Filter Equations.

I would like to see a general discussion of the math behind crossovers.
I have scoured the internet and cannot find anything on them! I need a low pass filter with 12db roll off at 40hz. 12db because i heard that that puts the phase 180 out, which is easy to correct by swapping wires. what sort of components will it need?
I cannot hink of anything else to ask, so i will check back often!
thanks guys.
two pole filter is 180 degree phase shift only at the -3db point.
It will reduce gradually as you move in towards the pass band and at about F/10 phase shift is almost 0degrees.
Look up National Semiconductors application notes AN779, oa21, oa26, oa27, oa28, oa29, Burr Brown ab034b, ab017c,
Plenty reading & maths in that lot.
good point. so an active filter which would go before the amp will be cheaper perhaps? but require an extra amp.
(i have the extra amp but am not nuts about taking it back to school. the nine cubic foot sub box is bad enough!)
In my room it sits behind and to the left. I have it crossed over at about 200 and it is clearly directional, plus boomy because of the poor quality materials and general construction. Maybe i do not need 30hz?
PE sells an active crossover simulator which i believe you wire in between your source and the amp. I could buy a pair of those as a low pass. A solution?