Video · Teachers Media
Sound through a medium: wobbling bubbles starter
Bubbles gently wobbling on a speaker cone are one of the most beautiful demonstrations in physics — a way of making standing waves and resonance visible. It looks like magic; it's the same physics as the wine glass, made visible by soap film.
About this video
A speaker cone driven by a signal generator vibrates at whatever frequency you send it. Place a shallow dish of soapy water on the speaker, or a soap film across a frame held above it, and the vibration transfers into the film. At most frequencies not much happens. At specific resonant frequencies, the film breaks into visible patterns — nodes and antinodes of a standing wave, geometrically arranged.
What makes this demonstration valuable is that pupils are used to sound being invisible. Suddenly they can see the frequency of a note; see how patterns change as frequency rises; see resonance as a specific frequency at which the film "chooses" to move. Pair the demonstration with an oscilloscope trace of the same signal and pupils have two representations of the same physics — the temporal wave on the screen and the spatial pattern on the film.
The classic historical antecedent is Chladni patterns — sand on a metal plate, bowed with a violin bow, forming beautiful geometric patterns at resonant frequencies. Bubble films do the same job with cheaper materials and a more forgiving setup.
What pupils should notice
- At most frequencies, the film looks the same. Resonance is specific.
- Different frequencies produce different patterns — higher frequency, more nodes.
- The patterns are geometric, not random. Physics is choosing them.
- The film has natural modes that depend on its shape and size — a square film, a circular film, a rectangular one all produce different families of patterns.
Discussion prompts
- What does this demonstration make visible that the wine-glass demonstration doesn't?
- How could you use pupil-generated data (photos of the patterns at different frequencies) as the basis for a written task?
- Where else in the curriculum do standing waves matter (musical instruments, laser cavities, quantum wave functions)?
Try this in your classroom
- If you have a speaker and signal generator, run the demonstration live with a soap film in a small wire frame.
- Show Chladni-plate videos and ask pupils to predict which patterns appear at higher versus lower frequencies.
- Ask pupils to photograph the patterns and label each with the frequency that produced it. The result is an evidence-based classroom display.