Harman Kardon NC headphones feature an exclusive acoustic system that integrates superb mechanical design, materials and electronics. User-exchangeable bows, self-adjusting housings, soft leather cups and slow-retention ear-cup foam ensure a comfortable fit and an optimal acoustic seal on the ear. The NC model employs the proprietary HARMAN closed-loop, active digital noise-cancelling technology developed for acoustic installations in luxury cars. The USB-rechargeable NC headphones have a battery that lasts up to 40 hours. Even when the battery dies, the headphones will still work when connected by the supplied bypass cable to a playback device. No other noise-cancelling headphones offer this passive playback mode.