Accelenation posted a review on the Noctua NF-S12 Quiet Fans

One way in which fan noise can be reduced is to reduce the speed at which the fan rotates. This approach not only reduces the amount of air disturbance, but also reduces some of the frequency components below that of human hearing. In addition, reducing fan speed also reduces bearing noise - a secondary emanation.

Noctua NF-S12 Quiet Fans Review