Outdoor Ultrasonic Bark Control Device: this Ultrasonic Bark Control System can hang in a tree, on a fence post, or anywhere outside. When the Ultrasonic Bark Control Birdhouse detects excessive barking, the microphone picks up the barking and activates the unit. This results in a safe high pitch ultrasonic sound that is audible to dogs, but not to humans. The offending dog hears the sound and will stop barking. Since it is unpleasant to the dog, the dog learns to stop barking!