Sennheiser HDR 175 Accessory RF Wireless Headphone for RS 175 System Closed back, circumaural headphone system. Connectivity Technology: Wireless High-quality Sennheiser transducers for exceptional sound Designed exclusively for use with the RS 175 RF Wireless Headphone system (sold separately). Frequency response : 17 - 22000 Hz. Maximum sound pressure level : 114 dB at 1kHz, 3 percentage THD Two year warranty when purchased from an authorized Sennheiser dealer. Connectivity technology: Radio Frequency wireless