When the outdoor bark control is within range of constant barking, an internal microphone picks up the sound and the unit is activated. The Pet safe Outdoor sound can only be heard by the dog. Startled by the high--pitched sound, the dog should stop barking, as it will associate its bark with this unpleasant noise. Perfect for dog training! Refer to Instruction Manual. It goes without saying we're a pet-loving brand. Right from the earliest days of PetSafe brand, our love of pets meant we all shared the same dream to make PetSafe brand one of the most trusted brands for pet owners the world over.