Any sound of frequencies above the limit of human hearing, that is, 20,000 hertz or more, is called ultrasound. Ultrasound or ultrasonic waves are generated by devices called transducers that convert electrical, mechanical, or hydraulic energy into ultrasonic energy. The intense ultrasound waves generated by these devices can pack enough energy to break kidney stones (a process called lithotripsy); destroy harmful tissues in the brain and force dust and grease off delicate metal surfaces. They can cause liquids such as oil and water that normally would not mix to do so.
Ultrasound is also used to measure the wall thickness of pipes; to detect flaws in metal castings, and in sonar devices to locate enemy ships and under-water obstacles. It is used to monitor the development of the fetus, diagnose tumours, gallstones and heart disease. A beam of tiny bursts or pulses of ultrasound is generated by a small probe which is moved over the surface of the body. Each pulse of ultrasound is reflected or absorbed depending on the characteristics of the tissues through which it passes. The probe picks up echoes reflected from the tissues. The echo pattern is analyzed by a computer that produces an image on a TV screen.