12-02-11: Dbphd
I'm not sure what's meant by fast in the LF range where wave lengths are long. Perhaps it means the sub is farther from the listener than the mains, but most processors let you account for speaker distances during setup.
Fast in the sense of rise time, which--regardless of frequency range--has to do with how quickly the cone accelerates, and perhaps more importantly, how quickly it stops. When you pluck a string bass for example, the fundamental plus its overtones are spread across the subwoofer, woofer, midrange were applicable, and tweeter. For ideal cohesion of just this single note, the cones of all four drivers should accelerate at the same rate; otherwise the overtones arrive at your ears before the fundamental. It stands to reason that a carbon fiber midrange or a beryllium dome tweeter with neodymium magnet is going to reach its excursion point faster than a 15" driver made of conventional materials with an average strength magnet seated in a less-than-rigid stamped frame.
If you look at the evolution of subwoofers, the emphasis has (generally speaking) evolved from cone diameter to absolutely massive magnets, heavy deep excursion cast frames, high excursion surrounds, and way more powerful internal amps powering much lighter, smaller diameter cones.
Even if you argue that rise time of the sub cone is irrelevant (and I think it's *very* relevant), you must at least be able to grasp that the higher amp power coupled with a stronger magnet and lighter diaphragm better controls diaphragm motion and makes the diaphragm stop when it's supposed to with less after-ring. This would result in cleaner bass more in keeping with the acceleration and stopping characteristics of the lighter midrange and tweeter, helping to prevent the sub from muddying the sound.
Also, a sub with a faster rise time also has higher frequency response which can be handy in blending with some L-R speakers. If a sub has a 12 dB/octave slope in its crossover, then if the crossover is set to the Dolby surround standard of 80Hz, its -6dB point is about 120 Hz, so the idea that a sub only need make a 40 Hz wave is simply not realistic.
