This Flowchart Shows What Is Happening in Diagram 1

You do not need to remember the details on this sheet for your exam but you could be asked to apply your knowledge to unfamiliar situations.

Sound travelling / % energy reflected
from / to
air / skin / 99%
air / water / 99%
water / skin / 0.1%
skin / muscle / 0.1%

Ultrasound scans work because some of the ultrasound energy is reflected each time it passes from one material into another. The proportion of the energy reflected depends on the difference between the speed of sound in each of the two materials and the density of the two materials. The table shows some typical values.

The diagrams below show two ultrasound scans being made. The scanner both sends and detects ultrasound as it is moved over the woman's abdomen.

This flowchart shows what is happening in Diagram 1.

1Copy and complete this flowchart to show what is happening in diagram 2.

2aHow much of the ultrasound energy will be reflected between stages B and C?

bAfter stage C how much of the ultrasound energy will be left to reflect from the fetus inside the woman's body?

cThe jelly used in diagram 2 is similar to water. How much of the ultrasound will be reflected between stages F and G?

dAfter stage G how much of the ultrasound energy will be left to reflect from the fetus inside the woman's body?

eExplain in your own words why jelly is used in ultrasound scans and why the scanner must be touching the jelly.

3The distance between the mother's skin and the back of the fetus' head is about 15cm. The speed of sound in muscle is approximately 1580m/s.

aHow long will it take an ultrasound wave to travel this distance and back again?

bSuggest why the time you have calculated may not be the correct value.

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The distance a wave has travelled can be worked out from its speed and the time it takes. Distance, speed and time are related by this equation:

distance = speed × time

The units normally used are metres, metres/second and seconds, respectively. However, for small distances it is sometimes more convenient to use other units. When using ultrasound for quality control, units of centimetres per microsecond (cm/µs) are often used. One microsecond is one millionth of a second (1 x 10−6s).

Material / Speed of sound (cm/µs)
aluminium / 0.63
steel / 0.59
polythene / 0.27

1A manufacturing company uses ultrasound to check that the polythene sheets they make are the correct thickness. If the sheet is the correct thickness, the ultrasound detector receives an echo after 3.6µs.

aExplain how the machine detects the thickness of the polythene.

bHow thick is the sheet supposed to be?

2The machine receives an echo after 3.8µs.

aIs the polythene sheet too thick or too thin? Explain your answer.

bBy how much is it too thick or too thin?

3The machine is adjusted to check the thickness of a thinner sheet. Should the detector receive the echo in a shorter or longer time than 3.6µs? Explain your answer.

4The same machine can be used to detect the thickness of steel plates. The machine receives an echo after 2.2µs if the steel is the correct thickness.

aHow thick should the steel be?

bDuring one test, the machine receives a faint echo after only 0.5µs as well as a strong echo at 2.2µs. Suggest what could have caused the faint echo and where it is.

A manufacturer makes sheet material that consists of a layer of polythene sandwiched between sheets of steel and aluminium. The material is manufactured in several different overall thicknesses. The trace below is from a final test of a sheet.

5What do the three peaks on the trace represent?

6Calculate the thickness of the steel layer.

This document may have been altered from the original.

1Copy and complete the table, using words from the box. You do not need to use all of the words.

Name / Type of sound / Frequency
ultrasound
infrasound

too high for humans to heartoo low for humans to hearabove 20 Hz

below 20 Hz above 20 kHzbelow 20 kHz

2aName two animals that use ultrasound.

bSuggest one reason why these animals use ultrasound.

3Ultrasound scans can be used to make images of babies before they are born. Suggest one reason why ultrasound scans are used by doctors.

4The speed of sound depends on the material the sound is travelling in. The table shows the speed of sound in sea water and fresh water.
A fishing boat uses sonar to work out how deep the sea is. It sends out a ‘ping’ of sound and detects the echo 2 seconds later. / Material / Speed of sound (m/s)
sea water / 1533
fresh water / 1493
aHow far has the sound travelled in the 2 seconds?
bHow deep is the water?
cIf the boat was on a lake instead of the sea how deep would the water be if the echo took 2 seconds to return?
dWhy is it important that the sonar machine has the information about what kind of water the boat is in? /

You will need this equation to help you to answer the questions:

speed =

This equation can be rearranged to give these equations:

distance = speed × timetime =

This document may have been altered from the original.

The table shows the speed of sound in different materials. You will also need this equation to help you to answer some of the questions.
speed = /

1Describe the similarities and differences between sound, ultrasound and infrasound.

2An anchored fishing boat uses sonar to work out how deep the sea is. It sends out a ‘ping’ of sound and detects the echo 2 seconds later.

aHow deep is the water?

bIf the boat was on a lake instead of the sea, how deep would the water be if the echo took 2 seconds to return?

cWhy is it important that the sonar machine has the information about what kind of water the boat is in?

dA little while later, the ultrasound machine detects two echoes from a ping, after 0.05 seconds and 0.7 seconds. Suggest what these echoes mean.

3Some ultrasound energy is reflected each time an ultrasound wave passes from one medium to another. Ultrasound is used to scan pregnant women, to check on the health of the baby (see diagram, right). The proportion of energy reflected depends on how different the speed of sound is between the two materials, and on the difference between their densities. Water, skin and muscle all have similar (but not identical) properties.

aExplain how an ultrasound scan works.

bExplain why a watery jelly is used when an ultrasound scan is made.

cWhy is it important that there are no bubbles of air trapped in the jelly?

dWhen doctors need to insert a metal needle into a particular blood vessel or organ, they can see where the needle is going using ultrasound. Suggest why metal objects show up on ultrasound scans.

Extra challenge

4The speed of sound in the sea varies because of changes in pressure and salinity. Sonar equipment assumes a standard speed of sound.

A ship is sailing in water 4km deep. What depth error would a 1% error in the speed of sound cause?

This document may have been altered from the original.