Have you ever wondered why people (or cats) are always waving their hands (or paws) around when playing a Theremin?
The Theremin is one of the earliest electronic music instruments, invented in 1928 by Ле́в Серге́евич Терме́н, westernized as Léon Theremin. Like many acoustic instruments, it offers two dimensions of control—volume and pitch—but the way they are controlled is what catches the eye.
First off, we should draw a distinction between the generation of sound (for electronic instruments, generally called the synthesizer) and the interface for controlling the synthesizer. The Theremin’s synthesizer is canonically a single simple oscillator, usually a sine (or in the case of the above video, a square wave).
A definitive Theremin uses capacitive antennas to vary pitch and volume, and many modern version (especially DIY kits) use optical sensors of some kind, usually a simple photoresistor or photodiode.
Here’s a way to think about capacitive sensing. Imagine an elevator crammed full of people—every one of them is desperate to get away from everyone else. When the elevator arrives at a big, open floor, everyone runs out of the elevator as fast as they can and spreads out across the whole floor. If we can measure the capacity of that floor compared to the elevator, we can do something useful with that information.
A wire has a capacity for holding electrons, just like the elevator. If you cram some electrons in there and then add a whole bunch more wire, you’ve changed the capacitance. Now it turns out your skin is pretty effective at holding electrons—the trick is that you don’t actually have to touch the wire to change its capacitance! Just moving your hand closer and farther away will do it, and that’s how you change pitch and volume on a Theremin. One of the unique affordances of the instrument is its high sensitivity, which allows you to easily add vibrato to a pitch, like moving your finger on the fretboard of a violin.
For some beautiful examples of play and accompaniment and notes on composition, check out this video. (Watch/listen to the bubbles at 12:02!)
Bonus! Recently, lost footage of Theremin himself performing at Stanford in 1991 was discovered on a VHS tape!