Seeing Sound: Vibrations on a Plate
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- Published: Friday, 30 November 2018 21:46
This week, a tweet by Simone Schnall of Cambridge University went viral on Twitter, sharing a video by British science presenter Steve Mould.
Coincidentally, this week also marks the birthday of Ernst Chladni, for whom this phenomenon is named- Chladni Figures.
Sand is sprinkled on top of a plate. As it vibrates, the sand traces out the pattern of node and antinodes, accumulating along the lines where the plate is at rest, and being driven away from the areas where the plate is moving up and down with the sound wave. In its simplest form, the plate would be clamped at the center and driven by the bow at one edge; as you change the bow position, you can excite different vibrational modes of the plates and form different patterns. But pressure elsewhere on the plate, even something as simple as pressing your thumb against the edge, can form a node and thus change the pattern of standing waves.
Of course, this can be made far more complex, and the formation of these patterns can be used to staudy how different objects vibrate in different conditions. This is still used today to help in the design of musical instruments.
If you are teaching this topic at UMD, consider using our own set of Chladni plates, and invite your students to try it for themselves. Or, for greater complexity (and volume), try the oscillator driven version. By using an audio oscillator to drive the plate from the center, a wide range of modes can be observed by carefully varying the driving frequency. For this version, we can also offer a wider variety of plate shapes, including a model of a violin back.
To learn more about Ernst Chladni, Chladni Plates, and the history of acoustics, visit these pages at the Smithsonian and at Cambridge's Whipple Museum.