The Echo Mapper

Sports day is in a week, and there is a fight brewing. The 100-metre sprint starts at the far wall of the playground and finishes at the assembly line — except nobody can agree that the track is actually 100 metres. One PT teacher paced it and said 96. Another swears it's 108. The only long tape in the school snapped last term and the new one hasn't arrived. If the track is wrong, every timing is wrong, and the inter-house record falls to a fluke. The headmaster wants a number by Friday, and he wants it defensible — "measured, not guessed." You have no tape long enough. What you do have is a wall, a clap, and the fact that sound travels at a known, steady speed. Here is the idea an engineer would reach for. Clap your hands facing that far wall and, a fraction of a second later, the clap comes back to you — an echo, your own sound bouncing off the wall. Sound covers ground at about 343 metres every second in ordinary air. So if you can measure how long the clap takes to fly to the wall and back, you can work the distance out: the sound made a round trip, out and back, at a speed you know. Time it, halve it, multiply by the speed — and you have measured a wall you never touched. The catch: at fifty metres, on a noisy playground, a single hand-clap's echo is faint and smeared, and the gap you're trying to time is under a third of a second — far too quick for a thumb on a stopwatch. So you will build an instrument to sharpen the clap and gather its echo, and you will use a rhythm trick to stretch that tiny time into something you can measure. That instrument is your clap-cone.