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.
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.