Higher tier: Measuring the speed of sound using the flash-bang method
A person fires a starting pistol.
A distant observer stood 200 metres away (measured using a 20 m tape measure or trundle wheel) records the time between seeing the flash of light from the starting pistol and hearing the sound.
The sound takes more time to cover the same distance as the light because sound travels much more slowly than light.
The speed of sound can be calculated using the equation:
Speed = \(\frac{\text{distance}}{\text{time}}\)
The main source of error in this experiment is reaction time when starting and stopping the stop clock.
The error is reduced by repeating the timing ten times and calculating the average time.
The starting pistol could be replaced by two blocks of wood as shown below.
Image caption, Click to see a step-by-step guide to measuring the speed of sound
WHAT YOU NEED: Two or three people, two blocks of wood, a stopwatch, a notepad and pen, and a large, open space.
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Example
Higher tier only: An observer 400 m away records a 1.2 s time difference between seeing the flash of a starting pistol and hearing the bang. Calculate the speed of sound.
Speed = \(\frac{\text{distance}}{\text{time}}\).
Distance = 400 m.
Time = 1.2 s.
Speed = \(\frac{400}{1.2}\)
Speed = 333 m/s.
The accepted value for the speed of sound in air is 340 m/s.
However, this experimental method is flawed because of human reaction time when using the stop clock.
This explains why the answer of 333 m/s is slightly below the accepted value for the speed of sound in air.