Ultrasound refers to sound waves with frequencies higher than the frequencies audible to the human ear. Frequency of sound is measured in hertz, where 1 hertz equals one cycle of vibration per second. Humans can hear sound with frequency ranging from 20 hertz to 20,000 hertz. There are animals such as bats who can hear sound waves with higher than 20,000 hertz.
Ultrasonic waves have shorter wavelengths. This causes them to be reflect or echo back more clearly when they meet some object or obstacle in their path. In comparison, sound waves of higher wave length bend round the objects in their path and do not produce clear echo. This ability of ultrasound is used in medical field to study internal parts of body without having to expose such parts to direct human vision. In this ways it is possible to obtain images of soft internal body parts that do not show up in x-rays scans.
The imaging of internal body parts called sonography or ultrasonography was first used by a Sottish obstetrician in 1958 for examining a foetus inside a womb. Sonography is now used for examining may other organs like lever, heart and kidney. A process called echocardiography uses ultrasound waves to study movement of heart and flow of blood through valves.
Ultrasound is also used for performing some active medical procedures, an addition of just passive collection of information. For example ultrasound is used destroy brain tumors and crush kidney stones.