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A set of Bluetooth headphones.
Have you ever wondered how noise-cancelling headphones work? © Unsplash/ C D-X

Noise-cancelling headphones

Used by plane and train passengers wanting to listen to radio, music or film without hearing background noises, active noise-cancelling (ANC) headphones are able to prevent outside noise from leaking through to the inside of headphones.

Active Noise-Cancelling (ANC) headphones are used mainly by train and plane passengers wanting to listen to radio, music or film without hearing background low-frequency engine and travel noises.

ANC headphones usually incorporate two microphones near each ear piece – one outside and one inside. The signal from the outside microphone is reversed and then amplified (or reduced) to the level that the noise is expected to leak through to the inside of the headphones. If perfectly reversed and exactly the same amplitude as the signal that leaks through, then silence results. The microphone inside the headphones checks that this happens correctly and alters the gain of the amplifier if needs be, while at the same time removing any wanted music generated by the headphones themselves.

A diagram showing how noise and anti-noise waves cancel each other out perfectly to provide silence.

Cancelling out unwanted sound waves

Although not perfect these headphones allow the user to hear their music free from distractions or create quiet space so that the wearer can sleep peacefully or work at their PC in a busy open plan environment.

Sharp sounds and voices pose a greater challenge to the designer. The latest digital ANC headphones employ a signal processor equipped with sound mapping software to analyse the noisy environment. When first switched on the listener hears the normal level of noise reduction followed a moment later by an even quieter ambience as the system maps the environment and responds accordingly.


This article has been adapted from "How does that work? Noise-cancelling headphones", which originally appeared in the print edition of Ingenia 66 (March 2016).

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