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View Full Version : The strange behaviour of ticking sound in a cup of hot coffee or coco


julius
29th October 2010, 07:00 AM
This is a question that I haven't been able to answer for years. It is about hot coffee and coco in (preferably) ceramic cups.

About ten years ago I was installing Windows and to ease the waiting I made myself a nice cup of hot coffee. While waiting and being bored I was ticking the bottom of the coffee cup with my spoon and I noticed the tone of the sound that produced got higher as I kept ticking. I had no clue why this would happen, and I'm still not sure. In the past 10 years I've had many discussions about it, even with a physicist, but I am still not entirely sure what causes the tone to get higher.

Some suggestions that I've heard so far are:

- The fluid is a suspension and the particles in the suspension are driven outwards by the sound waves, altering the density of the fluid (I don't think this is true)
- The fluid contains small air bubbles that are driven from the fluid by the sound waves, also altering the density of the fluid (plausible).

Some other things I discoverd.

- It works with coffee and chocolate milk, not with clear fluids like tea or water.
- It works best in ceramic cups.
- It kind of works with cold chocolate milk, but much better with hot chocolate or coffee.
- When the tone is at a peak level, you can bring it down again by stirring the fluid. The effect of stirring is getting less over time, perhaps because the temperature is also going down? No clue why!

I think the air bubbles explanation is the best so far, becuase that would explain why stirring causes the tone to go down again: new air is mixed into the fluid. However, other factors are involved, temperature for example.

So, WHY, would the sound created by ticking the bottom of a ceramic cup filled with hot coffee or coco get higher over time? Am I onto a new fundamental law of physics?

Howie Felterbush
29th October 2010, 07:13 AM
http://www.straightdope.com/columns/read/1029/when-i-stir-my-coffee-why-does-the-sound-gradually-change-pitch

Looks like bubbles may be the answer.

madurobob
29th October 2010, 07:37 AM
Since I was just making coffee, I tried this. Black coffee is a ceramic mug. Tapped the outside of the mug with a spoon. My tin ear noticed no change in pitch.

I didn't try tapping the inside of the mug - is that what I was suppose to do?

Howie Felterbush
29th October 2010, 07:51 AM
Since I was just making coffee, I tried this. Black coffee is a ceramic mug. Tapped the outside of the mug with a spoon. My tin ear noticed no change in pitch.

I didn't try tapping the inside of the mug - is that what I was suppose to do?

Yeah, the inside, I think.. Like the sound you get when you're stirring cream or sugar (or Jameson's:)) into the coffee.

I have noticed this change in sound as well, and since I use cream and sugar I figured it had something to to with them.

Olowkow
29th October 2010, 08:35 AM
Well, I just tried the experiment. Using Nescafe, Taster's Choice freeze dried coffee.

One cup with plain water, second cup with one teaspoon of coffee...no cream or sugar.

When there is nothing in the water, the spoon tapping produces a high pitch, ringing sound. When coffee is added, the tapping produces a dull thudding sound, no ringing. As the coffee dissolves, there is no question that the sound changes slowly back to what a cup of plain water sounds like: Hi pitch ringing.

This seems to have something to do with the suspension of large vs. microscopic particles in the liquid, in my opinion, essentially density.

ETA: Tapping inside vs outside makes no difference. Coffee made by using a drip or percolator coffee maker would not produce this effect, since the liquid does not contain any of the large undissolved bits as it does with instant coffee.

madurobob
29th October 2010, 08:56 AM
Well, that makes complete sense - undissolved stuff, or even bubbles, in the liquid interferes with the liquid transmitting sound waves. And, since I always drink my coffee black, I've never noticed the phenomenon.

I guess I'll have to make some hot chocolate from a power mix for lunch...

julius
29th October 2010, 10:58 AM
It happens when you tap the bottom of the inside of the cup with your spoon.

Have you tried stirring and then tapping again? Stirring brings the pitch down, but when you stirr several times, the pitch doesn't go down that far anymore. Not as far as the original pitch when the coffee was fresh :) How would you explain that?

I agree that air bubbles is the most plausible explanation, but what about the stirring multiple times and why would cold liquids not or hardly change in tone, while hot liquid do?

madurobob
29th October 2010, 12:56 PM
It happens when you tap the bottom of the inside of the cup with your spoon.

Have you tried stirring and then tapping again? Stirring brings the pitch down, but when you stirr several times, the pitch doesn't go down that far anymore. Not as far as the original pitch when the coffee was fresh :) How would you explain that?

I agree that air bubbles is the most plausible explanation, but what about the stirring multiple times and why would cold liquids not or hardly change in tone, while hot liquid do?

what have you got in the cup besides the coffee and how vigorously are you stirring?

I suspect if your stirring in a boatload of sugar that not all of it dissolves, and re stirring brings the undissolved crystals back off the bottom of the mug and into the solution again. I don't think this would be true of, say, a spoonful of milk, however. Heavy cream, on the other hand, often floats to the top and you have to stir it back in, changing the density of the coffee.

As for cold vs hot, its probably got to do with what they can hold in suspension. Ever try to dissolve sugar into a glass of cold tea?

TubbaBlubba
29th October 2010, 12:59 PM
Yeah, the inside, I think.. Like the sound you get when you're stirring cream or sugar (or Jameson's:)) into the coffee.

Well, hopefully you'd be stirring both sugar and Jameson's. Unless you're one of those horrible people who just put sugar on top of the cream.

jaydeehess
29th October 2010, 03:39 PM
Just a thought;

what about the movement of the liquid itself. Left alone the coffee will experience quite a bit of convection driven movement. You can see this by pouring a small amount of milk (not heavy cream) into the cup and watching the milk move about(no stirring).

If the convection currents alter the transmission of sound from cup to liquid then as the liquid settles down transmission gets better and there are fewer destructive wave interferences in the liquid. Stir the liquid and you get back to square one again except now the movement is caused by your mechanical input rather than convection.

It seems to me that this would affect higher frequencies more than lower ones and thus the pitch change.
Perhaps it works best with denser liquids and thus plain water or tea not so much as coffee or hot choc.

ETA: A test of this might be to heat the cup up first to the same temperature of the coffee being put into it. This would create less convective currents at least for a few seconds.

Jack by the hedge
29th October 2010, 04:01 PM
I remember noticing this years ago. When I made a cup of coffee and stirred it, the spoon hitting the sides of the mug would make a pinging sound which rose in pitch over a few seconds. I wondered if it might be something to do with the increasing dip in the middle of the rotating liquid, or the sides of the mug heating up. Never came up with a satisfactory answer.

Come to think of it I don't recall the effect being isolated to instant coffee. It might affect tea, too. I'm going to have to try it now...

quarky
29th October 2010, 04:10 PM
I like this, julius.

I notice similar, mundane oddities. For instance, at a certain thickness, a milk-shake in a tall glass will vibrate when you drag your finger along the rim of the glass. The whole surface becomes chaotic. You can also hear a note.

Didn't mean to hi-jack your question, but I'll tap my mug in the morning and report back.

Howie Felterbush
29th October 2010, 08:18 PM
Well, hopefully you'd be stirring both sugar and Jameson's. Unless you're one of those horrible people who just put sugar on top of the cream.


Guilty as charged. I like cream and sugar, and the coffee's gotta be really strong.

I like my metal black, but not my coffee.

jaydeehess
29th October 2010, 08:28 PM
I'm going to try it next week. I only drink coffee at work.
,,, and only coffee in my coffee for me.

macdoc
29th October 2010, 10:55 PM
notice similar, mundane oddities. For instance, at a certain thickness, a milk-shake in a tall glass will vibrate when you drag your finger along the rim of the glass. The whole surface becomes chaotic. You can also hear a note.
That just resonance...not at play here...hmmm then again ( see below )

Suspended solids most likely tho it could be an effect of the stirring reducing the impact of the spoon on the cup due to currents and maybe even reduced friction as the inner surface of the cup is fully wetted and the spoon and circulating liquid couple up..
A spoon tends to flutter in a moving liquid changing its impact profile and once swirling the spoon/liquid has its own inertia.

If you let that inertia take over instead of continuing to stir the sound will change as te fluid slows.

If I visualize we usually give a reasonably hard stir initially then reduce the energy input - one of those things you do unconsciously.

Just tried it with a partially full espresso cup.....might be the slope of the cup to the bottom....sound on the top a higher pitch where the cup is wider and thinner than the towards the bottom....maybe there is some resonance at play.

TubbaBlubba
30th October 2010, 04:43 AM
Guilty as charged. I like cream and sugar, and the coffee's gotta be really strong.

I like my metal black, but not my coffee.

I love black coffee, but the cream doesn't float properly on the coffee unless you sugar it richly (and the aroma of brown sugar blends really well with the whiskey and coffee).

Emet
30th October 2010, 07:27 AM
Guilty as charged. I like cream and sugar, and the coffee's gotta be really strong.


There is only one way to drink coffee-- I love my coffee full-bodied and black. But it was an acquired taste.

TubbaBlubba
30th October 2010, 07:49 AM
There is only one way to drink coffee-- I love my coffee full-bodied and black. But it was an acquired taste.

Definitely. I started with half milk, half coffee and gradually worked my way to espresso.

quarky
30th October 2010, 12:13 PM
Once you start looking for "Oooommmm", stuff gets weird. The universe has a way of reinforcing a wide variety of nonsense. The sub-atomic realm, even more so.

There may come a point wherein Occam was wrong. And the razor got dull.

(Shameless bait to encourage dialog)

phildonnia
31st October 2010, 06:26 AM
When you stir tea, why do tea leaves migrate to the center of the cup, when they're heavier than water?

TubbaBlubba
31st October 2010, 06:29 AM
When you stir tea, why do tea leaves migrate to the center of the cup, when they're heavier than water?

I was thinking "Because they're heavier than water."

The water can't push them around enough.

quarky
31st October 2010, 05:10 PM
When you stir tea, why do tea leaves migrate to the center of the cup, when they're heavier than water?

In a round container of contaminated water, the denser particulates will tend to conglomerate toward the center of the stirred flask. The fluffier stuff heads towards the perimeter surface of the containment.

(The bottom center of the contained flask; tornado filters)

macdoc
31st October 2010, 10:12 PM
Gold panning anyone?