I got this idea from reading about noise cancelling headphones a few months ago. You can read more about them here since Wikipedia can explain it better than I can http://en.wikipedia.org/wiki/Noise-cancelling_headphone

Basically I want to apply the same principle to cancel out fan noise in my computer case. I think it would be possible to do this with an inverting unity gain amplifier, microphone and a PC case speaker. It could be powered by the computer power supply and mounted inside the case.

Has anyone tried this?

I think it would be a great way to keep a computer quiet without sacrificing cooling ability. I know there are a lot of effective low noise heat sinks and fans but they cost $40 or more and won't work when a new socket comes out.