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.
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Robert is a science writer and visiting professor of science at Aston University.