What it shows
If you stand about 50 m from a large wall and clap so that each clap meets the echo of the one before, the time between claps equals the time sound takes for the round trip. The distance and time give the speed of sound as about 340 metres per second.
The science
Sound is reflected from hard surfaces and returns as an echo. If the distance to the wall is d, the sound travels 2d in the round trip, so speed v = 2d divided by the time t between a clap and its echo. Since t is only about 0.3 s for 50 m, it is hard to time one echo, so we time a number of claps, each made at the instant the echo of the previous one is heard. The speed of sound in air is about 331 m/s at 0 °C and rises about 0.6 m/s for each degree Celsius, so near 340 m/s on a warm day.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A big flat wall, school building or compound wall in an open ground
- A measuring tape or a 30 m rope marked in metres
- A stopwatch or phone timer
- A notebook
- A helper to count
- A calculator
Safety
For this project: Choose a quiet open place away from traffic and keep clear of roads. Do not go near water or unsafe walls. An adult must be present.
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 wall and measure the distance d from where you stand to the wall with the tape, at least 40 m, and write it down.
Stand facing the wall and clap once, and listen for the echo. If there is no clear echo, move to a better wall.
Clap again at the moment you hear the echo. Keep doing this to build a steady rhythm in which each clap coincides with the echo of the one before.
Once the rhythm is steady, your helper starts the stopwatch on one clap and counts 20 intervals, stopping at the 20th clap. Count intervals, not claps.
Divide the total time by 20 to get the time t for one round trip. Write it down.
Calculate the speed: v = 2d divided by t. Repeat the whole thing three times and take the average.
Compare your answer with 331 + 0.6 times the air temperature in °C. Note the temperature from a thermometer.
List what could cause errors, such as reaction time, distance error and wind, and say how you reduced each.
What you should see
The round trip time for 50 m comes to about 0.29 s per clap, and the calculated speed of sound lies close to 330 to 350 m/s, near the value expected for the day’s temperature.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is the distance doubled in the formula?
The sound goes to the wall and back, covering twice the distance.
2Why time 20 intervals instead of one?
It reduces the error caused by slow human reaction time.
3How does temperature affect the speed of sound?
It increases by about 0.6 m/s for every degree Celsius rise.
4Why is a minimum distance of about 17 m needed to hear a clear echo?
The ear can separate two sounds only when they are about 0.1 s apart, which is a path of about 34 m and so a distance of 17 m.
Take it further
Repeat in the morning and in the afternoon and plot the calculated 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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