Nippon America TUBO8 8" Ported Car Subwoofer Tube
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18inch LONG X 11inch DIAMETER, (WHICH INCLUDES THE TURBO VENT ON ONE SIDE OF THE TUBE).
THE WOOFER OPENING IS 8.25inch
AIR SPACE = .52 CU. FT.
The port that runs along the top of the tube is square along the top rather than round.