i'm currently playing through an aguilar db750, and i'm looking to trade in some gear for 2 new cabs with different levels of power handling. i'm still undecided, but let's say i go with a 212 and a 115. (600w and 400w respectively). since there are no individual output level controls on the speaker outs of the head, equal amounts of juice are being put into each cab, yes? so, how can i get the most out of the 212 without blowing the 15? or would i be stuck under-powering the 212 a bit for the sake of the 15? unless of course i'm totally misunderstanding how this works, which is a strong possibility. thanks for the help!

my understanding is this: lets say your head puts out 500 (i know nothing about aguliar) 250w will go to the 212 and 250w will go to the 115. although i could be completely wrong....

It depends on the impedance of the cabs. If they have same impedance lets say 8 ohms. They will both get 50% of the total load, but it is likely that the 2x 10" has 4 ohms (since there are possibly two 8 ohm speakers in parallel). In that case the 4 ohm cabs would get 2/3 of the total power and the 8 ohm 1/3. To get more technical. The total impedance will be 2.667 when we combine a 8 and 4 ohm cab. Now we assume 600 watt. Then using P = I2 * R we get P/R = I2 -> sqrt(600/2.667) = 15 Amps of total current. Now U = I * R -> 15 * 2.667 = 40 Volts We have the basics. I = U/R we will use this one the each speaker. For the 4 ohm we get 40/4 = 10A and the other one is 40/8 = 5A. Then using P = U * I we get 40 * 10 = 400 Watts for the 4 ohm speakers and 40 * 5 = 200 Watts for the 8 ohm speaker. Still awake I now there a an easier way the calculate this. I is reversely correlated the the impedance. So two times more impedance = 2 times less power, but I am not sure this is a scientific approach.

Why take the trouble to learn anything? Just keep spending the money down at the music store - they'll tell you what to buy.

Assuming that the nominal impedance of each cab is the same then they would each recieve equal wattage. In that situation a wise person would limit total output to the lesser of the two ratings. Of course, in a real-world application it will likely not be the RMS thermal rating that determines the maximum usefull volume of your cabs but rather maximum excursion. if you are runnign a bass-heavy EQ it is theoretically possible that the 212 with the greater power handling could begin to fart out or generally sound lousy before the 115. Besides the additional volume that would be ained by running the 212 at 600W instead of 400W would be negligible. Again, this is all from based on the assumption the the two cabs' impedances are the same, but if you like the way they sound together then I wouldn't worry too much about their power ratings being different. If you follow rule number 1 for bass amplification: Don't be an idiot. With the corollary: If it sounds like you are damaging something then you probably are. Then you should be fine.

With that amp you would push 375W RMS to both cabs if they are 8 Ohm each. I'd choose cabs that can handle that amount and not be worried about using or not using all the 2x12" can offer. Of course DB750 can handle 2Ohm load and pump 975W at it, so you could get 4 Ohm cabs. In theory you could blow a 400W speaker with this power (487,5W per cabinet), but in practice you will: 1) not turn the amps volume to the absolute maximum 2) not push constant 487,5 RMS power to the cab unless your bass lines have no groove and dynamics in them. Then you could also use the 4 Ohm 600W 2x12" as stand-alone when needed and could push it to it's limits when needed with your amp.

I don't think Aggies 1x15" come in 4 Ohm though, if that's where you are heading. You could also get: 2x12" in 4 Ohm 1x15" in 8 Ohm 2x10" in 8 Ohm That's a total of 2 Ohm if all put together and can handle all the power you can give them. Also there are several smaller combination possibilities with this selection (only the 2x12" being one and 2x10" plus 1x15" another for example). 2x12" with 1x15" would probably not work very well with this, since 1x15" gets less power, but you could try - might be a good solution!

And since you seem to like math, don't forget that cab efficiency is also important. A cab with 103 db efficiency will need half the power than a cab of 100db efficiency.

You are, but you're in good company, as most players don't. The power handling rating of a cabinet is the thermal rating of the voice coils, which has little relationship with how much power they can actually make use of. What does determine usable power is driver displacement, but unless you know exactly what driver(s) are being used and what their displacement (Vd) values are there's no way of knowing what the cab's capability is, short of just turning it up until it farts out. This, along with some of the other reasons already posted and others not mentioned, is why in the vast majority of cases you can't make an accurate assessment of how two different cabs will work together without actually trying them.

Unless its just published sensitivity which often is so far from actual sensitivity that no difference actually exists between the two cabs ; } Also, and as Bill has just noted, what matters more is often excursion abilities.

it's all about the math. i wouldn't worry too much about it. at a 400 watt rms rating the 15 should be ok. If both cabs are 8 ohms you won't be putting more than 400 watts into either. If the 212 happens to be 4 ohms and the 15 is an 8 ohm model then the 212 t will get twice the wattage as the 15 and at a load of 2.67 ohms the head will be happy. (IIRC, the 750 is stabel down to 2 ohms). you just have to try it to see how you like it. I have used many different cab combinations over the years and found most to be ok. occasionally you find that one cab overpowers the other substantially, but not as often as you'd think.

So, hypothetically, if someone were running a head that is 500W at 4 Ohms, when they run two 8 ohm cabs, both would receive 250W?