I'm building a two way OB (fullrange with a woofer assisting) and had a quick question regarding crossover points. My assumption is to keep the crossover point as low as possible and let the Fullrange do as much work as it can comfortably.
Is this correct? What other factors should I look at?
That's about right, but you don't want it too low or else dynamics and SPL will be limited, as well as risk damage to the FR if too much power is applied at LF. If you need high SPL for loud rock, then raise the xo to limit excursion. Just remember that the FR has its own natural rolloff, which will combine in steepness with any electrical xo you apply. You want the net freq response of the FR and the woofers to be flat throughout the xo band. But there may be baffle step and OB cancellation happening near your crossover freq also.
If you are not a head banger, you might consider playing the FR naked, with no electrical filter. Then it's own natural rolloff will be its high pass. Then adjust the woofer low pass filter to blend with the FR driver. The FR rolloff will be the sum of falling acoustic impedance, baffle step and OB side wave cancellation, so it will be pretty steep where these three combine. You have to be careful that you don't overdrive it. Usually you can hear bottoming before any damage, but that's risky. Proceed carefully until you get to know what it can handle safely. Some FR have very limited excursion. Others will have no problem if you mind their recommended power rating.
If you make the crossover freq equal to the baffle step freq then you can simplify the BSC by applying it only to the woofers. But you have to design the baffle width to complement the desired crossover freq. A 15" woofer needs 18" wide baffle whose BS is 250Hz, about right for most 6" FR drivers. If you do this, then the woofer low pass filter must remain 2nd order or lower.
Open Baffle also has LF cancellation effects to deal with. when the wavelength of a driver gets longer than the baffle width, the front and back waves from the same speaker cancel each other out, increasing with falling freq. Another factor you have to correct with xo. It happens at tha same freq as baffle step, so you can just make the BSC steeper. in a box speaker it would need 3-6db boost, but with ob, it might need 6-10dB depending on room, taste, acoustics, baffle width, baffle shape, etc.
Have you considered doing an active crossover? It can make all these adjustments very simple compared to doing a passive crossover.
