Sennheiser RS 165 RF Wireless Headphone System Closed back, circumaural headphone system with excellent digital wireless audio transmission delivering range up to 30 meters(100-foot). Connectivity Technology: Wireless High-quality Sennheiser transducers and a selectable Bass Boost mode for exceptional sound. Max. sound pressure level 106 dB at 1kHz, 3% THD. Signal-to-noise ratio - typ. 85 dBA at 1 Vrms . Frequency response - 18 - 21,000 Hz Multi-purpose transmitter with Analog Input - also functions as 'easy-charge' cradle and docking station for the headphones Carrier frequencies: 2.4  2.48 GHz. Multi-receiver transmission - transmitter supports up to two pairs of headphones simultaneously (optional HDR 165, sold separately) Troubleshoot-sound dropouts or no sound is due to headphones are out of the transmitters range, signal is shielded and there are interfering devices.Operating time : up to 18 hrs Two year warranty when purchased from an authorized Sennheiser dealer.