The ESS Pro 6 series was a large loudspeaker made by ESS (ElectroStatic Sound) of Cupertino, California. The speaker was very large—some referred to it as refrigerator-sized—and it was made of particle board 1½" thick, which added considerably to its weight. The design was different from previous variations of ESS speakers such as the AMT1 and AMT Rock Monitor series in that instead of one subwoofer there were two 12" ones, and instead of one large Air Motion Transformer there were four of them in a 180-degree array.