I don't hear much about dog whistles these days, except maybe in politics or cartoons. I was watching a children’s cartoon where the genius dog gives his adopted son a dog whistle to use in case of emergencies. That got me wondering, are there really things that produce a sound audible to dogs but not to people?
Francis Galton developed the first high-frequency sound device, back in the 1870s. He was a curious man, in all senses of the word. To test the limits of human hearing, he built a pipe whistle with a length that could be precisely adjusted to produce different frequencies. He was a half cousin to Charles Darwin and maybe that inspired him to also test animals at the London Zoo, as well as cats and dogs in the streets.
Humans and other mammals have tiny hairs in their ears that react to different frequencies and send electrical signals to their brains. For humans, the upper limit is around 20 kilohertz or about 20,000 vibrations per second, but this varies between individuals and drops off as adults age. Dogs can hear up to 45kHz, with smaller dogs usually being more sensitive. Many mammals can hear higher frequencies than humans—even cats.
I would imagine that good hearing must be important to the survival of mammals, especially those scuttling around at night. Two distinct ears enable them to identify the direction that sounds come from, which could be the difference between getting eaten or not. For smaller mammal heads, higher frequencies and their correspondingly shorter wavelengths are required for comparable resolution.
Some people still use dog whistles to train hunting dogs as they will respond to the sound, but the game will not. It seems to me that a high frequency whistle would be more for the novelty than the necessity, in most cases.
Speaking of novelty, at the end of the song “A Day in the Life” by the Beatles (from their Sergeant Pepper’s Lonely Hearts Club Band album), apparently there is a dog whistle. I have heard different arguments about whether it was there or not. Some say that it depends on the state of your sound equipment, whether it was the British or American version and whether or not it was vinyl or CD. I tried a version and my ten-year-old claimed she could hear the whistle (just after 5:00 into the song), although my dog did not show any reaction. Maybe I was just barking up the wrong tree.