Measuring the Speed of Waves

• Measuring the Speed of Waves

  • Sound waves in air are a type of longitudinal wave
  • The speed of sound can be measured

  Experiments to Determine 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 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
• Measuring the speed of sound directly between two points 6. The speed of sound can then be calculated using the equation:

  
• Method 2: Using Echoes

  
• Measuring the speed of sound using echoes

  1. A person stands about 50 m away from a wall (or cliff) using a trundle wheel to measure this distance
  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 they hear one of the claps and stops timing 20 claps later
  5. The process is then repeated and an average time calculated
• Measuring the speed of sound using echoes

  1. The distance travelled by the sound between each clap and echo will be (2 × 50) m
  2. The total distance travelled by sound during the 20 claps will be (20 × 2 × 50) m
  3. The speed of sound can be calculated from this distance and the time using the equation:

  
• Method 3: Using an Oscilloscope

  
• 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
• Measuring the speed of sound using an oscilloscope

  1. The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time difference between them
  2. This is repeated several times and an average time difference calculated
  3. The speed can then be calculated using the equation:

  
• Measuring Wave Speed in Water

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

  
• 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
• Creating ripples in water

  1. The average time and distance can then be used to calculate the wave speed using the equation:

  
• • Evidence for Energy Transfer
  • Waves transfer energy and information
  • Waves are described as oscillations or vibrations about a fixed point
  • For example, ripples cause particles of water to oscillate up and down
  • Sound waves cause particles of air to vibrate back and forth
• • Evidence for Energy Transfer
  • In all cases, waves transfer energy without transferring matter
  • For water waves, this means it is the wave and not the water (the matter) itself that travels
  • For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels
  • Objects floating on water provide evidence that waves only transfer energy and not matter