Audio Unlimited SPK-TRANS2 Dual Powered Transmitter for SPK-ROCK & SPK-VELO series (White) Powered by battery (take 4 AA batteries not included) or AC adapter. Fully compatible with all Audio Unlimited products Create a multi-room sound system Phase Loop Lock circuitry automatically locks in and keeps audio signal clear and strong Connect to your computer for remote listening 900 MHz transmitter sends audio signals without speaker wire