I have taken the liberty of transplanting this thread over to Fans where it might fit better, because it deserves good coverage as a topic all by its self.
At the end of the Vaccuum thread in HardDrives, Rusty075 wrote:
Actually CRT_Leech, I've been thinking about this idea for a while now.
In theory it wouldn't be that complicated: Since fan noise is of a constant frequency all you'd need is a DSP plugin to convert the noise 180 degrees out of phase. Your computer already has the hardware it needs; a soundcard with a line in for the mic's and a line out for the playback speakers.
But in reality its alot more complicated. The fan noise comes from too big a source to be damped by 1 speaker. You'd need an array of mic's and speakers working together. The system would require individual tuning based on its particular environment, and that tuning would be much more sensitive than what a hobbiest could do. Even the slightest imperfection in the sound wave interferences and instead of reducing the noise you make it twice as loud.
Somewhere I ran across a java app online that sends 1 frequency to your left speaker, and the inverse to your right. If you put the 2 within an inch of each other there was silence, but move 1 even a tiny bit and it got really loud.
One way this could be made to work would be to put the mic, speaker, and processing chip all inside the hub of the fan. By being within the source of the noise 1 speaker could cancel it out by itself. Of course this would require some seriously tiny electronics, and would probably result in a fan that costs $500 each.
Excellent concepts Rusty! I have stumbled onto the idea of using active noise cancellation under controlled conditions for computer noise control too. I think that with fans, you might be able to make a dense noise blocking duct (possibly as part of a cabinet type encloseure), put powerful fan/s inside it, and use active cancellation downstream from the fans, where you would have a realatively straightforward acoustic environment to work in. For example, you could put the mike closer to the noise source than the speaker enough to compensate for the processing delay. This might help greatly simplify the processing requirements. I haven't studied active cancellation enough to know what the real factors are, but it has to be simpler in a near ideal space. I also do remember from somewhere that the tech has been adapted to automotive exhaust systems, where you have a nice linear noise path and straightforward acoustics.
In some ways, one might expect the idea to be possibly fairly inexpensive, given that speakers and dsp's/mcu's are all very cheap fare these days. If you add it up, it might well be possible to build an "air pump" that uses a larger & faster fan, plus active cancellation, for maybe $100. The thing would ventilate your whole PC, and with strategic ducting, could cover the CPU HS too. In a big tall case, it might even live up in the top/rear space above the PSU, and could then be usefull to system integrators as well as DIY modders. This could be extra potent in the workstation market where the bigger bucks roll out, and the ergonomics demands would certainly justify the trouble.
OK, I'm done thinking out loud for now.