Sound is generally classed as ultrasonic when its frequency exceeds 20,000 hertz. This is the upper range of humans' ability to perceive, though the ultrasonic range of frequencies extends upward into the millions of hertz.
Hertz is a measure of frequency. A sound that arrives at its target with a frequency of one wave crest per second is vibrating at 1 hertz. The human ear can detect sound from about 20 to 20,000 hertz, above which is the ultrasonic range. Many animals, such as bats and dolphins, use ultrasonic frequencies for communication and echolocation, as very high-frequency sounds have a short wavelength and can be used to develop a high-resolution picture of the surrounding world.
Noninvasive medical scans require a higher resolution than can be achieved at frequencies heard by animals, so scanners designed for medical applications typically operate in the 1 megahertz to 20 megahertz range. In general, the resolution of an ultrasound image is greatest near the surface of the body because deeper tissues can only be reached with lower frequencies. Deep-tissue ultrasound scans must pass through the body's tissues, which increases the attenuation of the sound waves and forces the use of lower-frequency, and therefore lower-resolution, tones.