The SD-2 woofer was born of the need for a woofer as close to the performance of the SA series as possible but with significantly less mounting depth. We started with the same heavily tested and proven cone, surround, and spider assembly of the SA line. From there an all new motor was engineered using half the magnet height of the SA line combined with a solid back plate. The solid back plate allows the SD-2 line to be mounted directly against the enclosure wall eliminating the need for venting space behind the motor assembly -- this reduces effective depth requirement by another inch! The SD-2 line is still able to effectively vent the voice coil through 16 holes drilled around the basket perimeter as well as a vented voice coil former allowing it to handle it's rated 500-watts RMS with ease.