How a “silent” whistle makes a sound dogs can hear
A dog whistle is not silent; its tone is simply too high-pitched for most people to hear. Sound is vibration traveling through air, and frequency—the number of vibrations per second—sets its pitch. Frequency is measured in hertz. Human hearing usually tops out near 20,000 hertz, or 20 kilohertz, while dogs can hear some higher frequencies.
In many whistles, blowing air across a small opening sets a column of air vibrating. The opening and the whistle’s shape determine the pitch, much as the size of a musical instrument helps determine the notes it can produce. Some models let the user adjust the tone, though a high-frequency whistle is not automatically a louder one.
Why the tone can work at a distance
Because a person may not hear the whistle, it can seem as if a dog responds to nothing. The dog is detecting a sound outside the person’s hearing range. That can make the whistle useful as a distinct training cue, but it does not make it a magic command: the dog still has to learn what the sound means, and its response depends on training and circumstances.
“Ultrasonic” is a relative label here: it means above the usual upper limit of human hearing, not inaudible to every person under every condition. Hearing ranges vary, and a whistle’s tone and strength depend on its design and how it is blown. The practical oddity is that the handler may be unable to monitor the cue by ear even while a dog can hear it clearly.
