Measuring the Speed of Sound (CIE IGCSE Physics)

• 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

EXTENDED

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

• The table below shows some typical speeds of sound in solids, liquids and gases:

• 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
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. A second person has a stopwatch and starts timing when they hear one of the claps and stops timing when they hear the echo
4. The process is then repeated 20 times and an average time calculated
5. The distance travelled by the sound between each clap and echo will be (2 × 50) m
6. 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
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:

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