Bl: force factor

Bl is the product of the magnetic flux density in the gap, B, and the length of wire immersed in it, l. It is the conversion factor between electrical current and mechanical force: one ampere through a driver with Bl = 10 T·m produces 10 newtons on the cone.

It can be recovered from the other parameters:

Bl = \sqrt{\frac{2\pi f_s M_{ms} R_e}{Q_{es}}}

What a strong motor buys

A larger Bl means more force per ampere, so more efficiency. It also means stronger electrical braking of the cone, so a lower Qes and a lower Qts. The two effects are the same mechanism seen from two directions, and it is why high-efficiency drivers are nearly always low-Q drivers that want vented boxes.

What it costs

Bl is bought with magnet and with copper, which is to say with weight and money. There is also a ceiling: past a point, adding motor strength damps the cone so heavily that the response rolls off early, and you have traded bass extension for sensitivity. That trade is Hofmann's Iron Law again.

Why it is not constant

Bl is quoted as a single number, and Baffle's model treats it as one, but it falls as the coil leaves the magnet gap. This variation with displacement is one of the main sources of distortion in a loudspeaker, and it is the reason Xmax matters as much as it does. Modeling that properly requires large-signal parameters, a Klippel measurement or equivalent, which is beyond what a small-signal lumped-element simulator like this one attempts.