The Mysterious Device That Makes Styrofoam Float in Midair — Visualizing Sound Waves with a Kundt’s Tube
Here is the translated version:
I’m Ken Kuwako, a science trainer. Every day is an experiment.
Think sound is invisible? As it turns out, under the right conditions, sound waves can reveal themselves right before your eyes. On a recent visit to the Tsukuba Expo Center, I came across a device that does exactly that — tiny polystyrene beads floating up inside a tube, tracing the invisible shape of sound. No textbook needed. You just see it, feel it, and suddenly understand: sound is vibration.
What is a “standing wave,” and why does it appear inside a tube?
The device I encountered is called a Kundt’s tube — a closed tube at both ends. When sound is fed into it at just the right frequency, something remarkable happens: a standing wave forms inside.
A standing wave is what you get when a wave bounces back and forth inside a confined space and interferes with itself. Certain spots vibrate intensely — these are called antinodes — while other spots stay perfectly still, called nodes. Think of a jump rope swinging in a big arc and appearing almost frozen in place. That’s the idea.
Now, if you fill the tube with fine polystyrene beads beforehand, something magical happens: at the antinodes, where the air is vibrating wildly, the beads lift up and dance. At the nodes, they sit completely still. The result? The wave’s shape becomes visible. You can watch it in action in this video:
Change the frequency, change the shape
What makes this device even more fascinating is that you can tune the frequency. Increase it, and more waves fit inside the tube — more antinodes appear, and the pattern grows more complex. Lower it, and the pattern simplifies, with fewer, wider ripples.
This is resonance at work. A standing wave only forms cleanly when the length of the tube and the wavelength of the sound line up just right. Wind instruments like flutes and recorders use exactly this principle. By covering or uncovering holes, you’re effectively changing the tube’s length — and therefore which frequency resonates — which is how different notes are produced.
Seeing sound with your own eyes
The sound we hear every day is a longitudinal wave — microscopic fluctuations in air pressure traveling through space. Because these fluctuations are invisible, it’s easy to nod along when someone says “sound is a wave” without it really sinking in.
Kundt’s tube translates the invisible into the visible, using polystyrene beads as a kind of physical “display.” Beads rise at the antinodes, stay still at the nodes, and suddenly the wave is right there in front of you. See it once, and “sound is vibration” stops being something you memorized and becomes something you actually know.
The Tsukuba Expo Center is full of experiences like this — science you don’t just read about but genuinely feel. If you’re ever in the area, it’s well worth a visit.
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Science should be fun, surprising, and accessible to everyone! This site is full of hands-on experiments you can try at home, along with tips to make them work. Feel free to browse around!
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