When the PetSafe® Indoor Bark Control is within range of a barking dog, an internal microphone picks up the sound. The unit automatically emits an ultrasonic tone that can be heard by dogs but is silent to most humans. Startled by the high-pitched tone, the dog should stop barking. In time it will associate its bark with this unpleasant noise. When the dog stops barking, the ultrasonic tone also stops. It is effective up to 8 metres. Use 1 unit for 1 room or 2 units for multi-room coverage.