I fabricated A sub box about a year ago or so out of fiberglass and wood. Its for a 15" Kicker L7. It's about 3 Cubic Feet. It beats pretty hard right now, but I'm just trying to get a little more sound out of it. Could I use PVC pipe? If so what size (Length and Diameter) would work best? If not, what would work better? Pics are provided in the link below!

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4 inch port, tuned to 35 hertz, should be 6 1/4 inches long.