When the PetSafe® Deluxe Outdoor Bark Control is within range of a barking dog, an internal microphone picks up the sound and the unit is automatically activated. The Deluxe Outdoor Bark Control emits an ultrasonic sound that can be heard by dogs but is silent to most humans. Startled by the high-pitched sound, the dog should stop barking as it will associate its bark with this unpleasant noise. The digital timer can be programmed to turn the unit on and off at specific times during the day and night. The PetSafe® Deluxe Outdoor Bark Control is weatherproof and designed to easily hang from a tree or hook in your garden. It can also be placed on your garden ledge. Always ensure the unit is facing and within the 15 m range of the barking dog.