Originally Posted by
leech27
Any noise that's regular and periodic can be canceled out by a corresponding negative waveform.
Any noise that is non-constant (i.e. a person in a normal conversation) can't be canceled. The noise-canceling circuitry can't cancel out a noise if it cannot predict the shape of the incoming waveform.
This explanation is incorrect. They simply use a negative feedback circuit based on a microphone measuring ambient sounds. It's just imperfect, because the gain and the frequency response at the position of the microphone might not exactly match the frequency response of ambient sound attenuation inside the headphone ear cups. But there's no prediction of waveforms involved.