first time with a dual sub box, I desire a tuning of 30-31hz. the dementions here should be sitting at 16 cubic after displacement. all wood is 1.5" thick with port wood being .75" thick. having a shared port is throwing me off a bit as i dont have any calculators that are designed for such an enclosure. most of my math is done by hand on this design so i am uncertain if im off. 2 SP4 18's