Noctua NF-S12B ULN 120 x 25mm Fan (Ultra Low Noise - 500/700 RPM) Highly optimised impeller with Bevelled Blade Tips 2 speed settings for maximum quietness: Providing 700 and 500 rpm speed settings via the supplied adapter, the ULN (Ultra-Low-Noise) version is tailor made for serious silent enthusiasts who demand super-slow running speeds and utmost quietness of operation. Smooth Commutation Drive 2: The latest version of Noctuas advanced SCD drive system ensures superb running smoothness by eliminating torque variations and switching noise SSO-Bearing