Ultrasound is sound or vibrations with frequencies too high to
be audible by the human ear, above approximately 20 kilohertz.
Some sound that is ultrasound to the human ear can be heard by
animals, such as dogs. Ultrasound has industrial and medical
applications. Medical ultrasonography can visualise muscle and
soft tissue, making them useful for scanning the organs.
However, they will not penetrate bone so they are not a
substitute for conventional X-rays in many cases. 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.