I agree with Keith... or spend a few extra bucks and run a single 4" aeroport. That way, you're essentially building a sealed box, and sticking a tube in it lol... I'll show you how to calculate tuning and displacement for the port itself. In any case, this is how you calculate airspace, which is something you need to know whether doing sealed or ported: Length x width x depth (divided by) 1728. When you do this, make sure you take 1.5 inches off of each measurement to account for the fact that each wall is .75" thick, assuming that's what you'll use. So, say your box was 15" L, 18" W, and 7" D.... that'd give you 1890. Divided by 1728, that gives you 1.09 cubes. However, that is external... so you'd actually need to calculate for 13.5" L, 16.5" W, and 5.5" D..... that would give you 1225. Divided 1728, that gives you roughly .70 cubes. You can then factor in the displacement of your sub, which should be available on the product website. Those were just measurements I pulled out of my butt btw, not what you should actually build... I'd like you to understand what it is you're doing. If you need anymore help, lmk.... I'm sure I800 is full of knowledge as well.
