For reference, since you are limited in bandwidth:
Extremis: Centered around -1.25mm, around 9.4 TM at that point. Rises to around 10.2 TM at +4mm, and rises to a bit over 10TM at -6mm excursion. Falls sharply to 70% at +8.5mm, and -11mm, but suspension limits it to +8 and -7.5.
XXX: Centered right around 0, around 3.8 TM at that point. Rises to ~4.4TM at +7mm, and ~4.1 TM around -7mm. Falls to 70% at +/- 12mm. Suspension limited to +12, -10.
Excel: Center at 0, 7.7 TM at that point. Stays flat to +/-5mm, then a gradual rolloff with 70% occuring at +10, -11mm. Suspension limited to +9, -8.
CSS FR125: Center at 0, BL of 4.05. Flat to 2.5mm, rolloff to 70% at +/-6mm.
I was a bit curious seeing those BL curves, but after checking things this also shows up in the Dumax tests of drivers like the Brahma (Brahma Mk I Dumax is on Adire's site). And the Extremis Klippel analysis looks almost exactly the same as that of the Brahma on the PDF file, though it is a bit more extreme in variance. In case you don't have it on tap, the Brahma Mk I Dumax has around a 5% variance within +/- 7.5mm excursion, then past that BL rises to around the same value of the motor at center at around 15mm each way (the "ripple", if you will
). Enough of a variance to be notable as something other than a picnic table ]Coil length relative to the gaps is absolutely 100% critical.
If you make the coil slightly short, you develop a dip in the center - heck, you could make the coil short and create a twin camel hump out of the
curve.
If you make the coil slightly long, you start to create a curve that looks like... well, a nipple - one that shows a bump within a short range of excursion (where "more coils" are suspended in the gap), that then falls quickly down to a lower level where it plataus for a while (once the coil moves far enough to stabilize the number of coils in the gap) before rolling off like it normally would for an XBL^2 curve. You could create an XBL^2 driver with MORE intermodulation distortion than a traditional sub, if you wanted.
I can't speak to the design intent of those drivers you cite - I'll assume all the tests were run appropriately (I've talked to npdang about Klippel testing, and he's aware of the statistical centering issue, and how to mitigate it, and DUMAX tests on Adire's site were usually backed up with a second test for confirmation).
Bear in mind that there are other factors to consider when designing a sub - particularly with regard to impedence... the impedence you get is a function of how much wire you have. Remember JL's explaination as to why the W7 has such an odd impedence? Their goal was to create as perfect of a subwoofer as they could, and those impedences are what resulted.
The designer of the speakers you mention may have had that as a higher priority...
With our sub, we worked the dimensions to our advantage - our Re is only slightly high for a dual-2 - which should work well for most people, it's an easy load on an amp.
I attached two plots for you to view, direct from our FEMM and modelling tools:
BABL.jpg shows the designed BL curve, given our exact coil dimensions, all magnetics and hard parts considered.
BABL2.jpg shows a magnified view of the BL curve "ripple" - note the scale on the Y-axis!
