The US221-4 is a dual 21 inch, ultra-compact, direct radiating subwoofer that is designed to provide gut churning impact and solid musicality – not just infrasonic effects. Its 6 inch voice coils, unusually rigid cones, and innovative low-turbulence ports permit an optimal tuning to be achieved in an enclosure that is smaller than most dual 18 inch subwoofers. For overhead suspension, sixteen M10 suspension points allow either horizontal or vertical orientation. For ground stacking, the supplied vibration isolators may be threaded into any four M10 mounting points to prevent spurious vibrations and “walking”. When stacking enclosures, the vibration isolators nest in recesses surrounding the mounting points in the cabinet below. The input is mounted on a chamfer at the rear of the enclosure, allowing cable access without requiring additional space between the enclosure and a wall or stage. The US221-4 is perfectly suited for highly dynamic live performance venues and nightclubs, A/V screening rooms, and low frequency effects in theme parks. And, its small size allows it to be used under stages, built into small cavities, and surprisingly, in many situations where a double 18 inch subwoofer would normally be too large.