Noise cancellation headphone use electronics to reduce unwanted ambient sounds, such as acoustic noise, rumble, and so on.
A microphone placed near to the ear piece on the headphones detects unwanted noise. The electronics generate sound waves with the opposite polarity to the unwanted noise. The sound waves cancel each other and remove the unwanted noise.
Noise cancellation is most effective for low or medium frequencies and least effective for high frequencies.
Use your noise cancellation headphones to cancel unwanted engine noise in planes, ambient noise in trains, and noise from other traffic. They can help to reduce fatigue.