Ultrasound is merely sound (pressure waves) which cycles at such a high frequency that human ears cannot respond. We say our ears are not sensitive to ultrasound.

Because our ears don't respond, we can't hear ultrasound, but other animals can hear it, like cats, dogs, bats and dolphins. Bats and dolphins often call at ultrasonic frequencies, especially when they are using echolocation (sonar) calls.

What is the frequency of the highest sound that you can hear? Most people can't hear sounds of frequencies above 20kHz. Can you*? Click and hold on the notes below, and listen.

* the speaker of your computer was designed to produce sounds within the human hearing range, so will not reproduce the sounds above 8kHz very well. This demonstration gives you some idea of how sounds of different frequencies appear to us, and the frequency limits of human hearing sensitivity. The notes play a pure tone (sine wave) of the frequency shown, in kilohertz (cycles per second).