Active noise cancellation

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.

The idea is neat, but there would be some difficulties making it effective in comparison to noise-cancelling headphones.

Headphones are a controlled environment - the phase relationship between the driver in the phones and your ear does not change, so an appropriate amplitude anti-phase (inverted) version of environmental noise will effectively cancel out the unwanted sound. Because the phones are locked to your head, head movements don't change the phase relationship of the noise/anti-noise.

In free air, the phase relationships between the noise source, the anti-noise source and your ears will be much more complex, and will change as your head moves. That would produce "sweet spots" where the noise cancelling is happening, and moving your head would change the results to some degree - especially turning your head. Our binaural hearing system is very sensitive to phase relationships, we use them to locate sounds spatially with barely perceptible head movements. So basically the cancelling would tend to come & go as you moved - this may not matter if you got it working for where you normally sit.

More importantly the pickup microphone needs to be able to "hear" only the unwanted noise, not the cancelling "anti-noise". This is easy for headphones - the mic is on the outside where the noise is, the anti-noise goes into your ears. For the same sort of result with a computer case, you probably need the microphone inside the case, and the noise-cancelling speaker outside. Otherwise, you've got a "chicken & egg" situation, the microphone hears noise, the inverted signal cancels it, but if it's cancelled, the microphone can't hear it any more so it stops getting cancelled... etc.

It could be a fun idea to play with, you'd need to be able to vary the gain of the inverting amp to get the right amount of cancelling, and experiment with microphone placement and so forth.

Another possibility may be "masking", which uses a quirk of human sound perception to make a sound less noticeable by adding another more pleasant sound. This is like the old trick of overcoming sleep-disturbing noises by playing a tape of seaside surf sounds by the bed. Your mind tends to tune out irrelevant sounds, so something continuous like white noise (or the softer "pink" noise) blended in from a speaker in the case may paradoxically make the other sound less noticeable.

So there would have to be a separate speaker on the outside of the case -would it not be possible to use the stereo speakers connected to 'line out' if there was an adjustment slider in software?

I like the idea of masking 'in addition' selectable to 'waterfall, rain, rushing stream etc.' maybe. I'm toying with the idea of carrying further to make a 'piece of furniture' to slide a case into - could install acoustic foam on the inside. Need to address airflow, but ought to be able to get down close to zero effect noise - right?
If I go this route, would the noise canceling software (/hardware) be unnecessary?

Thanks :)

1 - Nope. You have several sources of noise inside the computer and the noise outside is different from the one inside. Also noise reflects and/or is being reduced by various objects, furniture, carpet, walls... The only thing that would work is wearing a set of noise-cancelling headphones, which induce a signal that cancels the noise *at your ears*... back to start.
2 - The foam thing would work to a certain degree, you need to cool the computer, which in turn requires some airflow... leading to noise. But it could definitely be a less audible noise than without the insulation.