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Ultrasound refers to sound waves with frequencies greater than the upper limit of human hearing, approximately 20 kilohertz. Some animals, such as dogs, dolphins, and bats, have an upper limit that is greater than that of the human
ear and thus can hear ultrasound.
Ultrasound has industrial and medical applications. Medical ultrasonography can visualise muscle and soft tissue, making them useful for scanning the
organs. Typical diagnostic ultrasound scanners operate in the frequency range of 2 to 10 megahertz. Powerful ultrasound may also be used to break up kidney
stones.
Ultrasonic cleaners, sometimes called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and
industrial parts. Home cleaners are available and costs range from approximately US $100.
Ultrasound in unusual materials can produce exotic phenomena such as sonoluminescence. These phenomena are being investigated partly because of the possibility of bubble fusion.
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