Ultrasound is produced by high frequency vibrations beyond the range of human hearing. The frequency of ultrasound is therefore greater than 20,000 hertz.
Ultrasound, like audible sounds, is transmitted by means of longitudinal waves.
Ultrasound is used in medicine to break up kidney stones and gall stones. It is also used, in a process known as medical scanning or imaging, to create images of an unborn baby in a womb.
A gel is placed between the ultrasound probe (transmitter/receiver unit) and the skin to allow ultrasound waves to be transmitted into and out of the body. A computer is used to build images of the unborn baby from the reflected ultrasound waves detected by the probe.