Noctua explains active noise cancellation tech
 

Computex — We probably should have scheduled our Computex meeting with Noctua earlier in the week, before my brain had been turned to mush after five days of wall-to-wall meetings and other events. We visited the company's booth primarily to see the active noise cancellation technology it's working on with RotoSub—and to get an explanation for exactly how it works. The details are pretty nerdy, and the physics is definitely over my head.

According to Noctua, the challenge with applying active noise cancellation to fans is the fact that they generate rotating pressure fields.

http://techreport.com/discussions.x/23076