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Dave, there are many factors to consider really.. such as some of the following:
the amp is only 400 watts (rated power) at peak output, continuous.
power output decreases drastically as you lower the volume, so usual power consumption is closer to probably about 50 watts RMS.
Impedance rise will be one factor in power output, but so will the enclosure type, and the specific frequency being reproduced by the speaker at the moment in question.
remember "4 ohms" is a nominal or average impedance. the actual load is reactive, and varies constantly. a "4 ohm" sub at it's Fs can drop to less than 2 ohms in an instant, and rise to as high as 8 or more ohms as the frequency rises toward the HPF point, or the sub's natural rolloff. whichever comes first.
music is also a very dynamic and transient sort of source material for an amplifier and speaker to reproduce, so that will also cause actual power to vary vastly from moment to moment, altering current draw by more than ten times the difference between crecendos and quieter passages. Now if that weren't enough, you also have unregulated power supplies to contend with, cabin gain, and if you're an SPL competitor, dealing with wave length calculations for wavelength point of execution figures at mic positioning distances, and other fun stuff like that.
Short of it is this: If you use the proper filters and enclosure for subs, impedance rise shouldn't be a major concern unless as I mentioned, you're competing for SPL events professionally and every tenth of a dB is life or death.

This is somewhat confusing then. People tell me running 400 to each sub will be perfect for the Tempest, but Impedance rise is never brought up in the discussion...it's just really disapointing knowing that the power I thought the amp would put it could be cut almost in half by impedance rise..I'm suprised this discussion has never been brought up before..

i believe mtx amps are rated at 12.5 volts and most car charging systems charge between 13.6-14.4...so the impedance rise may be overcome by a small increase in power from ur charging system. losing a total of 300 watts sounds like a lot to me tho.
TEDD

Two dvc 8ohm subs wired in parallel give a 2 ohm impedance. That's not my question though. Kiki helped a lot by explaining what an inductor does. I guess i'm just wondering if I do go with the MXA8001 like i planned..it puts out 800x1@2ohms, but would each sub only actually see around 250 after impedance rise without an 18 db octave? How much would that run me if its needed? Also, if I use one would my subs actually be seeing 400rms each like I was hoping for? I never hear impedance rise being considered when someone is picking out an amp. I hear that MTX amps are somewhat underrated so maybe even with the impedance rise i'll have sufficient power running to each sub?

find an amp rated at 2 ohms mono. when u have the tempests wired in series (i think..correct me if wrong) thats what the final impedance will be.

Finding an amp wont be too much of a problem for me. I would consider the JBL 1200.1 if they started selling the refrubs on sounddomain again, but other than that i'll prbobably bet the MTX MXA8001. I can get a refurb pretty cheap on ebay from a good store.

well being that Tempests are dual 8 ohm subs(i have one myself in a box tuned to 28hz), you will need to find an amp that puts out a decent amount of power at 4 ohms if you want to put some power to em... depending on how much you have...... look at the JL 500/1.... it puts out 500rms at all ohms and would give each of the Tempest 250rms(probably more since they are underrated).... but for the money ofcourse... the JBL 600.1 would be your best choice.... i plan on running it for my single Tempest and maybe adding another Tempest.. or if you really want to give em the power, see if the JBL 1200.1 is in stock at Sound Domain(also where you should purchase the 600.1). i hope i helped! :s_thumbup

hmmmm

An inductor, also known as a "coil" or "choke", is a coil of wire which may or may not have an iron or ferrite core. Inductors with an iron core are called "iron core inductors" while those without are called "air core inductors". The basic characteristics of both types is the same. As the frequency passing through an inductor increases, so does the inductive reactance of the coil. This rise in impedance at higher frequencies allows us to use the inductor as a filter that passes low frequencies but chokes off high ones. A speaker's voice coil is also a wire-wound coil that has inductive qualities like the chokes used in crossovers. Because of this, if you don't use the proper crossover for a subwoofer, you'll encounter an impedance rise as the frequencies passing through the voice coil rise.
If you have a properly designed enclosure, and use a steep Q LPF, like 18dB/octave which is sttper than the speaker's natural roll off, and have it set to about 60Hz, you shouldn't have any issues.
If you're competing strictly for SPL and have no concern about speaker or amplifier life or sound quality, then yes you can run at lower loads to compensate for impedance rise. You can also tune quarter wave enclosures and do many other tricks to squeeze everything out of the system you can, but that's strictly the sort of thing you do to win events in SPL with no plan to use the system for actual musicality.

If someone can comment and help my understanding of this concept a bit it would be nice. I will soon be running two tempests ported off an amp that puts out 800rmsx1@2 ohms. I was told to only expect 250 to each woofer because of impedance rise? This has never been brought to my attention before. I was also told that if I wanted to compete I should run at half the rated impedance and a good 4ohm amp is what i need to run them. This is all new to me so anything that can help my understanding a bit would be appreciated.