Ultrasounds are sound waves with frequencies higher than the upper audible limit of human hearing.Ultrasound is not different from 'normal' (audible) sound in its physical properties,only in that humans cannot hear it.This limit varies from person to person and is approximately 20 kilohertz (20,000 hertz) in healthy,young adults.Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.
Ultrasound is used in many different fields.Ultrasonic devices are used to detect objects and measure distances.Ultrasound imaging or sonography is often in medicine.In the nondestructive testing of products and structures,ultrasound is used to detect invisible flaws.Industrially,ultrasound is used for cleaning,mixing,and to accelerate chemical processes.Animals such as bats and porpoises use ultrasound for locating prey and obstacles.
Acoustics,the science of sound,starts as far back as Pythagoras in the 6th century BC,who wrote on the mathematical properties of stringed instruments.Sir Francis Galton constructed a whistle producing ultrasound in 1893.The first technological application of ultrasound was an attempt to detect submarines by Paul Langevin in 1917.The piezoelectric effect,discovered by Jacques and Pierre Curie in 1880,was useful in transducers to generate and detect ultrasonic waves in air and water. Echolocation in bats was discovered by Lazzaro Spallanzani in 1794,when he demonstrated that bats hunted and navigated by inaudible sound and not vision.
Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz."
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