Ten users at once, getting exactly the same session, with voiceover if desired, separate volume/intensity controls for each. Works with any light/sound system. Great for demos, seminars, research.  Expander features: Accepts light and sound input from any light/sound system. Stereo connectors for 10 headphones and 10 light frames (common ground, like our small system light frames, not the Pro version) Separate volume and intensity control for each user. AC adapter included, can handle 10 light frames turned all the way up. Inaudible coupling from light frames to headphones (that was tricky). Expanders can be daisy chained to handle more users. What you get: Expander chassis, AC adapter.