Rear Seat Expansion Kit with Helmet Kits or Headsets expands your two-person intercom system to 4 persons. These expansion packs are available in headset and helmet kit options for STX Stereo and Mono Intercoms, equipped with 16' cables to reach the back seats. The AlphaBass Headset features TRUE Carbon fiber, leather ear seals, Flex Boom Mic, M101 Red dynamic noise cancelling mic, and easy-to-use volume and bass control. Ultimate Mono and STX Headsets come with a Carbon Fiber style finish, Flex boom mic, M101 red dynamic noise cancelling mic, hard wired cabling, large comfortable gel ear seals, cloth ear covers and easy to use volume control. Meanwhile, Mono and STX Helmet Kits feature Alpha Audio speakers, Clear Hear speakers, Dynamic Noise Canceling mic and easy to install helmet speakers with peal and stick adhesive. Each kit includes two Headsets or Helmet Kits and two 16' intercom Cables. Compatible with various STX Stereo and Mono intercoms.