January 29, 2014
Mike Sterling for Canadian Community News
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Working with a couple of friends, I've been designing a musical instrument that looks like a harp, but isn't. It's based upon the logarithmic spiral and its involute. Here is a picture of a spiral and the involute. Imagine that the straight lines are strings.
A logarithmic spiral and its involute
There are 36 strings (more in the picture above) on the instrument we are designing The instrument will be made out of metal, either steel or aluminum. Because of this it will need pickups on the strings to amplify the vibrations and send them to a microprocessor and MIDI device finally wending their way home to an amplifier and speakers and finally the human ear.
Since it is MIDI, the sound can be of a piano, harp, trumpet or any other instrument we want to mimic. We only have 36 strings, but we can move up and down many more 12 string chromatic 'octaves', if we want, ranging from a low frequency to a high frequency which a young ear can hear.
Many pickup devices are magnetic The vibrating string disturbs a magnetic field and a varying voltage is produced, scaled, amplified and sent to the speakers. You see those on electrical guitars. Normally they use magnetic pickups, but piezo pickups are popular too.
It so happens that Madame Cure's husband and his brother discovered the piezoelectric effect in 1880. It lay around as an oddity for a long time until made use of by others who found applications for it.
Here is what it does:
If you have piezo crystals (and you can make them in your kitchen. See video in column 2), you can do some interesting things. If you apply pressure to them like a musical string can do, you will generate a voltage, a sizeable voltage. If you apply an outside voltage to them, they will distort. So they generate dual results. Distort them and they produce a voltage. Apply a voltage to them and they distort.
Some ignition devices use that voltage. If the thumb presses down on something that distorts the crystals, they respond with an arc and ignite a fuel.
You might want to make some of these crystals in your kitchen, if you have a child or grandchild who is the right age. (about 10 would be fine).
This is the sort of thing that escapes us because it is way, way back in some textbooks and when the year or semester ends, it has not been discussed. You cannot forget something you never knew. My recommendation for youngsters is to start at the end of a text book and work to the front, while the teacher is going the other way. Wave as you go by.. Back to front is much more interesting.
So what is the moral of this story? When you look at things like an electric guitar, do you ever ask yourself how it works? If not, then you are missing the charm of being human and having the ability to learn at any age.
Making Piezoelectric Crystals
Testing the Crystals
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Wednesday, January 29, 2014