Rear Horn Design Question

[johninCR]

In a rear horn I understand that it's important to have the horn's output and the driver's output in phase at their cutoff frequency to avoid cancellation at that acoustical crossover point.

My question is how do you go about choosing that point? Should you look at the driver's free air response and where it starts to decline, have the horn take over? OR Should you look at a drivers response in a sealed box the size of your compression chamber and make the point where the driver is say 3db down your horn cutoff point?

[pxr5]

try transmission line google search,
1/4 wavelength