When I get coffee in a coffee mug (pretty sure porcelain), and stir in sugar or whatever, the tapping of the spoon sounds “normal.”
When I make hot chocolate, and stir with a spoon, the mug sounds completely different - like there’s a hollow cavity in the mug’s walls. Or even if I tap the side of the mug, the sound has an entirely different “quality” than it does if it has water or coffee.
1.) Jearl Walker addressed this in his book The Flying Circus of Physics back in the 70s, and he hit it again when editing The Amateur Scientist for Scientific American. Only he observed it with lots of beverages:
2.) Later on, it showed up on a certain answer column:
3.) Later still, it showed up on this Message Board:
Bibliography to Walker’s article:
ON THE NOTE EMITTED FROM A MUG WHILE MIXING INSTANT COFFEE. W. E. Farrell, D. P. McKenzie and R. L. Parker in Proceedings of the Cambridge Philosophical Society, Vol. 65, Part 1, pages 365-367; January. 1969.
OBSERVATIONS OF AN EARLY MORNING CUP OF COFFEE. Vincent J. Schaefer in American Scientist, Vol. 59, No. 5, pages 534-535 September-October, 1971.
I suspect the current online incarnation might have more refs.
I don’t think that’s the effect I’m describing. It’s not pitch; it’s timbre. It’s roundness, or fullness. And the effect you describe (called the hot chocolate effect) should also occur in coffee if it’s in motion from stirring. The effect I’m describing does not.
It’s like the difference between knocking a stick against a table edge, or knocking a piece of rose wood against a table edge.
Well, you know what it is you’re looking for. The effect you describe sounded like the one Walker covered, and which I have heard in different beverages. I can’t honestly say I’ve detected a difference between hot chocolate and tea, so I’m not sure exactly what you’re describing.
I’ve noticed the same thing with heavier stoneware mugs. Different fluids seem to affect the resonance, not so much the pitch. I have 5 or 6 favorite hot drinks- same volume/mug/~temperature. I’ve always assumed it was substances with higher specific gravities within the mug dulling the resonance. After all, there is a marked change between empty and full of water so i’d imagine the effect would continue in that vein. Any weigh-ins from acoustic engineers (or anyone who understands the actual science) instead of my guessing?
I’m pretty sure mine’s not stoneware - it’s very smooth and white. I admit it could just have a lot of glaze, but I think it’s porcelain.
OK, here’s what I can tell you - I put a lot of crap in coffee - sugar and non-dairy creamer - which I’d think would be “suspended particles” as well. Maybe not - the sugar probably becomes actual liquid (right?) but the non-dairy creamer, anyhow.
Maybe I need to get the difference on a short digital movie. The sound difference is resonance. I’ve never shot a digital movie with my camera, but this question interests me enough.