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AQA GCSE Physics · 8463
AQA 8463 · 4.6.1.5 Waves for detection and exploration Check the specification (PDF) (opens in a new tab)
Ultrasound is sound at a frequency above the upper limit of human hearing. Like other sound waves, it needs a material to travel through. It can pass through body tissues or a solid object even when we cannot see inside them directly.
Imagine sending a short pulse of ultrasound into a material. When it reaches a boundary between two different media, such as two tissues or metal and the air in a crack, some of the wave is reflected. The rest can continue through the boundary. The reflected wave, or echo, travels back to a detector. A transducer can both send the pulse and detect its return.
The delay between sending and receiving an echo tells us how far away the boundary is, provided we know the speed of ultrasound in the material it travelled through. If that speed is and the round-trip time is , the boundary is a distance from the transducer. We divide by two because the pulse covers the distance twice: out to the boundary and back. If the wave travels through more than one medium, its speed may change, so a single speed must not be assumed for the whole path.
Echoes from different boundaries return at different times, allowing hidden structures or flaws to be located.
In a medical scan, the transducer sends pulses into the body. Boundaries between different tissues reflect some ultrasound back. Echoes from nearer boundaries return sooner than echoes from deeper ones. The scanner uses these return times to locate boundaries; measurements from many positions build up an image of internal structures. This is used, for example, to image a foetus during pregnancy or to examine organs. Ultrasound is non-ionising: unlike X-rays, it is not ionising radiation.
The same principle can reveal a hidden crack inside a metal component. A pulse sent into an uncracked piece can reflect from its far surface. If there is an internal crack, the boundary at the crack also produces an echo, which returns earlier than the far-surface echo. Its return time can be used to locate the flaw without cutting the component open.
Ultrasound has a frequency above human hearing. At a boundary between different media, it is partially reflected and partially transmitted. A transducer sends pulses and detects returning echoes. For wave speed in the material and round-trip time , boundary distance is .
Medical imaging: echo times locate tissue boundaries; repeated measurements build an image, such as a foetal scan.
Industrial imaging: an early echo can reveal an internal crack before the echo from the far surface.
For an echo calculation, use the time from sending the pulse to receiving it: the wave travels to the boundary and back, so divide the total distance by two.
For industrial testing, an echo from a crack arrives before the echo from the far side of the object.
State that ultrasound is partially reflected at a boundary between different media; some of the wave continues through.
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Last updated
Ultrasound
Sound with a frequency above the upper limit of human hearing, normally 20 kHz.
Transducer
A device that converts electrical signals into ultrasound waves and returning ultrasound waves into electrical signals.
Echo
A reflected sound wave that returns to a detector.
Put your knowledge into practice — try past paper questions for Physics
Ultrasound
Sound with a frequency above the upper limit of human hearing, normally 20 kHz.
Transducer
A device that converts electrical signals into ultrasound waves and returning ultrasound waves into electrical signals.
Echo
A reflected sound wave that returns to a detector.