A Question for the Physicists

I rarely watch what I’m doing when I pour coffee these days. Same goes for filling a glass of water, or a cup of milk for my daughter. And surprisingly, it doesn’t end in disaster very often. Why not? Because I’m listening. 

As I fill the container with liquid, the sound it makes gets higher and higher. At some point, I get the feeling it’s high enough, and I stop. Most of the time, the liquid is just about 3/4–1” from the top of the container. I’m surprised at how consistent it is, and how seldom I get coffee on my hands or on the floor.

So here’s my question: what am I using to gauge this process? I can’t just be listening for a specific pitch, since a coffee cup and a water bottle resonate at very different frequencies. There must be a metric that holds steady across containers of all shapes, sizes, and materials, and across methods and rates of filling.

Help me out, physicists! What is it? Rate of change? Difference in rate of change? Some ratio between beginning and ending frequency? I must know!

4 thoughts on “A Question for the Physicists

  1. Great thought experiment! I’ve often wondered the same with my metal water bottles with a narrow opening, often cannot see the liquid.

    I’m no physicist, however I believe it’s due to the change in pitch relative to how much liquid the is in the container.

    The negative space remaining in the container is your key factor. A empty container will reverberate with a lower frequency while a mostly full container a higher frequency.

    A good analogy here may be a wind instrument?

  2. Do you feel like you have other senses heightened since you have less of your brain devoted to visual processing? The brain & body do amazing work of substitution for areas that are not working at their best.

    1. Not heightened on the physical, sensory side, but my brain is learning to get all the information it can from the remaining senses. I’m learning to use hearing more, especially. The annoying downside is that I find it hard to NOT pay attention to noise when I want to, like when I’m trying to fall asleep.

  3. An answer from Kristin’s high school physics teacher on Facebook:
    “My thought is the glass is a “closed-end tube” which has a resonance frequency that rises as the air-filled portion of the tube becomes shorter. That’s why piccolos are shorter than flutes which are shorter than bassoons. The splashing of the water has many frequencies, and the tube resonates to and magnifies the one that corresponds to its length at that moment. It’s the length of the tube. I suspect the other variables (diameter, material of glass, rate of fill, temp, etc.) are second-order effects.”
    So it looks like it IS absolute pitch we’re listening for, since the pitch is almost entirely a function of the height of the negative space left in the cylinder. No fancy ratios or functions of rate of change necessary. Good to know!

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