Generally you have no concern about it in a 1 or 2 sub setup, unless you play way below your tuning. The time when it is most likely to occur is when you put your subs in rows of 3 or more. Here's a quick sketchup I did as an example for you: http://i172.photobuc...ngissuesbox.jpg
hey that looks like mine lol and yeah cuz of the low tunning and the fact that like Autruche said its a row of three with 1 port like the pic and if i put a piece of wood behind motor i would loose displacement