Nippon America TUBO12 12" Ported Car Subwoofer Tube
21inch LONG X 16inch DIAMETER, (WHICH INCLUDES THE TURBO VENT ON ONE SIDE OF THE TUBE).
THE WOOFER OPENING IS FOR A 12inch WOOFER.
AIR SPACE = 1.37 CU. FT.
The port that runs along the top of the tube is square along the top rather than round.