Noise cancelling headphones are helpful to obstruct out sound in noisy environments similar to trains, buses, and public places. The technical assumption that is worked out is that this device copies the noise effect from outside and then they create these noise effect go out of phase. It can be also compared to that of swimming upstream. There are two types of headphones designed to decrease the intrusion of the noise that is produced in the background. They are the passive noise canceling headphones and the active noise cancelling headphones.