When the PetSafe® Indoor Bark Control microphone picks up the sound of your dog's bark, the unit emits an ultrasonic tone that can be heard by your dog, but it is silent to most people. Startled by the high-pitched tone, the dog should stop barking, as it will associate its bark with the unpleasant noise. For dogs of any size, this bark control unit uses a high-frequency sound that works for dogs small to large within hearing range. PetSafe® brand is here to help you and your pet live happy together™.