What it shows
Sound takes time to reach a wall and return. If you clap so that each clap meets the echo of the last, the time between claps is the echo time, so speed = 2 × distance ÷ time.
The science
Sound travels in air at about 340 m/s. A clap travels to the wall and back, a total distance 2d, in a time t = 2d / v. If the claps are repeated so that every clap coincides with the echo of the previous one, the interval between claps equals this time. Timing a long run of claps, such as 20, reduces the human reaction-time error. The measurement shows v = 2d / t.
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 flat wall or building at least 50 m away across an empty ground
- A measuring tape or a long rope marked in metres (or a trundle wheel)
- A phone stopwatch
- Two flat wooden blocks or a pair of cymbals to clap
- A helper
- A notebook
Safety
For this project: Choose a quiet open ground away from traffic. An adult must supervise.
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.
Stand at a measured distance d from a large wall, about 60 m, and mark it clearly.
Clap once and listen for the echo; you should hear a distinct second sound after a short delay.
Now clap again exactly when you hear the echo, then again with the next echo, building a steady rhythm.
Ask the helper to start the stopwatch on one clap and stop it after 20 claps (counting the first as zero).
Compute the time between claps t = total time / 20.
Calculate the speed of sound v = 2d / t.
Repeat at 40 m and at 80 m, and see that t changes in proportion to the distance.
Compare your value with 343 m/s at 20 degrees C.
What you should see
The clap interval comes out as about 0.35 s at 60 m. The speed of sound is found to be 330 to 350 m/s, and the time interval grows in proportion to the distance to the wall.
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?
The repetition of a sound after it is reflected from a distant surface.
2What is the minimum distance to hear a separate echo?
About 17 m, because the ear can separate sounds only 0.1 s apart.
3Why time 20 claps instead of one?
It shrinks the effect of reaction time on the average interval.
4Where do bats and ships use echoes?
Bats for hunting in the dark and ships in sonar to measure the depth of the sea.
Take it further
Use a phone voice recorder and a free sound editor to measure the delay between a clap and its echo directly.
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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