Rear Seat Expansion Kit with Helmet Kits or Headsets allows expansion of two-person to 4-person intercom system, available in headset and helmet versions for STX Stereo and Mono Intercoms including 16' cables. Notable features of AlphaBass Headset include TRUE carbon fiber, leather ear seals, Flex Boom Mic, M101 Red dynamic noise cancelling mic and easy to control volume and bass. Ultimate Mono and STX Headset includes a carbon fiber style finish, Flex boom mic, M101 red dynamic noise cancelling mic, hard-wired cabling, large comfortable gel ear seals and easy volume control. Mono and STX Helmet Kit provides Alpha Audio speakers, superior highs and lows, clear hear speakers, a dynamic noise canceling mic and easily installed helmet speakers. Each kit includes two headsets or helmet kits and two 16' intercom cables. Compatible with various STX and Rugged Radios MONO intercoms.