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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 100 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, either made by hands or by banging two wooden blocks together.

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).

Measure the distance to the wall using a 20 m tape measure or a trundle wheel.

Record the distance in a suitable table.

Stand beside the person who is going to clap.

Start a stop clock when the clap is made and stop it when you hear the echo.

Repeat ten times and calculate the average time to hear the echo.

Results

Distance to wall =

Total distance travelled = 2 x distance to wall

Time to hear echo:

Speed of sound = m/s.

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.

Example

A girl stands 102 m away from a wall and claps two wooden blocks together.

She hears an echo after 0.6 s.

Calculate the speed of sound in air.

Answer

The sound travels 102 m to the wall and 102 m back to the girl, so the total distance travelled by the sound = 102 + 102 = 204 m.

Time = 0.6 s.

Speed = 340 m/s.

The speed of sound in air is 340 m/s.