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. The ultrasonic sound can be heard by dogs but is silent to most humans. Startled by the high-pitched sound that is safe and effective, the dog should stop barking, as it will associate its bark with this unpleasant noise. When the dog stops barking the ultrasonic sound also stops. Programmable timers let you curb barking at certain times of day, i.e. after school as children pass by. Setting the timer(s) also helps conserve battery life. The Deluxe Outdoor Bark Control includes a timer for two off/on presets. It is effective up to 50 feet.