Metal plate resonance experiments using only household items

I had seen a few experiments showing resonance on a metal plate, there are lots of examples on YouTube. It's also called a "Chladni plate".

I had the idea of trying to reproduce such an experiment using really basic means.

So instead of fancy equipment, I'm using a tablet computer, some computer speakers, a cookie sheet, and some salt.


The first thing to figure out is what frequencies the cookie sheet will resonate at. I installed the "Spectrum Analyze" app on my Android tablet. I tapped the cookie sheet and watched the spectrum display to see which frequencies the cookie sheet liked to resonate at.


Zooming in, and moving the graph around, I could see a particularly strong one was at 178 Hz.

Next I used an app called "Frequency generator" on the tablet to make that frequency of sound.


I supported the cookie sheet above a computer speaker. The speaker is facing up and I turned it up loud while the 178 Hz tone was playing. Spreading some salt on the cookie sheet revealed the modes of vibration.

The cookie sheet has a standing wave pattern on it, where some parts deflect upward while other parts deflect down. The areas in between move very little.

The salt in areas of strong vibration tends to bounce away from those areas and settle where vibrations are weaker. So the lines formed by the salt show the "nodes" of the vibration — areas between the stronger vibrations.


Trying a lower frequency. The area of the cookie sheet is divided into three, left top corner, middle and right 40%. Basically, the left top corner and the right side move up at the same time that the remaining area (middle to bottom left corner) moves down, and vice versa, and the salt settles in between.


An even lower frequency, this time the cookie sheet is just two zones.


I tried higher frequencies, but the computer speakers were not powerful enough to get the salt to move much. So I switched to using my stereo amplifier and a small stereo speaker.


This worked better. Higher frequencies produce more intricate patterns.


Another pattern.


And another. But this is as high a frequency as I was able to go.


I had turned up the power more at about 700 Hz. I couldn't see any deflection in the speaker, but at some point the sound changed and something smelled burnt.

When I turned the sound off, and pushed the middle of the speaker, I could hear the speaker's voice coil scratching against the magnet. The speaker was shot.

So I tore it open to see the damage. The voice coil was blackened (it's normally red magnet wire). The heat had caused it to distort, and then it rubbed against the magnet.


So I didn't want to go to higher frequencies, but I figured I could get more patterns by using a larger sheet of metal. This one is the top part of an old clothes dryer.

I also switched from salt to sand so it would show up better against the white surface.


Another pattern.


And another...


.. and another.

It's really cool how, after dialling in a frequency, the pattern slowly emerges in the sand.

I wasn't able to get results as clean or spectacular as other experiments on YouTube, but the cool thing is, I was able to do these experiments without using any scientific equipment - using just regular household items!



More physics related articles:

The hui game. Also
known as gee-haw stick
Building an air raid siren

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