W-Cone (U.S. Patent #6,496,590)
The W-Cone is a unit-body cone assembly that delivers astonishing cone stiffness with minimal mass. The shape also provides superb torsional rigidity, which is critical to maintaining voice coil alignment at the suspension limits.
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).
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 1.24g/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.