6.1.11 Ultrasound

Ultrasound (Physics only)

When ultrasound reaches a boundary between two media, they are partially reflected back.
The remainder of the waves continue and pass through.
A receiver next to the emitter can record the reflected waves.
- The speed of the waves are constant, so measuring the time between emission and detection can show the distance from the source.
- Can be used for imaging under surfaces.
- A crack in a metal block will cause some waves to reflect earlier than the rest, so will show up.
- Scanning of the human fetus also uses ultrasound for non-invasive imaging.

Infrasound

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Infrasound is the opposite of ultrasound – it is a sound wave with a frequency lower than 20Hz – also known as seismic waves. There are two: P and S waves.

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Key Points

This is used to explore the Earth's core.
P waves are longitudinal and can pass through solids and liquids.
S waves are transverse and only pass through solids (these move slower too).
On the opposite side of the Earth to an earthquake, only P waves are detected, suggesting the core of the Earth is liquid – hence no S waves can penetrate it.

Sonar

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A pulse of ultrasound is sent below a ship, and the time taken for it to reflect and reach the ship can be used to calculate the depth.

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Key Points

This is used to work out whether there is a shoal of fish below the ship.
Or how far the seabed is below the ship.

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