Infrared & RF Systems
Infrared (IR) listening systems transmit sound by invisible light beams. The infrared signal is limited to line of sight transmission, so there is no spill over from room to room. This makes it a quite secure form of transmission used in conference rooms and courtrooms where confidentiality is a concern. It is not suitable for outdoor use because sunlight and bright incandescent light can be a cause of interference. Some units also have issues with plasma and jumbo flat-screen TVs.
RF Systems use a radio frequency transmission. These are able to penetrate walls allowing the receivers to be moved from room to room without loss of signal.