Perfect example. While just about any pair of decent cabs will sound OK when stacked, if you want really exceptional performance, you have to sweat the details.
In the case of Greenboy's cabs, the boxes are carefully designed to be complimentary, even to the point that the second cab is given a reversed baffle when used for stacking- so that the mid and HF drivers line up for optimal dispersion. Testing has proved that the placement for the smoothest and best response comes down to fractions, not inches.
This is the way really great, not just good, speaker systems are designed, whether we're talking about a bass rig, a concert sound system, or home audio. For anyone who's had the privilege of auditioning a really first-rate setup, it's well worth the extra effort.
Coming from a product manager at a very prestigious and well-known speaker manufacturer, this so far has been the only complete and true thought mentioned here yet without getting bogged down and lost in the details and what-ifs.
The statement that a GOOD cab should be, self-contained, and well engineered to ITSELF, is in fact the only consistent guarantee in the whole mess. Just like different size/type drivers are used, configured, and crossed over (internally) to create a flat, balanced, and well covering speaker, the same consideration needs to be given to the stack.
As soon as you put one speaker on top of another, you start dicking with the output of the drivers with regard to the balance of one cabinet to the other. Not because they physically affect each other, (at least they shouldn't) but the output of one can, and will play a factor in the other.
I've attached an example of the mapping of a Line Array system I was working out yesterday. Take a look at the nice big old green area, and then look closely at the line array modules top to bottom. Two speakers, of any kind, should lend themselves to the other speaker in the general area otherwise, they WILL conflict! Thats to say that the output SHOULD be balanced and additive to the other cabinets in the array. If you were to say, through in an unbalanced, or unique, cab in there, those small red interference lines will and do, imbalance the field output.
The reason cabinets with different size drivers in them receive cross-overs is to control the output of each driver to handle the correct frequency range that they are designed to take care of (HF regions, MF regions, or our favorite, LF Regions).
In summary here, and staying really simple, if you have a 410 cab on top of a 115 cab (or any other mixed sized driver combination), you are INDUCING conflicts between the cabinets! This is you doing it, not the cabinets doing it by themselves. Think of it as if you just put a baffle between a 15" driver and 4 10" drivers in the same box and expected the output field to be even and equal. They will NOT be. If you physically separate those cabs, you will start to see less interference in the frequency response of the cabinets cause they produce less phase interference against each other.
Long story short, if your two different size cabs produce EXACTLY the same SPL, at exactly the same Frequency Response, at the same power consumption you should be JUST fine as far as the human ear is concerned. However, if your FR is in any way different (usually the case with different size drivers, check your cabinet spec sheets), you can and will start creating weird interactions at any random location inside the directional field of the driver combination, ie. your listening area in front of the speakers. This is what creates the weird phasing and changes in tone. You'll notice that the issues change as you move from left to right and near field to far field. I would HIGHLY suggest, unless you're an audio engineer who is going to put in a dsp unit to correctly crossover your cabs together and apply the corrections via IIR or FIR filters to flatten both cabs together (not your "EQ" on your head which will affect each cab collectively), you should take the guesswork out of the whole thing and go with the same driver size in all the Cabs. Even though this still isnt perfect, its a lot less risky than assuming or guessing. The next best thing is to reach out to the manufacturer and see if they can provide you with the field relationship information on mixed sized driver cabinets. Any good manufacturer selling a product FAMILY, should have done that information and testing themselves already to ensure their products can be used together seamlessly. However, if someone IS interested in mixing different size cabs (by the same manufacturer and model family...) let me know and I would be GLAD to help guide you to the right DSP you'll need and provide the right corrections to be "perfectly" matched. Im staying away from mixing manufacturers and driver types, and all that craziness, at least without getting paid! Lol.
Just my two cents! Btw, I'm going to be "mixing" the Hartke HyDrive 115 and 410 together here next month in the chamber and getting some dsp settings to maximize the rig for my own use. I'll let you guys know how that turns out if anyone is interested! I'll show you the corrections I use to get a good crossover. I'm suspecting the 410's should only need a small low-pass and the 115 a decent hi-pass to eliminate the mixing. Remember guys, save your EQ for the Room! Not your speakers!
I'm just trying to be nice and not poop on anyone who has a mixed cab rig.
.
