Ultrasound

Ultrasound is sound of 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.

See also physics, light.