We’ve already looked at standing wave patterns that are created when a reflected wave interferes with an incoming wave. It looks like the wave is fluctuating in place, when really it’s just an optical illusion of two waves interfering with each other. The point is, this effect are created at specific frequencies called harmonics, and now it’s time to learn about vibrational modes using a really cool experiment by Ernst Chladni.

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Ernst Florens Friedrich Chladni (1756-1827) is considered to be the ‘father of acoustics’. He was fascinated by vibrating things like plates and gases, and his experiments resulted in two new musical instruments to be developed.

When Chladni first did these vibrating plate experiments (as shown in the video below), he used glass plates instead of metal. He was also one of the first to figure out how to calculate the speed of sound through a gas. And it will completely blow your mind. Chladni patterns are formed with a metal plate covered in regular table salt is vibrated through different frequencies.

There are different ways of vibrating the plate – the easiest is by banging it, but this gives you only one frequency and usually makes a mess of the salt. You can alternatively bow the edge of the plate (clamped to a table) with a bass fiddle bow and specific points to get various frequencies… but you will need to practice to get this method to work.

These patterns can also be formed by setting the metal plate on a mechanical driver (like a speaker) controlled by a signal generator. (This way you don’t have to practice your bowing!). The patterns you get this way are different from the bowing patterns, since you are vibrating it from the center instead of the edge.

Click here to go to next lesson on Tacoma Narrows.

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