Rear Seat Expansion Kit with Helmet Kits or Headsets can expand a two-person intercom system to four persons. Available options are for STX Stereo and Mono Intercoms using a 16' cable. AlphaBass Headset has carbon fiber, leather ear seals, Flex Boom Mic, M101 Red dynamic noise cancelling mic, and controls for volume and bass. Ultimate Mono and STX Headsets have a carbon fiber style finish, flex boom mic, M101 dynamic noise cancelling mic, hard wired cabling, large comfortable gel ear seals, cloth ear covers, and volume control. Mono and STX Helmet Kits have Alpha Audio speakers, Clear Hear speakers, dynamic noise cancelling mic, are easy to install with peal and stick adhesive. Compatibility with STX and Mono intercoms.