Farris404

Hello, I need help on the port volume. I have a 15" bl with a 1500.1 kicker mono amp and Will be building a 4 cubic foot box and would like it tuned to like 35 hz. I don't understand what 12-16 square inches per cubic foot means in the recomendations. I have always used/seen cubic inches for a port valume. so if someone could "please" help me out asap that would be great. Thanks alot. Farris.