Both the Indoor and Outdoor Bark Deterrents are activated by sound (dog barking) and emit an Ultrasonic Tone that only dogs can hear. Dog's find this sound unpleasant and associate it with their barking behaviour.
Most dogs find the sound annoying and associate their barking with the unpleasant sound. The end result...no more barking! These devices work best when the dog is approaching the front of the device to bark.