Active Noise Control

Active Noise Control (ANC) is a method of reducing unwanted noise by actively generating an anti-sound, cancelling out the noise. The noise, like any sound, is composed of alternating compression and rarefaction phases, which the human ear perceive as sound. By creating an inverted sound, with a rarefaction phase during the noise's compression phase and vice versa, the noise pressure wave is cancelled out, reducing the noise.

Traditional approach

ANC has been around for a long time, and the basic principle is illustrated below. A microphone listens to the noise, inverts the phase and plays the anti-sound using one or more speakers.

This approach has some technical challenges. The most obvious one is the need for a separate anti-noise speaker components. This is especially troublesome in applications with space constrains. Another challenge has to do with sound physics of fans. The noise from a noise source, such as a fan, is by its nature a dipole.
The sound from a traditional sound generators is by nature a monopole.

This creates a situation where (due to phase misalignment) the system only reduces noise on one side of the fan.

To solve this problem a second set of speakers has to be put on the opposite side of the fan.