I did not have any intention to attach speakers to them or modify it any way to make it work with PC to cancel noise. The headphone I am talking about has built in mic, and I just wanted to wear the headphones as it is, when I was sleeping. Would it cancel out the noise of a PC humming away?
Ah, sorry, I missed the original context. Oddly enough, I just noticed a "review" of noise-cancelling headphones today at http://www.barefeats.com/bose.html
. They say that the $300 Bose QuietComfort headsets work really well, at least on airplanes. (For that much money, they'd better work well!) The author, later in the article, does comment that he used them to take a nap in a noisy "office/home" environment, although there is no specific mention of any computers involved.
As far as frequency response goes, an outfit like Bose might have specs available on their website. I believe good headphones can go down to around 5-10 (?) Hz or so, which is probably good for covering computer noise. I'd think the limiting factor on the frequency response would be the microphones they use. Of course, any audio device is going to have a gradual drop-off near its limits, so a device rated to 10 Hz, for example, might not attentuate (or emit) at as well as in the middle of its design range. But I'd think most computer noise, coming from fans, would be higher frequencies that should fall in the working range of a unit like the Bose. That's just my guess, though.
Say, wouldn't sleeping with bulky headphones on be problematic? Earbud-style units would be better, but I haven't seen any noise cancelling versions of those yet. I second the idea of earplugs or something similar; cheap and proven effective, with good frequency response. Or, if you have to wear the big over-the-ear headphones, you could hold them in place with a liberal application of duct tape.
On a less related note, I noticed that the Bose headphones in the review require two AAA batteries. I wonder if they could work up a kinetic drive system or a solar recharger like the newer watches... Having lots of gadgets that all require batteries is a real hassle.