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P11.4 More about Waves

GCSE Combined Science · AQA · Chapter P11: Wave Properties

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how sound waves are made, why sound needs a medium, and three ways to measure the speed of sound in air. 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 measure the speed of sound in air with two microphones and an oscilloscope, including reading a time base

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.
  • Oscilloscope: A device that shows electrical signals (e.g. from microphones) as traces on a screen; the time base gives the time for each horizontal square.
  • Speed of sound: About 330–340 m/s in air (it depends on temperature); faster in liquids and faster still in solids.

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 = 66 m. Time for 25 intervals between claps = 10.0 s, so t = 10.0 ÷ 25 = 0.40 s
v = 2d ÷ t = (2 × 66) ÷ 0.40 = 132 ÷ 0.40

Answer: v = 330 m/s

Worked example: how far away is the storm?

Thunder is heard 5.0 s after the lightning is seen. The light arrives almost instantly, and the sound travels at 340 m/s.
distance = speed × time = 340 × 5.0 = 1700 m (no doubling – the sound only travels one way)

Answer: 1.7 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.

Timing sound electronically

Reaction time limits any stopwatch method. In the laboratory, a loudspeaker makes short clicks and two microphones a measured distance d apart feed a dual-trace oscilloscope. The pulse reaches microphone B a little later than microphone A, so the two traces are shifted sideways on the screen.

The time base tells you the time for one horizontal square (division), e.g. 1 ms per division. Count the squares between the two pulses and multiply.

Worked example: speed of sound with an oscilloscope

The microphones are 1.36 m apart. Time base = 1 ms per division. The pulses are 4.0 divisions apart.
time t = 4.0 × 1 ms = 4.0 ms = 0.0040 s
v = d ÷ t = 1.36 ÷ 0.0040

Answer: v = 340 m/s

  • see the bang, hear the bang (field); Main source of error reaction time ≈ 0.2 s in a time of about 1 s; How good? rough
  • repeated echoes from a wall; Main source of error reaction time, spread over many intervals; How good? better
  • two microphones and an oscilloscope; Main source of error reading the trace positions on the screen; How good? best: no reaction time

Exam tip: Always convert: cm to m and ms to s before dividing. 1 ms = 0.001 s.

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.
  • Forgetting the time base: count the divisions between the two pulses, then multiply by the time per division and convert ms to s.
  • 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 glass 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 still works and can be seen because light travels through a vacuum.

Question 2. A student stands 85 m from a tall wall. The echo of her clap returns after 0.50 s. Calculate the speed of sound.

Answer: 340 m/s

Working: v = 2d ÷ t = (2 × 85) ÷ 0.50 = 170 ÷ 0.50 = 340 m/s.

Question 3. Thunder is heard 3.5 s after the lightning is seen. The speed of sound is 340 m/s. How far away was the lightning?

Answer: 1190 m

Working: distance = 340 × 3.5 = 1190 m (about 1.2 km).

Related lessons

Continue in the full course

This lesson is section P11.4 (AGCSC4004) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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