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.