Dog whistle

A dog whistle (also known as silent whistle or Galton's whistle) is a type of whistle that emits sound in the ultrasonic range, which people cannot hear but some other animals can, including dogs and domestic cats, and is used in their training. It was invented in 1876 by Francis Galton and is mentioned in his book Inquiries into Human Faculty and its Development, in which he describes experiments to test the range of frequencies that could be heard by various animals, such as a house cat.