<P> Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing . Ultrasound is no different from' normal' (audible) sound in its physical properties, except 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 . </P> <P> Ultrasound is used in many different fields . Ultrasonic devices are used to detect objects and measure distances . Ultrasound imaging or sonography is often used 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 . Scientist are also studying ultrasound using graphene diaphragms as a method of communication . </P> <P> 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 . Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound and not vision . 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 . 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 . </P> <P> Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz ." In air at atmospheric pressure ultrasonic waves have wavelengths of 1.9 cm or less . </P>

What is the difference between ultrasonic and ultrasound