Ultrasound is the term used for vibrations at higher frequencies than the human ear can hear. Generally, the upper end of the range of human hearing occurs at 20 kHz, or 20,000 vibrations per second. Since the speed of ultrasonic waves is the same as the speed of sound (1130 ft/s in warm air and standard atmospheric pressure), the wavelength of these vibrations is extremely short. Ultrasonic waves, like all sound waves, are the pressure waves of a displacement front. However, in the case of ultrasonic waves, the displacements are coming too quickly for the human ear to distinguish them.

Ultrasonic vibrations in nature are usually emitted by animals such as dolphins and bats, although some pure crystal minerals can be mechanically induced to vibrate at ultrasonic frequencies as well. The ultrasonic imaging used by bats and dolphins is also known as sonar, and some human...