The UM-15's will give up before the UM-18's will, and so it will start to distort earlier too, and ultimately place a limit on how loud the system will go before they become damaged or start sounding ugly.
...Unless you de-tune them to compensate for that difference, which is what I do with my fronts; this of course causes them to play quieter / less excursion so you don't over stress them.
Originally Posted by aaks38
compensates for the phase variation.
It won't, as the two drivers will have a slightly different response for any given input signal.
It is unavoidable in a mixed system.
I have 5 different subwoofer drivers in my system: the SAB-24, MalX-21's, LMS-18's, UM-15's and PA-460's.
It's not "ideal" but it still sounds really good, and is still loud as hell.
Too bad TC and Exodus went bankrupt before I could finalize. Is what it is...
Luckily though humans don't hear phase, they hear SPL.
I mean, everything causes a phase shift, moving your head 1 inch is all it takes to shift the phase of all drivers based on the changed distance "ray" paths, and changing seats or walking around the room? Forget about it!!!
It happens in every system, even if you had all the same subwoofers. Only one seat can be golden, the rest will just be average.
I mean I suppose you "could" put a nearfield subwoofer behind every seat in the room, that would help (up to a point). It's been done before!
Or you could implement a double bass array with quarter-wave placement.
but in summary: Adding more subwoofers, is always a good thing...