Dog whistles emit sounds in the high frequency range. Humans can only hear up to 10 000Hz, but dogs have much better hearing. Since dog whistles are silent to humans, but loud to dogs, they are perfect for training dogs.
App can also be connected to a Bluetooth speaker to produce a louder sound if dogs are further away

This Dog Whistle app can emit high frequency sounds anywhere in the range from 5 000 to 22,000 Hz.