The Thermos is a Flute: The Science of the Rising Pitch When Pouring Water

I’m Ken Kuwako, your Science Trainer. Every day is an experiment.

Have you ever listened closely to the “Tock-tock-tock…” sound as you pour boiling water from a kettle into a thermos? That simple, everyday sound actually hides a scientific mystery about “sound” that you learn in middle school science. Today, I’ll be talking about the fascinating science of sound and vibration, inspired by a phenomenon I noticed in my own lab.

The Laws of Physics Hidden in the Changing Sound

I was in the lab, pouring hot water into my thermos to make plain hot water (sayu), when I was intrigued by how the sound of the pouring water was gradually getting higher in pitch, so I recorded a video. Here it is:

At first, I wondered if the sound was caused by the hot water itself vibrating as it filled the bottle. However, if that were the case, as the water level increases, the mass would also increase, which should make it harder to vibrate, causing the pitch to get lower. Think about a large bell versus a small bell; the larger (heavier) one produces a lower pitch. The same logic applies. Yet, the sound was clearly getting higher. This led me to conclude that a different mechanism must be responsible for the rising pitch.

What is “Air Column Resonance”? An Experiment with Test Tubes

This brought to mind two sound phenomena involving test tubes that we cover in middle school science experiments:

The sound produced like a whistle when you blow across the mouth of a test tube filled with water.

The sound produced when you tap the test tube with a glass rod or similar object.

Both produce sound, but the results when the water level (volume) changes are exactly opposite. When the water level is higher, the former phenomenon produces a higher pitch, while the latter produces a lower pitch.

In the latter case (tapping), the sound gets lower because the added water increases the mass, slowing the vibration. The former case (blowing), however, is called Air Column Resonance. This is a mechanism where the column of air inside the test tube vibrates to produce sound. As the water level rises, the air space becomes narrower (shorter). An air column has the property that the shorter it becomes, the higher the frequency of vibration (and thus the higher the pitch), and the longer it becomes, the lower the frequency of vibration (and the lower the pitch).

This is the same principle used in vertical flutes, like recorders, where you change the pitch by covering holes to shorten or lengthen the path of the air.

The Thermos is Just Like a Flute!

Let’s apply this law of Air Column Resonance to our thermos phenomenon.

The hot water dropping into the thermos shakes the water surface, and that vibration causes the air inside the thermos to vibrate. The column of air between the mouth of the thermos and the water surface is acting just like the air column in a test tube, vibrating and producing sound. As you continue to pour water, the water level inside the thermos naturally rises. Consequently, the length of the air column (the column of air from the mouth to the water surface) gradually shortens.

The air column shortens

↓

The frequency increases

↓

The pitch gets higher

And there you have it. The reason the sound gets higher as you pour hot water into a thermos is that the thermos is acting like a flute, playing a tone by changing the length of its air column. People who are visually impaired can intuitively use this same scientific mechanism to judge whether a cup is about to overflow just by the sound. It’s fascinating that a simple act like pouring water is governed by such clear laws of physics!

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