title: What is the theory behind active noise cancellation?
body: <p>In a previous question, I had asked <a href="https://stackoverflow.com/questions/10290862/by-reversing-the-polarity-of-a-time-domain-signal-can-i-cancel-the-source-audio">Why can't I simply negate the source time domain amplitude values to produce a destructive noise signal?</a></p>
  
  <p>One of the posters said that while simply producing a inverses polarity (negated) signal will work in theory, in practice it is not possible</p>
  
  <p>So I am asking, what is the fundamental approach (in a sort of semi technical way) to active noise cancellation?</p>
  
  <p>Secondly, why are most literature on this topic in frequency domain?</p>