This is something I've never really understood - - In WinISD, If I model a sub (Shiva) with 1 port, 4" dia, 5ft3 tuned to 20hz, length = 13.62. 2 ports in the same config, all else being equal, yields "vent length 30.17". I have asked before, and all responses say that that's 30" for EACH port. Now, looking at some recent production subs, like SVS's box types or the VTF-3, there are multiple ports, but not nearly enough room for vents longer than maybe 18" each. Of course I don't know the driver parameters, but would those parameters be enough to be able to have short multi ports?