Real loudspeakers often have an inductance or impedance peak that can complicate crossover design
A Zobel filter is a circuit placed in parallel with the driver to cancel out the reactance that causes the changing impedance with frequency. Mathematically, this circuit has a reactance that is the complex conjugate of the reactance of the loudspeaker, cancelling out all but the resistance.
R-C Zobel
+ ------+-----+
| | +
Rz |
| Woofer (Re and Le)
Cz |
| |
- ------+-----+
Assuming Le (loudspeaker electrical inductance in Henries) is constant (it isn't) one would calculate:
Rz = Re
Cz = Le/Re^2
RLC - resonant peak filter
+ ------+-----+
| | +
R |
| |
L Tweeter
| |
C |
| |
- ------+-----+
This is a series RLC Circuit (parts order does not matter) typically used to suppress the impedance peak of a midrange or tweeter
R = Re * (1+Qes/Qms)
C= 1/(2*pi*Re*Qes*Fs)
L=Qes*Re/Fs