Harman Kardon BT 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 lightweight, comfortable headphones employ AAC (Advanced Audio Coding) and apt-X coding for the highest-quality wireless audio, and incorporate a built-in microphone and playback buttons. The USB-rechargeable BT headphones have a battery that lasts up to 40 hours. And unlike competitive Bluetooth headphones, the headphones will still work when connected by the supplied bypass cable to a playback device, even when the headphones" battery dies. Features: Bass tube Sonic clarity and accuracy even at low volume levels User-exchangeable metal bows Self-adjusting housings User-detachable, soft leather ear pads Slow-retention premium foam 2D folding mechanism Detachable cable with metal connectors Premium carrying case Frequency Range - 2.4GHz - 2.4835GHz Transmitter Power - <3.0mW