Dual 18-inch active subwoofer. The SRX928S is a dual 18-inch active subwoofer designed to complement SRX900 full-range sound reinforcement systems. The subwoofer houses two 18-inch JBL 2279-1 woofers, which feature Differential Drive®, JBL's dual voice coil, double gap technology, to deliver better heat dissipation, lower power compression, and a wider dynamic range. wider than conventional single coil designs..PZA MORE SPECS