The Petsafe Ultrasonic bark control uses a microphone to pick up the sound of your dog's bark, the unit then emits an ultrasonic tone that can be heard by your dog but is silent to most humans. Startled by the high-pitched tone the dog should stop barking as it will associate its bark with the unpleasant noise.
Please note that these types of deterrents are best suited to timid dogs as there is a high chance a stubborn dog would ignore it.
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