how would you go about calculating impedence rise??

In Winisd pro, under Help->contents->tenchincal, it explains how loudspeakers are modeled as equivalent electrical circuits.. You will see that the winisd model isn't all that complicated.. For a better explination, as well as notes on other factors that the winisd model does not include, I'd recommend looking for good books on the subject..

I dobt there is an equation, My program BassBoxPro shows you graphs of the impedance curve across the frequency range!

but this programs retails for $1800 not like Free WINISD ect/

As for calculating I'm not too sure, but here's a discussion taken from TermPro as to how to measure impedance curves vs frequency:

http://www.termpro.com/cgi-bin/ubb/ultimat...ic;f=6;t=000818

"posted by EMU 09/02/03
--------------------------------------------------------------------------------
Here's the easiest way to do it. Invite a bunch of buddies with DMM's to your house for a few brews. Then take one DMM, and place in series with the sub. Set this one to read current. Take a second DMM and place it in parallel with the sub. Set this one to read voltage. Now do a burp at very low levels, just enough so you can see the cone move about 1/8". Read voltage, read current. Divide voltage by current and hazzah. Do it at 38 Hz, swap boxes and do it again at 52 Hz.

BTW, a subwoofer cannot handle as much power when it is played near the box's tuning frequency. The reduced cone motion means reduced cooling. Impedance rises, effeciency also rises. There is nothing wrong here and nothing should need to be corrected. "