Back EMF causes current flow in opposite direction to one that would cause motion of the membrane in given direction. This current will produce force moving membrane in opposite direction - braking movement completely, like shorted motor. Except, it is not completely shorted - voice coil impedance is in series with it.
Resistive impedance of the speaker's voice coil is about 2/3 of the rated impedance. Since this resistance is in series with the back EMF it will reduce overall damping factor to 1.5 max. We don't want to make it much worse than that, but DF=15 amp would make it only 10% worse.
DF specification might be important for other reasons but for the reason of the damping is pretty much useless.
Resistive impedance of the speaker's voice coil is about 2/3 of the rated impedance. Since this resistance is in series with the back EMF it will reduce overall damping factor to 1.5 max. We don't want to make it much worse than that, but DF=15 amp would make it only 10% worse.
DF specification might be important for other reasons but for the reason of the damping is pretty much useless.