Does sound travel faster in warm or cold air?

I was asked this by the principal of a school I was visiting during Science Week last year. I gave him the short answer… it travels faster through warm air.

Technically that is correct.. it does travel faster through warm air… the molecules in the warm air are more “excited” and will vibrate more easily. Sound needs vibration in order to work so the sound is carried more easily through the air with the more excited molecules than through air with more “still” molecules (cold air).

A good way to think of it is to imagine a line of dominoes. The air molecules are the dominoes.

Sound makes air molecules around the source vibrate and hit off the next molecule which vibrates and hits of the next (just like the dominoes hitting off each other) and the chain keeps going until the sound reaches your ear… and then the vibrations get carried on to your middle and inner ear until they are changed to electrical pulses that are sent to the brain!

And there was me thinking I was keeping this simple… back to the dominoes… just keep thinking of it like a string of dominoes. Actually that is not quite true…. for the domino model to really mimic the movement of sound you have to arrange the dominoes in concentric circles, not in straight lines. Sound travels outwards from the source in all directions.

So there you have it sound does travel faster in warm air BUT it may appear to travel farther in cold air.

This is how that works…

…if the air close to the ground is colder than the air above it then sound waves travelling upwards will be bent downwards. This is called Refraction. These refracted sound waves can act to amplify the sound to someone standing far away.

“Sound” about right to you?

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