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The science of sound waves with frequencies above those that humans can hear (>20 kHz). With modern piezoelectric techniques (see piezoelectricity), ultrasonic waves having frequencies above 24 kHz can be readily generated with high efficiency and intensity in solids and liquids, and exhibit the normal wave properties of reflection, refraction, and diffraction. They can be used as investigative tools or for concentrating large amounts mechanical energy. Low-power waves are used in thickness gauging and holography, high-power waves in surgery and for industrial homogenization, cleaning, and machining. See also ultrasound scanning.

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