If you’ve ever tried listening to music on a flight, you’ll be familiar with battling the constant droning of the jet engines. Noise-cancelling headphones can reduce this ambient sound, helping you to enjoy listening to music or watching an in-flight movie. There are two ways to achieve this effect, known as active and passive noise reduction. The latter is the simplest method, as it blocks sound waves with layers of high-density foam. This is good for masking high frequency sounds such as a loud bang, but the headphones will struggle to stop resonances at a lower frequency.
Active noise-cancelling headphones also use special materials, but go one step further and create their own sound waves too. Tucked inside the earpiece is a small microphone that detects ambient sound and feeds it to a digital processor, which analyses the sound wave’s composition. It uses this information to create a sound wave that’s the complete opposite of the one it analysed. This ‘anti-sound’ wave has the same sized peaks and troughs as the background noise, but they are inverted.
These anti-sound waves are then played back from a small speaker in the ear cup, actively blocking the ambient sound waves through a phenomenon known as destructive interference. When the incoming sound wave is at a peak, the anti-sound wave is at a trough, and the sum of these two waves adds to zero, resulting in minimal external sound reaching the wearer’s ears.
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