top of page

12.4 More About Waves

GCSE Physics · AQA · Chapter 12: Wave Properties

A free GCSE Physics lesson for AQA (8463) from OBA School: how sound waves are made, why sound needs a medium, measuring the speed of sound with echoes, and apparatus for investigating waves. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how a vibrating object produces sound waves in air
  • Explain why sound cannot travel through a vacuum
  • Use echo timings to calculate the speed of sound (speed = 2d ÷ t)
  • Describe how to investigate waves with a ripple tank, a stretched string and a signal generator with a loudspeaker

Key words

  • Sound wave: A longitudinal wave of compressions and rarefactions made by a vibrating object and passed on through a medium.
  • Signal generator: An electrical supply whose frequency (and amplitude) can be set with a dial; used to drive a loudspeaker or vibration generator.
  • Vacuum: A space with no particles in it. Sound cannot travel through a vacuum.
  • Echo: A sound heard after it has reflected from a hard, smooth surface such as a wall or cliff.
  • Vibration generator: A device that makes a string or rod vibrate at the frequency set on a signal generator.
  • Loop: One section of a vibrating string between two points that do not move. One loop is half a wavelength long.

How sound waves are made

Every sound comes from something vibrating: a guitar string, your vocal cords, a bee’s wings, a loudspeaker cone. As the object moves forwards it pushes the layer of air next to it, squashing it (a compression). As it moves back it leaves a region of lower pressure (a rarefaction). Each layer of air pushes and pulls on the next, so the vibrations pass outwards. The air particles only oscillate backwards and forwards; it is the energy that travels.

Turning up the frequency on the signal generator makes the cone vibrate more times per second, so more compressions are sent out each second. Sound is a longitudinal wave because the air oscillates parallel to the direction of energy transfer.

  • air (20 °C); Typical speed of sound about 340 m/s (330–345 m/s depending on temperature)
  • water; Typical speed of sound about 1500 m/s
  • steel; Typical speed of sound about 5000–6000 m/s

Key fact: Sound usually travels faster in liquids than in gases, and faster still in solids, because the particles are closer together and pass the vibrations on more quickly.

Sound cannot travel through a vacuum

Sound needs particles to pass the vibrations along. An electric bell is hung inside a sealed glass jar and switched on. A vacuum pump then removes the air:

Worked example: exam-style answer (3 marks): explain the bell-jar result

As the air is pumped out the sound gets quieter and then cannot be heard. (1)
Sound waves need a medium (particles) to travel through, and there are fewer and fewer air particles to pass on the vibrations. (1)
The bell can still be seen working because light is an electromagnetic wave and does not need a medium. (1)

This is why astronauts on a spacewalk talk by radio: there is no air in space to carry sound.

Using echoes to measure the speed of sound

Sound reflects well from large hard, smooth surfaces. If you clap in front of a tall wall you hear an echo a short time later, because the sound has travelled to the wall and back.

speed of sound v (m/s) = total distance 2d (m) ÷ time t (s)

One echo takes only a fraction of a second, so it is hard to time with a stopwatch. A better method: clap repeatedly so that each clap is made just as the echo of the previous one is heard. A partner times a large number of clap intervals, e.g. 20, and divides by the number of intervals to get t.

Worked example: speed of sound from repeated claps

Distance to wall d = 85 m. Time for 20 intervals between claps = 10.0 s, so t = 10.0 ÷ 20 = 0.50 s
v = 2d ÷ t = (2 × 85) ÷ 0.50 = 170 ÷ 0.50

Answer: v = 340 m/s

Worked example: how far away is the storm?

Thunder is heard 3.0 s after the lightning is seen. The light arrives almost instantly, and the sound travels at 340 m/s.
distance = speed × time = 340 × 3.0 = 1020 m (no doubling – the sound only travels one way)

Answer: about 1.0 km

Exam tip: Only double the distance when the sound goes there and back (echoes, sonar, ultrasound). For a one-way journey, use distance = speed × time.

Apparatus for investigating waves

  • water waves; Apparatus ripple tank, lamp, vibrating bar or ruler, ruler, stopwatch; What you can measure f (count waves in 10 s), λ (shadows), speed (time a wave across the tank)
  • waves in a solid; Apparatus stretched string or elastic cord, vibration generator, signal generator, pulley, masses, metre ruler; What you can measure f (from the signal generator), λ (length of loops), speed v = f λ
  • sound waves; Apparatus signal generator and loudspeaker; microphone and oscilloscope; What you can measure f (dial or oscilloscope), changes in loudness and pitch

On a stretched string the vibration generator sends waves along the string. They reflect from the pulley and the string settles into a pattern of still loops. Adjust the frequency until the loops are sharp. Each loop is half a wavelength, so with n loops on a vibrating length l: λ = 2l ÷ n.

Worked example: speed of waves on a string

A vibrating length of 1.20 m shows 3 loops when the frequency is 75 Hz.
one loop = 1.20 ÷ 3 = 0.40 m, so λ = 2 × 0.40 = 0.80 m
v = f λ = 75 × 0.80

Answer: v = 60 m/s

Raising the frequency gives more loops (a shorter wavelength), but as long as the tension and string are unchanged the speed stays the same. The full method is in the required practical section (12.8).

Common mistakes

  • Forgetting to double the distance for an echo: the sound goes to the wall and back.
  • Doubling the distance for a one-way journey such as thunder.
  • Taking one loop on a string as a whole wavelength. One loop is half a wavelength.
  • Saying the bell in the bell jar stops working. It keeps ringing – the sound just cannot reach you without air.

Revision questions

Try these, then check your answers below each one.

Question 1. An electric bell rings inside a jar. The air is pumped out. Why can the bell no longer be heard? (Choose from: sound needs a medium, and there are no particles to pass on the vibrations, the bell stops working in a vacuum, light cannot travel through a vacuum, the glass absorbs all the sound.)

Answer: sound needs a medium, and there are no particles to pass on the vibrations

Working: Sound is a mechanical wave: without air particles the vibrations cannot be passed on. The bell is still ringing.

Question 2. A student stands 68 m from a tall wall. An echo of her clap returns after 0.40 s. Calculate the speed of sound.

Answer: 340 m/s

Working: v = 2d ÷ t = (2 × 68) ÷ 0.40 = 136 ÷ 0.40 = 340 m/s.

Question 3. Thunder is heard 6.0 s after the lightning is seen. The speed of sound is 340 m/s. How far away was the lightning?

Answer: 2040 m

Working: distance = 340 × 6.0 = 2040 m (about 2 km).

Related lessons

Continue in the full course

This lesson is section 12.4 (AGCPH1204) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

bottom of page