Use a broken speaker, bits of wire, and tape to prepare a coneless voice coil driver, then use it to generate standing waves on a sheet of metal, making sound visible. Magic!
My knowledgeable friend Robin once said that you don’t need to worry about having too big an audio amplifier, because speakers are usually damaged by under-powered amps working too hard and clipping the signal, creating rough square waves with too much power. I learned that this is true when I melted a speaker’s coil by running a strong 20Hz signal through it, to drive a vortex cannon (MAKE Volume 15, page 114).
On the bright side, I now had a nice speaker magnet to use as the foundation for something else I wanted to try, a Chladni plate!
Early acoustics researchers Robert Hooke and Ernst Chladni (CLOD-knee) found that fine powders sprinkled on a vibrating plate would settle in patterns that showed how the plate was vibrating. They got their glass and metal plates vibrating for their experiments by running a violin bow across the edges. In our updated version, we’ll generate the vibrations using a voice coil driver, which is basically a speaker without the cone.
Steps
Step #1: Tear down the speaker.
Next
- The first step is to tear apart a speaker to get at the good bits. We’ll need the magnet assembly and the former, which is the cylinder that the voice coil wraps around.
- Cut through the speaker cone around its outside edge and around the dust cap in the center. Remove the cone.
Conclusion
Operating Instructions
1. Sprinkle some powder across the plate.
2. Starting at a signal of a few hundred hertz, slowly turn the amplifier power up until the powder starts to vibrate. Adjust the frequency and volume until patterns appear!
To reduce friction, I periodically rub graphite powder over the plate and brush off any excess. Then my other powder slides around on this slick surface like a cat in roller skates.
Clean patterns will appear for only those frequencies that resonate with the plate. On smaller plates only very high frequencies will show a stable pattern of nodes and antinodes; on larger plates, lower frequencies will resonate as well. On a large plate with a high frequency, you’ll see a detailed pattern across the plate.
On a round plate, you’ll mostly get concentric circles, with the number of circles indicating the ratio of the driver frequency to the plate’s natural fundamental frequency. In such cases, the driver is playing a harmonic (or multiple) of the lowest frequency that the plate produces naturally when you strike it. With some frequencies, you’ll see a serpentine pattern on the round plate.
On a square plate, or a plate in the shape of a violin or other complex shape, the resonances are more complex and interesting.
If you want a permanent display of your vibrational patterns, photography is the way to go. Although if you have an extremely effective filter mask and a high tolerance for a horrible mess, I would think that laser printer toner would make a nice pattern, and then, using a heat gun from below, you could fuse it to the metal for a permanent display. I haven’t tried this, however. Also note that laser toner is extremely bad for your lungs, laundry, and household harmony.
Keep Your Powder Dry
Any fine powder will work, but the finer the powder, the more sensitive it will be to vibrations, enabling it to work at lower volumes. If you’re using a coarse powder, you may need to turn up the volume quite a bit before it bounces into place.
If the powder is sticky or overly fine, it may not want to bounce at all, but instead may stick to the plate and ignore even the most abusive volume levels.
Some powders, such as salt or sugar, will absorb moisture from the air and melt (especially here in Texas), making a terrible mess. Gelatin and graphite powder are both very fine, but tend to stick to the plate over time. Fine white sand should work nicely, with minimal mess.
The traditional superfine substance of choice is Lycopodium powder, which is the spore of a particular fern. This powder is used by magicians and pyrotechnicians as a flash powder, and it can be found at chemical supply houses.
This project first appeared in MAKE Volume 16, page 122.
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Have you tried any other methods, somewhere in between these two? I have been dying to try this and now you caught my attention. I would like something as simple as your idea but on a larger size plate.