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By utilizing space wasted in conventional speakers, this ground-breaking innovation controls the W7's massive excursion without sacrificing precious cone area.
One of the first things you notice about a W7 is that something is "missing"... the mounting flange. Of course, this is actually not the case. The mounting flange is simply hidden beneath the surround and is made accessible for mounting purposes by detaching the outer edge of the surround and moving the roll to the inside (a pretty neat little trick). Apart from the obvious benefits of amazing your friends as you pull the surround off your speaker, there is a serious technical issue that led us in this design direction: Effective Piston Area ("Sd"). This is essentially the speaker's "cylinder bore", to use an automotive engine analogy, and is calculated by measuring the diameter of the diaphragm including one-half of the surround roll-width. In other words, from the top-center of the surround on one side to top-center of the surround on the other side.
The displacement capability of a speaker is determined by this piston area times the speaker's excursion capability. Displacement of air is directly linked to output potential. Therefore, the more air a speaker can ultimately displace, the louder it can play. That being said, there is a big difference between piston area and excursion: piston area doesn't need power to make it happen. This means that by making a larger piston, you are directly improving displacement for a given amount of excursion and, therefore, making your speaker more efficient. This is not the only factor that governs efficiency, but it is a major one.
To make a speaker have more excursion capability not only requires a motor design that can deliver more stroke, but also requires a surround rugged enough to handle the demands of longer excursions and controlled enough to keep everything lined up properly. If the surround's roll-width is not adequately large, its behavior (compliance) is not linear over the useful stroke of the woofer and it is more likely to fatigue and fail. For this reason, speakers with longer excursion capability generally need larger surround rolls (we won't comment on the ones that use large rolls strictly for cosmetic effect).
The problem with big surrounds is that they begin to encroach on the effective piston area of the driver. For example, a typical 12-inch woofer with a medium-sized roll has an effective piston area of 81.52 square inches. Compare this to a fat-surround 12-inch woofer which has a piston area of 69.07 square inches (15.2 % less effective piston area than the medium-size roll.) To overcome this loss, the fat-surround woofer has to produce more excursion to displace the same air as the woofer with the medium surround (and will require more power to do so).
OverRoll™ technology neatly sidesteps this compromise by allowing us to make full use of the entire footprint of the speaker, placing the surround further to the outside than in a conventional woofer. This means that we can use a large roll for all its benefits without sacrificing cone area (in fact, the 12W7 has 1% more piston area than the medium-surround conventional woofer). By maximizing the effective piston/total footprint ratio, we can deliver more output for a given excursion and outside frame diameter. This means that the prodigious excursion advantage of the W7 can be put to full use enhancing output, rather than making up for lost piston area.
The technology also provides a geometry advantage on the outside edge of the surround roll, allowing for more linear operation. A further benefit is that the mounting holes are inherently sealed by the surround, resulting in an improved box seal.