# Measuring the speed of sound using echoes

##### Class practical

Echoes are used outdoors to estimate the speed of sound. Good weather has to be ordered at the same time as the equipment!

#### Apparatus and materials

Stopwatch

Large reflecting surface, preferably outdoors

#### Health & Safety and Technical notes

Obviously, you want strong echoes from one reflecting surface and not several!

#### Procedure

a The experimenter stands as far away as possible from a large reflecting wall and claps their hands rapidly at a regular rate.

b This rate is adjusted until each clap just coincides with the return of an echo of its predecessor, or until clap and echo are heard as equally spaced.

c Use a stopwatch to find the time between claps, t. Make a rough measurement of distance to the wall, s. Thus the speed of sound, v = 2s/t in the first case.

#### Teaching notes

1 Students are far more likely to grasp and to remember how to get the estimated speed of sound if you can arrange for them to undertake this experiment in pairs.

2 Newton used echoes to estimate the speed of sound, in an outdoor corridor at Trinity College, Cambridge. It is alleged that the sound he produced was able to lift a door knocker at the far end of the corridor.

3 Discuss why a rough measurement of the distance is adequate.

This experiment was safety-checked in July 2006