Dog whistle emit sounds in the high frequency range. Humans can only hear up to 20 kHz, but dogs have much better hearing. Since dog whistle are silent to humans, but loud to dogs, they are perfect for training dogs. Dog whistle - Dog Trainer is easy to use app.

Dog Whistle app can emit high frequency sounds anywhere in the range from 100 to 22,000 Hz (Depends on device's speaker).