When the Outdoor Bark Control is within range of a barking dog, an internal microphone picks up the sound and the unit is automatically activated. The Petsafe Outdoor Bark Control emits an ultrasonic sound. Startled by the high-pitched sound, the dog should stop barking as it will associate its bark with this unpleasant noise. The Petsafe Outdoor Bark Control has 3 range levels and a test mode. It is effective up to 15 metres.