Dayton Audio ND91-8 3-1 - 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).