After doing a lot of plane travel with the annoying drone of the engines, Joanne Manaster found herself in need of some noise cancelling headphones. According to her post on PsiVid:

“Although I understand the physics (and accompanying math-specifically trigonometry) of how the earphones work, I was uncertain if the technology was sophisticated enough to cancel ambient noise for the average flyer (at a reasonable price).”

So just how do they work? James May at HeadSqueeze has a great video to explain it: