Waves: Ultrasound

Properties of Ultrasound Waves

Ultrasound is a type of sound wave that has a frequency of above 20,000 Hz, which is higher than the human hearing range.
Like all sound waves, ultrasound is a type of longitudinal wave, meaning the vibrations occur in the same direction as the direction of energy transfer.
The speed of ultrasound in air is approximately 343 metres per second, but it travels faster in liquids and solids.
Ultrasound waves can be produced using piezoelectric materials which vibrate when an electric current is applied to them.

Medical imaging: Ultrasound scans are commonly used in medicine to create images of the inside of the body, such as foetal scans during pregnancy.
Sonar: Ships and submarines use ultrasound waves to detect the depth of the sea or locate underwater objects.
Cleaning: High frequencies ultrasound waves are sometimes used to clean delicate equipment in industries such as jewellery or electronics.
Industrial testing: Ultrasound waves can be used to detect flaws in metal structures. They reflect back if there is a flaw, allowing for early detection of potential issues.

Reflection: When an ultrasound wave reaches a boundary between two different materials, some of the wave is absorbed and some is reflected back.
The time taken for the reflected wave to reach the detector can be used to determine how far away the boundary is.
The difference in speed of ultrasound in different mediums can result in a change in direction, a process known as refraction.

An echo is the reflection of an ultrasound wave back to its source. This principle is used in both sonar and ultrasound scans.
In the pulse-echo technique, pulses of ultrasound are transmitted and the echoes are detected and timed. The distance to an interface (boundary) can be calculated by multiplying the time taken by the speed of ultrasound.