Sound with frequencies above about 20,000 Hz (20 kHz) is known as ultrasound. Such sounds are not audible to humans. Ultrasound has many important industrial and biomedical applications.
Medical ultrasound is important for the diagnostic imaging of internal body structures such as joints, blood vessels, and internal organs. The examination of pregnant women using ultrasound is now a routine procedure.
Ultrasound can be used in nonndestructive testing, for locating flaws in materials and for measuring the thickness of objects. Frequencies of 2 to 10 MHz are common for examining metal, plastics and aerospace composites. Lower frequency ultrasound (50–500 kHz) can be used for materials such as wood, concrete and cement.
Ultrasonic sensors are widely used in automatic door openers and security detection systems.