This unit is an improved version of the highly praised 18W/8535-00 midwoofer. A new aluminum chassis and an updated cone as wells as a few other details are introduced. These updates improves mechanical stability and sound performance. High quality magnet system design with patented Symmetric Drive(SD-1) continues to be key feature. Patented Symmetrical Drive Motor Design 38mm Voice Coil Low Damping SBR Rubber Surround Coated Air Dried Paper/Carbon Fibre Cone Low-Loss linear suspension Aluminium Chassis Replacement for: 18W/8535-00, 18W853500, 18W 8535 00, 18W 853500, 18W8535, 18W/853500 Box suggestions: 0.55 to 0.75 cubic foot (15 to 20 ltrs) sealed with 20% filling for an F3 of 55Hz 1.0 cubic foot with 2″ diameter port by 7″ long for an F3 of 38Hz