It creates an indoor bark-free zone for one or more dogs without a collar. No collar needed, freestanding or can attach to doors, windows, tables, etc. 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.