Key Stage 3 science · Physics – Waves

KS3 water waves and sound waves: a guide for parents

In Key Stage 3 physics, pupils move from “waves are wobbly lines” to a proper model of how waves carry energy. They study waves on water, which can bounce off barriers and add together or cancel out, and then sound waves, whose frequency is measured in hertz. They also find out why sound needs something to travel through, how echoes and absorption work, and why sound moves at very different speeds in air, water and solids.

Water waves and sound waves, explained

The most important idea is that a wave transfers energy from one place to another without carrying the material along with it. Drop a pebble in a pond and ripples spread outwards, but a floating leaf mostly bobs up and down and stays roughly where it was. In the simple model taught at this stage, the water surface moves up and down at right angles to the direction the wave is travelling. This is called a transverse wave. Pupils learn the vocabulary for describing it: the crest (the top), the trough (the bottom), the amplitude (how far the water rises above its resting level) and the wavelength (the distance from one crest to the next).

Water waves can be reflected: when they hit a barrier they bounce back, with the angle they leave at matching the angle they arrived at. When two waves meet, they simply add together, a process called superposition. Where two crests arrive at the same moment, the water rises higher than either wave alone. Where a crest meets a trough of the same size, they cancel and the water stays level. Afterwards each wave carries on as if nothing had happened. Schools often show this with a shallow ripple tank, a clear tray of water lit from above with its shadows projected on to a screen.

Sound is also a wave, but a different kind. It is made by something vibrating, and its frequency is the number of vibrations each second, measured in hertz (Hz). A frequency of 200 Hz means 200 complete waves passing a point every second. High frequency sounds have a high pitch, such as a whistle, and low frequency sounds have a low pitch, such as a bass drum. Loudness is a separate idea: it depends on the amplitude, meaning how big the vibrations are. A quiet whistle and a loud whistle can have exactly the same frequency.

Unlike light, sound needs a medium, which is a substance made of particles, to travel through. The vibrations are passed from particle to particle, so in a vacuum there is nothing to carry them and there is no sound. Because the particles in a solid are packed closely, they pass vibrations on faster than in a liquid, and liquids faster than gases. Sound travels at about 340 metres per second in air, about 1,500 metres per second in water and several thousand metres per second in solids such as steel.

When sound meets a surface it can be reflected, absorbed or both. Hard, smooth surfaces such as tiles, brick and glass reflect most of the sound, which is why a bathroom sounds echoey. If the reflected sound reaches your ears a moment after the original, you hear an echo. Soft, rough or porous materials such as curtains, carpet and foam absorb sound, turning its energy into a tiny amount of heat, so a room full of furnishings sounds deadened. Pupils use the speed of sound and the time for an echo to return to work out distances.

Words your child will hear

Transverse wave: A wave where the vibrations are at right angles to the direction the wave travels, like ripples on water.
Superposition: What happens when waves meet: they add together, or cancel out if a crest meets a trough.
Frequency: The number of waves passing a point each second, measured in hertz (Hz).
Amplitude: The biggest displacement from the resting position. For sound, a bigger amplitude means a louder sound.
Medium: The material a wave travels through. Sound can pass through solids, liquids and gases but not a vacuum.
Echo: Sound that has been reflected from a hard surface and arrives back a moment after the original.
Absorption: When a material takes in the energy of a wave instead of reflecting it, as soft furnishings do with sound.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about waves on water as undulations which travel through water with transverse motion; these waves can be reflected, and add or cancel – superposition
Pupils should be taught about frequencies of sound waves, measured in hertz (Hz); echoes, reflection and absorption of sound
Pupils should be taught that sound needs a medium to travel, and about the speed of sound in air, in water and in solids

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A pupil claps once in front of a large, flat wall and hears the echo 1.5 seconds later. The speed of sound in air is 340 m/s. How far away is the wall?

Use the relationship between distance, speed and time: distance = speed × time. The sound has travelled for 1.5 seconds at 340 m/s.
Calculate the total distance the sound travelled.
```
distance = 340 × 1.5
         = 510 m
```
Think about what that distance represents. The sound went from the pupil to the wall and then all the way back again, so 510 m is the there-and-back distance, not the distance to the wall.
Halve it to find the distance to the wall.
```
510 ÷ 2 = 255 m
```
Check it makes sense. A wall 255 m away is roughly two and a half football pitches off, which is why you would hear a clear gap before the echo. An echo only sounds separate from the original if the gap is more than about a tenth of a second, which needs a reflecting surface roughly 17 m or more away.

The wall is 255 m away.

Common mistakes and how to help

Thinking the water itself travels along with a wave

Waves rolling up a beach look like moving water, and the word “wave” suggests something is being carried from one place to another.

What helps: Watch a floating cork, toy duck or leaf in a bath as a ripple passes. It bobs and returns. The wave carries energy across the surface, and each bit of water mostly moves up and down.

Believing a high-pitched sound must be a loud one

High and loud both feel like “more”, and in everyday speech people mix up “high” and “loud”.

What helps: Compare a quiet whistle with a loud bass drum. Frequency decides pitch; amplitude decides loudness. A tuning fork struck gently and then hard keeps the same pitch but gets louder.

Expecting sound to travel faster in air than in water or solids

Sounds heard through a wall or underwater seem muffled, which feels like slower or weaker, and air feels like the natural home of sound.

What helps: The muffling comes from reflection and absorption, not speed. Put an ear on a table while someone taps the other end, and the tap is clear. Closer particles pass vibrations on more quickly.

Thinking sound can travel through space

Space films are full of loud explosions, and children often imagine sound as something that simply “flies through” empty space.

What helps: Ask what would be there to vibrate. With no particles, there is nothing to pass the vibration on. Many schools show a bell ringing in a jar and fading as the air is pumped out.

How to help at home

Ripples in a bowl. Stand a washing-up bowl or deep baking tray with a little water on a towel. Tap the surface gently with a finger, then with two fingers at once, and watch ripples bounce off the sides and cross each other. Look for the moments when they seem to add up or flatten out.
A string telephone. Join two paper cups with a long piece of string pulled tight, and talk into one while a friend listens at the other. Then let the string go slack and see what happens. Sound passing along a solid string shows that solids carry sound well.
An echo hunt around the house. Clap in a tiled bathroom, a bedroom with curtains and carpet, and a large empty space such as a stairwell or hall. Which rooms sound lively and which sound dead? Talk about which surfaces reflect sound and which absorb it.
Twang a ruler. Hold a plastic ruler firmly on the edge of a table, with most of it hanging over, and flick the free end. Shorten the overhanging part and flick again. The vibrations get faster and the pitch rises, which is frequency in action.
Count the storm. During a thunderstorm, count the seconds between a flash of lightning and the thunder. Sound covers roughly a kilometre every three seconds, so ask your child to work out how far away the storm is. Stay indoors while you do it.

Leads on to

Key Stage 4: Properties of waves
Key Stage 4: The electromagnetic spectrum