After generating a bass planer wave front traveling towards the rear wall then one uses digital delay to cause the two subwoofers located on the rear wall emit a 180 degree out of phase wave front that is timed to cancel out the bass wave before it reflects off of the rear wall and causes standing wave problems. This is how I have my system set up, the only problem being that the room is L shaped and the cancellation is incomplete. A cubic or rectangular shaped room can yield extraordinarily flat bass response at all locations in the room due to the suppression of room mode energization.
Scotty
