It's all about the process of elimination. Place the Audio Source NDHD1AS Noise Cancelling Headphones on your head. Turn the noise cancel switch on. Suddenly all that ambient noise around you seems to have vanished. The noise still exists but your headphone has the ability to electronically eliminate it. This lets you listen to your music more comfortably in practically any environment.
AudioSource's NCHD1AS on-ear noise canceling headphones combine excellent sound quality and value. The comfortable on-ear design incorporates advanced technologies to reduce ambient noise while producing great sound quality, features you have come to expect from AudioSource. The adjustable headband and thickly-padded earcups provide long-lasting comfort, and a high degree of natural noise isolation. The cable has a standard 3.5mm mini plug for connecting to MP3 players, CD players, and other consumer devices, and an airplane adapter is included for use with in-flight entertainment systems. The headphones operate on a single AAA battery for hours of enjoyment.
Unlike most earphones that use passive sound isolation, these use Active Noise Cancellation Technology to significantly reduce ambient noise that exists all around you. It's like the world is your living room.