Rear Seat Expansion Kit with Helmet Kits or Headsets is designed to expand two-person intercom system to four. It comes with headset and helmet kit options for STX Stereo and Mono Intercoms and feature 16' cables to reach the back seats. The AlphaBass headset is equipped with carbon fiber, leather ear seals, Flex Boom Mic, M101 Red noise-cancelling mic, and has accessible volume and bass control. The Ultimate Mono and STX headset features carbon fiber style finish, a noise-cancelling mic, hard-wired cabling, large gel ear seals, cloth ear covers, and volume control. The STX Helmet Kit features Alpha Audio and Clear Hear speakers, noise cancelling mic, and is easily installed. Also, it includes 16' intercom cables and works with both STX Stereo High Fidelity Intercom, and Rugged Radios Mono intercoms.