This is by far the simplest of subwoofer designs. The woofer plays directly
into your cabin and doesn't require a box. Most of the time, the speakers
are mounted to the rear deck of the car, though one could put the drivers
in the doors, if you used smaller drivers.
Pros:
Low-cost design
Simple installation - NO BOX!
Since there's no box, the subwoofer doesn't use much trunk space.
Cons:
Requires more power than other designs.
Few subwoofers are designed for free-air use
One must make absolute certain the rear wave does not cancel the front
wave. Basically, you'll spend lots of time sealing the trunk from the
passenger area.