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Hot chocolate effect

As a child, I made a "discovery": After stirring my hot chocolate, the sound of my spoon tapping the bottom or the edge of the cup became increasingly high-pitched.

So, I made various hypotheses, some more accurate than others, until one day I decided to get to the bottom of it: has this effect been documented?

Yes, indeed! It is the "allasonic effect," sometimes called the "hot chocolate effect" although it also works with coffee, tea, or any hot beverage. This phenomenon was first documented in 1982 by Frank S. Crawford. It is a wave mechanics phenomenon: the variations in the speed of sound in a liquid due to the presence of gas bubbles.

According to the dedicated Wikipedia page: The wavelength being fixed by the size of the cup, the frequency of the sound (thus the "note") depends on its speed in the liquid. The presence of air bubbles (compressible) in the liquid (incompressible) greatly reduces this speed. Therefore, the sound seems lower (i.e., lower frequency), then higher when there are no more bubbles.

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