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Measuring the Speed of Sound (Cambridge O Level Physics)

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Measuring the Speed of Sound

  • There are several experiments that can be carried out to determine the speed of sound
  • Three methods are described below
    • The apparatus for each experiment is given in bold

Method 1: Measuring Sound Between Two Points

Measuring the Speed of Sound Using a Loud Noise

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 100 m apart
  2. The distance between them is measured using a trundle wheel
  3. One person has two wooden blocks, which they bang together above their head
  4. The second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear 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 space of space sound space equals fraction numerator space distance space travelled over denominator time space taken end fraction

Method 2: Using an Oscilloscope

Measuring the Speed of Sound Using Microphones

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 using a tape measure to measure the distance
  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 space of space sound space equals fraction numerator space distance space between space microphones over denominator time space between space peaks end fraction

Measuring Wave Speed in Water

  • Ripples on water surfaces are used to model transverse waves
    • The speed of these water waves can be measured

Ripples on Water

Creating ripples in water

  1. Choose a calm flat water surface such as a lake or a swimming pool
  2. Two people stand a few metres apart using a tape measure to measure this distance
  3. One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple
  4. The second person then starts a stopwatch to time how long it takes for the first ripple to get to them
  5. The experiment is then repeated 10 times and an average value for the time is calculated
  6. The average time and distance can then be used to calculate the wave speed using the equation:

average space speed space equals fraction numerator space distance space moved over denominator time space taken end fraction

Examiner Tip

When you are answering questions about methods to measure waves, the question could ask you to comment on the accuracy of the measurements

  • In the case of measuring the speed of sound:
    • Method 2 is the most accurate because the timing is done automatically
    • 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 (for example, use greater distances)

  • For example, if a manual stopwatch is being used there could be variation in the time measured which can be up to 0.2 seconds due to a person's reaction time
  • The time interval could be as little as 0.3 seconds for sound travelling in air
  • This means that the variation due to the stopwatch readings has a big influence on the results and they may not be reliable

Speed of Sound in Air

  • Sound waves travel at a speed of about 340 m/s in air at room temperature
    • The higher the air temperature, the greater the speed of sound
  • The speed of sound in air varies from 330 – 350 m/s

Speed of Sound in Materials

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

  • Some typical speeds of sound in solids, liquids and gases are:
    • Solids almost equal to 5000 m/s
    • Liquids almost equal to 1500 m/s
    • Gases almost equal to 350 m/s

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    Leander

    Author: Leander

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.