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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.Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 k Hz." In air at atmospheric pressure ultrasonic waves have wavelengths of 1.9 cm or less.

Ultrasonic inspection has progressed from manual methods to computerized systems that automate much of the process.Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats.This includes many groups of moths, beetles, praying mantids and lacewings.Upon hearing a bat, some insects will make evasive manoeuvres to escape being caught. In the order Clupeiformes, members of the subfamily Alosinae (shad), have been shown to be able to detect sounds up to 180 k Hz, while the other subfamilies (e.g. An ultrasonic level or sensing system requires no contact with the target.For many processes in the medical, pharmaceutical, military and general industries this is an advantage over inline sensors that may contaminate the liquids inside a vessel or tube or that may be clogged by the product. The principle behind a pulsed-ultrasonic technology is that the transmit signal consists of short bursts of ultrasonic energy.Francis Galton in 1893 invented the Galton whistle, an adjustable whistle which produced ultrasound, which he used to measure the hearing range of humans and other animals, demonstrating that many animals could hear sounds above the hearing range of humans.The first technological application of ultrasound was an attempt to detect submarines by Paul Langevin in 1917.In fluid mechanics, fluid flow can be measured using an ultrasonic flow meter.Ultrasonic testing is a type of nondestructive testing commonly used to find flaws in materials and to measure the thickness of objects.The upper frequency limit in humans (approximately 20 k Hz) is due to limitations of the middle ear.Auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear.

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