that transmit, not on a given radio
frequency, but by infrared
. This technology
is vastly cheaper than the 2.4 Ghz
radio versions, and functions adequately well in most circumstances. If placed correctly, it can cover a large room.
However, its main drawback is that, being based on infrared technology, it is unable to pass through walls, and so cannot be used with the reciever in a different room as the headphone, unless the base station is aimed at an open doorway. However, they are perfectly sufficient for use with a computer, since you can move around the room freely, and use the computer with no headphone wire to get tangled up in.
Most versions are powered by an AC adaptor for the base, and batteries for the headphone unit. The version I use uses 2 AAA batteries, and usually lasts for at least 3 weeks of intermittent listening.