Freestanding unit uses ultrasonic sound to automatically correct excessive barking. No collar needed. Detects barking up to 25 feet away. The unit can be left freestanding or can be adhered to door frames, window frames, etc. When the microphone picks up the sound of your dog's bark, the unit emits an ultrasonic tone that can be heard by your dog, but is silent to most humans. Startled by the high-pitched tone, the dog should stop barking, as it will associate its bark with this unpleasant noise.