- large flat wall outside, with space in front of it 50m or more e.g. gym wall of a school
- tape measure
- optional: wooden blocks
- paper, pen
- calculator and stop watch e.g. on a phone

Gather 40m or further from the wall, and demonstrate by a loud clap, or one bang of the wooden blocks together, that there is an echo. The sound of the clap/blocks, reaches the wall and bounces back to us, so that there is a delay between the initial sound made and the sound heard after it bounces off the wall.

By timing how long the delay is and measuring how far away the wall is, we can measure how fast sound travels i.e. the speed of sound.

Show students how long a metre is using the tape measure, then ask them to see how long their stride must be to measure a metre. Once their stride is calibrated to a meter, ask them to pace out the number of strides (or metres) to the wall.

Write down this number [52m and 58m for two of our students]. Double the number to find out how far the sound must travel to the wall and back [110m for us].

Meanwhile, another student needs to bang the wood blocks together so that the echo from the first bang coincides with the echo from the second bang. Ask them to keep banging the wood at this rate, so each bang coincides with the echo from the previous bang. The time between the bangs is the time it takes the sound to travel to the wall and back.

To be somewhat accurate in how long it takes the sound to travel the distance, ask the student to keep banging the blocks together at the same rate, while another students times 10 bangs of the blocks. [Our students measured 3.42 seconds for 10 bangs.]

Divide this number by 10 to find the time for one bang i.e. the time for sound to travel to the wall and back [0.342 seconds for us].

Now do some math: if the sound travels x metres to the wall and back, and takes y seconds, the sound is travelling at x/y metres in one second - this is the calculated speed of sound. [We calculated 110/0.342 = 321.6 metres per second, approximated to 320m/sec.]

The actual speed of sound in air is 343m/sec, so this method is not bad for calculating the speed of sound.