Two passive radiators per enclosure is reccomended: however you wont hear much difference when playing music or movies between using one or two each enclosure. When I built the sub, I meausured with just one PR first, adding a second one made it a bit cleaner when playing a 20Hz test tone. When playing music, there was no difference. (it looks kickass though with a PR on each side, three cones facing your... mmmm...) I would reccomend buy one if you cant afford two, and if you really want, you could easily cut another hole in the futre to add a second PR. Internal volume is 50L - if you like I can get you the enclosure dimensions when I get home from work. Let me know and I'll post them on this message board.