I guess that would depend on how far away you are from the source
if I remember my training correctly. you measure the distance from one ear to the other and calculate the wavelength. the idea being, that any wavelength larger than your ear seperation will not be able to localize since it arrives at both ears simultaneously.
I think it somewhere in the 1500-2000 hz area.
if anyone else has a theory, dont take my word on it, lol.
Haha, well I guess I should clarify... The reason I'm asking is because I want to place some midbass drivers in the rear doors, and I want to know the where to set the low pass frequency so they dont screw up the imaging.
Look up a thread called midbass arrays by Patrick Bateman. I'd say 80Hz is about right for a sub, depending on install and any noises/vibration will make localization worse. For the midbass you can run them higher long as you have midbass output in the front, that thread will explain but it is a long and great thread lol. In general you can use around the same midbass output in the rear, as the fronts are closer to you they will sound louder (in a typical install). This can beef up your midbass, but you need the stage to come from the front to make it work right....so first you need some MB in front or it will simply sound like the MB is from the rear because it is.