Active audio noise cancellation technology works by generating an identical audio noise as the noise you want to remove, but with its phase inverted. When both are “added,” the noise is cancelled by the “counter noise.” Several high-end headphones use this technology.
Noctua is researching for a while how this technology could be applied to computer fans, and during this Computex the company demonstrated its “RotoSub” solution. It consists of adding a metal ring around the frame of the fan and several electromagnets. A controller then turns on and off the electromagnets, moving the metallic ring and generating a counter noise to cancel the noise generated by the fan.
This technology is still in its experimental phase, but in the video below you have a better idea of how it works. You can see that, with the controller turned on, the flow of air through the fan still produces a “whooshing” sound, but the more high-pitched noise generated by the fan bearing is gone.