first off impedance doesnt just rise. impedance rise is most often refered to in SPL competitions because they play their subs at a certain frequency. they measure what the load rises to and then can add more subs, add more power, or whatever they want.
on music impedance rises and falls. its called a "reactive" load. all the numbers im about to spout off are arbitrary(they dont mean **** except to make this example work). say you wire your sub to 2 ohms. using a multimeter with the sub at rest it will read ~2 ohms. now, using that same sub hook the miultimeter to the coil(s) and push down on the cone. what happens? the impedance CHANGES. and chances are it rose. now say you put that sub in a box and played a 30hz tone. you measure the impedance across the terminals while the tone is playing and most likley(depending on what sub you have, what enclosure type you have, and a few other factors) you will see an impadance HIGHER than 2 ohms, lets say 4 ohms for shits and giggles. now you play a 60hz tone. read the impedance again and you will most likley see an impedance LOWER than 4 ohms and possibly(again, depending on sub,enclosure, blah blah) lower than 2 ohms.
the real fun starts when you play a tone and add in kick drum or play two tones at once. ive played with reading impedance on music a bit and my "4" ohm woofer read as high as 16 ohms on music. now keep in mind multi-meters arent exactly the eaisest things to get an accurate impedance read on, so the actual impedance might have only been as high as ~12 ohms constant with a fast peak to 16 ohms. BUT that same 4 ohm woofer also read as low as 1.5 ohms.
everyone needs to realize that impedance is NOT a one way street. it rises and falls. the ONLY time you can use rise to your advantage(aka running a 1 ohm stable amp at .25 ohm nominal) is in the lanes.
oh, and one more thing. you may "set" you amp to put out 1200W at 2 ohms. but in reality you wont see more than 1/4-1/3 of that much power on music(except decaf or boosted songs). setting your gain with a -3 or -7DB tone can help you make up for this, but BEWARE OF CLIPPING. invest in an O-Scope(preferably) or SMD-DD1 to make sure you arent sending a clipped signal to your sub or speakers.
Matt
