Ultrasound & Infrasound

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.

Sound and seismic words

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

How deep is the sea?

A ship sends a sonar pulse straight down and the echo returns 0.40 s later. Sound travels at 1500 m/s in seawater. How deep is the water, in metres? Think about how far the pulse actually travelled.

Now the other way round

The same ship moves to where the seabed is 450 m below it. Sound still travels at 1500 m/s in seawater. How long after the pulse is sent does the echo come back, in seconds?

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?

This is a cut-through of the Earth: a thin outer ring (the crust), a thick layer beneath it (the mantle), a large circle inside that (the outer core) and a small circle at the very centre (the inner core). S-waves are transverse, so they cannot pass through a liquid. Tap the layer that stops them.

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

The same method as the sonar, run thousands of times a second. Put it in order.

• The probe sends a short pulse of ultrasound into the body
• The pulse reaches a boundary between two different tissues
• Part of the pulse is reflected back towards the probe, and the rest carries on
• The probe detects the returning pulse and the machine times how long it took
• The time and the speed of sound in tissue give the distance to that boundary
• Thousands of these distances are assembled into an image on the screen

Three decisions

Three situations where the physics decides what you do.

• A sonographer squeezes cold gel onto someone's skin before starting a scan. Why is that necessary rather than just unpleasant?
• A hospital needs to check on a developing foetus. Why ultrasound rather than X-rays?
• A survey ship gets an echo back in a fifth of the time it did an hour ago, at the same settings. What has changed?

Seeing with echoes

Explain how sending out a wave and waiting for it to come back can tell you about something you cannot see. Use both a sonar or scanning example and the seismic evidence for the Earth's core.

• Explain what happens when a wave meets a boundary between two materials
• Describe how the time an echo takes is turned into a distance, and why the time has to be halved
• Explain what the S-wave shadow zone shows about the outer core, and why S-waves in particular
• Say why P-waves still get through, and what their curved paths tell you
• Finish with the idea that links all of it: a wave that fails to arrive is evidence too