Why does the clinking sound go down in pitch when I stir sugar into my coffee? © Getty Images

Why does the clinking sound go down in pitch when I stir sugar into my coffee?

Clink, clink. The symphony of coffee cups you hear in a café has to do with the sweet stuff changing the liquid it dissolves into.

Asked by: Julius CentikPiešťany, Slovakia  

Advertisement

When adding sugar into coffee – or any hot drink – the ‘clink’ of the spoon striking the mug’s sides becomes lower in pitch. The explanation is surprisingly subtle, and lies in the effect of the sugar on the liquid.  

The rough surface of the sugar granules traps molecules of air dissolved in the liquid, creating tiny bubbles. These make the liquid in the mug more ‘squashable’ – the technical term is ‘compressible’ – allowing it to mop up more of the energy in the sound waves created by the spoon, which in turn makes them travel more slowly through the liquid.  

As the speed of the sound waves in the mug is proportional to their frequency, the reduced speed lowers their frequency, which we hear as a drop in pitch.

Read more:


Advertisement

Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.