The PetSafe Ultrasonic Indoor Bark Control system automatically deters excessive barking without the use of a collar. An internal microphone picks up the sound a barking dog from up to 8 meters away. 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. The system includes 2 indoor bark control units (for multi-room coverage), 2 mounting strips, 4 3-Volt batteries & operating guide.