Active noise cancellation | What is it & How it works

External noise from the surroundings is nullified to give you perfect silence while you are using your headphones. This process is called an Active noise cancellation 

Active noise cancellation is now available on almost every earphone and headset. You can find the answer here if you wonder how this noise-cancellation feature works in devices. 

There is a small mic on your headset/earphone where your ear is. This mic captures the coming noise from the surroundings. Create an inversion of this noise signal and feed it to your ears. These signals cancel out and give you complete silence. All this happens within a very short time.

You also have passive noise cancellation, where your earphone/headset cancels noise out with its outer coverings. This can be achieved by using sound-insulating materials as your headphone covers. combining both, you will get near the "perfect silence" experience.