RMS Speed
Properties of Matter

Measuring the speed of sound using echoes

Practical Activity for 14-16 PRACTICAL PHYISCS

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

Read our standard health & safety guidance


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

Procedure

  1. The experimenter stands as far away as possible from a large reflecting wall and claps their hands rapidly at a regular rate.
  2. 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.
  3. 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 = 2 s/t in the first case.

Teaching Notes

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

This experiment was safety-tested in July 2006

RMS Speed
is a feature of the Kinetic Theory of Gases Model
is used in analyses relating to RMS/Mean Free Path
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