I think it would be helpful if you stated what your objectives are; numbers or just street pounding. From the way it sounds you are a street beating kind of guy. If so, I have seen alot of the bandpass guru's recommending 2.5 cubes sealed for 18's and generally a 3:1 ratio on the ported section. That being the case you would go with 7.5 cubes per 18 on the ported section and absolutely as much port as you can muster. Probably something like 20-25 square inches of port per cube on the ported section tuned anywhere between 45 and 50 hz. By no means am I any kind of an expert so I am hoping some of the more experienced bandpass experts will chime in. If I were you, I would pop over to the FloridaSPL forums as there seems to be more information about 4th order builds on that site.
Originally posted by rival904:
Currently got a box built for 4 15s,
2 cu.ft sealed per sub
6cu.ft ported per sub(after displacements.
Port is only 8.75 tall, and 46.50 wide(internal) and tuned to 45hz.
I am considering rebaffling for 4 18s, the sealed chamber would stay the same, and the ported might shrink down to 5.75 per sub or so.
Would an 18 do well in that size of a box or should I just stick to 15s?
Subs will be either DD 3515s or DD 3518s.