Infrared systems work on the same principle as FM systems, but use infrared light instead of radio waves to transmit sound. The transmitter converts sound signals into light and beams those to the receiver, which then translates the light signal back into sound.
An advantage to infrared systems is the fact that their signal is unable to pass through walls as it does with FM systems, eliminating competing broadcasts that might hamper the listener and preventing confidential information from being disseminated. They are particularly useful in courtrooms and large movie theaters.