what the hell is everyone in this thread talking about?!?!?!
i don't think this box is for spl. the slot ported one he linked to is tuned to 27hz, not an spl box!
the linked box has 20in^2 of port area. decent for a 12" but could be more.
a 3" round port has about 10" of port area, waaaay too small. port noise for sure.
raising tuning will increase peak spl (theoretically).
if you want to do round ports, it should be at least two 3" ports, if possible two 4" ports.
if you do a long box like the one in the link you could probably handle the 30" length needed for a 32Hz tuning frequency( with two 4" ports).