How to Measure the Speed of Sound With Two Lumps of Wood ...

... a stop watch and a convenient wall.

Step 1: The Idea

Speed is how far something travels in a known time speed = distance / time.

In everyday life, speed is generally measured in either miles per hour (or kilometres per hour; most of the world is metric). Higher speeds, especially in a scientific or engineering context, are usually measured in metres per second (m/s, ms-1), and that is what we will use in this Instructable.

We will measure the time it takes for sound to travel a known distance by using rhythmic echoes.

Step 2: The Equipment

To time echoes accurately, we need a short pulse of loud sound. Two solid lumps of wood are ideal.

To generate an echo, we need a hard flat surface in a fairly uncluttered space. A wall in a school playground is perfect.

We need to time the echo's journey, so we need a stopwatch. Actually, we are going to use several stopwatches to eliminate human error: a small group of Science Clubbers will each use a stopwatch, and an average will be taken of their results,

To find the distance? part of the equation, we need to know how far we are from the wall. This should be measured in metres, either with the school-child's favourite, the click wheel, or with a dirty great tape measure.

Step 3: The Procedure

The person generating the sound should smack the lumps of wood together. After a short time, an echo will be heard. You should immediately notice that the time between SMACK and ECHO is too short to measure manually, so we will make it longer.

The wood-smacker needs to create a rhythm, so that they generate a SMACK at the exact moment they hear the ECHO. Listeners close to the wood-smacker should not be able to tell SMACK from ECHO.

As soon as the wood-smacker has his rhythm, the timers should get ready. Working together, they all start their stop-watches on the same SMACK/CRACK sound. They then count ten more SMACK/CRACKS, and stop the stop watches as soon as they hear the tenth SMACK/CRACK after the starting one. That is, eleven SMACK/CRACKS, with ten time intervals in between.

The group has, therefore, effectively timed the sound travelling over a distance ten times greater than the distance to the wall and back.

Step 4: The Maths.

First, we need a consensus on time. Take all the times recorded by the timers, add them up, and divide that by the number of people timing.

Then we need to know how far the sound travelled: measure the distance from wood-smacker to wall, and multiply it by twenty (double it for the journey there and back, then make it ten-fold larger for the ten time-intervals measured). Don't forget, you can set the distance at a mathematically-convenient distance - the sums are a lot easier if you stand fifty metres from the wall than if you stand forty-seven metres from the wall.

Divide the distance (in metres) by the average time measured (in seconds) to calculate the speed of the sound.

For example
We stood 50m from the wall that provided the echo, so the total distance travelled by the sound was 50x20 = 1000 metres.

Mean time measured = 2.89s

1000 / 2.89 ~ 346m/s

Pretty close, for a bunch of 10 year-olds.