Simple! An 8000X, 8000XU1, and 8000XU2 (Liberty & MegaVox) include a built-in transmitter - as designated by the X. This transmitter operates on the 902 - 928 MHz frequency, with 100 user-selectable channels. The AIR wireless companion speakers have a built-in receiver which, when set to the same channel as the transmitter, picks up the audio signal.