Sound waves are longitudinal waves propagated through a
material by the transfer of kinetic energy from one molecule to another
(a) in solids
and liquids by intermolecular forces and collisions,
(b) in gases by intermolecular
collisions alone.
Hence the velocity of sound is greater in solids and liquids than in
gases (the intermolecular forces in a gas are very small or zero).
The velocity of sound
in a gas will thus be slightly less than the root mean square velocity of the gas molecules
themselves. It will increase with increasing temperature since this will give a larger molecular
velocity.
The table below gives the velocity of sound in a number of materials.
Material | Velocity of sound (ms-1) | Material | Velocity of sound (ms-1) | |
Air (273 K) | 330 | Aluminium | 5100 | |
Water (298 K) | 1430 | Copper | 3650 | |
Steel | 5060 | Iron | 5130 | |
Vulcanised rubber | 54 | Glass | 4000 - 5500 | |
Granite (293 K) | 6000 | Pine | 3313 | |
Hydrogen (273 K) | 1286 | Oak | 3837 | |
Lead | 1230 | Elm | 4108 |
It is for this reason that the velocity of sound at high altitude is low since the air there is cooler. The speed of an aircraft relative to the speed of sound (its Mach number) is therefore greater when it flies at high altitude even though its actual speed may be the same as that before it began to climb. The change in the velocity of sound with temperature also explains why an instrument such as a flute becomes sharp when taken into a warm concert hall, the frequency change being greater than any effects due to the expansion of the instrument.