I heard "Good Vibrations" today, and was reminded of the Theremin.
Cool. Maybe I'll take lessons like... Lennin! Anyone here ever play one?
The theremin is unique among musical instruments in that it is played without physical contact. The musician (occasionally referred to as a thereminist) stands in front of the instrument and moves his or her hands in the proximity of two metal antennas. The distance from one antenna determines frequency (pitch), and the distance from the other controls amplitude (volume). Most frequently, the right hand controls the pitch and the left controls the volume, although some performers reverse this arrangement. Some low cost theremins use a conventional, knob operated volume control and have only the pitch antenna.
The theremin uses the heterodyne principle to generate an audio signal. The instrument's circuitry includes two radio frequency oscillators. One oscillator operates at a fixed frequency. The frequency of the other oscillator is controlled by the performer's distance from the pitch control antenna. The performer's hand acts as the grounded plate (the performer's body being the connection to ground) of a variable capacitor in an L-C (inductance-capacitance) circuit. The difference between the frequencies of the two oscillators at each moment allows the creation of a difference tone in the audio frequency range, resulting in audio signals that are amplified and sent to a loudspeaker.
To control volume, the performer's other hand acts as the grounded plate of another variable capacitor. In this case, the capacitor detunes another oscillator, which affects the amplifier circuit. The distance between the performer's hand and the volume control antenna determines the capacitor's value, which regulates the theremin's volume.