Using Winisd To Calculate Impedence Curve

Sounds off to me.

What driver, power, and box is it? I will model it and either screenshot it, or let you know what I come up with.

 

no, that sounds perfectly normal actually.

If you model up a sub free-air, that subwoofer will have a large rise in it's impedance corresponding to it's resonant frequency (Fs).

If you model the sub up in a sealed box, that resonant frequency will shift, as it does in an enclosure. You are changing the way the cone can respond to the voice coil, therefore putting more resistance on the voice coil moving, therefore changing it's movement in the magnetic gap in different frequencies, which effects it's electrical current flow at different frequencies.

If you model the sub up in a ported box, you'll see something even more interesting.
You'll see one impedance peak below the tuning frequency. Then you'll see a big dip (almost all the way down to the Re value even) right at your tuning frequency (actually it's not too far off from Re for bit of a range, right around the tuning frequency, between the resonant peaks), and then above the tuning frequency you'll see an even larger impedance spike.
The higher you raise your tuning frequency, you'll see the lower frequency, slightly smaller spike shift upwards and taller a tiny bit, but the one to watch is that higher frequency impedance spike, as it grows significantly taller (and also moving slightly up the frequency spectrum) as you move your tuning frequency upwards.

All related to how the enclosure affects cone motion...
Which therefore affects how the voice coil is actually moving in the magnetic gap...
Which therefore presents different impedances to the amp at various frequencies in various enclosures. B)