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Noise-cancelling headphones work by listening to the sounds around the wearer and generating an opposing signal to counter them. Tiny microphones detect the steady hum of an aeroplane cabin or a passing train, and the headphones instantly produce a sound wave that is the mirror image of the incoming noise. When the two waves meet, their peaks and troughs cancel out, leaving the listener with relative quiet.

The headphones reduce background noise by producing a wave that is the —oppositeechoamplifier of the unwanted sound, so that when the two combine they —cancelstrengthenrecord one another.