🌞 A warm day
The ground heats the air next to it, so the air near the ground is warmer than the air above. Sound travels more slowly in the cooler air higher up, so it bends upwards, away from the ground. Sounds do not carry as far along the ground.
Light and Sound » Sound Waves
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 3.23, 3.24P
- How a vibrating object makes a sound wave, and why sound is a longitudinal wave
- Why sound needs a material to travel through
- How sound is reflected to make an echo, and how to do echo calculations
- How sound is refracted when its speed changes
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseHow a sound wave is made
Every sound starts with something vibrating. A drum skin vibrates when you hit it. A ruler held over the edge of a desk vibrates when you twang it. Your vocal cords vibrate when you speak. If you stop the vibration, for example by resting your hand on the drum skin, the sound stops too.
Here is how a drum makes a sound wave in air:
- The drum skin moves outwards. It pushes the air particles next to it closer together. This makes a compression.
- The drum skin moves back. The air particles next to it are now more spread out. This makes a rarefaction.
- The skin keeps vibrating, so it sends out compression after rarefaction after compression.
- Each layer of air pushes on the next layer, so the pattern travels away from the drum.
- When the pattern reaches your ear, it makes your eardrum vibrate and you hear the sound.
The air particles vibrate backwards and forwards along the same line that the sound is travelling. That is why sound waves are longitudinal waves. Learn that sentence. Notice that the air does not travel from the drum to your ear. Each particle only vibrates about its own position. It is the energy that travels.
Key terms:
- Sound wave: a longitudinal wave made by a vibrating object and passed on by the vibrating particles of a material.
- Medium: the material that a wave travels through, such as air, water or steel.
- Vacuum: a space with no particles in it.
- Echo: a reflected sound that is heard a short time after the original sound.
Sound needs a medium
A sound wave is passed on by particles pushing on their neighbours. With no particles there is nothing to vibrate, so sound cannot travel through a vacuum. The bell jar experiment shows this.
- An electric bell is hung inside a bell jar, which is a thick glass dome. The bell is switched on and can be heard ringing clearly.
- A vacuum pump is switched on. It slowly takes the air out of the jar.
- The ringing gets quieter and quieter until it can hardly be heard.
- The hammer can still be seen hitting the bell, so the bell is still vibrating.
- When the air is let back in, the sound returns.
The bell vibrates all the time. The only thing that changed was the air. With almost no air particles left, the vibrations cannot be passed on. You can still see the bell, which shows that light can travel through a vacuum but sound cannot.
Sound travels through solids, liquids and gases. It travels at about 340 m/s in air. It is faster in liquids, about 1500 m/s in water, and faster still in solids, because the particles are closer together and pass the vibrations on more quickly.
Reflection of sound: echoes
Like all waves, sound can be reflected. When a sound wave hits a hard, flat surface such as a brick wall or a rock face, it bounces off. If the surface is far enough away, you hear the reflected sound separately, a moment after the original. This is an echo.
Hard, smooth surfaces reflect sound well. Soft surfaces such as curtains, carpets and cushions absorb most of the sound instead. That is why an empty room sounds echoey, and the same room sounds quieter once it is furnished.
Echo questions use the equation you already know: speed = distance ÷ time. There is one trap. The sound travels to the surface and back again, so the distance travelled by the sound is twice the distance to the surface.
Worked example 1
Question: A walker shouts towards a rock face 170 m away. The speed of sound in air is 340 m/s. Calculate the time before she hears the echo.
distance travelled by the sound = 2 × 170 = 340 m
time = distance ÷ speed = 340 ÷ 340
time = 1.0 s
Worked example 2
Question: A boy claps his hands in front of a tall building. He hears the echo 1.5 s later. The speed of sound in air is 340 m/s. Calculate the distance from the boy to the building.
distance travelled by the sound = speed × time = 340 × 1.5 = 510 m
This is there and back, so halve it.
distance to the building = 510 ÷ 2 = 255 m
Worked example 3
Question: A ship sends a pulse of sound straight down to the sea bed. The echo returns 0.4 s later. The speed of sound in sea water is 1500 m/s. Calculate the depth of the sea.
distance travelled by the sound = speed × time = 1500 × 0.4 = 600 m
depth of the sea = 600 ÷ 2 = 300 m
| You are given | You want | What to do |
|---|---|---|
| Distance to the surface | Time for the echo | Double the distance first, then divide by the speed |
| Time for the echo | Distance to the surface | Multiply speed by time, then halve the answer |
Refraction of sound
Sound can also be refracted. Sound travels faster in warm air than in cold air. So when sound passes at an angle between layers of warm and cold air, its speed changes and it changes direction. It bends towards the layer where it travels more slowly.
🌙 A still, clear night
The ground cools quickly, so the air near the ground is colder than the air above. Sound travels more slowly in the cold air near the ground, so sound heading upwards bends back down. This is why distant sounds, such as traffic or a train, can often be heard more clearly at night.
Refraction also happens when sound passes from one material into another, for example from air into water, because its speed changes at the boundary.
Paper 2 only
The frequency range for human hearing is 20 Hz to 20 000 Hz. Learn both numbers with the unit. 20 000 Hz is the same as 20 kHz. A sound with a frequency below 20 Hz or above 20 000 Hz is still a sound wave, but humans cannot hear it. The upper limit also gets lower as people get older.
Sound waves at a glance
| What you need to know | How we know |
|---|---|
| Sound is made by vibrations | Stop a drum skin vibrating and the sound stops |
| Sound is a longitudinal wave | Particles vibrate along the direction the sound travels, making compressions and rarefactions |
| Sound needs a medium | The bell jar: the sound fades as the air is pumped out |
| Sound can be reflected | Echoes from walls and rock faces |
| Sound can be refracted | Sound bends between layers of warm and cold air |
Common mistakes
- Forgetting that an echo travels there and back. Double the distance, or halve your answer, every time.
- Writing that sound is a transverse wave. Sound is longitudinal. Light is transverse.
- Saying the bell stops ringing in the bell jar. The bell keeps vibrating. There are too few air particles to carry the sound.
- Saying air particles travel from the source to your ear. They only vibrate backwards and forwards. Energy is what travels.
- Paper 2 only: writing the hearing range as '20 to 20 000' with no unit, or as 20 kHz to 20 000 kHz. It is 20 Hz to 20 000 Hz.
Exam-style question
A sailor on a small boat sounds a horn. The sound travels through the air to a cliff, and the sailor hears an echo.
(a) State the type of wave that a sound wave is. [1 mark]
(b) Describe how the vibrating horn produces a sound wave in the air. [2 marks]
(c) The sailor hears the echo 2.4 s after sounding the horn. The speed of sound in air is 340 m/s. Calculate the distance from the boat to the cliff. [3 marks]
(d) Explain why the horn could not be heard through a vacuum. [2 marks]
(e) Sound waves can also be refracted. State what happens to a sound wave to cause refraction. [1 mark]
Model answer
(a) Longitudinal (1).
(b) The vibrating horn pushes the air particles next to it closer together and then lets them spread out, making compressions and rarefactions (1). The air particles vibrate backwards and forwards and pass the vibration on to the next layer of air (1).
(c) distance travelled by the sound = speed × time = 340 × 2.4 (1) = 816 m (1). distance to the cliff = 816 ÷ 2 = 408 m (1).
(d) Sound is passed on by vibrating particles (1). A vacuum has no particles, so there is nothing to carry the vibrations (1).
(e) Its speed changes (1).
Exam tip
In part (c), write the line 'distance to the cliff = 816 ÷ 2' clearly. If you forget to halve, you still get the first two marks for 340 × 2.4 = 816 m, but only if the examiner can see your working. Whenever you see the word echo, think 'there and back'.