What it shows
Sound travels at a finite speed, about 340 metres every second in air. A clap that returns as an echo from a far wall lets you work out this speed with only a stopwatch and a tape.
The science
Speed equals distance divided by time. When you clap, the sound goes to the wall and back, a distance of 2d. If you clap again each time you hear the echo, the time between claps is the time for the round trip. Timing many claps gives a more accurate time than timing one. At 30 degrees C, sound travels at about 349 m per second, and the speed rises by about 0.6 m per second for each degree rise in the temperature.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A tall wall or building at least 60 m away in an open field
- A measuring tape or 50 m rope, or a map for the distance
- A stopwatch or phone timer
- Two wooden blocks or pans to clap
- A friend to time
- A notebook
- A calculator
Safety
For this project: Do the test in a safe open ground away from roads and ask an adult to come along. Do not shout or clap near anyone’s ears.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Choose a large flat wall at least 60 m away and measure the distance d from where you stand to the wall with the rope or tape.
Stand facing the wall and clap the blocks once. Listen for the echo that comes back.
Now clap again exactly when you hear the echo, and keep going so that each clap coincides with the echo of the one before.
A friend starts the stopwatch at a clap and counts 20 claps, and then stops the watch at the 20th clap. A common mistake is to count the first clap as one when it should be zero.
Divide the total time by 20 to find the time for one round trip.
Calculate the speed as 2 x d divided by this time.
Repeat three times, and at a different distance, and find the mean.
Compare with the value of about 331 + 0.6 x temperature in degrees C metres per second and find the percentage error.
What you should see
A measured speed of sound within about 5 to 10 per cent of 340 m per second, with the answer improving when 20 echo intervals are timed together.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is an echo?
A sound that is reflected back from a hard surface.
2Why time 20 claps instead of one?
It reduces the effect of the small error in starting and stopping the watch.
3Why is the distance doubled?
The sound goes to the wall and comes back.
4Why is the minimum distance for an echo about 17 m?
Our ear needs at least 0.1 second between the sound and its echo, and sound covers 34 m in that time there and back.
Take it further
Repeat on a hot afternoon and a cool morning and plot the speed against the temperature.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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