Ultrasound consists of a series of extremely high-frequency waves of sound, which may be bounced off of objects to form an image in three dimensions, according to ICU Sonography. Ultrasound scanners are used to noninvasively scan the interior of many objects including the human body.
Sound is a mechanical wave, which must propagate through a medium of some kind. A standing sound wave consists of alternating, regularly spaced areas of compression and rarefaction. This pattern of waves passes by a given point at whatever the speed of sound is in the medium transmitting the wave. The crests and troughs of the wave arrive at their target point at a rate determined by the speed of the wave and the distance, or wavelength, between the crests. The rate of arrivals is referred to as the wave's frequency, and it is measured in arrivals per second, or hertz, explains Dr. Beatrice Hoffmann.
An emitted sound can be tuned to a short wavelength that, given a constant speed, raises the frequency of the wave, notes Dr. Hoffmann. Humans perceive frequencies of between 20 hertz and 20,000 hertz as sound. Waves operating above that range are inaudible to humans, though sensitive diagnostic equipment can interpret the echoes from ultrasonic frequencies to build up a picture of surfaces the sound has encountered.