Some headphones incorporate active noise cancellation. This is a technology that was originally created to reduce the noise pollution to workers in the air force. Noise cancellation allows you to isolate yourself from the environment and only hear the sound coming from the headphones.
It is an ideal system for long journeys by train or airplane and to isolate you from noisy environments. However, it is not recommended to use it while walking down the street or riding a bicycle, as it can be dangerous. For this reason, some headphones allow you to turn active noise cancellation on and off.
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