Hi, sorry form my redundant input, but when you use the approach I described below you get exactly what winisd (or other speaker cad) calculates: Example: Eminence Delta-12LF in 3.853 ft3 box tuned to 43Hz: one square port 5"x5" require 5.53" length "two" ports 2.5"x5" (same area) require 6.45" length BECAUSE: length of one port 2.5"x5" in 3.853/"two"=1.9265 ft3 for 43Hz is 6.45" This is exact and valid approach. V.