Detailed Information:
The
more excursion and motor force a speaker has, the more important cone
rigidity becomes. The acceleration forces are extreme, requiring the
cone to withstand rapid changes in speed and direction without
deformation. Deformation not only leads to distortion, but can also
affect the speaker's mechanical integrity by allowing the voice coil to
go out of alignment and rub on the top-plate and the pole-piece of the
motor.
There are several approaches to enhancing cone rigidity. The
obvious ones are using a thicker material and/or a stiffer material. In
recent years, several manufacturers have used composite cone materials
(Kevlar®, fiberglass, etc.) or metals (aluminum, magnesium, titanium
alloys). The use of these exotic materials is typically accompanied by
marketing claims that the material chosen has exceptional
stiffness-to-mass characteristics. These are true statements, but can
be misleading. While these materials have excellent stiffness-to-mass
properties (compared to paper or poly), they are not lighter than paper
or poly in practice. This means that their use accepts the compromise
of added moving mass on the design. This leads to efficiency penalties
and suspension complications (it's harder to keep a heavy mass aligned
properly).
A simple poly cone diaphragm, while sufficient for lower power
designs, would not remain rigid under the demands that the W7 design
requires. Our engineering team knew that high levels of cone rigidity
would be needed, but they focused on achieving rigidity without a huge
weight penalty. This ultimately led to the design we call the W-Cone.
The W-Cone assembly achieves its rigidity through architectural means,
rather than through inherently stiff materials. The design addresses
the stiffness issue by using two lightweight mineral-filled
polypropylene skins,
bonded together at the perimeter and the center of the assembly. The
lower skin's cross-section is shaped like a 'W', hence the name, and
provides incredible rigidity when bonded to the dished upper skin. The
effect is not unlike the trusses of a bridge or the unit-body
construction of a modern automobile. In addition to the overall
rigidity benefit, the lower skin's shape distributes the force
generated by the coil and motor more evenly than a typical diaphragm.
The force is not only applied to the apex but also distributed to the
perimeter of the outer diaphragm for more linear behavior. A further
benefit of the W-Cone is that the upper skin (the one in contact with
the listening environment), is isolated from the high air-pressure
gradients of the enclosure, further reducing deformation (and
distortion).
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As a point of comparison, the
W-Cone assembly of a 12W7 is 32% lighter than a typical aluminum-alloy
12-inch cone. If analyzed in terms of weight per square inch of piston
area, the W7 cone-body weighs 1.24/sq.in., compared to
1.45g/sq.in. for an aluminum-alloy cone and 1.66g/sq.in. for a
titanium-alloy cone.
So why polypropylene? As stated above, our patented W-Cone technology
achieves all of the benefits of more exotic materials while better
suiting the unique nature of the W7. Since the W7 surround is
detachable, the moving system (including the diaphragm) is subject to
mechanical stress unseen in conventional designs. Because the user can
tug on the cone while manipulating the surround, the cone must be able
to handle this without buckling or deforming. Paper, metal or brittle
composite cones would not handle this well. Our two-skin unit-body cone
design achieves outstanding axial and torsional stiffness to withstand
all kinds of abuse, and will remain largely unaffected and unblemished.
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