real-world noise cancelling

[televator]

So, I got this weird idea and I'm wondering how plausible it is. My computer can make some annoying fan-noise when the processor is working real hard, and I want to get rid of it (the noise, that is).

If I set up small mic behind my computer, run it through a mixer that reverses the phase, and send that signal directly through some desktop speakers, will that cancel out the real noise coming from the computer? Is phase cancellation only a recording phenomenon? If this idea is totally dumb, anyone have other suggestions for dampening such sound?

word.

[inverseroom]

You totally need to try this...

[jmoose]

You could just buy a new quieter fan for the PC...but that wouldn't be anywhere near as interesting!

I doubt it'll work though. To achieve total cancellation you need to have the exact same sound at the same amplitude. By putting a mic on it and sending it through speakers it'll change the characteristics & frequncies of that sound and its phase & time relationships with the original. So...it'll probably just make it louder!

But 'ya never know.

[Phiz]

To achieve total cancellation you need to have the exact same sound at the same amplitude.

You not only need the right sound (frequencies) and amplitude, but you also need to be 180 degrees out of phase. This is quite hard to do over a volume that is large relative to the wavelength of the sound. Thus, your proposed solution will only work on very low frequencies where the different distances between the sound sources, microphone, speakers, and your ears are all small enough that the phase relationship will be mostly preserved.

Also, beware of feedback. You could blow your tweeters out by trying this without some caution.

[eeldip]

you should do this with your voice... you know, memorize the sound of the computer, replicate it perfectly, then sing what the computer does out of phase.

that would be a super trick. then you could memorize what people are probably gonna say and cancel them.

[televator]

Unbelievably, I found a ny times article about someone doing just what I was suggesting. However, I'm really not very handy and would never be able to do what this professor did. and it sounds like my simpler idea won't work. funny, though.

here's part of the article:

"The constant drone of a computer cooling fan can be annoying. But a professor at Brigham Young University has taken an unusual step to mute this noise: more noise, produced in just the right quantities from tiny loudspeakers that surround the fan.

?We make anti-noise,? said Scott D. Sommerfeldt, a physicist who created a noise suppression system with his students. It is the latest example of a technology called active noise reduction, or noise cancellation, well known from its use in headphones designed to block out the low rumble of jet engines.

The sound waves engineered by Dr. Sommerfeldt are out of phase with sound waves from the fan and thus they cancel each other out, substantially reducing fan noise.

Dr. Sommerfeldt?s system has four miniature speakers and four even tinier microphones set in a ring around the computer fan. The microphones and other sensors detect the noise of the fan blades and, with the help of digital signal processing and algorithms, radiate opposing tones from the speakers. The whole system can be tucked into the same space that a conventional computer cooling fan would occupy."

[Professor]

Well the article at least points towards a couple of key ideas. Yes, it could work. Yeah, it would probably work better with multiple mic-speaker units, and with some kind of active DSP. But it also needs to be very close to the source - and that's the important point for you if you choose to pursue the experiment.
I could see building a little device with a tiny mic on one side of a circuit board, wired to a little chip-amp to a small speaker wired out of phase on the other side of the circuit board. It wouldn't be as high-tech, but also wouldn't require a research grant to build it and would get you closer than simply trying to go mic -> mixer -> computer speaker.
Conveniently, that article gave you the name of the professor and I did a quick google search and found a paper he submitted to the Acoustical Society of America here that happens to include a picture (though a very small one). Perhaps you can find better shots on the guy's personal webpage.

Of course, you'd also be amazed at what a 3-5 foot long piece of plastic dryer duct will do to control the sound of a noisy fan, especially if you redirect the air flow out a window or into a closet or even under your bed.

-Jeremy

[inverseroom]

About getting a quieter fan...go buy a SilenX. I got one for my Akai multitracker and the noise floor of my studio instantly dropped by a mile.