Active noise cancellation project

Just phase shift original noise or characterize signal and invert it. FFT?

Update 13th Dec 2016:
Use smartphone mic and processing power to send noise cancelling signal to headphone. See reddit on why there are no noise cancelling apps:

...biggest one is that your cell phone microphone is monaural while noise cancelling headphone have to have two independent pickup channels to effectively cancel out the outside noise

App on Google Play: Noise Canceller