Many, many years ago I was teaching gifted and talented seventh graders. One of the units of study was “Sight and Sound.” We did cool physics-related experiments, including setting up a darkroom in the basement of the gym, building pinhole cameras, and developing our own photos (back before the days of MDS sheets and fears of lawsuits over anything chemical-related)…
…turning an old piano into a hand bell-like instrument using popsicle sticks, fishing line and violin bow resin, building a walk-in camera obscura, and programming a Bach chorale into four old Tandy 1000 computers. In short, it explored two of my favorite things – photography and music.
One of the things we created was a simple laser oscilloscope. I had seen this at Discovery Place in Charlotte, and wanted to replicated it in my classroom. In the days before cheap laser pointers, I had bought a helium-neon laser for some of our class projects. Using that laser, an old speaker, and a music synthesizer, we were able to construct something that would work.
Fast forward 25 years or so…
Earlier this summer I was playing with our cats, using a toy laser pointer. The cats go crazy chasing the little red dot. It occurred to me that I could recreate my project from long ago with this simple pointer.
The first thing I needed was a pair of old speakers. I put out a call on various social media to see if anyone had an old pair with which they would part. I actually got lots of responses. Most went something like this, “My husband has a pair of speakers he’s not using anymore. I’d love for you to take them…” However, when I spoke with said husbands, the speakers weren’t as available as the wife would have liked for them to be.
One of my online friends suggested Goodwill, so this week I went in search of components. I scored a pair of monstrous Fisher speakers for only $10. Standing 3 ft tall each, these would do nicely.
At a local hobby shop I found an assortment of small, round mirrors.
The final component was a pair of spring clamps to hold the laser in the proper orientation. I found these for $1 each. The spring clamps would not only hold the laser in place, but also hold down the little button on top. They also had a hole where I could through a tripod screw and attach with a wingnut. I already had a couple of small Gorillapods that would work nicely. Looks like I was in business.
I glued a couple of the mirrors directly to the speaker. The idea is that a laser shining into one of these speakers will pick up the oscillations of the speaker, and trace those on the wall, or where ever the laser is shining.
Using the second clamp and another mirror, I set up the system so that the laser would be reflected onto a white(ish) door. I gathered together a stereo amp to drive the speakers, and some gear to start generating tones.
Aligning the laser light and the mirrors so that it would reflect on the wall proved trickier than I had thought. This would have been much easier with an optics bench, or at the very least with something more stable than Gorillapods and spring clamps. Every time I unclipped the laser to turn it off and save the battery, I had to re-align everything. I’m sure there is a better way to do this, but for a proof-of-concept it worked OK.
First, I used a tone generator app I had found on my iPhone. This produced a simple sine wave, which resulted in an oval projected on the wall.
First off I noticed that some distortion was being introduced by line noise and the speaker itself. I knew my results wouldn’t be perfect, but I still got some cool patterns. Increasing the volume increased the size of the oval, and changing the frequency altered the shape.
I fired up Garageband on my Macbook and loaded the AUSampler instrument. This was as close to a sine wave generator as I had on the Mac. I found other tone generator apps, but I needed something that would also work with a MIDI keyboard.
On the keyboard I was able to play various intervals and demonstrate how that affected the pattern. Basically, the more complex the pattern, the greater the dissonance of the interval. A rotating pattern indicates that the notes are slightly out of tune.
Here’s an octave…
…a perfect fifth…
…a perfect fourth…
…and a major third…
You can see how complex the patter for the third is. To our ears, a major third sounds like close harmony. However, in early music it was considered a terrible dissonance, and musical intervals were mainly unisons, octaves, fourths and fifths.
These patterns can also be used to demonstrate Well Temperment. The diatonic scale system used in Western music has an inherent flaw, based on the physics of the overtone series. The upshot is that if you tune a piano perfectly to play in the key of C, it will sound out of tune if you try to play it in any other key. To overcome this, the flaw is distributed throughout the scale. Fifths are tuned slightly narrow, and fourths slightly wide. The major third bears the brunt of the error, and can be as much as seven beats per second out of tune on a well-tuned piano.
On the oscilloscope, the pattern for the fifths tends to rotate counter clockwise, indicated that they are somewhat out of tune, and the fourths tend to rotate clockwise. The thirds spin like crazy.
The entire time I was putting this together I was shooting video and doing a narration. I pieced all of this together as a YouTube video. The first version I posted was too long, and had too much “talking head” narration. I trimmed it down a bit, but it was still 12 minutes long. Here’s the video…
When I Googled “laser oscilloscope” I found lots of videos and instructions on creating one of these from an old hard drive. Apparently the actuator arm of the drive moves up and down like a speaker when a tonal source is run through it. However, it only moves in one direction, rather than oscillating. A second mirror that spins or moves back and forth is needed to spread out the wave. I may try building one of these at some point.
The original oscilloscope at Discovery Place used a spinning mirror to show the waves. However, I think I prefer the circular Lissajous patterns. Regardless, it was fun putting this together and reliving some of my early teaching career. Now I, too, have a set of speakers that Laura thinks are available for someone else’s project.