It is a special kind of whistle which emits ultrasonic waves between 20,000 Hz and 40,000 Hz. These vibrations are not audible to human beings, but can easily be perceived by the dogs. Thus, dogs can be given special training to hear the sound from Galton's whistle. It is very useful, if an intruder enters someone's place. The whistle can be blown, which will alert the dog who may pounce on the unaware intruder.