Stroboscopic melodica reed imaging
What is stroboscopic imaging?
A “strobe light” is a light source that delivers a very bright and very brief pulse of light. MIT’s ‘Doc’ Edgerton pioneered the technique of using a strobe light to ‘freeze’ fast-moving objects - like bullets- in mid-flight.
Flash a strobe light on an object that is rotating, like a wheel or fan blade, at precisely one flash per rotation, and you can make that object appear to be stationary. This also works on an object that is cycling back and forth, like a guitar string: flash the strobe once per cycle and you can make the cycling object appear to be frozen in place.
I was very curious to see what could be learned by applying the stroboscopic effect to melodica reeds, especially since I’m not sure it’s ever been done before. This might make it possible to answer questions I’m interested in answering, such as:
Can more than one vibration mode be seen (i.e. higher harmonics)
What are the upper and lower bounds of reed motion and what determines this?
How much does the reed deflect from blowing pressure alone, vs. bernoulli effects?
Can this be a useful teaching tool?
Can this be used to confirm the accuracy of my tuning software?
Will there be any surprises?
Test apparatus
I started with one of the reed plates from a retired Hohner 32. I took a small clear plastic box and drilled a hole for a blow tube port. Then I carefully hot-glued it over the end of the reed plates, enclosing 3 reeds. I taped over all but the one closest to the end, which happened to be a G#4. Then I insulated the bottom of the reed plate with some kapton tape and ran some nichrome wire along it. I wrapped some of that wire around the plastic cover. This is necessary to prevent condensation from clouding the walls of the viewing port during the imaging.
Then I brought in my trusty video microscope, a microphone connected to a digital audio interface, a pair of high brightness LED’s, an LED driver connected to an output from my computer, and a voltage supply to run the heater wire.
I used an oscilloscope to inspect the signal going to the LED’s. The pulse width (time the LED’s are on) couldn’t be more than a few hundred microseconds or the image would be too blurry.
I modified my melodica-tuning software to drive the strobes based on the frequency of the detected tones, and programmed it to drive the strobe at a frequency about 1 cycle-per-second faster than the measured frequency. This would have the effect of making the reed appear to move up and down about once per second (when in reality it was vibrating hundreds of times per second).
Results
I was able to obtain some very nice strobe images!!! Here’s the video I produced. Here is one result obtained from screen shots from the videos:
Observations
1) The most interesting result was not one that I expected: I discovered I could change the frequency of the note by changing the size of my oral cavity (by moving my tongue back and forwards in my mouth)!!! Gianluca Barbosa had told me this would happen and I didn’t believe him! That’s a topic for another day!
2) I’m not sure if this was an artifact of the strobe or not, but I thought I saw some evidence of a higher vibration mode (because some parts of the reed where blurry and others were not!). So I may investigate this further.
3) This setup is only partially representative of how reeds behave in a melodica. The reed I tested is totally open to the air underneath, unlike in a melodica where there would be a chamber both above and below. This has a profound effect on pitch and amplitude. It’s a pretty complicated thing to understand… maybe I’ll look into that later, as well!!!
Any ideas for further studies?
COMMENTS WELCOME!!!
