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I've noticed that when stirring my tea, as I stir faster the tone generated by the stirring goes up. Why is that? Is it something to do with the Doppler Effect?

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Thanks for the responses guys - Victor's and Carl's seem like good answers to the question. Unfortunately I realise I may have described the situation wrong - I think I may actually be hearing the pitch go up as I stir at a generally constant speed. It may have something to do with some accumulating increase in the velocity of the tea. But as that's my stupid mistake I'll tick Victor's response to this particular question.

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  • $\begingroup$ You mean just the noise of the stirring or beating the spoon to the cup wall as well? $\endgroup$ – Victor Pira Jun 14 '15 at 11:41
  • $\begingroup$ I think I mean the scraping of the spoon along the bottom of the cup. $\endgroup$ – And Finally Jun 14 '15 at 11:46
  • $\begingroup$ In that case the answer should be obvious: you are increasing the rate at which the spoon vibrates. Just like (revealing my age) playing an LP at 45 rpm. $\endgroup$ – Carl Witthoft Jun 14 '15 at 12:12
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I would go for this: Imagine the bottom of the cup as a saw. The noise or chattering of the spoon jumping on the sawteeth is higher the faster spoon moves. Those "sawteeth" on the cup bottom are very small, but the principle is the same. Therefore the faster stirring the higher pitch.

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Actually the pitch obtained by striking a coffee cup is a function of how full the cup is with coffee - the fuller the cup, the lower the pitch. You can convince yourself of this with a simple experiment just by changing the amount of coffee (water will work) and tapping the outside of the cup with a spoon.

But when you stir a cup of coffee with a fixed amount, the circulation leads to a vortex that causes the coffee to rise up the inside wall of the cup - effectively leading to the same result as if you had more coffee in the cup. This explains why stirring seems to lower the pitch as you stir.

What I cannot explain - and what is somewhat counter-intuitive is why the pitch goes down. I would suppose at first this was a standing wave, but with a standing wave, and shallower unexposed space, you would expect pitch to go up!

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  • $\begingroup$ Note this answer is consistent with your latest edit. The pitch does go down with faster stirring. $\endgroup$ – docscience Jun 14 '15 at 23:10
  • $\begingroup$ Thanks, I like the vortex explanation! But in my case the pitch goes up. $\endgroup$ – And Finally Jun 15 '15 at 11:09
  • $\begingroup$ @AndFinally Did you try the experiment of progressively filling the coffee cup to different levels and tapping the outside (no stirring)? Also when you say "pitch goes up", you mean higher frequency, right? $\endgroup$ – docscience Jun 15 '15 at 13:58
  • $\begingroup$ Yes, I mean the note goes higher up the musical scale. I'll try that test tonight and report back. $\endgroup$ – And Finally Jun 15 '15 at 15:13

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