From the Manufacturer

The C7-SUB is a bass-reflex subwoofer specifically designed for use with the C7-TOP providing dynamic performance with high impulse program, a tight full sound and high efficiency. Its extra large bass-reflex tunnel guarantees high sound pressure while minimizing power compression and air turbulence noise. The C7-SUB cabinet is constructed from marine plywood and has an impact resistant paint finish, steel handles and an M20 threaded flange on the top to accept a loudspeaker stand. The front of the subwoofer cabinet is fitted with catches at the top and bottom for securing an optional wooden lid, and mounted on the rear panel are ratchet strap guide plates, two hinge plates and four heavy duty wheels.