Shipping January 2001 and available in 10-, 12- and 15-inch models, the GTi subwoofers handle 3000, 4000, and 5000 watts peak, respectively. Optimized for use in small sealed or vented enclosures, the W15GTi performs flawlessly in a 1.5-cubic-foot sealed box, while the W12GTi requires 1 cubic foot, and the W10GTi a mere .75 cubic foot.

At the core of these high-excursion and massive-power-handling subwoofers, which received a 2000 CES Innovations Award, is JBL's Differential Drive Design (U.S. patent no. 5,748,760 and other patents pending). Originally created by JBL Professional, the patented Differential Drive Design (DDD) was adapted to the automotive environment by JBL Mobile's engineers in Northridge, CA. For the first time, Differential Drive technology has been applied to a long-excursion woofer, and the result is one-way linear excursion (XMAX) that significantly exceeds the XMAX of competing subs (more than 20 mm for the 12- and 15-inch and more than 17 mm for the 10-inch).

"The GTi XMAX spec is outstanding in two respects," explains Andy Wehmeyer, Product Marketing Manager for JBL Mobile Systems. "First, it's the best in the industry; second, it's real. Subwoofer marketing runs amok with exaggerated XMAX claims. Our specification is derived from measurable, verifiable motor dimensions."

The series' high XMAX is possible because the subs have two voice coils in two separate gaps, each responsible for half-wave reproduction at high excursion. This design topology allows one coil to fully leave the gap at maximum excursion. This provides greater acoustical output, with lower power compression and less distortion, than the topology of conventional subwoofers.

"These new GTi subwoofers are better than any other sub available for use in cars," claims Wehmeyer. "That probably sounds like everybody else's claim, but the evaluation of even the most complicated subwoofer is based on three fairly simple criteria: How accurately does it reproduce the input signal, how loud will it play, and how much power will it handle? In the case of GTi, the answers are as simple as the questions: better, louder, and a whole lot more."

Differential Drive Design is an important advancement in subwoofer motor technology. The dual-coil design, which uses two 3-inch-diameter voice coils, results in significantly higher power handling through increased surface area, and maintains the low-moving mass, which is not possible with single large-diameter voice coils. Because the subs feature two coils in two separate gaps, they provide twice the heat-dissipating surface area of single-coil subs with similar coil and gap dimensions. This offers a distinct advantage over conventional dual-coil designs, in which one coil is wound on top of the other in the same gap, and each coil layer heats an adjacent layer until the outermost layer transfers heat to the woofer's top plate. Because of the dual inner-hung positioning of the voice coils, JBL's DDD maximizes the woofer's linear excursion, which is important for SPL operation and sealed-enclosure applications. The GTi coils are biased inward, toward the center of the motor structure, creating more coil "innerhang" and, thus, nearly twice the linear excursion of conventional motor designs (in which the coil is centered in the gap). With moderate input power, both the GTi's coils drive the woofer in the same direction and operate in reverse electrical and magnetic polarity, creating less distortion than conventional subs do. However, at higher excursions, one coil can leave the gap while the other coil drives the cone to the excursion maximum.

JBL has also utilized its Symmetrical Field Geometry‘ (SFG) technology in the DDD to create a magnetic field that is absolutely symmetrical. This second generation of JBL's SFG significantly reduces distortion at high excursion. In this application, the two voice coils work together to produce large linear excursion by operating within summing magnetic circuits of opposite polarity. Since each coil works within its own gap, the design ensures precise linear control.

Using its proprietary Finite Element Analysis‘ (FEA) software, JBL carefully designed the cone, suspension, and frame to optimize the new GTi subwoofers' DDD magnetic system. In the end, JBL opted for a spider made of NPC (a blend of Nomex‘, polyester, and cotton) because of its strength and durability. Also new to the GTi Series are interwoven speaker leads (on W12GTi and W15GTi), ensuring long life and noise-free operation at high excursion.

The subs also feature nitrile-butylene rubber surrounds. With a blend of elasticity and internal damping, the surrounds are resistant to ultraviolet radiation and remain durable even in extreme humidity and temperatures. The oversize roll offers long-throw linearity to enable extreme cone movement while maintaining the proper amount of control and stability at excursion limits.

Formed in a proprietary manufacturing process, the Kevlar-impregnated paper woofer cones used in the series provide exceptional strength-to-weight ratio. The material also adds more stiffness to relieve stress concentrations that may accumulate under high-power and high-pressure applications, such as sealed boxes.

In order to house such an innovative motor structure, JBL needed to design a completely new frame. Long-suspension travel with maximum distance between the spider landing and the motor-mounting surface needed to be considered, along with the weight of the motor itself (approximately 35 pounds). The resulting die-cast-steel frame has precisely machined surfaces that provide maximum rigidity. This thick-wall construction offers maximum support to the motor while its fully vented design provides maximum movement of air behind the cone and spider for motor cooling. The design also allows maximum rear-window area to minimize acoustic compression of the cone's movement at high excursion.

Link for more info: www.jbl.com