An Impedance Equalization Circuit is used to counteract the rising impedance of a voice coil caused by inductive reactance. The cause of this impedance rise is due to the speaker's voice coil inductance (Le).

For a midwoofer you calculate a Lowpass Butterworth 2nd order filter with a cutoff frequency at 4.5 kHz with the nominal impedance Re (from datasheet) of 6.0 ohm

The calculated inductor is 0.21 mH for 4.5 kHz cutoff frequency.

Impedancecurve Fullrange driver without Zobel compensation.

Take a look at the impedance plot of this fullrange speaker. The impedance at 4.5 kHz is not 6.0 ohm, but 9.3 ohm!

At this impedance you inductor has to be 0.33 mH and not 0.21 mH
This wrong inductor of 0.21 mH would create a crossover frequency of 2.9 kHz and not as assumed a crossover point of 4.5 kHz

The importance to use impedance compensation should be clear now...
Re = Ohms R = Ohms
Le = mH C = uf

the crossover filter works better, because the driver impedance is more stable.

Impedancecurve Fullrange driver with Zobel compensation.

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