The QZ900, latest addition to the Koss Quiet Zone line - adds a new level of performance and comfort. Designed to monitor outside noise received through specially mounted microphones in each ear cup. Koss QZ900 circuit introduces a sound wave which is 180° out of phase with the incoming signal. The result is "zero earth" at the point of the eardrum and dramatic reduction of most low frequency distractions, resulting in deep rich lows and sparkling highs, noise canceling ability so good you"ll need less volume for greater efficiency and less distortion from overdriving the tiny amplifier in your portable electronics. QZ900 operates with or without active noise canceling circuit engaged. Features a removable cord and an on line volume control and 1 standard AAA battery which provides approximately 50 hours of active noise cancellation.