Dayton Audio PS220-8 8" Point Source Full-Range Neo Driver - Manual (Page 4)

The inductance components Le (x) and Bl(i) of most drivers have a strong asymmetric
characteristic. If the voice coil moves towards the back plate the inductance usually increases since
the magnetic field generated by the current in the voice coil has a lower magnetic resistance due to
the shorter air path.
The nonlinear inductance Le(x) has two nonlinear effects. First the variation of the electrical
impedance with voice coil displacement x affects the input current of the driver. Here the nonlinear
source of distortion is the multiplication of displacement and current. The second effect is the
generation of a reluctance force which may be interpreted as an electromagnetic motor force
proportional to the squared input current.
The flux modulation Bl(i) has two effects too. On the electrical side the back EMF Bl(i)*v produces
nonlinear distortion due to the multiplication of current i and velocity v. On the mechanical side the
driving force F = Bl(i)*i comprises a nonlinear term due to the squared current i. This force produces
similar effects as the variable term Le(x).