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What causes sound to change pitch while stirring a cup of coffee?
Wiki User
∙ 15y ago
I have been looking into this and i think the following may combine to produce this effect. Either/and/or. 1. As you stir the liquid is forced up at the sides and down in the middle. This increases the area in contact with the liquid. 2. As the hot liquid is poured into the cup/mug, it gradually heats the ceramic which changes the pitch. To test this, pour hot water into a cold cup/mug and tap a spoon on the lip. While the liquid remains still, the pitch gradually goes up as the ceramic heats up. You could also try different materials to see what happens, e.g. plastic or glass. Put your results here if you like. Try hot and cold liquids also and try stirring them to see if there is a change in pitch. Also bear in mind research has apparently been done on this, see the post below: http://www.physicsforums.com/archive/index.php/t-18996.html tiny-tim
Mar8-08, 07:37 PM
Something else: why does the pitch sound different when you tap a spoon to the bottom of a cup with spinning (hot) liquid compared to when the liquid is not spinning?
You can actually hear the pitch change when you first stir it very well, start tapping until the vortex slows down.. it's my dad's favorite scientific experiment ;)
Hi Monique!
A very similar issue was examined in the Proceedings of the Cambridge Philosophical Society, volume 65, page 365 (1967), in an experimental research paper entitled:
On note emitted from a jug while mixing instant coffee.
The authors were Farrell W.E. (whose usual field seems to be plate tectonics), Mackenzie D.P., and Parker R L.
Unfortunately, my library has no volumes before volume 77, but from memory they discovered that the note depends on the density of air molecules trapped against the coffee grains - which changes as you stir it!
Well … it was the sixties … and that sort of thing was important … :smile:
(I know this is almost a duplicate of a post in another thread
- but I thought it was important)
Wiki User
∙ 15y ago
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