Powering all of this is an equally well-conceived motor system. JL Audio employs about as sophisticated engineering technology as any of the companies in the subwoofer business. This means they employ not only F.E.A. (Finite Element Analysis) CAD software for magnetic, thermal and mechanical design of woofers, but also have developed over a period of time simulation software written internally. All of this comes into play with the design of the motor structure, which for the JL 18W3 includes using dual-stacked magnets, each 122 mm in diameter and 22 mm in height. A brightly polished forged T-yoke and 11mm front plate conduct the magnetic field and focus it into the gap area. The pole structure part of the T-yoke includes the 12 lateral vents and an 11/16" diameter center vent that are an integral part of the VRC+ system. This structure includes a 9/16" height bumpout ("Powerbump" in JL speak) to accommodate long rearward excursions. With a substantial BL of 20.3 TM (measured with the two coils in series), the motor structure drives a 2.5" diameter 4-layer dual-winding voice coil wound on a Kapton former. Each winding is 1.67 ohms, or 3.35 ohms when connected in series (as I did when I measured the parameters of the JL 18W3).
T/S parameters were measured using the LinearX LMS analyzer and the new LMS4 Windows software (a big improvement over the original DOS version) and LEAP CAD software. Impedance was made using a LinearX VIBox (current source method), which means that the analyzer was connected directly to an amplifier driving the woofer. Rather than creating a single impedance curve, the procedure produces separate voltage and current curves at a 1V drive level that are divided (V/I=Z), using the LMS post processing menu. Once the free-air and test box impedance curves were created, the two curves were transferred to the LEAP program to calculate the T/S parameters. The resulting parameters and computer box simulation data are given in the Data Chart with the LEAP sealed/vented box simulation graph shown in Figure 1.
The measured parameters shown in the Data Chart were used to produce two different box simulations, a 3 cubic ft. sealed box with no fill material and a 4.75 cubic ft. vented box tuned to 26 Hz. Both box sizes are recommended by JL in their literature, but many other combinations are possible. With a Qts in the vicinity of 0.4, this driver could be made to work in almost any style box, but I tend to think that the best and most compact would be the 3 cubic ft. sealed box, which produces an F3 of 43 Hz with a moderately damped box "Q" (Qtc) of about 0.78. This was with no fill material.
If you fill the box with 50% R-19 type fiberglass, the Qtc drops to 0.72. While the lower Qtc also means more excursion, this really isn't a problem for the JL 18W3. The maximum linear operating level was 118.8 dB where the driver reached its maximum linear excursion limit of Xmax + 15% (14.7 mm), but was reaching a voice coil temperature that indicates that this woofer is more likely to be thermally limited than excursion limited. However, given the extent of the cooling system in this monster woofer, I definitely wouldn't want my ears, or yours, in the car when you finally heated it up sufficiently to fail. This is a very powerful device and should be treated with respect.