Your dimensions are wrong. L*L*L=L^3. So your 1938 is cubic inches of space, which is irrelevant for the question at hand. A 17"x6" port is 102sqin of port area. The length of the port is a variable dependent on the port area and net volume of the box through a series of equations.
For a tune of 35Hz, that is a 8.2cuft net box. Perfect for 3 12's, if not a little big. Gross will be like 9.5cuft plus driver and wood displacement. What kind of 12's are they?
Download WinISD. Its anecho response graphs are perfect world and therefore, often inaccurate. Some of the other information in WinISD can be helpful, though. Most importantly, it is a user friendly port calculator. You can set the net volume and change any of the other variables (length, tune, port size) and it will calculate all of the other variables for you.
I'd explain how I design boxes, but It'd be a short novel with a lot of algebra and a bit of calculus. Math is a wonderful tool in the speaker world. It allows you to maximize and optimize volume, port area, and tune with the amount of space available. I used to charge people for box plans over on CA.com. Then people saw that I was making some pocket change from it and started doing the same. The difference is I put a lot more time into it to optimize the box for their application. However, people who need people to draw them box plans usually dont care about all that junk so my competition beat me out of the business by getting the job done, reguardless of how well it was actually done.
Last edited by CBFranger; 01-12-2009 at 09:05 AM.