How does it work? 1. Use the ultrasonic sound device to deter dog barking - the microphone will pick up the ultrasonic sound which automatically activates the device. The sounds produced can only be heard by dogs and are silent to humans. 2. Use the Low level Mode if the dog does not respond, then adjust the device to another level accordingly. 3. Level 1 (15KHZ-30KHZ) applies to the large dog. Level 2 (20KHZ) applies to the docile dog. Level 3 (30KHZ) applies to the smaller dog.