# The Speed of Sound

Sound seems to travel very fast, when we talk to each other it seems as if we hear the sound as soon as the lips of the other person move.
In fact it does take the sound some time to travel and you can tell this if you watch a cricket match or see a pile driver banging in a pile. You will notice that you see the bat hit the ball or the pile driver hit the pile before you hear the sound.

This means that:

Sound travels much more slowly than light.

This is why people timing sprints will always look for the puff of smoke from the starting pistol and not wait until they hear the bang before they start their stopwatches.

You can measure the speed of sound by a very simple experiment. Two people stand at least 100m apart, one has a starting pistol and the other a stopwatch. The person with the gun fires it and the one with the watch starts it when they see the smoke and stops it when they hear the bang. The distance between them is found and the speed of sound worked out. The experiment should be done a few times to get an average result.

Safety: PROTECT YOUR EARS IF YOU ARE STANDING CLOSE TO THE STARTING PISTOL. A PAIR OF CLAPPER BOARDS MAY BE USED INSTEAD IF THE STARTING PISTOL IS FORBIDDEN

Note: using the echo from a building effectively increases the distance the sound has travelled and so makes the experiment more accurate.

The speed of sound in air is about 320 m/s. In water it is 1500 m/s and in steel it is 6000 m/s!

The reason for the difference in the speed of sound in the different materials is that sound waves are carried by molecules banging into each other and in air the molecules are far apart and not held together firmly. In steel the molecules are held together tightly and so the vibrations pass through steel much faster.

Two things affect the speed of sound in air
(i) the wind - if the wind is blowing in the same direction that the sound is travelling then the speed of sound is increased.
(ii) the temperature - the colder the air the slower the sound travels.

In 1887 Mach devised a scale based on the speed of sound it is still used today when describing the speed of aircraft.
A plane travelling at the speed of sound is said to have a velocity of Mach 1. Many military jets have speeds of Mach 2 or more - twice the speed of sound.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB