Thanks Ivan - perfect answer, and exactly what I presumed. So if I check the specs of any major brands mid drivers highest xover point, the spacing will be or should be 1/4 wave or less? Is that an absolute design rule?
Also, if we apply this 1/4 wave rule to our sub placement - we could just line them up any where on the ground +/- a few feet (4' @ 75hz) and be ok..... but we don't.
Back to the original question - would an asyemtrical design be any better simply becuase it could never exhibit any of the symetrical "2 points/same band" issues as you move to the widest point of coverage where the distance becomes greater - even if the drivers are 1/4 wave or less?
There is a bit of a "grey area".
1/4" is (should) be the design target. If it is a little larger-then there will be more interference.
How much more? It depends. How much is tolerable? it depends.
When some says "turn it down". Is 1dB enough? You "turned it down-but not enough for them to notice.
what about 10? Would 3 do it, or maybe 5.
As I often say, there is no simple answer that is correct.
But you need a guideline.
Many people are simply not aware of the "patterns" they are creating by placing subs where they do.
Sometimes you have no choice.
Often times there is no "right for everybody" position. It can be good for some people, but bad for others.
That is why it is ALWAYS best (no matter the manufacturer or product) to use a SINGLE sub and SINGLE full range cabinet.
As soon as you start to add more-you start to add problems.
But sometimes the end result is better than the problems it creates.
What IS important-is to understand how the interactions work, the penalties for doing so etc.
This way at least you have an educated idea of what to do and what might be the best compromise.
If you don't understand what if going to happen with various placements, then you are just "hoping" for something that may or may not happen.
VERY OFTEN, the result of adding more speakers is NOT AT ALL what most people think (or would like) will happen.
It is MUCH more science than voodoo. An accurate model (that is taking phase into account) will show these interactions.
A simple "amplitude" model will not show this and "assumes" (wrongly) that everything is "playing nice" with each other.