Some sounds are so high-pitched (those with frequencies above 20,000 Hz) that our ears cannot detect them. They are known as ultrasounds. Many animals, like dogs and bats, can hear some ultrasounds. Ultrasound is often used in medicine because, unlike X-rays, the sound waves do not damage the body's organs. An ultrasound scanner beams very high-pitched sound waves into the body. The echoes received from the bodies organs or a baby in the womb, are analysed by a computer to form an image.

Echolocation

Some animals, including bats, whales and dolphins, make calls and listen to the echoes of those calls that return from certain objects around them. They use these echoes to locate and identify the objects. This technique, called echolocation, is used by these animals both for getting around and for hunting—extremely useful in the darkness of the oceans (for whales and dolphins) or for flying at night (for bats).
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Non-destructive testing

Ultrasound can also be used to inspect materials and check whether there are internal flaws in them. This is called non-destructive testing (NDT). Metals (welded joints for example), plastics, concrete and wood are all materials that can be inspected.

Sonar

Another common use of ultrasound is in sonar (Sound Navigation And Ranging). An ultrasonic pulse is sent out in a certain direction. If an object lies in its path, the pulse will be reflected back as an echo and can be detected. By measuring the difference in time between transmission and the echo being received, the distance can be calculated. Ships use sonar to find shoals of fish, or other ships.

Cleaning

At high frequencies, ultrasound can be used along as a precision cleaning device: high-energy pulses passed through a liquid can dislodge tiny particles of dirt. Jewellery, lenses, watches, dental instruments and surgical instruments may all be cleaned in this way.