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Stunning Discoveries in the Theory of Sound

an amazing resonance experiment

a/k/a the chaldean figures (1787)

These wonderful diagrams are to be found in Entdeckungen über die Theorie des Klanges (Discoveries in the Theory of Sound), a late 18th-century work by German physicist and musician Ernst Chladni (1756–1827), in which he details his experiments — for which he is sometimes labelled the “father of acoustics” — with vibrating plates and nodal patterns. More than a century earlier the English scientist Robert Hooke had run a violin bow along the edge of a flour-covered glass plate and observed strange patterns forming.

These patterns were caused by “nodal lines”, the still areas of an otherwise vibrating plate. Chladni perfected these initial experiments in the theory of sound  by Hooke (using mostly sand this time) and introduced them systematically in his 1787 book, providing a significant contribution to the understanding of acoustic phenomena and how musical instruments functioned. Such patterns are now commonly termed “Chladni figures“.

Max Planck Institute

So, what are we seeing here?

This demonstration in the theory of sound is by a prolific YouTube user who goes by the handle brusspup. I’ve been enjoying his amazing visual illusions for a few years – and I’m not the only one! His videos have wracked up tens of millions of views.

But this one isn’t an illusion. Rather, it’s a clever way to reveal patterns not normally visible to our senses. And it traces back to the 18th – and even the 17th – century and a somewhat obscure scientist.

Ernst Chladni was a German-born Hungarian physicist and musician who did pioneering work in acoustics and also in the study of meteorites.

In fact, in 1794, he was the first to publish the outlandish idea that meteorites were extraterrestrial in origin, a proposal for which he was ridiculed. At the time they were thought to be of volcanic origin. But we all know who got the last laugh on that one. He was vindicated within ten years – within his lifetime – when a dramatic meteor shower left hard-to-dispute meteoritic evidence all over a French town.

But the phenomenon concerning the theory of sound  seen in the video is the one for which Chladni is perhaps best known. It is a technique to reveal the complex patterns of vibration in a rigid surface.

Modern versions of the demonstration tend to use modern equipment such as loudspeakers and signal generators with adjustable frequency. In the video, as the frequency is altered we are able to see how the patterns in the plate assume various intricate shapes. The sand is pushed away from the areas of vibration and gathers in the places where the surface remains motionless (the nodal lines).

The beautiful patterns that emerge are now called Chladni figures, although Chladni was actually building on earlier experiments performed by Robert Hooke, who, in 1680, observed these nodal patterns in vibrating glass plates.

Thus, Chladni, like so many scientists, was “standing on the shoulders of giants.” And, in his case, not just any giant, but the very giant to whom Isaac Newton was writing when he famously explained how he was able to see further than others who had come before him.

Scientific American

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