The KPT-684-SMA is a high output bass-reflex subwoofer featuring advanced low-frequency driver design.THX®-Approved the KPT-684-SMA is a high output bass-reflex subwoofer featuring advanced low-frequency driver design. Utilizing two 18-inch drivers that gain their high efficiency through the use of large geometrically optimized magnet structures and lightweight cones. The enclosure is constructed of one-inch void-free birch plywood and features internal cross braces at all cabinet joints.The KPT-684-SMA subwoofer increases the flexibility of the proven KPT-684 by adding internal aluminum bracing and fly points for reliable suspension.