How does noise cancellation work in headphones?

Is there an easy way to add noise cancellation to existing ordinary headphones?

• I have a pair of in ear headphones that I adore. It's fairly easy to find a new set w/ cancellation but I was hoping to find an 'adapter' of sorts, a tiny box that would go between my headphones and my laptop. I realize that most systems have the microphone on/near the headphones and a separate box like this will be less effective. But it seems like a reasonably useful gadget to have for anyone who has high end headphones they don't want to part with. Edit: The situation I want the most is in an airplane where I need to remove the droning sound of the engines. I wouldn't think that situation would require microphones near my ears...

• Answer:

  No. There is no adapter box that will turn ordinary headphones into noise cancelling headphones. Unfortunately, active noise reduction (noise cancellation) requires a microphone to be built into each headphone near the ear; picking up what the ear will hear is critical in cancelling the noise. If the microphones were placed in a separate adapter box, it would be nearly impossible to produce the required inverted signal in step with when the noise signal would be arriving at the wearer's ears.