Operating principles

A theremin is unique among musical instruments in that it is performed without being touched by the operator. The musician 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. Additionally, some theremins use a volume dial and have only one antenna.

A 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 other is a variable frequency oscillator, the frequency of which is controlled by the performer's distance from the frequency 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 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.