From Corsair:Ultra Low Noise — Hybrid Silent Fan Control
The Corsair AX850's energy-efficient design allows it to operate reliably with minimal cooling, and hence very low noise levels. The AX850's Hybrid Silent Fan Control technology offers three modes of cooling, depending on the load level. This includes a fanless mode of operation (0 rpm) up to about 20% load, a quiet, low fan speed cooling mode for typical loads up to around 50% load, and a higher performance cooling mode for more demanding loads. Noise levels still remain low even when fan speeds start to climb, thanks to the use of a premium-quality 120mm San Ace fan from industry-leader, Sanyo Denki.