Limits of audibility
The range of frequencies that humans can hear is generally considered to be between 20 Hz and 20,000 Hz. This range can vary slightly from person to person, but it is generally accepted that any sound outside of this range is considered inaudible.
Ultrasonic sound and Infrasonic sound
Ultrasonic sound is sound waves that have a frequency above the range of human hearing (above 20 kHz). Ultrasonic sound waves are used in sonar and medical applications.
Infrasonic sound is sound waves that have a frequency below the range of human hearing (below 20 Hz). Infrasonic sound waves are used in seismic and meteorological applications.
Ultrasonic sound:
Bats use ultrasonic sound to navigate and hunt prey in the dark. A dog whistle is an example of an ultrasonic sound.
Infrasonic sound:
Infrasonic sound is sound that has a frequency lower than 20 Hz, which humans cannot hear. Earthquakes and volcanic eruptions often produce infrasonic sound.