnetic structure
providing an annular magnetic air gap, a cone body having a
periphery and a central throat with a voice coil at the throat
extending into the air gap, a flexible surround and a flexible
spider for respectively mounting the periphery and voice coil for
free axial movement, the cone body being formed of hollow spheres
of glass in a hardenable binder of plastic capable of wetting the
glass.
11. The combination as claimed in claim 10 in which the plastic is
an epoxy.
12. The combination as claimed in claim 10 in which the spheres
have a diameter lying within the range of 10.mu. inches and
500.mu.inches.
13. In a loudspeaker the combination comprising a frame and a
magnetic structure providing an annular magnetic air gap, a cone
assembly having a periphery and a central throat with a voice coil
at the throat extending into the air gap, a flexible surround and a
flexible spider for respectively mounting the periphery and voice
coil for free axial movement, the cone body being formed of hard
and stiff plastic material forming a continuously smooth and
uninterrupted front surface and including radial and circular
axially-extending ribs which are integral with the back surface of
the cone body.
14. The combination as claimed in claim 13 in which the cone body
is formed of acrylic plastic.
15. The combination as claimed in claim 13 in which the cone body
is formed of polycarbonate plastic.
16. In a loudspeaker having a frame and a magnetic structure
providing an annular magnetic air gap, a cone assembly which
comprises a cone body forming a convexly curved front surface and
having a periphery and a central throat with a voice coil at the
throat extending into the air gap, a flexible surround and a
flexible spider for mounting the periphery and voice coil for free
axial movement, and a rigid cap enclosing the throat and forming a
concavely curved front surface.
17. The combination as claimed in claim 16 in which the axial
thickness of said cap varies as an inverse function of the radius.
Description
Of all electronic components, loudspeakers have, in the last half
century, changed the least. During all of that time a loudspeaker
has conventionally employed a cone of paper flexibly mounted at the
periphery for axial movement and with a voice coil at the center
operating in an annular magnetic air gap. In some respects paper
has been a happy choice since it is readily available, cheap and
easily worked and since it provides good strength in conical
configuration combined with a high degree of dampening which is
inherent in its fibrous nature. In the great bulk of loudspeakers
which are of small size intended for use in television sets,
portable radios and the like, manufacture has been so highly
competitive that the designs have been frozen with concentration
upon high production techniques and with little effort or expense
being devoted to improving the quality of reproduction. Even in the
case of speakers of larger size intended for "high fidelity"
reproduction, and in which a higher price has financed continued
developmental effort, improvement in fidelity has lagged woefully
behind the improvement in associated circuit elements such as
amplifiers.
Investigation of loudspeakers having paper cones provides ample
evidence of inherent drawbacks. Primarily, the cone does not act as
a unit or as a true piston in the movement of air. Instead, the
cone is subject to "breakup" in which localized areas of the cone
vibrate independently. One type of breakup may be descriptively
referred to as "Chinese gong" vibration in which the periphery of
the cone does not remain in a planar locus but is distorted out of
a plane in the form of a sine wave so that adjacent portions of the
cone periphery vibrate out of phase with one another. One result of
this type of vibration is the deformation of the central portion of
the cone out of its normal circular contour into an indeterminant,
and transient, elliptical shape.
A second type of localized breakup may be conveniently referred to
as "bed-sheet effect" in which the body of the cone departs from
its conical geometry and develops traveling waves. Where the waves
extend radially the type of localized breakup may be referred to as
"plumber's helper" vibration or "oil canning," with the waves being
analogous to those set up when a pebble is dropped into a smooth
body of water.
The effect of such localized vibration or "breakup" is to produce a
response curve which is non-linear and which has peaks and valleys
at various points within and beyond the audible range.
Efforts have been made in the past to treat the paper body of a
cone to increase its rigidity, to make it waterproof or fire
resistant, but such treatments have almost always had the effect of
degrading performance. Also efforts have been made to form speaker
cones of plastic material but the results have been disappointing
or have resulted in an increase in manufacturing cost which cannot
be tolerated in such a highly competitive market.
Consequently, it is an object of the present invention to provide a
cone for a loudspeaker which avoids use of paper, employing plastic
instead, and which overcomes the operating disadvantages of paper
cones as well as the operating and cost disadvantages of prior
plastic cones. More specifically it is an object to provide an
improved cone for a loudspeaker which provides more faithful
reproduction at low cost and particularly in sizes of speakers
which have not, in the past, been noted for their fidelity and
linearity. More specifically, it is an object to provide a cone for
a loudspeaker which is substantially free of "breakup" or localized
vibration and which acts, as a unit, over the entire spectrum of
reproduction with true piston-like effect.
In this connection it is an object to provide a cone for a
loudspeaker which has a substantially constant shear function, that
is, one in which the resistance to shearing stress in the axial
direction is substantially constant at all radii so that the axial
force which is developed in the voice coil is transmitted, in the
form of axial shear stress, outwardly to the periphery of the cone
with minimum time delay, minimum loss of energy and minimum
localized deformation of the cone body. It is a related object to
provide a cone having a rigid cap which encloses the central
opening and in which the cap, too, has a substantially constant
shear function at all radial points.
It is a still further object to provide a cone having such
desirable characteristics in either one of two embodiments, a first
embodiment in which the cone, which may be smooth surfaced, is
formed of an emulsion of small hollow glass spheres, referred to,
for example, as "Micro-baloons," in a plastic binder of epoxy or
the like and a second version in which the cone is formed of
acrylic plastic or the like in extremely thin section and with
integral reinforcing ribs arranged both radially and circularly for
the purpose of providing light weight combined with a substantially
constant shear function and to act as gates for easy molding
capability. In this connection it is a general object to provide a
cone in which distribution of the material of construction is
optimized to combine the quality of rigidity and lightness.
It is yet another object to provide a speaker cone assembly in
which a cone body having a substantially constant shear function is
engaged, at its periphery, by a flexible surround which may be
easily assembled to the periphery of the cone or which may be
integrally formed with it, and with the surround being shaped to
provide reinforcement of the edge of the cone against the "Chinese
gong" type of vibration.
It is a still further object of the present invention to provide a
loudspeaker cone assembly in which the surround is of varying width
thereby to reduce the likelihood of resonant reflections, back into
the cone, from the region of rigid attachment of the surround to
the speaker frame.
It is a still another object of the present invention to provide a
speaker cone which not only provides improved reproduction but
which is well suited to manufacture on high production basis and
which is more stable and durable than conventional paper cones.
More specifically it is an object to provide a cone which is
impervious to extremes of temperature and humidity, presence of
radiation, fire, water and other hazards and which is therefore
well suited for both military and non-military uses.
It is yet another object of the present invention to provide a cone
which enables reproduction with a minimum amount of distortion and
with good power handling and sound dispersal characteristics and
which may be employed to upgrade the performance of the small, low
cost type of speaker usually used in portable radios and TV
sets.
Other objectives and advantages of the invention will become
apparent upon reading the attached detailed description and upon
reference to the drawings in which:
FIG. 1 is an axial section taken through a loudspeaker constructed
in accordance with the present invention.
FIG. 2 is a fragmentary enlarged view showing the joint between the
cone body and surround.
FIG. 3 is an enlarged axial section taken through one side of the
cone body to show the variation in axial thickness as a function of
radius.
FIG. 4 is an enlarged axial section taken through the cap showing
the variation in axial thickness as a function of radius.
FIG. 5 is a greatly enlarged cross section of a portion of the cone
body showing the hollow spheres of glass in a binder of epoxy
plastic.
FIG. 5a is a fragmentary section showing the alternative use of
glass fabric.
FIG. 6 is a cross sectional view showing a mold for molding the
cone body of FIG. 1.
FIGS. 7, 8 and 9 are a series of figures showing the mixing and
applying of the glass-epoxy emulsion to the mold in paste form.
FIG. 10 shows a cross section of a modified cone body looking along
the line 10--10 in FIG. 11.
FIG. 11 shows the appearance of the underside of the cone body
looking along the line 11--11 in FIG. 10.
FIG. 12 is a fragmentary section taken through an injection type
mold for molding the cone body of FIG. 10.
FIG. 13 is a fragmentary section taken through a modified mold for
simultaneous injection of hard-curing plastic for the cone body and
soft-curing plastic for the surround and spider thereby to produce
a cone body having integral flexible mountings, taken along line
13--13 in FIG. 14.
FIG. 14 is a plan view of the mold shown in FIG. 13 and looking
along the line 14--14 therein.
FIG. 15 is a face view, looking along the line 15--15 in FIG. 1,
showing a circular cone body in a frame of square configuration to
produce a variable width surround.
FIG. 16 is a fragmentary section looking along the line 16--16 in
FIG. 1 showing a spider of square configuration.
FIG. 17 is a section taken through the spider and looking along the
line 17--17 in FIG. 16.
While the invention has been described in connection with certain
preferred embodiments, it will be understood that I do not intend
to be limited by the embodiments shown but intend, on the contrary,
to cover the various alternative and equivalent forms of the
invention included within the spirit and scope of the appended
claims.
Turning now to FIG. 1 there is shown a loudspeaker 10 having a
basket or frame 11 and a mounting flange 12. Secured to the frame
is a magnetic structure which consists of a permanent magnet 13,
which may be of annular shape, having a pole piece in the form of a
first stack of annular laminations 14 on one side and a pole piece
formed by a second stack of discshaped laminations 15 on the other
side. The magnet 13, which may be of the ceramic type employing
powdered ferrite is axially polarized so that the stacks of
laminations 14, 15 are oppositely poled. Mounted upon the stack of
laminations 15 is a central pole piece 16 consisting of a series of
concentric laminations, including an outer lamination 17. The stack
14 and outer lamination 17 form, between them, a main annular air
gap 20. An auxiliary lamination 21 of Z-shaped cross section
cooperates with the edge of the lamination 17 to form a second or
auxiliary gap 22 for a purpose which will appear.
In carrying out the present invention a cone body 30 is provided
which may be of circular shape having a periphery 31 and throat 32
with an annular convex front surface 33 and back surface 34.
Surrounding the periphery 31 of the cone body is a flexible
surround 40, which may in its simplest aspect be annular, having an
outer edge 41 and an inner edge having a portion 42 which meets the
periphery of the cone body at right angles and forming an axially
facing internal ledge, or shoulder, 43 into which the periphery of
the cone body may be forcibly snapped and cemented in position. The
right-angular relationship serves to reinforce the presented edge
of the cone. Alternatively, the collar may be integral with the
surround. The surround 40 may be made of flexible material
impervious to air, for example, butyl rubber, or of cloth
impregnated with a resilient plastic such as a phenolic resin. Or
tightly woven cloth without impregnation may be used.
In accordance with the present invention the cone body 30, as shown
in FIG. 3, has an effective axial thickness which varies generally
inversely with the radius thereby to provide a substantially
constant shear function. More specifically in accordance with the
invention the cone body is curved, preferably in accordance with an
exponential function so that the surface angles sharply inwardly at
the throat and approaches a plane at the periphery. Thus, referring
to FIG. 3, the effective axial thickness at a radius r1 is
indicated by the distance a1. Because of the curvature the
effective axial thickness at the lesser radius r2 is increased to
an amount a2 which is greater than a1. Taking this a step further,
the effective axial thickness at the radius r3 is an amount a3
which is a maximum. Thus, even though the cone body may be of
constant thickness, normal to the surface, the thickness in the
axial direction is not constant but varies as an inverse function
of the radius, being a minimum in regions of large radius and a
maximum at the throat where the radius is small.
This inverse relationship tends to produce a shear transmitting
capability which remains substantially constant for all radii. The
shear function, which may be denoted by the letter "U" denotes the
product of the axial thickness multiplied by the circumferential
dimension at the particular radius. The circumferential dimension
for a given radius is the radius multiplied by 2.pi.. Thus, in the
illustrated embodiment, the axial thickness is such that the
following quantities are approximately equal:
a.sub.1 2.pi.r.sub.1 = a.sub.2 2.pi.r.sub.2 = a.sub.3
2.pi.r.sub.3
In carrying out the invention the three quantities need not be
precisely equal to one another; it suffices that they generally
approximate one another to signify optimum usage of material, which
is to be contrasted with conventional loudspeakers where the shear
capability varies widely from the periphery of the speaker body to
the throat.
Further in accordance with the present invention a rigid cap is
provided to enclose the throat, the cap having an axial thickness
which is greater toward the center than toward its periphery
thereby tending to more nearly equalize the shear function within
the cap. Thus, referring to FIGS. 1, 3 and 4, the cap indicated at
50 has a periphery 51 and a center 52 with a front surface 53 and a
rear surface 54, the cap being preferably so shaped as to provide a
convex annular surface 53 terminating at an upraised point at the
center. As shown in FIG. 4, the cap has an axial thickness b1 at
the radius r3 and a greater thickness b2 at the lesser radius r4,
the thickness thus varying approximately inversely with the radius
as in the case of the cone body. The result is to produce a shear
function which remains more nearly constant over the radius of the
cap than would be the case, say, if the cap were formed in shallow
dome shape as is more conventional.
The periphery 51 of the cap seats in the throat 32 of the cone
body. Inserted into the throat, and suitably cemented therein, is a
voice coil form 60 of cylindrical shape having a main winding 61
which extends into the main annular air gap 20, with terminals 62,
63, as well as an auxiliary winding 65, having terminals 66, 67,
the latter winding being positioned in the auxiliary annular air
gap 22. The winding 61 may be conventional or it may be of
flat-wound ribbon for improved efficiency and heat transfer. The
purpose of the auxiliary winding 65 is to produce a motional
negative feedback signal as is discussed in greater detail in the
co-pending application mentioned above. For the purpose of guiding
the coils in the air gaps with pure axial motion, a spider 70 is
provided which is formed of flexible material having an inner edge
71 which engages, and supports, the throat portion 32 of the cone
body as well as a flange portion 72 which seats upon the speaker
frame. The spider may be formed of the same material as the
surround 40.
In accordance with one of the aspects of the present invention the
cone body 30 is formed of hollow spheres or balloons of glass bound
in a plastic having the wetting and other characteristics of epoxy.
More specifically in accordance with the present invention an epoxy
plastic, with hardener added, and heated to a flowable, viscous
consistency, is mixed with the glass balloons in such a way as to
insure complete wetting with a minimum amount of epoxy and with
reduced likelihood of air voids. The epoxy-glass mix is then
discharged into a mold, the mold is closed to define a cone of
desired shape, and any excess mix is expressed in the form of
"flash." Heat is then applied to the mold to cure or harden the
mix, following which the mold is opened to remove the cone which,
upon removal of the flash, is ready for mounting within the
surround and spider. The net result is to produce a uniform
cellular structure 80 as shown in FIG. 5 consisting of closely
spaced balloons 81 held together with a plastic binder 82, the mix
being homogeneous, extremely strong, smoothly surfaced and with a
uniform density on the order of 0.9, or lower, depending upon the
size and wall thickness of the balloons and the amount of plastic
required for wetting.
Referring to FIG. 6 there is shown a simplified mold of a type
which may be used in practicing the invention. FIGS. 7, 8 and 9
diagrammatically illustrate the manner in which the epoxy and glass
balloons are brought together and applied to the mold in measured
quantity.
The mold, indicated at 90, is of two-piece construction made up of
a lower mold section 91, and an upper mold section 92 which are
keyed together by a pin 93. The mold cavity is formed by the facing
surfaces 94, 95 with final closure being determined by an annular
stop surface 96. The surfaces 94, 95 are preferably surfaced with a
parting agent such as silicone or tetrafluoroethylene (Teflon). The
unit may be heated by electric heating coils 97, 98, with the lower
section preferably kept at a cooler temperature to prevent
premature setting when the mix is poured into the lower section and
until the mold is closed. In a production setup the sections 91, 92
may be hydraulically opened and closed.
In mixing the epoxy and glass balloons, the epoxy with hardener
added, is deposited in measured quantity in a dispenser 100, the
body of epoxy being indicated at 101. A typical epoxy plastic which
may be used for this purpose is chemically identified as a
cyclo-aliphatic resin. A hardener which is chemically
n-phenolene-diamine is commercially obtainable from Union Carbide
Corporation, New York, New York. Reference is made to the
literature for additional information on epoxy plastics. It will be
understood that any of the commonly available epoxies and hardeners
capable of wetting glass, of flowable consistency, and having an
appropriate hardening time may be used. It is preferred to choose
an epoxy and hardener combination which, upon slight heating, to
120.degree. F will have a viscosity which does not exceed 200
centipoises.
The glass balloons preferably have a diameter which may range from
10.mu. inches to 500.mu. inches with a wall thickness of about
2.0.mu. inches. Such balloons are obtainable commercially from
Emerson and Cummings, Inc. whose address is Canton, Massachusetts,
identified as their catalog 915-RU-30n 7- 69. The balloons are also
commercially available from 3M Company whose address is St. Paul,
Minnesota. The balloons should preferably be formed of glass of
hard composition, such as "flint," "crown" or "crystal" glass.
A body of glass balloons 102 is deposited above the epoxy as shown
in FIG. 8. The epoxy is drawn by capillary attraction between the
glass balloons until all have been wetted and absorbed into the
body forming a viscous yet slightly "pastey" consistency. For
discharging the mix from the vessel, a cover 103 is applied secured
by clamps 104 and connected to an air line 105 having a valve 106
and timer 107. The valve 106 is connected to a source of pressure
108 as well as a separately operable vacuum pump 109. Air is first
purged from the mix by operating the vacuum pump. Following this,
pressure from the source 108 is admitted by timed opening of the
valve 106. Using calibrated amounts of epoxy and glass heated to a
calibrated temperature and using a calibrated orifice 110, which
may be on the order of 1/8 inch diameter, a measured amount of mix
is deposited upon the lower die member 91. The upper die member 92
is then seated (FIG. 6) with any excess mix escaping as flash,
following which the mold is heated to a sufficient temperature and
for a sufficient time, to produce "kick-over." The mold shown in
FIG. 6 is by way of example only and an automated mold of
commercially available design may be employed in a high production
setup with induction heating and an automatically controlled
temperature cycle. Moreover, an epoxy and hardener may be chosen
which produce "kick-over" as rapidly as the state of the art
permits. The cap 50 may be made of the same material as the cone
body formed in a similar mold having a cavity of cap shape.
After removal of the molding flash, and after stamping out the
central annulus portion, indicated at 99, with a suitable set of
dies, the cone may be pressed, and cemented, in seated position
within the surround 40 and the voice coil support 60 may be
cemented in place in the central opening (see FIGS. 2 and 3). The
cap 50 may, however, be omitted until a final assembly step in
order to provide access for insertion of a centering shim, of paper
or the like, around the pole lamination 17 (see FIG. 1) to insure
entering of the coils 61, 65 in their respective magnetic gaps.
As a further aspect of the present invention the glass, instead of
being present in the cone body 30 (FIG. 1) in the form of balloons,
(as shown in FIG. 5) may be present in the form of reinforcing
glass cloth or roving G, as illustrated in FIG. 5a. The cloth in a
practical case may be quite thin having a thickness of 2 to 4 mils,
while the cone has a thickness of 5 mils. The glass cloth G may be
placed in the mold manually and the dispenser 100 may be used, as
described, to dispense the epoxy.
Tests made upon a loudspeaker employing a cone utilizing glass
balloons or glass cloth bound with epoxy as described above, show
that fidelity of reproduction is obtainable in various known types
of cabinets which is far beyond that obtainable using speakers with
conventional cones made of paper. The cone, utilizing a constant
shear function and of stiff, light construction operates without
breakup in the audible range and with true piston-like action.
Utilizing "strobe" lighting and other test facilities it is found
that a speaker employing the improved cone is capable of operating
at high power levels over, and substantially beyond, the audible
range without localized breakup of the type above identified as
"Chinese gong" effect in which the outer edge of the cone is
distorted into a sine wave configuration. That is particularly
attributed to the unusual edge stiffness resulting from the
structural characteristics of the glass balloons. Use of balloons
which are perfectly spherical is found to produce better results
than where the cells are faceted as is the case in connection with
plastic expanded, closed cell films of various kinds.
It is also believed that suppression of "Chinese gone" vibration is
assisted by use of the rigid circular cap 50 which is intimately
cemented to the throat of the cone and which insures that the
throat of the cone will remain perfectly circular at all times. One
effect, not widely recognized, of the "Chinese gong" type vibration
is that it tends to deform the throat of the cone from a true
circle into eliptical configuration, at least at certain
frequencies. By using a cap of rigid construction the reinforcing
effect is felt all the way out to the edge of the cone. The
separate effect known as "oil-canning" is also overcome by the
present cone construction.
A further explanation of the response achieved by the present
speaker design may be found in the high velocity of sound
propagation in the cone body and the attenuation of reflections of
the sound from the periphery of the cone assembly back to the voice
coil. Using present procedures the velocity of propagation in the
cone body is on the order of four times the velocity of propagation
in conventionally used paper. In speakers employing conventional
cones and surrounds, the wave which is transmitted within the body
of the cone passes radially, more or less freely, until the region
of anchoring of the surround is reached which provides an abrupt
change in impedance, resulting in a reflection of the wave back
into the cone where phase differences and resonances encourage the
setting up of localized vibration. Using the present cone with a
high velocity of transmission, any reflected wave resulting in
resonance causes resonance to occur well outside of the audible
range. The net effect of the foregoing is to produce a response
curve which is remarkably linear, that is, substantially free of
the peaks and valleys which are encountered, and taken as a matter
of course, in conventional designs of speakers.
It is found, too, that cone-air coupling is remarkably efficient
and that the off-axis dispersion characteristic is improved
compared to that of conventional speakers. It is believed that this
is due to the fact that the cone, free of localized "breakup", acts
as a single source with piston action, plus the combination of the
convexly arcuate, annular surface 33 of the cone body and the
concavely arcuate annular surface 53 of the cap. Indeed, more
reliance is placed upon the cap 50 as a transducer than is the case
of speakers of conventional design. The cap because of its shape
and rigidity exhibits a high degree of stiffness with no
possibility of breakup even at the highest frequencies which it may
be called upon to reproduce.
It is found that a speaker constructed as described above, and
mounted in an appropriate cabinet, is not only capable of high
fidelity reproduction with small speaker size but is capable of
providing improved reproduction when mounted in less-than-optimum
enclosures as, for example, encountered in portable radios and
television sets where the speakers used in the past have generally
been of poor quality. Moreover, speakers of the present design are
ideally suited for applications requiring waterproofness and
fireproofness. The epoxy-glass mix is inherently waterproof so that
the speaker is ideal for outdoor usage, either for music
reproduction or for paging purposes. The inherent fireproofness of
the cone material, as contrasted with paper, makes it possible to
use speakers of the present design in military apparatus or in
other equipment where flammability is considered a possible hazard.
The surround and spider may be made of any suitable materials which
possess the desired waterproofness and fire resistance as well as
providing the necessary flexibility over a wide temperature
range.
The above paragraphs have been directed toward the description of a
preferred embodiment of speaker. The invention in certain of its
aspects, however, is not limited to use of the particular cone
material and the constant shear characteristic may, if desired, be
obtained in a cone which employs a hard, dimensionally stable and
relatively dense plastic but which is molded in extremely thin
section with appropriate integral ribbing to provide the necessary
rigidity for piston-like action. An alternate form of cone body is
set forth in FIGS. 10 and 11 which show the cone body which may be
conveniently made of a high quality acrylic plastic which is
moldable under pressure in thin section, with the flow of the
molten plastic being facilitated by a radial and circular pattern
of ribbing. Turning to FIG. 10 a cone body 130 is shown having a
periphery 131 and a throat 132 and with a front surface 133 and a
rear surface 134. Integrally molded in the rear surface of the cone
body are a series of radial ribs 135 which may, for example, be six
in number and which are interconnected by a circular rib 136. In a
typical case where acrylic plastic is used having a density of
approximately 1.2 it may be molded in a thickness of 0.012 inch
with ribs having a width on the order of 0.030 inch. The ribs are
made of such height as to produce a substantially constant shear
characteristic at various radii within the body. It will be
recalled in connection with FIG. 3 that determination of the area
in axial shear at any given radius was a simple matter of
multiplying the axial thickness times the radius multiplied by a
constant 2, such product being generally constant at all radii. The
same is true of the structure shown in FIG. 10, except that the
axial cross sectional area of all of the ribs at the particular
radius must be added to the axial cross section area of the
portions between the ribs. It is desirable, in any event, for the
ribs to be so proportioned that the shear characteristic is
substantially constant for all radii except, of course, for the
circular rib or ribs which define regions of maximum shear
resistance.
The cap 150, which is centered within the throat is of similar
construction, having a periphery 151, a central point 152, a front
surface 153, rear surface 154 and radial ribs 155.
In the case of the embodiment shown in FIGS. 10 and 11, attachment
to the surround and to the voice coil form and spider are made
substantially the same way as shown in FIGS. 1-3. The rib 136 is
located at a sufficient radius, i.e., sufficiently close to the
periphery, so that the rib assists in suppressing any tendency
toward "Chinese gong" vibration.
The use of acrylic plastic is preferred for the embodiments of
FIGS. 10 and 11 because of its extreme hardness and because of the
high velocity of conducted sound, both of these factors being
enhanced as a result of the molding process. Such a plastic is
commercially available under the trademark PLEXIGLAS from Rohm and
Haas Chemical Co. whose address is Independence Mall, Philadelphia,
Pennsylvania. However, chemically different plastics having similar
physical and molding characteristics are usable as, for example,
polycarbonates. Such plastics are available commercially from
General Electric Company, New York, New York identified, for
example, as "LEXAN 500."
Because of the extreme thinness of section it is not possible to
employ a mold of the type illustrated in FIG. 6. Instead it is
necessary to use an injection type mold having a cavity
corresponding to the shape of the final product and into which the
molten plastic is injected, at one or more points, under high
pressure and at a relatively high temperature. Injection molding
machines capable of producing the cone body described in connection
with FIGS. 10 and 11 are readily available manufactured by
Cincinnati Company whose address is Cincinnati, Ohio or the Arburg
Company of Germany whose local address is Chicago, Illinois. During
the injection process the radial and peripheral ribs are utilized
to insure equalized distribution of the viscous plastic. FIG. 12
illustrates fragmentarily an injection type mold 160 having an
upper member 161 and a lower member 162 and with an inlet orifice
163 at a junction between a radial rib 135 and the circular rib
136, one orifice being used at each rib junction, making a total of
six. The plastic injected under pressure into the orifices flows
through the rib channels which act as gates for the feeding and
spreading of the viscous plastic throughout the structure, thereby
assuring the molding of a complete cone body, notwithstanding the
thin section.
It is one of the features of the present invention that the
flexible surround and flexible spider may be integrally molded with
the stiff cone as part of the same molding operation. This involves
use of a socalled dual-injection mold as shown at 170 in FIG. 13,
the mold being made up of an upper section 171, a central section
172, and a lower section 173. In addition to a series of inlet
orifices 174 which inject "hard-cure" plastic, there is a separate
set of injection orifices 175 which inject "soft-cure" plastic for
the surround and a third set of injection orifices 176 which inject
"soft-cure" plastic for the spider. The viscosity pressure and
orifice size may be adjusted so as to produce an integral joint
between the hard and flexible portions of the cone assembly. An
injection molding press capable of such dual injection is
manufactured by the Arburg Company mentioned above.
As an alternative to simultaneous injection, a cone body 130 such
as shown in FIGS. 10 and 11 may be pre-molded and interposed in
position in a mold such as that shown at 170 in FIGS. 13, following
which "soft-cure" plastic may be injected through orifices 175, 176
to provide the integrally-jointed surround and spider.
It is one of the still further features of the present invention
that the mounting flange 12 of the frame or basket 11 of the
speaker shown in FIG. 1 need not be circular or provide a surround
of constant width. On the contrary, the mounting flange may be of
square or rectangular cross section as shown at 12a in FIG. 15 with
the surround, indicated at 40a, having a width which varies, being
a maximum w1 at the corner positions and a minimum w2 at the side
portions. Similarly, the spider 70 need not be round but may be of
square configuration as shown in FIG. 16 to provide a variable
spider radius. In both cases the effect of the variable radius is
to avoid resonance effects due to reflections within the surround
or spider which might produce a peak or a valley in the response
curve.
It is a still further feature of the invention that the leads 62,
63 from the voice coil 61, instead of being provided as loose
pigtails, may be integrally molded or woven in the spider as shown
in FIGS. 17, the same being true of the leads 66, 67 from the
auxiliary coil 65. Moreover, while the main and auxiliary coils 61,
65 may be close wound of conventional wire, it is possible to make
them flat wound employing ribbon, for example, a thin ribbon of
aluminum on the order of 0.0015 inch in thickness and 0.5 inch in
width with adjacent convolutions insulated by anodization of the
aluminum to a thickness on the order of 0.0001 inch and the further
additional insulation value of the bonding glue.
In the following claims the term "exponential" is applied to the
cone body 30 to define its curvature. This term is a general one to
denote a curve which produces a smooth variation from periphery to
throat and which causes the surface of the cone to be deeply angled
at the throat while approaching a plane at the periphery. The exact
value of the exponent is not critical if this condition is met.
However, it is preferred to utilize a surface in which the exponent
lies within the range of 2 to 4. Nor need the curve be rigorously
exponential in shape since a hyperbolic curve, or even a circular
or eliptical section falls sufficiently close to an exponential
curve as to bring about the desired result, provided that the axial
section, including rib cross section, at each radius, is maintained
substantially constant as discussed.
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