Free air really means "really big box that is not perfectly sealed" the back wave of the speaker still has to be isolated from the front wave with a baffle.
EG; Mounting the subs on the rear deck and using the trunk of a car as the box.
The better the trunk is sealed from the pass. compartment, [rear wave isolated from the front wave] the better the bass will be.
If done properly free air subs can sound very good, if not done properly they will sound like crap and will not last very long.