1. Find a field with a wall, or a tall, wide building at one end of it. The school car park will work fine – there’s a brick wall at the North end which reflects the sound back to the experimenters very well.
2. Pace out 100m from the wall. Thank you, ladies.
3. Bang two wooden blocks together sharply and listen for the echo. It sounds like the crack of a cricket ball being struck.
4. Bang the blocks together again at exactly the same time as you hear the echo You might have to practise this to get it exactly right.
5. Start a stopwatch and time how many bangs you make in one minute.
6. Repeat twice more and average for accuracy.
Thanks to the Tang-Pedersen twins for their help.
• Number of bangs in 60s = 96 (average of 3 times)
• Time for the sound to travel there and back once = 60/96 s =o.625s
• Distance travelled by the sound waves there and back = 2 x 100m
• Given that : Speed = distance/time, the speed of sound in air = 200m/0.625s = 320m/s.
The actual value is closer to 343 m/s or 1236 km/h, which increases with increasing temperature.
Breaking the sound barrier. What happens if the aircraft we’re flying in is going as fast or faster than the speed of sound in air? This site is a little bit high powered, but there are a few nice animations to help you to see what’s going on.