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Active noise cancellation works by using microphones to detect surrounding sound and producing an opposing sound wave that reduces parts of that noise.
The technology is particularly effective against steady, low-frequency sounds such as aircraft-engine hum or air-conditioning noise. Sudden voices and irregular sounds can be more difficult to cancel completely.
That's why putting on noise-cancelling headphones can make an airplane cabin dramatically quieter without making the world completely silent.
Quick takeaway: Noise cancellation doesn't create silence—it uses sound to reduce sound.
© IEEE Spectrum