Xmax: linear excursion limit

Xmax is the distance the cone can travel from rest before the motor stops behaving linearly. The usual definition is the overhang: half the difference between the voice coil winding height and the magnet gap height, so that the coil remains fully immersed in the gap's magnetic field.

Past Xmax the driver does not stop or break. It gets progressively less linear: Bl falls as the coil leaves the gap, the suspension stiffens, and distortion rises quickly. Xmax is the point where the behavior stops matching the model, which is exactly why Baffle draws it as a limit line.

Why it binds at low frequencies

For constant output, excursion quadruples for each halving of frequency. A driver that is nowhere near its limit at 100 Hz can be well past it at 25 Hz at the same listening level. This is why almost every maximum-SPL curve slopes steeply upward at the bottom: excursion, not power, is what runs out first.

One-way or peak-to-peak

Conventions differ, which makes datasheet comparison hazardous. Baffle uses one-way peak excursion throughout: a quoted Xmax of 6 mm means 6 mm from rest in each direction, a 12 mm peak-to-peak total. Some manufacturers quote the peak-to-peak figure, which looks twice as good. Others quote an Xmax "with 10% distortion" figure, which is larger again and is not the same quantity at all.

The vented-box trap

In a vented box the cone is barely restrained below the tuning frequency: the port stops loading it and the cone flaps almost freely. A signal at 15 Hz (a door slam, a record warp, a film effect) can drive a driver past Xmax at a volume that sounds moderate. See subsonic unloading. A high-pass filter below tuning is the standard remedy, and it is not optional on a vented subwoofer.