GBC11-12134 Guardian Outdoor Bark Control, Ultrasonic New.How it works When the outdoor bark control is within range if constant barking, an internal microphone picks up the sound and the unit is activated- the guardian outdoor bark control emits an ultrasonic sound can be heard only from the dog. Started by the high pitched sound the dog's barking will be deterred, as it will associate it's bark with this unpleasant noise. The Guardian Outdoor Bark control has 4 setting including 3 sensitive Levels and the test mode and is effective up to 50 feet.