How sound waves travel

Sound waves are longitudinal waves. They need a medium to travel through. They cause particles of the medium to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases. The speed of sound depends on the medium through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuum because there are no particles to carry the vibrations.

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 (d) is measured in metres (m)
  • time (t) is measured in seconds (s)

Example calculation

An observer 400 m away records a 1.3 s time difference between seeing the hand signal and hearing the bang of the starting pistol.

v = \frac{d}{t}

v = 400 \div 1.3

310 m/s to 2 significant figures

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 and also the error in reading such a small time interval is very high. 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 accurate – this explains why the answer of 310 m/s is slightly below the accepted value for the speed of sound in air.