Sound turns on the transmitter; which might be a good way to hear a baby waking up. The transmitter can be monitored with any FM radio; Walkman etc. Simply tune the receiver to a clear spot on the FM band (between 88 and 108 MHz) and then tune in the transmitter. A neat way to catch a thief in another room or garage. The volume of sound needed to trigger the circuit is adjustable; can be set to very sensitive pickup.. The two stage transmitter has good range and the circuit powers down unless it "hears" sounds; reactivates with a sound.