How does noise cancellation work in headphones?

What are some tips and resources for making custom active noise cancelling headphones?

• Essentially, is there a way to cancel a quiet ticking noise, given close proximity to that noise?  Here's the full, detailed context. Relatively rarely, people suddenly develop a muscle spasm at the soft palate at the top, rear of the mouth. This produces a ticking or popping noise in their ears (like a metronome). This is a real noise that can persist indefinitely and can even be heard by others if they listen carefully right by the ears of the person (the ticking seems to come from the ears, even to the person themself). The cause is often unknown, as with "essential palatal myoclonus." (For some people, Botox injections into the palate can stop the spasm, but there is unfortunately no great treatment.) The frequency can vary, but my understanding is that it's usually 1 to 3 ticks per second and is for the most part steady. Outside the ears, it's at most as loud as an analog wristwatch, audible a couple feet away in total silence. I'm not familiar with audio technology, but I've been curious whether noise cancellation technology could be of use for giving people breaks from the ticking. Given the quiet sound profile, does anyone have thoughts on this? If you think it might work, any advice on how to get started exploring? I don't guess this would have a great shot, but are there any sensitive off-the-shelf noise-canceling headphones that would be easy to hack and turn their ambient noise microphones inward? Or else, any suggestions on a going about creating a very rough test experiment (programming software is fine, but could use pointers for the microphones/speakers hardware part)? Any other creative ideas appreciated.

• Answer:

  The frequency of repetition of the sound is less important than the instantaneous frequency characteristics of the sound itself. Active noise cancelling headphones become less effective as frequency increases...and if the sound is like a watch ticking it will likely be very quiet but broadband in nature. It is likely that noise cancelling headphones could reduce the sound for those around the subject but in their normal state would not likely do anything for the subject. Given that the sound is produced in the soft pallet, It's most prominent path to the ear is the eustachian tube. In order to to stand any chance of using phase cancellation techniques like in noise canceling headphones, you'd have to be able to measure the sound field with a mic at the relevant point inside the head. Added to this...any physical sensation created by this tick will focus the subjects attention which will subjectively amplify the auditory sensation too. Having said this...I have seen some very inventive ideas to overcome such problems... keep investigating.