Noise cancel headphone reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control (ANC). Essentially, this involves using a microphones, placed near the ear, and electronic circuitry which generates an "antinoise" sound wave with the opposite polarity of the sound wave arriving at the microphones. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphones.
Noise cancellation headphones makes it possible to enjoy music without raising the volume excessively. It can also help a passenger sleep in a noisy vehicle such as an airliner.
These headphones hush ambient noise by creating antinoise that obviates the noise at your ear. They don't eliminate noise, but the better models significantly reduce the whoosh of airplanes' air-conditioning systems. (They're somewhat less effective at quieting the background din on trains and buses.) Noise cancellation headphones come in all forms, from full size to earbuds. Since you no longer have to crank up the volume to overcome background noise, this type of headphones lets you listen at lower levels, which leads to reduced ear fatigue. You'll also hear more low-level detail in your music.