This is a D.I.Y. Subwoofer in our home theater it is in what you call an infinite baffle concrete "box". It uses 4 18" RCF Subs. It pack an impressive puch, powered by a crown xls 402:
link to pics http://public.fotki.com/playazinc1/sub_in_our_theater/
link to pics http://public.fotki.com/playazinc1/sub_in_our_theater/