what is xmax?
in a typical subwoofer, the voice coil is taller than the gap created between the pole piece and the top plate.
the distance that the coil "overhangs" the gap is called the "overhang". :-)
when the sub is powered with a signal and starts to move forward, the overhang descreases until the coil is flush with the gap. that is where some companies specify the "xmax"--the amount of the overhang.
however, at that point, the entire gap still has coil in it, so there is plenty of room to "keep it moving further". as the coil continues to move forward, it will eventually leave the gap altogether. when there is no longer any coil in the gap, there is no magnetic interaction, so it is completely out of force at that point. force = 0% at that point.
somewhere in the middle, usually when the coil is about 1/3 of the way into the gap, the force = about 70% of the level where the coil is completely filling the gap. based on distortion measurements that point also correlates to about 10% distortion. so that point is about where xmax is specified for many drivers.
example: let's assume the coil is 20mm long and the gap is 10 mm high. 20-10 = 10mm, 10mm/2 (because of overhang on the top and the bottom) = 5mm. that is the amount by which the coil will overhang the gap on each side. so using the overhang method, xmax = 5mm.
however once the coil moves 5 mm in either direction, it is still in the gap. so it can keep going about 1/3 of the gap before it starts to decline in force and increase in distortion. 30% of 10mm is 3mm, so in addition to the 5mm it can go another 3mm for a total of 8mm xmax.
so one company could say the driver has xmax = 5mm and another could call the same driver xmax = 8mm. however, they could both be describing the same driver. so it is important to know which method is being used to describe the xmax when making comparisons.