The SCS-321 is a self-powered cardioid sub-bass loudspeaker, designed to match the superb transient response of an HLA Hybrid Line Array. It extends system low frequency response, projection and directional control to 28Hz.

Three  21-inch long-excursion drivers are employed in a cardioid configuration using on-board Powersoft DSP and amplification. The drivers have (.7Q) sealed enclosures for optimum transient response.