Hot Chocolate affects Ceramic Acoustics

[bup]

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.

What gives?

[Rowanchilde]

Subscribes to thread

I have often wondered the same thing. Although it’s not just hot chocolate…

[CalMeacham]

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:

http://jesseenterprises.net/amsci/1977/11/1977-11-fs.html

Late one night, escaping for a few minutes from work on his doctoral dissertation, my friend John Hudak was idly mixing instant coffee into a mug of hot water, clinking his metal spoon on the bottom of the mug as he stirred. The pitch of the clinking noise fell abruptly as the powder went into the water, meaning that the sound waves had become longer and had decreased in frequency. As Hudak stirred for the time required to mix the powder the frequency slowly increased to its former value. He obtained the same results when he added a spoonful of a powdered cream substitute. Indeed, any powder dissolved in the hot water caused the same type of frequency shift.

Stretching of vortex columns when cold milk or cream is poured into hot coffee

Before any powder is added a particular note is heard from the clinking because the spoon excites resonant standing waves in the column of water and in the rim of the mug. The waves are similar to the resonant waves in an organ pipe that are responsible for the sounds you hear from the pipe. The harmonic frequencies in the coffee cup, however, are a bit more difficult to calculate than those in the organ pipe. The frequencies depend on the thickness, radius, density and elasticity of the rim, the radius of the column of liquid and the speed of sound in the liquid.

I first thought the powder would lie on top of the water and thus would have an influence in determining the resonant frequencies of the mug. It turns out, however, that any powder lying on the water only damps the oscillations in the mug, thereby muffling the clinking noise. It does not change the resonant frequencies. Moreover, the powder dissolves rather quickly.

When the powder first dissolves, tiny air bubbles are released from the powder grains. The speed of sound is considerably less in air (about 340 meters per second) than in water (about 1,450), so that the bubbles lower the speed of sound in the liquid. If the volume of the bubbles is a hundredth the volume of the liquid, the velocity of sound is reduced 30 times. As a result the frequency of any standing wave is lower when the bubbles are released. As the bubbles gradually burst, the speed of sound (and hence the frequency of the standing wave produced by the clinking) returns to normal.

A similar change in frequency can be observed in a freshly poured glass of beer. As the air bubbles in the beer (not the ones on top of it) dissipate, the pitch of the sound of a spoon tapped against the side of the glass rises slowly.

2.) Later on, it showed up on a certain answer column:

When I stir my coffee, why does the sound gradually change pitch?

3.) Later still, it showed up on this Message Board:

Pitch shift in stirring coffee

Bibliography to Walker’s article:

Bibliography

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.

[bup]

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.

[TriPolar]

I don’t drink coffee, so never cared about it’s physics. Hot chocolate would have a lot of suspended particles, is that the same with coffee?

[CalMeacham]

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.

[Chronos]

My guess would be viscosity-- Cocoa is more viscous than coffee, right?

[boyhowdy]

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?

[bup]

@boyhowdy

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.