Ultrasound & Infrasound
Send a pulse, time the echo, and you know what is down there. It works on the seabed, inside a steel casting, inside a person, and, with the waves an earthquake makes, all the way to the centre of the Earth.
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Above and below hearing [Physics only] 🔊
**This whole module is [Physics only] content.** If you are taking Combined Science you will not be examined on it. A healthy young human ear picks up sound between about **20 Hz and 20 000 Hz**. Sound exists on both sides of that window, and both sides turn out to be useful. **Ultrasound** is sound above 20 000 Hz. **Infrasound** is sound below 20 Hz. Neither is a different kind of thing from ordinary sound: they are longitudinal waves like any other, just out of reach. And the reason they matter is one idea you will use three times in this module: **a wave that meets a boundary tells you what is on the other side of it.**
The words you need 🗂️
Six terms, and the last two are the ones that unlock the Earth:
Which one is ultrasound? ❓
Human hearing runs from about 20 Hz to 20 000 Hz. Which of these frequencies is ultrasound?
- 45 000 Hz
- 18 000 Hz
- 500 Hz
- 15 Hz
Four things a wave can do at a boundary 🚧
When a wave reaches the place where one material meets another, four things can happen, usually several at once: **Reflected** - it bounces back. This is the one every technique in this module is built on: the reflection is the signal. **Refracted** - it carries on into the new material but **changes direction, because its speed changes**. If it arrives straight on, along the normal, the speed still changes but the direction does not. **Transmitted** - it passes through and keeps going. **Absorbed** - its energy is taken up by the material, usually ending as heat. **Which of these dominates depends on how different the two materials are.** Two similar tissues let most of the wave through with a weak reflection; tissue against air reflects almost all of it, which is why a hospital sonographer puts gel on the skin - to get rid of the layer of air that would send the pulse straight back.
Match each behaviour to what it means
- Reflected
- Refracted
- Transmitted
- Absorbed
- it bounces back off the boundary, and the returning pulse is what gets measured
- it enters the new material and changes direction, because its speed changes
- it passes into the new material and carries on
- its energy is taken up by the material, usually ending up as heat
How deep is the sea? 🔢
An interactive activity.
Now the other way round ⏱️
An interactive activity.
Two waves from one earthquake 🌍
An earthquake sends out both kinds at once, and they behave differently. That difference is the only reason anybody knows what the middle of the Earth is made of.
Where do the S-waves stop? 👆
An interactive activity.
What does the S-wave shadow prove? ✅
After a large earthquake, S-waves are detected across a wide area but never on the far side of the Earth. Select the TWO conclusions that follow.
- Part of the Earth's core must be liquid, because S-waves cannot pass through a liquid
- Where a wave is NOT detected can be evidence just as strong as where it is
- The earthquake produced no S-waves at all on that occasion
- The centre of the Earth must be hollow
Write the evidence out 📝
S-waves are _____ waves, so they can travel through solids but not through a _____. After a large earthquake they are never detected on the far side of the Earth, which tells us the outer core is not _____. P-waves DO get through, because they are _____ waves. Their paths also curve, because the waves are _____ as their speed changes with depth.
How a scan is built 🩺
An interactive activity.
Three decisions 🧭
An interactive activity.
Your turn ✍️
An interactive activity.