The Sound Of Instant Coffee

[Eddy-B]

I have been wondering why the pitch of my coffee mug changes when I stir the coffee. I did some searches on the net, but did not yet find an answer that would completely satisfy me.

So, since i KNOW there are some total geeks in the OPU community i'm posting the question here:

Fill up a mug with hot water. Add instant coffee (or the other way around, it doesn't really matter) and tap the bottom of the mug with a spoon. Now stir the coffee and notice the pitch go down dramaticly. Stop stirring and keep tapping the bottom of the mug and notice the pitch going up, first fast then slower until it reaches its original sound.
Once you stir it again the effect can be generated over and over.


A few possible solutions i found:

• Some people state this happens only when you use a microwaved cup of water and then add instant coffee. The water is supersaturated (it contains more oxygen and nitrogen then it can actually hold) -> i don't know about you, but i don't usually use a microwave to heat up my coffee.. and the effect is quite audible when using just normally heated water
• some would say this has to do with the speed at wich the coffee is stirred, changing the speed at which the sound moves through the liquid. Myth busted: after a couple minutes i can stir and tap all i want, the pitch of the mug doesn't change much anymore.. the effect is gone. Besides: it doesn't work with water only - i've tested this!
• some would say this has to do with air trapped inside the liquid as it is attached to small particles of unmelted coffee, and as you stir the air gets released and changing the pitch. Also busted: when i have my mug filled up the pitch is high, then i stir and within a second or 2 the pitch goes down like 2 or 3 octaves, then when i stop stirring it goes slowly back up. This process is repeatable.... does air magicly re-attach itself inside my coffee
• yet another explaination was the difference in temperature between the inside and the outside of the mug, and the accompanying stresses would change the pitch. This would certainly explain why the effect is gone after a couple minutes... BUT why is the effect repeatable a dozen times in a row? does my coffee cool down and heat up again while i stir

As you can see: i have no conclusive answer yet. Some of the theories are plausible, but so far i can contradict all of them using the above observations.

[CK9]

possibility: poor solubility of instant coffee.

If the instant coffee is acting a lot like my chocolate milk mix, a fration of the particles are re-solving themselves.  That is, after you mix them, the particles start clumping together to undo your efforts to mix them with the liquid.  This would creat particles of a different size, changing how the sound waves are interrupted.

Stirring forces the particles to dissolve, thereby removing the interruption.

Let's see someone try to disprove that!

[Hooman]

I was thinking something along the same lines as CK9.

Remember that the speed of sound depends on the density of the medium it is travelling through. It may just be to do with how much is currently disolved in the liquid. If you stop stirring, it may start to settle out to the bottom of the cup.


Since I know you'll enojoy some numbers, here's a few from a book off my shelf:
Approximate speed of sound in (at "typical" density and temperature)...
    Air: 344 m/s
    Water: 1478 m/s
    Steel 5064 m/s

Also, if you're ever out on a cold winter morning, and you see someone slam their car door down the street, you might notice a slight delay before you hear the sound.
 

[Sirbomber]

Are you sure you didn't mean to post this in the "Stupid Questions" thread...?
Sure, I don't remember my chemistry too well, but particles don't just "fall out" out of solution.  If a solution is saturated and extra particles are added, particles will constantly dissolve/un-dissolve (yay unscientific terms) while an equilibrium is attempted to be reached.  Particles don't just reform on the bottom of the cup and stay there, they immediately dissolve again while other particles fall out of solution.

As for why stirring helps dissolve the particles on the bottom of the cup, stirring increases the surface area available for the solution to dissolve the particles.  It's easier to dissolve a sugar cube by submerging it in water than it is to pour water on it.
Or something... Like I said, it's been awhile.  

I'd provide a drawing, but what am I?  A science textbook?

Point is, you're probably just hitting the bottom of the cup in different places or with different parts of the spoon.  Or you're just crazy.

[BlackBox]

Well, I would tend to think that instant coffee might act more like a suspension than a solution (who knows given what they probably make it out of compared to "real" coffee). So you could very well have large particles settling out.

I would think that the density would be the biggest factor. Frequency of a vibrating string is inversely proportional to the square root of the density (in simple terms, the denser the string, the lower the pitch)

Stirring it is going to cause the density of the fluid to increase (more coffee particles are entering the fluid phase) thus the pitch could decrease due to this.

Another possibility is that there is some kind of resonant effect, for example, perhaps the stirring is causing interference patterns which may mask higher harmonics (i.e. destructive interference) or amplify lower harmonics (constructive interference) as it is being stirred.
I don't put a lot of stock in this theory (and I am no expert on this subject anyhow, I'm not going to college for physics so I think the change in density seems to be a more likely thing).

Try this with something that absolutely fully dissolves into solution and then try stirring it and see if you get a similar effect. (i.e. salt into water). If the solute is completely dissolved in the solution and the density hypothesis is true then you should see no change in pitch as you stir the solution (since the density stays the same). Otherwise, we're on the wrong track.

[Eddy-B]

I know the speed of sound through a medium depends upon its density. That would support the idea that stirring would change the density, while letting the coffee settle would change it back. But does this explain why the effect goes away after a few minutes ?

I will test this with my coffee tonite, to see how this effects the sound.. because the odd thing is that after a few minutes i cannot distinguish any resonant sound, other then the tapping itself (a ticking sound).

Has any of you tried it out ??

[CK9]

hehehe!  I had the same idea as two people here who are more than likely smarter than me!  ^_^  I so happy!

I'm not quite sure what you mean by "goes away"

Is ther any way you can elaborate on it?


I don't drink coffee myself, stuff is nastey to my pallet.

[Eddy-B]

I'm not quite sure what you mean by "goes away"
 

At first it works fine: you stir, tap the cup/mug and the sound will go from low to high.
Over time (2~3 minutes) you keep repeating this, it will start from a higher pitch every time, until finally after a few minutes there's no obvious pitch change anymore.
 

[CK9]

could be that the particles finally fully dissolve...

[BlackBox]

That'd be my guess (and then at that point, there's no change in density).

[Eddy-B]

I believe we may have solved this

[CK9]

Isn't it strange what catches our interests?

[Hidiot]

No, this has actually been an interesting topic.

[Eddy-B]

I have mentioned you can find this topic on several other forum and discussion boards, but there was no answer that satisfied me completely..

[Freeza-CII]

If i am right this same effect can be found with a sause pan like a copper bottom aluminum type kinda the cheap thin ones.  And some water dont stir the water but tap on the bottum of the pan and tilt the bottom of the pan.  You dont need very much water for this.  But youll find that the moving water distorts the pitch in a kinda oscialting fasion.

[Eddy-B]

That's an easy one (and totally different): the pitch at which the object (the pan in this case) resonates is determined by the amount of liquod in it.

The same effect can be observed with a glas-organ (wineglasses partially filled with water).
Also try this: take a beer bottle, take a few sips and blow into the hole. At a certain angle you can produce a high pitched sound. The more you empty the bottle, the lower the pitch. When you're about halfway you can change the sound somewhat by tilting the bottle and blowing. This changes the distance between the opening and the reflecting surface (the lowest point of the water).