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Description  |
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TECHNICAL FIELD
This invention generally relates to audio transducers. More particularly,
the invention relates to improvements in the design of a transducer having
a cylindrical or partially cylindrical arcuate diaphragm defined by a
cross-sectional profile projected on an axis to define a generally
cylindrical diaphragm.
BACKGROUND OF THE ART
U.S. Pat. Nos. 4,584,439 and 4,903,308, and pending U.S. patent application
Ser. Nos. 07/499,492 filed Mar. 29, 1990; 07/436,914 filed Nov. 14, 1989;
07/708,924 filed Apr. 11, 1991; and 07/730,172 filed Jul. 12, 1991, are
incorporated herein by reference, as they disclose variations and
refinements of an audio transducer having a diaphragm that can be
generally described as "cylindrical" in the broadest sense of the term.
That is, the diaphragm is defined by a two-dimensional cross-sectional
profile that is projected on an axis to form a three-dimensional diaphragm
having a constant cross-section. The cross-sectional profile need not be
circular but may be an open or closed polygon or curve. These cylindrical
diaphragms may generally be formed from flat sheets that are curved so
that all lines normal to the curved surface remain perpendicular to the
axis of projection. The diaphragms in the disclosed patents typically
include a pair of tangentially abutting circular or semi-circular
cross-sectional tube-shaped webs.
In operation, these cylindrical diaphragm transducers generate sound by a
"rolling motion" in which an electromagnetic coil attached to the
diaphragm interacts with a fixed magnetic field to move in a direction
perpendicular to the axis of projection of the diaphragm. Each of various
portions of the diaphragm accommodate the coil motion relative to a fixed
frame by selectively tightening and loosening its radius of curvature to
achieve the rolling motion.
While the transducers of the above-referenced applications and patents are
reasonably efficient, with a relatively flat frequency response over a
large bandwidth of approximately 5 octaves, there remains a need for
additional improvements in the performance criteria of efficiency,
bandwidth and response flatness. In addition, there is a need to reduce
manufacturing costs and to further increase product quality by simplifying
the manufacture of such a device.
SUMMARY OF THE INVENTION
The primary object of this invention is to provide an improved transducer
having features that independently and in concert overcome the
difficulties and shortcomings of the prior art and which fulfills the
aforementioned needs.
This object may be satisfied by providing a transducer having a cylindrical
diaphragm and one or more of the following improvements: an asymmetric or
unbalanced diaphragm, a monopolar diaphragm, an S-shaped diaphragm,
opposed and spaced-apart diaphragm webs for omnipolar output, an etched
coil, an electrostatic drive element, a three-lobed diaphragm, and molded
contoured diaphragms with stiffening ridges.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional schematic top view of a prior art transducer.
FIG. 2 is a sectional schematic top view of a transducer having an
asymmetric diaphragm in accordance with one embodiment of the present
invention.
FIG. 3 is a sectional schematic top view of a symmetrical monopolar
transducer in accordance with a second embodiment of the present
invention.
FIG. 4 is a sectional schematic top view of an asymmetric monopolar
transducer in accordance with a third embodiment of the present invention.
FIG. 5 is a sectional schematic top view of a transducer having a balanced
S-shaped diaphragm in accordance with a fourth embodiment of the present
invention.
FIG. 6 is a sectional schematic top view of a transducer having an
unbalanced S-shaped diaphragm in accordance with a fifth embodiment of the
present invention.
FIG. 7 is a sectional schematic top view of a transducer having a balanced
truncated S-shaped diaphragm in accordance with a sixth embodiment of the
present invention.
FIG. 8 is a sectional schematic top view of a transducer having an
unbalanced partially truncated S-shaped diaphragm in accordance with a
seventh embodiment of the present invention.
FIG. 9 is a sectional schematic top view of a transducer having an
unbalanced partially truncated S-shaped diaphragm in accordance with an
eighth embodiment of the present invention.
FIG. 10 is a fragmentary perspective view of a transducer having an etched
coil in accordance with a ninth embodiment of the present invention.
FIG. 11 is an enlarged cross-sectional view taken along line 11--11 of FIG.
10.
FIG. 12 is a perspective view of an electrostatic transducer having a
cylindrical diaphragm in accordance with a tenth embodiment of the present
invention.
FIG. 13 is a schematic cross-sectional top view taken along line 13--13 of
FIG. 12.
FIG. 14 is an enlarged partial cross-sectional view taken along line 13--13
of FIG. 12.
FIG. 15 is an enlarged cross-sectional view taken along line 15--15 of FIG.
14.
FIG. 16 is an enlarged fragmentary perspective view of the electrostatic
drive portion of the transducer of FIG. 12.
FIG. 17 is a schematic cross-sectional top view of an electrostatic
transducer having multiple drive elements in accordance with an eleventh
embodiment of the present invention.
FIG. 18 is a schematic cross-sectional top view of a low frequency
transducer having three cylindrical diaphragm lobes in accordance with a
twelfth embodiment of the present invention.
FIG. 19 is a fragmentary perspective view of an omnipolar transducer in
accordance with a thirteenth embodiment of the present invention, with
magnet means omitted.
FIG. 20 is a cross-sectional schematic top view taken along line 20--20 of
FIG. 19.
FIG. 21 is a top schematic view of the transducer of FIG. 19 showing the
operation thereof.
FIG. 22 is a schematic cross-sectional top view of an omnipolar transducer
having an electrostatic drive in accordance with a fourteenth embodiment
of the present invention.
FIG. 23 is a perspective view of a molded diaphragm in accordance with a
fifteenth embodiment of the present invention.
FIG. 24 is a perspective view of an alternative molded diaphragm.
FIG. 25 is a perspective view of an alternative molded diaphragm.
FIG. 26 is a perspective view of an alternative molded diaphragm.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a schematic cross-sectional view of a prior art
transducer illustrated in FIG. 1 of U.S. Pat. No. 4,903,308 to Paddock et
al. The prior art transducer 10 includes a rigid frame 12 carrying magnets
14. A symmetrical two-lobed "figure-eight" shaped diaphragm 16 has two
intercoupled circular sections tangentially abutting at a central expanse
18 that carries an electromagnetic coil. In this schematic view, the
diaphragm is viewed along its axis of projection to show its
cross-sectional profile. The remote ends of each web are connected to
opposite ends of the frame 12. The transducer 10 is bilaterally
symmetrical, giving it predictable and balanced acoustic properties.
However, any acoustic faults in any one portion of the diaphragm are thus
likely to occur in corresponding symmetrical portions, with the
undesirable consequences of such faults being magnified multi-fold.
It will be appreciated that unless otherwise specified, the actual
construction details of the prior art transducer of FIG. 1 and transducer
designs described below are identical to what is disclosed in U.S. Pat.
No. 4,903,308. Additional information for constructing these transducer
designs can be found in U.S. Pat. No. 4,584,439.
FIG. 2 shows an asymmetric transducer 20 having a frame 22, magnets 24
secured to the frame, and an asymmetrical "figure-eight" shaped diaphragm
26 secured at its remote ends to the frame. One generally circular first
lobe 27 of the diaphragm 26 is larger than an adjacent smaller second lobe
28, with the lobes tangentially abutting at a central expanse 29 between
the magnets 24. The two lobes are interconnected at the central expanse.
While the asymmetric diaphragm 26 may be formed of a single uniform
material, it is preferred that the two lobes be formed of materials having
different thicknesses and flexibility properties. Because of spring forces
in the diaphragm, it tends to return to a centered position in the absence
of external forces. Preferably, the lobes have similar spring constants to
provide a net balanced spring force, and so that the central expanse
naturally follows a straight path during the rolling motion of the
diaphragm. This may be achieved by selecting a thicker and stiffer
material for the larger first lobe 27 than for the smaller second lobe 28.
Alternatively, the entire diaphragm may be formed of a single sheet of
material with molded stiffening ridges to provide needed rigidity, as will
be discussed below. The diaphragm preferably is provided with damping
means such as damping strips adhered to the inner concave surface of the
diaphragm (as shown in the '308 patent) or alternative damping means as
described below.
FIG. 3 shows a monopolar transducer 30 having a frame 32 carrying magnets
34 and having a flexible cylindrical diaphragm 36 attached to the frame.
The diaphragm 36 is formed in a "numeral-three" profile with a pair of
semi-circular lobes 37a, 37b attached at their distal ends to the frame
and tangentially abutting at a central expanse 38 disposed between the
magnets 34. The transducer 30 is a monopolar design and generates sound
from only one side, so that it may be attached to a large flat surface,
such as a wall or the front of a speaker cabinet (not shown), with the
convex lobes projecting away from the surface. Because of the inherent
tendency of the semi-circular lobes of the diaphragm 36 to straighten out,
the lobes are securely glued together at the central expanse so that the
diaphragm retains its shape while at rest. Also, the lobes may be
preformed in the curved state so that they remain curved when unstressed.
FIG. 4 shows a transducer 40 having a "numeral-three" shaped diaphragm 46
similar to that of diaphragm 36 shown in FIG. 3, but with asymmetrically
shaped lobes. Consequently, the transducer achieves the advantages of
asymmetry in a monopolar design.
FIG. 5 shows a transducer 50 having a frame 52 with magnets 54 attached to
the frame. A substantially S-shaped diaphragm 56 attached to the frame has
two substantially semi-circular lobes, with each lobe being convex outward
away from opposite sides of the frame. The lobes are joined at a central
expanse 58 between the magnets 54. Because of the inherent tendency of an
S-shaped diaphragm to straighten out to a flattened state, the diaphragm
56 is preferably molded to its desired S-shape so that it retains its
shape at rest without internal stresses. In contrast to the diaphragms
previously described, the diaphragm 56 may be constructed of a continuous
single sheet or multi-layer sheet which forms both lobes, rather than two
separate and distinct sheets (multi-layer or otherwise) which are
interconnected at the central expanse to form the two lobes.
FIGS. 6-9 illustrate variations of the S-shaped diaphragm. FIG. 6 shows a
transducer 60 having a substantially S-shaped diaphragm 66 with lobes of
different sizes analogous to the asymmetrical transducers shown in FIGS. 2
and 4.
FIG. 7 shows a transducer 70 having a substantially S-shaped diaphragm 76
in which each lobe forms a quarter circle, as opposed to the semi-circular
lobes illustrated in FIG. 5. The transducer 70 has some similarity to the
bipolar transducer disclosed in U.S. Pat. No. 4,584,439 to Paddock.
FIG. 8 shows a transducer 80 having a generally S-shaped diaphragm 86 with
a forward-facing semi-circular lobe 87 having a first radius and a
rearward facing quarter-circle lobe 88 having a second radius smaller than
the first radius. As discussed above with respect to the asymmetrical
transducer of FIG. 2, the different lobes are preferably formed of
materials having different stiffness and other mechanical properties to
achieve a balanced rolling motion.
FIG. 9 shows a transducer 90 having an S-shaped diaphragm 96 similar to
that of FIG. 8, except that it has a semi-circular front lobe 97 with a
radius smaller than a quarter-circle rear lobe 98. It is also contemplated
that the embodiments of FIGS. 8 and 9 may be rotated by 180 degrees so
that the quarter-circle lobe of either embodiment faces forward.
FIG. 10 shows a modified version of the transducer 10 of FIG. 1, with an
etched coil assembly 100 attached to the diaphragm 16 at the central
expanse 18. These modifications may be employed in any of the diaphragm
profiles disclosed or suggested above. As shown in FIG. 11, the coil
assembly 100 is formed in a multi-layer laminated design like that used
for production of conventional two-sided printed circuit boards. A thin
substrate 102 formed of a glass epoxy material or others such as Kapton
common to printed circuit boards includes a pair of conductive coils 104
etched from copper foil laminated to opposite sides of the substrate 102.
The substrate may range upward from 0.0025 inch thick, with 0,005 inch
being preferred. One ounce copper foil provides adequate current carrying
capacity, with trace widths of between 0,004-0,010 inch for the long
vertical traces; the short transverse traces may be somewhat wider.
Overall impedance of the coil may be varied by adjusting the width of the
transverse traces. In the preferred embodiment, each coil is capable of
carrying 2 amps of current continuously. Because the assembly is commonly
fabricated for very stressful manufacturing processor, it is not
susceptible to delamination at temperatures that occur in an audio
transducer environment.
Each coil 104 includes a trace end contact 106 suitable for attachment to
wiring 108 (shown in FIG. 10) that connects to an amplifier output. A
metallized through-hole 110 defined in the substrate 102 permits the
connection of the inner terminus of one coil to the inner terminus of the
other coil on the opposite side of the substrate. As a result, there is no
need for lead wires to provide a crossover for connecting to the interior
of the coil. Also, the number of turns is effectively doubled, with the
current flowing in one orbital direction. The coil assembly 100 is
preferably adhered to inner diaphragm edges 112 as shown in FIG. 11 to
allow the coils 104 to remain exposed to air for heat dissipation. The
etched coil assembly may also be used in conjunction with any of the
asymmetrical, S-shaped or monopolar embodiments shown in FIGS. 2 through
9.
FIG. 12 shows an electrostatic transducer 120 having a cylindrical
diaphragm 122 with a substantially "figure-eight" profile, similar to the
prior art transducer 10 shown in FIG. 1. The electromagnetic drive system
of the prior art device is replaced by an electrostatic drive. In the
electrostatic transducer 120 of FIG. 12, a highly charged filament 124 is
attached to the diaphragm at the central expanse and runs the full height
of the diaphragm without interruption. The filament is electrically
connected to a high voltage of about 2-10 kv, and remains constantly
charged during operation. A set of conductive rods 128 is fixed to the
transducer frame 12 and connected to the variable signal outputs of an
amplifier 129. The charged filament 124 is thereby electrostatically
attracted to and repulsed by the variably charged rods with a force
sufficient to create motion in the diaphragm for generating sound.
FIG. 13 shows the electrostatic transducer 120 in cross-section. To achieve
a balanced, controlled diaphragm motion, the drive rods 128 are arranged
in a rectangular array. Each drive rod runs parallel to the projection
axis of the diaphragm 16. A left front drive rod 128a and right front
drive rod 128b are positioned adjacent the central expanse 18 on opposite
sides thereof and generally forward of the filament 124. The front drive
rods 128a and 128b are electrically connected together and are connected
to a first amplifier output line 131. A left rear drive rod 128c and right
rear drive rod 128d are similarly positioned on opposite sides of the
central expanse, but to the rear of the filament 124. The rear drive rods
128c and 128d are electrically connected to each other and to a second
amplifier output line 133, with the amplifier being connected to an input
signal and creating a variable potential voltage difference between the
front and rear drive rod pairs.
The charged filament 124 is preferably sandwiched between the tangentially
abutting diaphragm lobes. In embodiments having S-shaped diaphragm
profiles, such as those shown in FIGS. 5-9, the filament may be attached
to one side of the diaphragm or laminated between layers of a multi-layer
diaphragm.
As shown in FIG. 14, the filament 124 includes a conductive core 130
surrounded by an insulating cladding layer 132. The core is preferably
formed of graphite-impregnated thread or other electrically conductive
material to retain a charge. The cladding layer 132 is preferably formed
of a thin tube of glass or other dielectric material that is not
susceptible to dielectric breakdown at high voltages in the range of up to
5-10 kv. Without the cladding layer, the conductive core would be
susceptible to arcing at high voltage, leading to ozone generation and
other related problems. While a voltage of 2 kv may be adequate to achieve
acceptable performance, higher voltages will provide commensurate
increases in speaker efficiency, reducing amplifier cost and power
requirements.
As shown in FIG. 14, one or more rod retention clips 136 may be used to
laterally interconnect rods 128a, 128b, 128c, 128d. The clip 136 is formed
of insulating material, such as a resilient plastic, to mechanically align
the rods 128 and to eliminate unwanted vibrations thereof. The clip 136
defines a set of rod apertures 138 through which rods 128a-d are
received.. The clip defines a central space 144 for receiving the charged
filament 124 and to permit a range of motion. Because the clip completely
encircles the charged filament, the filament must be threaded through each
clip prior to lamination with the diaphragm. Alternatively, the clip may
be U-shaped so that it may be installed after the filament is laminated
with the diaphragm and may further include flexible snap connections for
receiving the rods without requiring the rods to be threaded through the
apertures 138. To prevent vibration and loosening, the rods are preferably
adhesively attached to the clip after assembly.
FIG. 15 further shows the clip 136 in a vertically aligned relationship
with the diaphragm 16. The diaphragm defines an oblong or rectangular
aperture 146 that is sufficiently large to provide clearance for the clip
136 and so that the diaphragm may vibrate in a sufficiently wide range of
motion to generate sound without contacting the clip.
FIG. 16 shows a central portion of the diaphragm 16 in which two clips 136
are attached to rods 128a, 182b, 128c, 128d to provide alignment. This
approach is useful for very tall transducers, an application to which the
electrostatic approach is particularly well suited. Many clips are
employed in a tall transducer, with the clips being spaced apart by 3 to 6
inches. An electromagnetic coil driven speaker of this type suffers from
increasing impedance as the coil length is extended. Thus, a transducer
several feet tall must be manufactured in several distinct sections.
However, the electrostatic transducer has no such limitations.
FIG. 17 shows an electrostatic transducer 150 using ganged components for
improved efficiency. The transducer 150 has three charged filaments 124a,
124b and 124c mounted on an enlarged central expanse 152 of the diaphragm
16. Drive rods 128a-128h are arranged in pairs in alternation with the
filaments, with the members of each pair being positioned in opposite
sides of the central expanse 152. So that all of the components act in
concert to provide efficient, high output sound, the central filament 124b
is charged to a high voltage polarity opposite that of filaments 124a and
124c. Drive rods 128a, 128b, 128e and 128f are connected to a first output
131 of amplifier 129; rods 128c, 128d, 128g and 128h are connected to the
opposite amplifier output 133. The ganged approach illustrated in FIG. 17
is shown as having three filaments, but it is contemplated that this
number may be two, four or more.
The electrostatic drive construction is illustrated in conjunction with a
symmetrical bipolar "figure-eight" profile diaphragm, as shown in FIGS.
1-13. However, the electrostatic principle may be applied to any
transducer having a cylindrical diaphragm, such as those illustrated in
FIGS. 2-9. The ganged construction illustrated in FIG. 17 has a similarly
wide applicability and need not be limited to the illustrated embodiment.
FIG. 18 shows a low range transducer 160 having a three-lobed diaphragm
162. The transducer 160 includes a frame 164 supporting three sets of
magnets 166. The diaphragm 162 includes two primary peripheral lobes 170,
172 formed of a flexible material, as used in two-lobed diaphragms of the
prior art. A central lobe 174 has a smaller radius than the peripheral
lobes 170, 172 and tangentially abuts each peripheral lobe at a respective
central expanse 176, 178 that carries a coil for production of sound
generally in the manner disclosed in the prior art. With the peripheral
magnets being oriented in similar polarity and the central magnets
oriented oppositely, the coils attached to each central expanse 176, 178
are connected in opposite polarity so that both coils act in concert to
create a synchronized driving motion.
The transducer 160 may be configured as a woofer for producing primarily
low frequency sounds, or alternatively may serve as a wide bandwidth
device with a frequency range extending to substantially lower frequencies
than would be possible with a two-lobed diaphragm.
For use as a woofer only, the central lobe material may be a relatively
heavy and stiff material for maximum efficiency. The central area behaves
as a piston and generates low frequency sound in concert with the
peripheral lobes 170, 172, which operate in a rolling motion, as described
in the prior art. Because the central lobe 174 functions ideally as a
piston, wave motion across the central lobe is undesirable and may be
controlled through use of a damping material such as felt, which may be
attached to the entire inner surface of the central lobe 174.
For the transducer 160 to function as a wide bandwidth device, the central
lobe 174 is formed of a thin, flexible material that may be appreciably
thinner than the flexible material forming the peripheral lobes 170 and
172. Such a thin material will be sufficiently rigid at low frequencies
due to the tighter radius in which it is bent. At low frequencies, the
full range transducer 160 operates essentially as the woofer embodiment
discussed above. At high frequencies, the central lobe responds flexibly
to wave motion. Accordingly, the central lobe 174 must be damped adjacent
to one central expanse 176 by a pair of felt strips 182, 184 attached to
the interior of the central lobe 174. Without such damping, each central
expanse would function as a separate sound source with the sound generated
by each objectionably interfering with that generated by the other.
Alternatively, to avoid interference, the input to one of the coils may be
electronically filtered to eliminate interference-generating high
frequencies.
FIG. 19 shows a compression omnipole wave generator transducer 190 having
opposed semi-cylindrical diaphragms 192, 194 with opposed, central
coil-carrying portions 196a, 196b. Distal edge portions of the diaphragms
are mounted to a frame 198. An electromagnetic coil 200 is attached to the
diaphragm and forms a series of adjacent loops, each one of which runs up
the first diaphragm 192 and down the second diaphragm 194. Accordingly, at
any given time, all current flowing through the coil is flowing in a
single direction in the wire portions of the coil 200 attached to the
first diaphragm 192, while the current is flowing in the opposite
direction through all the wire portions of the coil attached to the second
diaphragm 194.
FIG. 20 shows a cross-sectional schematic view of the omnipole transducer
190, which has magnets 202, 204 attached to the frame 198 within the
respective diaphragms 192, 194. The magnets are oriented in similar
polarity so that the north pole of the first magnet 202 is directly
opposite the north pole of the second magnet 204, with the south poles
being similarly opposed. While the coil 200 is securely adhered to the
diaphragms where the vertical wire portions run adjacent the magnet
structures, the coil 200 includes slack upper and lower loops 206, 208 to
permit the central coil-carrying portions 196 of the diaphragm freely to
move toward and away from each other as a varying current passes through
the coil.
In FIG. 21, the diaphragms 192, 194 (shown in solid lines) are shown in the
extended position more closely spaced than when in the flexed positions
192', 194' (shown in dashed lines). This opposed motion creates
compression and rarefaction of air within the space between the
diaphragms. Consequently, acoustic waves 212 are emitted from the space
between the diaphragms in a widely dispersed pattern on each side of the
transducer. The combination of the acoustic waves, which constructively
interact with each other as they emanate from the front and rear, gives
the transducer an omnipolar response. In other words, the sound pressure
generated by the transducer in a response to a given signal does not
appreciably vary as the listener moves in a horizontal 360 degree circle
centered on the transducer. The transducer 190 may be constructed in a
vertically elongated configuration to create an effective omnipolar line
source, that is, one that emulates a theoretical radially-pulsing
cylinder.
Alternatively, as shown in FIG. 22, an electrostatic omnipole transducer
220 may be constructed according to the principles of the electrostatic
transducer of FIG. 13. The electrostatic omnipole transducer 220 has
similarly charged planar elements 222, 224 attached respectively to
diaphragms 192, 194. The planar elements are wired to a high voltage power
supply (not shown). A central plate 228 occupies the line of symmetry
between the diaphragms and is connected to a first amplifier output 230. A
pair of similar outer plates 232, 234 are positioned symmetrically within
the respective diaphragms 192, 194 and are each electrically connected to
a second amplifier output 238. The central plate 228 experiences balanced
forces, making substantial reinforcement unnecessary. The outer plates
232, 234 may be secured along their height to the frame 198.
Alternatively, all the plates may be replaced by similarly connected
vertical rods, as shown in the embodiment of FIG. 13.
In any cylindrical diaphragm system such as those disclosed above, as well
as those of the prior art, it is necessary to control the flexibility and
resonances of the diaphragms. In the bipolar cylindrical transducer 10
illustrated in FIG. 1, as well as in many of the other transducers
disclosed herein, a wide frequency range is achievable. However, this
range is limited at the high and low ends by contrary factors.
For theoretically ideal, efficient high frequency response, the central
expanse 18 should approach infinitely low mass and high rigidity so that
it may move crisply and responsively to an input signal of a limited
power. The distance between the central expanse 18 and the diaphragm ends
attached to the frame 12 is sufficiently long compared to the wavelength
of high frequency vibrations that such waves are damped within the
diaphragm well before they reach the diaphragm outer edges and have an
opportunity to reflect back and interfere with subsequently generated
waves. Also, because the diaphragm moves only a very small amount to
generate high frequencies, flexibility is not critical.
At low frequencies, on the other hand, the diaphragm moves an appreciable
amount, requiring flexibility. Furthermore, the long wavelengths involved
may propagate within the diaphragm to the frame and reflect back to
interfere with subsequently generated waves, creating unacceptable
resonances at various frequencies if left undamped. Therefore, the ideal
diaphragm for producing low frequencies is thick, non-resonant and
flexible. In the prior art, these contrary objectives of high- and
low-frequency production have been reconciled with reasonable success
because rigidity for high frequency production is essential only near the
central expanse, while flexibility and wave damping is necessary only in
the diaphragm regions remote from the central expanse.
FIG. 23 shows a contoured diaphragm 240 in "numeral-three" configuration
for a monopole transducer, To provide rigidity near the central expanse 18
and flexibility near the remote end 244, each lobe of the diaphragm 240 is
molded from a single sheet of thermoformable plastic with a set of raised
ridges 246. These ridges are broad and gently contoured near the remote
end 244 to permit flexibility, and are narrow and more sharply contoured
near the central expanse 18 to provide rigidity, even with a thin,
otherwise flexible material. The ridges also have a taller profile near
the central expanse and a lower profile near the remote end 244.
Additional rigidity enhancing narrow ridges 248 may be positioned adjacent
the central expanse for additional rigidity.
The contoured diaphragm 240 is preferably vacuum-formed onto a cylindrical
form (not shown) shaped like the desired resulting diaphragm. This
provides a diaphragm that is stress-free when at rest. If the diaphragm
were formed in a generally planar position, it would become stressed as it
was curved into the final cylindrical form. When so formed, it would have
an outer surface in tension and inner surface in compression, resulting in
different wave propagation rates.
Each ridge 246, 248 has a tapered end 252 adjacent the central expanse 18
so that waves propagating from the central expanse through the diaphragm
do not appreciably reflect off the leading edge of the ridge. The ridges
provide for controllability of the diaphragm's flexibility without the
time-consuming and efficiency-impairing addition of mass, such as the
damping strips shown in the prior art. The ridges need not have a regular
or symmetrical appearance. In fact, a designer may analyze a prototype
diaphragm for undesirable resonances and selectively place ridges to
eliminate the resonances. For instance, a region showing excessive
flexibility may be provided with narrower, taller, more rigid ridges.
Other contemplated variations are illustrated in FIGS. 24-26. FIG. 24 shows
a diaphragm 256 having a plurality of parallel linear ridges 258 molded
therein. Each ridge 258 spans nearly the entire distance between the
central expanse 18 and one of the remote ends 244. Each ridge is gently
tapered at its ends to avoid reflections of propagating waves caused by
abrupt transitions.
FIG. 25 shows a diaphragm 260 having parallel ridges 264 in an alternating
arrangement, with full length ridges as shown in FIG. 24 being
interspersed with shorter ridges to provide a wider transitional zone
between the ridge-free areas and the ridge areas. FIG. 26 shows a
diaphragm 270 providing a similar effect, but with intermediate length
ridges 272 of the same length being positioned alternately proximate to
and distal from the central expanse 18.
Any or all of the above features and improvements may be employed in
embodiments also including features of the prior transducers. For
instance, the diaphragm may include support or suspension members such as
elastic cords or tab cut-outs folded from the diaphragm and adhered to the
magnet or frame structure. Also, the diaphragm may be formed of either
single or multiple layers of different materials and may also include
adhesive damping strips applied to selected regions of the diaphragm inner
surface. It should also be noted that the narrow magnet spacing of the
prior systems is preferred; the schematic drawings in this application
show a wider magnet gap to facilitate illustration.
Having illustrated and described the principles of my invention by what is
presently a preferred embodiment, it should be apparent to those persons
skilled in the art that the illustrated embodiment may be modified without
departing from such principles. For instance, while the contoured
diaphragms of FIGS. 23-26 are illustrated in the context of monopole
transducers, the contours may similarly be applied in asymmetrical,
S-shaped, or dipolar transducers. The various features and improvements
disclosed herein may be combined in many combinations, such as a preferred
embodiment having an electrostatic drive with an S-shaped diaphragm having
a small radius semi-circular front lobe and a large radius quarter-circle
rear lobe, and having contours molded into the diaphragm to provide added
rigidity to the rear lobe. Innumerable other permutations of the features
disclosed herein are contemplated to provide alternative embodiments. I
claim as my invention not only the illustrated embodiment, but all such
modifications, variations and equivalents thereof as come within the true
spirit and scope of the following claims.
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