... Materials, (environmental protection) 8.Dual channel for two car audio sources9.Compatible with car entertainment, PC, TV, DVD, IPOD, MP4/MP3,10 ... .Personal volume control13. Numbers of headphones can be used simultaneously in the carSpecifications:1) Carrier frequency:a) A source: 2.8MHz ... ) Frequency response: 30 ~ 20,000Hz9) Power supply: 2 AAA battery car Infrared wireless headphone