Vincent Manze, of Ilam School, asks :-

Why does sound travel faster in solids than in air?

John Campbell, a solid-state physicist at the University of Canterbury, responded.

Sound travels by atoms and molecules bashing against their neighbours. In a solid or liquid the atoms and molecules are touching their neighbours which is shown by the fact that we cannot easily compress solids or liquids. Thus it is easy for them to pass on the extra vibrations due to sound.

If we turn a volume of solid or liquid into a gas the molecules now occupy about 700 times the volume. This is close to 1000 (which is 10 x 10 x 10) so the molecules in a gas are about ten times a molecular diameter apart. In order for the sound to be transported to neighbouring molecules the molecules themselves have to travel about ten diameters before making contact with a neighbour.

Hence sound in gas travels a lot slower than in liquids or solids, for example 330 metres per second in air as compared to 1450 metres per second in fresh water and 5000 metres per second in aluminium.

Note that in a gas each molecule has on average the same energy, so the lighter the gas molecule the faster is its speed on average. This means that a light molecule gets to its neighbour in a shorter time so in general the speed of sound is faster in gases made up of light molecules than that of heavier molecules. For example the speed of sound is 965 meters per second in helium (light), 435 metres per second in neon (intermediate) and 319 metres per second in argon (heavier).

The speed of a gas molecule between collisions is faster than the speed of sound in that gas. For example, at room temperature the typical speed of a hydrogen molecule is 184,000 meters per second whereas the speed of sound in hydrogen is only 1284 metres per second.