Dayton Audio ND90-8 3-1 - Manual (Page 9)

Asymmetrical nonlinearities produce not only second- and higher-order distortions but also a dc-part in
the displacement by rectifying low frequency components.
For an asymmetric stiffness characteristic the dc-components moves the voice coil for any excitation
signal in the direction of the stiffness minimum.
For an asymmetric force factor characteristic the dc-component depends on the frequency of the
excitation signal. A sinusoidal tone below resonance (f<fS) would generate or force moving the voice
coil always in the force factor maximum. This effect is most welcome for stabilizing voice coil position.
However, above the resonance frequency (f>fS) would generate a dc-component moving the voice coil
in the force factor minimum and may cause severe stability problems.
For an asymmetric inductance characteristic the dc-component moves the voice coil for any excitation
signal in the direction of the inductance maximum.
Please note that the dynamically generated DC-components cause interactions between the driver
nonlinearities. An optimal rest position of the coil in the gap may be destroyed by an asymmetric
compliance or inductance characteristic at higher amplitudes. The module "Large Signal Simulation
(SIM)" allows systematic investigation of the complicated behaviour.
Bl Symmetry xb(x)
This curve shows the symmetry point in the nonlinear Bl-curve where a negative and positive
displacement x=xpeak will produce the same force factor
Bl(xb(x) + x) = Bl(xb(x) – x).
If the shift xb(x) is independent on the displacement amplitude x then the force factor asymmetry is
caused by an offset of the voice coil position and can be simply compensated.
If the optimal shift xb(x) varies with the displacement amplitude x then the force factor asymmetry is
caused by an asymmetrical geometry of the magnetic field and can not completely be compensated by
coil shifting.