The Califone HIR-TX1 is a wireless infrared USB transmitter. There is no limit to the number of listeners so long as they are within the 20' transmission range with a clear line of sight between the headphone and transmitter. It has a USB transmitter with 10" semi-rigid USB extender. It is compatible with Mac or Windows. Use with HIR-HP1 Headphones. Use Califone's Wireless Infrared USB Transmitter in your group learning sessions without the mess of cords. This transmitter delivers an infrared signal up to 20' away with a clear line of sight to the headphones. Incorporate lessons from your computer with ease. Use with Califone's Infrared Stereo/Mono Headphones.