Measuring the speed of sound in air and water

The air is made up of many tiny particles. When sound is created, the air particles vibrate and collide with each other, causing the vibrations to pass between air particles. The vibrating particles pass the sound through to a person's ear and vibrate the ear drum.

Light travels much faster than sound through air. For example, a person fires a starting pistol and raises their hand in the air at the same time. A distant observer stood 400 metres (m) away records the time between seeing the action (the light reaches the time keeper immediately) and hearing the sound (which takes more time to cover the same distance).

The speed of sound can be calculated using the equation:

speed = \frac{distance}{time}

v = \frac{d}{t}

This is when:

  • speed (v) is measured in metres per second (m/s)
  • distance (s) is measured in metres (m)
  • time (t) is measured in seconds (s)

Example

An observer 400 m away records a 1.2 s time difference between seeing the hand signal and hearing the bang of the starting pistol.

v = \frac{d}{t}

v = 400 \div 1.2

v = 333~m/s \ (3 \ sf)

The accepted value for the speed of sound in air is 330 m/s.

However, this experimental method is flawed as humans do not use stop clocks identically to one another. One person might stop the timer a fraction of a second later than another person. The values recorded will be dependent on the reaction time of the observer, and will not be entirely accurate. This explains why the answer of 333 m/s is slightly above the accepted value for the speed of sound in air.