Reference woofers offer higher efficiency and higher performance. INFINITY uses progressive spiders to provide the best possible control at excursion limits compared to the much more commonly used linear spiders. The benefit is a much more controlled woofer punch with much tighter bass characteristics. With high-rolling surrounds, patented cone design, and long excursion, Reference woofers deliver deep, powerful bass—with greater precision than conventional woofers. And the exclusive SSI™ (Smart Selectable Impedance) switch lets you choose either 2 or 4 ohm impedance for maximum setup flexibility. It's everything you want in a woofer..PZA MORE SPECS