Wireless infrared (IR) headphones use IR data transmission technology. Infrared refers to a band of electromagnetic radiation just outside the visible spectrum of light, and for this reason, it is sometimes called infrared light, although the term light is more often applied to only those wavelengths visible to the human eye. Wireless IR headphones receive a signal, in the form of pulses of infrared radiation, from a transmitter that is connected to the output of some other device, such as a television, computer, mp3 player, or stereo. This signal carries data in much the same way that radio waves carry data, and the headphones convert the data into sound.
Infrared technology is used in a number of short range wireless devices, such as the ubiquitous television remote. Devices of this type operate in one of two ways, by line of sight or by a somewhat more versatile diffuse mode. Infrared radiation does not penetrate buildings or remain viable over long distances. This is why most television remotes, which generally operate on a line of sight basis, have to be pointed directly at the television from relatively close range to work. Both the transmitter and the receiver must be able to "see" each other. Generally, the range on such devices is around 20 feet (6 meters).
Diffuse mode, which is sometimes called scatter mode by some manufacturers, works by broadcasting the infrared data signal in all directions at once. This allows for the receiver to pick up the signal without a direct line of sight to the transmitter. Wireless IR headphones often use this type of infrared transmission so that the user can move about without fear of losing the signal. This type of infrared transmission still has a very short range, however, and generally only works if the transmitter and receiver are in the same room. Walls and closed doors will block the signal.
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The term infrared itself can be somewhat confusing, as it is used to describe a number of different products and technologies. Many people are familiar with infrared cooking appliances or heat lamps. While these appliances do employ the same wavelengths of energy used in wireless IR headphones and other such devices, the power behind infrared data transmissions is much lower and poses no danger. Wearing wireless IR headphones will not cause a feeling of warmth or damage the user in any way, as is sometimes mistakenly believed.
This type of technology is popular for applications like wireless IR headphones and remote controls for because of its limitations, not in spite of them. The fact that the transmission is blocked by walls makes it difficult to interfere with or eavesdrop on the data stream. Radio interference from other devices like cell phones, baby monitors or televisions that use long range or penetrating electromagnetic frequencies cannot produce interference, allowing infrared devices to provide a very clean signal with good sound transmission properties.