When the PetSafe® Outdoor Bark Control is within range of a barking dog, an internal microphone picks up the sound and the unit is automatically activated. Emits an ultrasonic sound that can be heard by dogs but is silent to humans. Startled by the high-pitched sound that is safe and effective, the dog should stop barking, as it will associate its bark with this unpleasant noise. When the dog stops barking the ultrasonic sound also stops. 3 sensitivity levels to choose from that indicate what barking level activates the unit and a test mode. It is effective up to 50 feet/ 15 meters. Operates on a 9 volt battery (sold separately). Designed to hang or mount on a tree, wall or fence post. Safe and effective for all sizes and breeds of dogs. Durable and weather proof. Two color LED for good/low battery indication.