Dog whistles exploit the difference between the hearing range of a dog and that of a human. They use a pitch that is too high for humans to hear but is high enough for dogs to hear. Longer whistles have lower pitches, so in order to get a higher sound, dog whistles use a very short tube (only about an inch long and possibly shorter).
Because humans cannot hear in the range of a dog whistle, the person who is blowing it will hear only the hissing of the air through the whistle. As a result, dog owners can train their dogs while other people are around without disturbing them. A dog whistle is effective at getting a dog's attention because there are very few sounds that occur at that range in a dog's hearing. Despite the name, dog whistles work on cats as well.
The human hearing range extends from 20 Hz to 20 kHz. A dog's hearing range, on the other hand, normally extends to 45 kHz, while a cat can hear as high as 64 kHz. Because of this difference, dog whistles are constructed to make sounds between 23 kHz and 54 kHz.
Dog whistles are also known as silent whistles or Galton's whistles.