The range of electromagnetic frequencies above the audio range and below visible light. 30KHz and 300GHz Blue Tooth Bluetooth provides up to 720 Kbps data transfer within a range of 30feet. Bluetooth transmits in 2.4GHz band and uses a frequency hopping spread spectrum technique that changes its signal 1600 times per second. An invisible band of radiation at the lower end of the electromagnetic spectrum. It starts at the middle of the microwave spectrum and goes up to the beginning of visible light.
Infrared transmission requires an unobstructed line of sight between transmitter and receiver.
No costly rewiring is necessary. Up to 256 different addresses are available. If you want more than one device to respond to the same signal, simply set them to the same addresses. The conversion of spoken words into computer text. Speech is first digitized and then matched against a dictionary.