CIE IGCSE Physics

Revision Notes

3.4.2 The Speed of Sound

Experiments to Determine the Speed of Sound

  • In your IGCSE exam, you might be asked to describe a method of measuring the speed of sound
  • When giving your method, try and include the following things:
    • List all of the apparatus that you will need
    • Choose a suitable (realistic) distance over which you will measure the sound
    • Describe how you will measure this distance
    • Explain how you will produce a loud enough sound
    • Explain how you will time the sound (and how the timer will be started and stopped)
    • Explain how you will calculate the speed (give an equation)
    • State that you will then repeat the experiment several times and take an average
  • Three methods for carrying out this experiment are given below:

Method 1

sound-method-1, IGCSE & GCSE Physics revision notes

Measuring the speed of sound directly between two points

  1. Two people stand a distance of around 100m apart
  2. The distance between them is measured using a trundle wheel
  3. One of the people has two wooden blocks, which he bangs together above his head
  4. The second person has a stopwatch which he starts when he sees the first person banging the blocks together and stops when he hears the sound
  5. This is then repeated several times and an average value is taken for the time
  6. The speed of sound can then be calculated using the equation:

Speed of sound equation (2), IGCSE & GCSE Physics revision notes

trundle-wheel, IGCSE & GCSE Physics revision notes

A trundle wheel can be used to measure large distances

Method 2

sound-method-2, IGCSE & GCSE Physics revision notes

Measuring the speed of sound using echoes

  1. A person stands about 50m away from a wall (or cliff). This distance is measured using a trundle wheel
  2. The person claps two wooden blocks together and listens for the echo
  3. The person then starts to clap the blocks together repeatedly, in rhythm with the echoes
  4. A second person has a stopwatch and starts timing when he hears one of the claps and stops timing 20 claps later
  5. The process is then repeated and an average time calculated
  6. The distance travelled by the sound between each clap and echo will be (2 x 50) m
  7. The total distance travelled by sound during the 20 claps will be (20 x 2 x 50) m
  8. The speed of sound can be calculated from this distance and the time using the equation:

Speed of sound equation (2), IGCSE & GCSE Physics revision notes

Method 3

sound-method-3, IGCSE & GCSE Physics revision notes

Measuring the speed of sound using an oscilloscope

  1. Two microphones are connected to an oscilloscope and placed about 5 m apart
    (This distance can be measured using a tape measure)
  2. The oscilloscope is set up so that it triggers when the first microphone detects a sound, and the time base is adjusted so that the sound arriving at both microphones can be seen on the screen
  3. Two wooden blocks are used to make a large clap next to the first microphone
  4. The oscilloscope is then used to determine the time at which the clap reaches each microphone, and the time difference between them
  5. This is repeated several times and an average time difference calculated
  6. The speed can then be calculated using the equation:

Speed of sound equation (2), IGCSE & GCSE Physics revision notes

Exam Tip

Method 3 is the most accurate method (because the timing is done automatically) whilst method 1 is the least accurate (because the time interval is very short).

Whilst this may not be too important when giving a method, you should be able to explain why each method is accurate or inaccurate, and suggest ways of making them better (use bigger distances).

Extended Only

Speed of Sound by Medium

  • Sound travels at different speeds in different mediums:
    • It travels fastest in solids
    • Slowest in gases

Speed of sound table, IGCSE & GCSE Physics revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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