The human ear responds to sounds with frequencies in the range from 20 Hz to 20,000 Hz. This is called the audible range of the human ear. Examples of vibrating sources that produce sound in the audible range of frequencies are drums, guitar strings, tuning fork, human vocal cords and diaphragms of loudspeakers.
In audible sound waves whose frequencies are less than 20 Hz are in the infrasonic range. Sources of infrasonic waves include earthquakes, thunder, volcanoes and waves produced by vibrating heavy machinery. The hearing ranges of elephants and whales extend into the infrasonic region.
Frequencies above 20,000 Hz are in the ultrasonic range. The audible range of dogs, cats, moths and mice extends into ultrasound frequencies. They can hear very high frequencies that humans cannot.