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Entries in sound (3)


Optical Siren

For the last several years Ranjit Bhatnagar has made a different musical instrument each day in February. His Optical Siren from 2012's batch caught me attention.

Inspired by Ranjit's siren, I decided to make a larger one for my show. It ended up being fairly straight forward.

The thing in the back is a cordless drill. I cut a circle, using aviation snips, out of a piece of perforated aluminum plate that I bought from Online Metals -- Aluminum 3003-H14 Perforated Sheet Round Hole 0.063" Thick (0.125" dia. holes) 0.1875" stagger. The edges were a bit ragged, even after sanding, so I covered the edges in a couple of layers of duct tape to cushion.

I had to drill out the center a little to put in a screw.

The detector is a optical diode from DigiKey. I got mine from Radio Shack but they don't sell them anymore. I connected it in series to 1/8 inch mini plug (headphone plug) and a 9 V battery. I plugged headphone plug into a small Radio Shack amplifier. You can see this construction as it is similar to the set up for the Communicating with Light arrangement from the Exploratorium.  Remember the photodiode is a diode, and so it will work much better one way than the other. You may have to swap it.  

If you have a fluorescent light in the room, you will hear a buzzing right away. The photodiode works as a switch. The more light that hits it, the lower the resistance, letting more current flow from the battery. Fluorescent lights blink on and off hundreds of times a second, so the amplifier is getting hundreds of pulses of electricity a second. These pulses cause the speaker cone to move out and in, making a compression and an expansion in the air. In a chain reaction or domino way, these compressions and expansions move through the air until they get to your ear where it interprets them as sound.

More compressions per secnod are higher pitches and fewer compressions are lower pitches. Bigger compressiosn or expansions are louder sounds.

The photodiode is so sensitive that even the fluctuation in the intensity of an incandescent light bulb will make a tone.

The main idea though is to create the flashes of light using the spinning disk. Start the drill and place the diode near the spinning disk. When the photodiode is near the center, the pitch is lower. When it is near the edge, it is higher. Why?


Communicating with Light

Connect an LED to the output of an audio source in such a way that the LED flashes with the source. Then attach an applified speaker to a solar cell and presto you are communicating via light.



The Exploratorium has a series of books called "Snacks" which explain how to make cool science experiments. Unlike some other books, though, the experiments actually work, and also unlike other some other books their books come with correct explanations of the science behind them. Check out a selection at

You can find the books at most big public libraries. See a list at Amazon here.

Oh, full disclosure, I work/volunteer with the Exploratorium, but I've never worked on a Snack book.


Singing Rod

The singing rod works via stick-slip forced vibration leading to resonance similar to a wine glass. However, the natural frequency of the rod is higher than the wine glass, so you need rosin to make your fingers more sticky rather than more slipprey like in a wine glass. This is rather more detailed than you will probably want.

One note, the first site at the Whelmers suggests that the singing rod is vibrating transversely. I'm pretty sure that's not the case. Put the end in water and you will see the water making concentric circles which looks quite a bit different than what a tuning fork looks like in water.