Speed of sound

Although sound travels quite fast, it is still possible to measure its speed in air. To do this, you need to measure the time it takes a sound to travel a measured distance. To reduce errors, particularly timing errors, you should either:

  • use a large distance (preferably over 50 metres), or
  • use an electronic timer or data logger to record the time taken

Clap-echo method

This method involves measuring the time taken for you to hear an echo from a sharp clap. You stand a long distance from a wall, clap, and listen for the echo. The distance travelled is twice the distance from you to the wall (because the sound has to travel to the wall and back).

One way to reduce timing errors in this method is to clap in time to the echoes. This means that the time between each clap is the journey time for the sound. You then measure the time for 11 claps, which is the time for 10 journeys by the sound. This time can then be used to calculate an average time for the sound to travel to the wall and back.

Using microphones and data logger

A data logger can measure and record the time taken for sound to reach two microphones. Unlike the clap-echo method, these can be quite close together.

A ringing bell emits sound waves from left to right, towards two spaced microphones. The distance between them is labelled 'Measured distance (d)' and they are connected to a laptop computer with '0.010 Sec' on the screen, next to the formua: 'speed of sound = measured distance/time on computer'Using a data logger and microphones to measure the speed of sound

For example, two microphones are 3.4 m apart. The data logger recorded a time of 0.01 s for the sound to travel between the microphones.

average speed = distance travelled ÷ time taken

= 3.4 ÷ 0.01 = 340 m/s

Sound through different materials

Sound travels faster through liquids and solids than it does through air and other gases. The table gives some examples.

SubstanceSpeed of sound
Air343 m/s
Water1493 m/s
Steel5130 m/s

This is because the particles of gases are further apart than liquids and finally solids. Sound waves move more slowly when particles are further apart.

Bang Goes the Theory presenters Jem and Dallas use a 340m plastic tubing coiled to experience the speed of sound