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!