Here's what the seller says:
Well, like all drivers, the harder you drive, the greater the distortion; the linear range is +/- 12mm, but the mechanical range is far higher. However, it is only at very low frequencies that the cone excursion will get to be high for a given power level and distortion is very inaudible anyway at these frequencies, (as I said - this applies to all speakers - no matter how good!) The drivers will go astoundingly loud and the enclosure should be as small as you can make it for the best results, (which is difficult actually as the port tube, if you want a ported design, must be folded around inside the box). We are going to do an enclosure, but not yet. The other auction, if you look closely is for TWO drivers, but the nice piccies are on the single driver ebay entry
