Originally posted by Kevin C Brown:
I always thought that the reason why you wanted to be (at least) one octave up from the - 3 dB point, was that if that point is too close to the slope itself the crossover provides (where the mains are still audible), that you can get the effect of cascading filters.
That's correct. It's what I meant by 'range where the first part of the slope can be controlled better'.
But, if your speaker begins to roll off at 40hz and it rolls off at 12db / octave, you can use a 40hz 2nd order highpass and match it with a 40hz 4th order lowpass on the sub and the result should be a symetrical 4th order xover. In order for that to work you need to have control of the xover slope and point and know where and how rapidly your mains fade away, so it's not always easy.
Some THX systems do this at 80hz with good results though. Assuming there is no max SPL issue of course, which is less likely at 80hz.
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