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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 - 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
    Commented Jun 14, 2015 at 11:41
  • $\begingroup$ I think I mean the scraping of the spoon along the bottom of the cup. $\endgroup$
    – And Finally
    Commented Jun 14, 2015 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
    Commented Jun 14, 2015 at 12:12
  • $\begingroup$ What are you atually stirring? I mean, do you stirr a mixture of water and sugar or instant coffe or coffe whitener? Are you stirring a homogenous solution to start with or you stirr to mix thigs in water? $\endgroup$
    – nasu
    Commented Jan 9, 2021 at 4:58

3 Answers 3

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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
    Commented Jun 14, 2015 at 23:10
  • $\begingroup$ Thanks, I like the vortex explanation! But in my case the pitch goes up. $\endgroup$
    – And Finally
    Commented Jun 15, 2015 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
    Commented Jun 15, 2015 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
    Commented Jun 15, 2015 at 15:13
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Actually.... Regardless of the volume of the "bell" changing due to the vortex created, (its a very small factor), from what I understand, its actually the movement of the tea, or coffee itself. That movement changes the frequency of the sound wave created as it travels through the liquid. I'm not sure if its technically the same as the Doppler effect but it could be.

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