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Rising pitch in moving coffee?

  1. Nov 23, 2006 #1
    I don't know which mentor to address this question to so I'm trying it here.

    After I have stirred my coffee I always tap the spoon on the lip of the cup. As I do so the pitch keeps rising until the coffee is no longer moving. I've been curious about this for years and hope somebody can answer the question. Why does the pitch rise in moving coffee and stop in stilled coffee.

    Thank you,
  2. jcsd
  3. Nov 24, 2006 #2


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    I never noticed this, but not using sugar I very rarely have a spoon.

    If this occurs, then on the ringing cup the sides move in and out so that the round cup becomes oval. The rotating fluid might generate a out of phase condition retuning enegy to a different point on the cup surface from which the energy was adsorbed. The net effect of this would be a change in impedance, changing the ring frequency.
  4. Nov 24, 2006 #3


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    I'm quite sure this is the result of the level of the fluid at the side of the cup. If you do the same thing with a cup in which the fluid is not in motion, and pour out some fluid between rings, you get the same effect. With stirred fluid, the effect is taking place because centrifugal force is throwing the liquid against the side of the cup, where the liquid is forced upward, covering more of the side of the cup. Since the material of the cup is what does the actual ringing, only the level at which the fluid makes contact with the cup matters. So stirring the coffe ahs the same effect as having more or deeper coffe in the cup.
  5. Nov 25, 2006 #4
    Thank you for that answer. I've suspected as much but thought the difference of the level of the coffee was too small to make a difference in sound. By the way, I've found that this pinging sound only occurs in better quality ceramic mugs. Maybe I'll try it with slower moving syrup and see if the level might be more noticeable. Thanks again.

  6. Nov 25, 2006 #5


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    You way want to do a search on this, it has been been discuseed before a year or 2 ago.
  7. Nov 25, 2006 #6


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    Why don't you develop a hypothesis, test it and get back to us?
    It would be pretty simple to vary factors such as the level of the liquid and get reproducible results.
  8. Nov 26, 2006 #7
    I may experiment a little but I'm no scientist, I'm just a retired 68 year old woman who has a curiosity about physics and related puzzles. I still find it hard to believe that the level of the coffee has anything to do with it since the level change is not even noticeable. But, I'll have to take your word for it. To those of you who answered thanks again.
  9. Nov 26, 2006 #8
    You're certainly no scientist, but it's not too late to become one. Don't take his word for it, try adding the extra mm of milk to prove that the level isn't responsible (or is)!
  10. Nov 27, 2006 #9
    You guys don't give up easily do you? I guess that's what separates scientists from lay people like me. Okay, you forced me to try an easy experiment. While tapping the cup with the spoon I slowly added more coffee and sure enough the tone gradually deepened. Duh!

    Maybe one day I'll come back and tell you about a dream I had for an antigravity machine using a crystal with a 37 degree orientation and a small electrical charge. Sounds crazy but the dream sure seemed real.

    Thanks again guys

  11. Nov 28, 2006 #10
    Honestly I would think the sound is "deeper" when the liquid is moving, because when it's moving the atoms and jostling around more. This would cause any higher frequency motion to be swept away into the "noise" of the current. Hence the resonant frequency is lower.

  12. Dec 1, 2006 #11
    Thank you niko. That answer makes the most sense to me. As I said, the level change is not even noticable and yet the pitch keep rising as the coffee slows down. I've been asking this question for years, now I believe I know.

    A greatful Nantz
  13. Dec 2, 2006 #12
    I've noticed this effect myself with certain brands of instant hot chocolate. My theory is that the stirring process changes the distribution of suspended particles in the liquid, and that as they settle the speed of sound in different areas changes. The reason I suggest this is that the pitch change doesn't seem to take place in water -- it must depend on the composition of the liquid in some way.
  14. Dec 2, 2006 #13
    Yeah i also think it was something to do with the density of the fluid, if you stir at a constant rate you can see that the vortex maintains the same level in the cup yet still hear the pitch of the sound change. Once you have stirred for a length of time the pitch seems to become constant.
  15. Jan 14, 2007 #14
    Density - yeah. That gets my vote, because you're dissolving the sugar as you stir.

    If you let it calm down and then start stirring again, there isn't the same rise in pitch.
  16. Jan 14, 2007 #15


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  17. Mar 4, 2007 #16
    An experiment

    I too have noticed the phenomenon and this thread finally stirred up enough interest for me to try an experiment.

    I intuitively didn't feel the fluid level change as the vortex slowed was enough to warrant what I perceived as a substantial pitch change. But as an intellectually honest coffee drinker I wanted to test the theory.

    I used my favorite mug that I knew from experience exhibited the phenomenon every morning without fail.

    Since I didn't want to waste a lot of perfectly good coffee, I resorted to using tap water.

    I first filled my mug to about 80% capacity to allow enough freeboard for vigorous stirring without spilling. I stirred vigorously until I had a nice vortex in motion and began lightly tapping the edge of the cup just as I normally do. To my consternation, the pitch of the ringing cup did not change!

    Like any good kitchen counter scientist, I examined the variables for clues as to why the test failed to yield the expected results. That's when I realized what I had done. I had used cold tap water! I quickly formulated an alternative hypothesis --

    I tested the hypothesis by repeating the initial test with cold water and not stirring. I did not perceive any change in pitch as I tapped for approximately 30 seconds. I then ran the tap water until it was at its hottest which closely approximates the temperature of coffee fresh from the pot and again filled the cup to about 80% capacity. I did not stir before I began tapping the edge of the cup and, Eureka! the pitch deepened as I tapped.

    Subsequent experiments that began with a cup that was preheated by immersion in hot water for a period of time adequate to ensure maximum heat absorption followed by filling with cold water yielded similar results with the only difference being that the ringing rose in pitch as the water and cup exchanged heat. (just as my hypothesis predicted)

    Therefore, I propose that the perceived change in ringing pitch of a freshly poured cup of coffee tapped on its edge by a spoon is due to changes in the resonant qualities of the cup as it absorbs heat from the coffee fresh from the pot.

    Anyway, that's my theory and I'm sticking to it.

  18. Mar 4, 2007 #17
    Good work!

    Regards "resonant qualities": Usually the size of something is proportional to the temperature, and the frequency inversely proportional to the size. So you'd expect the frequency to be rising as the mug cools down (which, likely being primarily due to evaporation, would occur as the heat of the mug is absorbed by the coffee).
  19. Mar 5, 2007 #18


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    That makes sense.

    Cups are normally fairly good insulators. When you fill the cup with a hot liquid a thermal gradient through the thickness of the cup will build up slowly. As it does that, the inside of the cup expands more than the outside.

    This will result in stresses in the cup. In the tangential direction these will be compression in the hot part and tension at the cold part. These stresses are large - they are big enough to shatter the cup, if it is cracked before you fill it with hot liquid (been there, done that!)

    If the cup was initally at room temperature, at first the temperature gradient will not be uniform - it will be steeper near the inside. So most of the material in the cup will be in tension. This will raise the vibration frequency of the cup in the same way as a guitar string increases frequency if the tension (i.e. the internal stress) in increased.

    Some ideas for more experiments to confirm or deny this:

    1) Try it with a metal container, e.g. an empty food can or a small saucepan. The temperature gradient through the metal will be small because it is a good conductor of heat, therefore the thermal stress will be small and there should be no change of pitch.

    2) Try warming a cup for a while in a warm oven (set to less than 100 degC!!) till it is at uniform temperature (that may take a hour or more) and then full with cold water. The pitch should drop, because this time most of the cup will be in compression not tension.

    The theory about the rotation speed of liquid changing the frequency is physically sensible but the effect would be too small. The frequency of vibration (judging by the sound) is about 500 to 1000 Hz. The frequency of rotation of the coffee is only 1 or 2 rev/sec.

    If the two frequencies were closer together there could be an effect - for example water flowing through a high pressure fire hose, where the flow does create dangerous vibrations in the hose unless it is handled correctly.
    Last edited: Mar 5, 2007
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