The Kappa Series subwoofers feature SSIâ„¢ (Selectable Smart Impedance), a HARMAN-exclusive impedance selector, that lets you choose a 2- or 4-ohm impedance at the flip of a switch. SSIâ„¢ (Selectable Smart Impedance) Oversized aero-cooled voice coil Acoustically damped glass-fiber cone Premium screw-down terminals