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Explain this (changing pitch in coffee mug)
- Thread starter Keith_W
- Start date
shear thickening (oobleck) is more common in dispersions with very high content of the dispersed phase. Milk is actually quite diluted and is very slightly shear thinning (the opposite). I have no idea of how this can influence sound propagation, or if it actually does. Just wanted to point out that assuming milk is an homogeneous liquid is not accurate.
OP
Keith_W
Major Contributor
- Thread Starter
- #23
Anyway, I highly doubt if milk has much non-Newtonian behaviour in this setting although I would fall off my chair in amazement if it did. I would LOOOVE it to be true because it would be so cool! The experiment could be replicated by creating a real non-Newtonian fluid, e.g. cornstarch dissolved in water, and observing if it demonstrated this behaviour.
Anyway, to recap:
- I believe that the first experiment (with coffee) has been explained to my satisfaction. Standing waves in a coffee mug, changing speed of sound in liquid due to rising bubbles affecting density.
- the second experiment with frothed milk which demonstrates the opposite effect has not yet been fully explained. The spectrogram shows selective attenuation of lower frequency ringing when bubbles are present, suggesting the bubbles have a dampening effect on vibrations selective for longer wavelengths. One explanation is Oobleck's phenomenon (which I am honestly skeptical about in this setting).
There is another phenomenon we are not really explaining. If we beat more air into the milk, the volume of the liquid increases. It is the same as filling the glass with more liquid, albeit a less dense liquid. If we simply topped the glass up with more milk, the pitch should decrease. Increasing the volume by aerating the milk seems to make the pitch increase. I am twisting my brain in knots trying to work this one out.
Anyway, to recap:
- I believe that the first experiment (with coffee) has been explained to my satisfaction. Standing waves in a coffee mug, changing speed of sound in liquid due to rising bubbles affecting density.
- the second experiment with frothed milk which demonstrates the opposite effect has not yet been fully explained. The spectrogram shows selective attenuation of lower frequency ringing when bubbles are present, suggesting the bubbles have a dampening effect on vibrations selective for longer wavelengths. One explanation is Oobleck's phenomenon (which I am honestly skeptical about in this setting).
There is another phenomenon we are not really explaining. If we beat more air into the milk, the volume of the liquid increases. It is the same as filling the glass with more liquid, albeit a less dense liquid. If we simply topped the glass up with more milk, the pitch should decrease. Increasing the volume by aerating the milk seems to make the pitch increase. I am twisting my brain in knots trying to work this one out.
OP
Keith_W
Major Contributor
- Thread Starter
- #24
My friends are still going at it in the discussion group. To test whether the different densities of liquids change the pitch, I performed this experiment:
I used a syringe to measure out exactly 100mL of soy milk, water, and methylated spirits. Do you hear a difference in pitch? I think I do. But that could be expectation bias at play!
(EDIT) I converted the above video into .WAV and analysed it in Audacity's spectrogram.
All 3 are exactly the same.
This is a spectrogram of the first experiment (with coffee). I can clearly see the rising frequency peak, but I can not see n1.
I used a syringe to measure out exactly 100mL of soy milk, water, and methylated spirits. Do you hear a difference in pitch? I think I do. But that could be expectation bias at play!
(EDIT) I converted the above video into .WAV and analysed it in Audacity's spectrogram.
All 3 are exactly the same.
This is a spectrogram of the first experiment (with coffee). I can clearly see the rising frequency peak, but I can not see n1.
Last edited:
OP
Keith_W
Major Contributor
- Thread Starter
- #25
I suspect that you are all bored of my experiments by now 
I was a bit surprised by the lack of result with the above experiment, so I repeated it. My theory was that the glass I used was too thick and did not resonate enough. So I repeated the experiment with champagne glasses.
This time, there is a control - 3 empty champagne glasses to show that there is no difference in pitch between them due to manufacturing. Note that the way I hit the glass produces a different tone, if I strike and hold, it dampens the resonance immediately. If I strike it at various heights on the glass, the pitch sounds different. So I took care to strike the rim of the glass with the same force and remove the spoon immediately.
All 3 glasses were filled with exactly 100mL of liquid using the syringe seen in the video. The soy milk was frothy when it was poured. I let it settle for about 5 minutes before taking this video.
This is the result:
And on Audacity's spectrogram:
Soy Milk: the resonance at about 1400-1500 Hz seen in the other two is absent. This contributes to the lower pitch. Also, there is much less ringing. This implies that a more dense liquid dampens resonance more. Perhaps this might be evidence of non-Newtonian behaviour @MCH?
Water: noticeably higher pitch than Methanol, with the peak occurring at 1500Hz vs. 1400Hz. Also, much less ringing than Methanol (look at the length of the tail at 3kHz). Given that c = f * lambda, this implies that the speed of sound is higher in water than it is in Methanol. Which is what we expect, since water is more dense.
So we are back to the square one ... is there a good explanation for the resonance damping effect of soy milk?
I was a bit surprised by the lack of result with the above experiment, so I repeated it. My theory was that the glass I used was too thick and did not resonate enough. So I repeated the experiment with champagne glasses.
This time, there is a control - 3 empty champagne glasses to show that there is no difference in pitch between them due to manufacturing. Note that the way I hit the glass produces a different tone, if I strike and hold, it dampens the resonance immediately. If I strike it at various heights on the glass, the pitch sounds different. So I took care to strike the rim of the glass with the same force and remove the spoon immediately.
All 3 glasses were filled with exactly 100mL of liquid using the syringe seen in the video. The soy milk was frothy when it was poured. I let it settle for about 5 minutes before taking this video.
This is the result:
And on Audacity's spectrogram:
Soy Milk: the resonance at about 1400-1500 Hz seen in the other two is absent. This contributes to the lower pitch. Also, there is much less ringing. This implies that a more dense liquid dampens resonance more. Perhaps this might be evidence of non-Newtonian behaviour @MCH?
Water: noticeably higher pitch than Methanol, with the peak occurring at 1500Hz vs. 1400Hz. Also, much less ringing than Methanol (look at the length of the tail at 3kHz). Given that c = f * lambda, this implies that the speed of sound is higher in water than it is in Methanol. Which is what we expect, since water is more dense.
So we are back to the square one ... is there a good explanation for the resonance damping effect of soy milk?
Could be because of the higher viscosity. You can try cooking oil to see if it behaves similarly to soy milk.
For your reading enjoyment, I found this paper plus a demo/presentation by another guy based on the research of the paper.
Curvature
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I see a harmonic at 3.3kHz shifting up to over 4kHz.
This is a spectrogram of the first experiment (with coffee). I can clearly see the rising frequency peak, but I can not see n1.
Fun thread
I'm away from my main machine and used foobar for my spectrograms.