The document discusses using active noise cancellation to control noise from CPU fans. It begins by explaining how increasing heat generation in electronics requires fans for cooling but fans produce noise. Active noise cancellation uses a reference sensor to detect fan noise which is then used by a controller to generate antinoise from a loudspeaker to destructively interfere with and reduce fan noise. The document describes installing an active noise cancellation system on a CPU fan with an infrared sensor as the reference, a digital signal processor as the controller, and a loudspeaker and error microphone in a duct. It achieves over 30dB of noise reduction at various frequencies of the tonal fan noise. The conclusion states active noise cancellation effectively reduces CPU fan noise and heat,