Sennheiser HDR 165 Accessory RF Wireless Headphone for RS 165 System Closed back, circumaural headphone system. Connectivity Technology: Wireless High-quality Sennheiser transducers for exceptional sound Designed exclusively for use with the RS 165 RF Wireless Headphone system (sold separately) Two year warranty when purchased from an authorized Sennheiser dealer. Connectivity technology: Radio Frequency wireless