Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

## Tuesday, 5 July 2011

### Faster Than The Speed of Sound

Sound is everywhere around us, some noises are far away, and some are nearer. One of the only times we really seem to think about its speed though is when lightening strikes. Then, we all count the gap until the thunder, a rudimentary experiment to see how far away the thunderstorm is (3 seconds is about one kilometre away). It’s based on the fact that light travels a lot faster than sound.

But how fast does sound go exactly? Well, it depends on different things. Have you ever noticed that when you’re underwater at a swimming pool how loud and clear all the other underwater noises are? Well that’s because sound travels much faster underwater than in air. This is because sound is transferred though the molecules one to the next, and in water (and in solids), the molecules are closer together, so it’s faster to transfer the noise.

You may have also noticed that when night falls, you can hear distant train and traffic noises louder and clearer. Now I’ve always thought that this was just because everyone shut up and now you could hear it better, but there is a more scientific reason too. As night falls, the ground temperature falls quickly, but the air temperatures higher up stay warmer. Sound waves travel faster in warmer air, as the molecules have more motion to them, so you hear the far-off noises better at night.

Once we know the exact speed of sound in various mediums (such as hot air, cold air and water), we can start to use sound to measure things instead of us going to get the measurement ourselves. The only other thing we need to know is that sound echoes, which anyone who likes shouting in large spaces will be able to tell you!

One of the first instances of this was developed for war use. RADAR uses microwaves to measure how far away other aircraft are, giving you the heads up when one is on its way. It does this by recording the time it takes for the wave to echo off the enemy plane and back to you, and knowing the speed of the wave you can easily work out distance.

In peace-time we transferred this knowledge to echo-sounding, which is used by boats either to chart the bottom of the sea, or to find shoals of fish to catch. Sound waves are used here as there’s no need to hide! The exact same principle has been taken on today by building surveyors, who now only need a little black box that sends and receives the pulse to measure the dimensions of a room.