Pitch shift in stirring coffee

As to the change in the pitch of your spoon hitting your cup mentioned in When I stir my coffee, why does the sound gradually change pitch? , the ball was seriously dropped. No attempt to recreate the effect, no experiment, just a hip-shot response.
I have therefore filled in this deficiency by conducting the following experiment:

I got out three coffee mugs of differing types, and one by one added hot WATER (no powders), stirred them, and listened to the pitch of the spoon/mug interactions. They did indeed rise in pitch in each case. I then let them sit a few minutes and tried again: no change.

I did note a reversion to the original pitch upon putting COLD water in the mugs.The pitch of the mug without anything in it, of course, differs from that with anything in the mug.

Conclusion: The change in pitch is caused by the change in temperature of the mug, Jearl Walker’s half-hearted guess be damned. (I would welcome a defense of his response, given second hand as it was.)

Anyone attempting to replicate this experiment would find best results using a mug with heavy walls, like the old Buffalo-type diner coffee mugs, presumably because the added mass and thickness takes longer to heat, slowing the effect.

I’ll have to go look this up, but IIRC, Jearl Walker included this in his book The Flying Circus of Physics, which in most cases gives references to articles on the topics (although not always, I find). In a lot of cases like this one, there’s a reference to an article in, say, the American Journal of Physics. So someone may already have done this experiment (so it’s not necessarily guesswork). Check the references in TFCOP. I will when I get home.

hammerbach, I assume that the “hot” and “cold” water you’re using just comes from your kitchen tap. If I recall correctly (which I might not, so I hope someone else will step up and agree/disagree), the aerator in your tap aerates hot water better than cold water. Therefore, the hot tap water starts out with more tiny air bubbles in it. So your experiment may not have adequately controlled for precisely the factor that Jearl Walker believes causes the pitch change, leading to a faulty conclusion.

Experimental verification
A possible verification of this idea would be to start with cold tap water and heat it on the stove before adding it to the mug. Compare the resultant pitch change with that from hot tap water and cold tap water. I’ll give this a try when I get home.

If this idea pans out, then I’d say that Jearl Walker stands vindicated (in part), but Cecil remains accused of dropping the ball.

Nope… Tap water heated up. I think hot water is in fact less capable of holding dissolved air (I have been instructed to re-aerate water after pastuerization for making wine). A more precise version of this experiment would call for two water samples, both boiled and one cooled to provide hot and cold non-aerated water. I’m pretty confident that it won’t make a difference, but not to be accused of hip-shooting when I have a little time I’ll repeat the experiment with pre-boiled water as described.

Another way to control for this would be to add cream and sugar to observe the effect. I have never observed this when preparing my coffee, but admittedly, I have not been specifically looking for the additives to change the effect. I’ve only noticed that the effect happens whether I put in additives or not. (Whiskey doesn’t seem to effect it either…)

You will probably find as I did that for a pronounced effect, you will have to get the water pretty hot, although not boiling, and that the thicker the mug, the slower the change takes place.

Also, note that the change reverses when cold water is put into the hot mug… I think that these factors adequately control for the water, which is from the same source in each case. If the effect was caused by the change in relative density of the water only, I would expect the change to happen more quickly in a thicker mug which could provide a much better heat sink than the thin mug. But the opposite is true. (Anybody out there with a heat-transfer background ? I’d welcome a bit of help/correction here…)

According to my copy of The Flying Circus…, Jearl Walker’s references are W. Bragg’s 1968 book The World of Sound, p. 158, and – God help us – an article entitled “On the Note Emitted from a Mg while Mixing Instant Coffee” by W.E. Farrell, D.P. McKenzie, and R.L. Parker, which appeared in the Proceedings of the Cambridge Philosophical Society, vol. 65, p. 365 (1969). Not noted by Walker (since it appeared after his book came out) is an article on the topic that appeared in the American Journal of Physics, vol 50, #5, p. 398.