A video of this demonstration is available at this link.

The theremin gets its name from Léon Theremin (born Lev Sergeivich Termen), the Russian physicist and electronics engineer who invented it in 1919. A Wikipedia entry about him is here. What is unique and extremely interesting about this instrument is that the musician plays it without touching it. Hence one of the original names given it, etherphone. (The other was thereminvox.) The instrument shown in the photo above is a Moog Etherwave® theremin, assembled from a kit. The theremin has two antennas – the loop antenna protruding horizontally at left, for volume control, and the vertical antenna on the right, for pitch control. Each of these antennas forms, essentially, half a capacitor, and one of the player’s hands constitutes the other half. By moving your hands in the vicinity of the two antennas, you can control the volume and pitch of the sound produced by the instrument. With the proper hand control, it is possible to play melodies on the theremin, and professional thereminists do so. The one who developed the first method for playing the theremin, and who established it as an instrument on which serious music could be played, was Clara Rockmore. We have an interesting recording of conversations with Clara Rockmore, Nadia Reisenberg (her sister, a concert pianist), Robert Moog (inventor of the Moog synthesizer) and Thomas Ray, led by Robert Sherman (Nadia’s son and Clara’s nephew, and host of “The Listening Room” on WQXR in New York City), with performances by Clara and Nadia interspersed, both on videotape and DVD. The DVD also contains lessons by Lydia Kavina, performances of three of Ms. Kavina’s own compositions, a brief biography of Leon Theremin, and an appearance by Bob Moog. You can view descriptions of the tape and the DVD here (tape) and here (DVD).

The theremin is easy to set up. All it needs is an extension cord for power, and a 1/4-inch phone plug to a cable with the appropriate “Y” adapter and connectors to connect to the audio jacks on the computer lectern in the lecture hall. (It is possible to connect to the 3.5-mm stereo phone cable or jack, as well. When only HDMI is available (as is now the case in Broida 1640), an HDMI adapter is available.) When you first turn the instrument on, if you are standing away from it, you should hear no sound. When you bring your right hand within 18 to 24 inches of the pitch antenna, you should hear a low pitch, which increases as you bring your hand closer to the antenna. The volume should begin to decrease when you bring your left hand to within about a foot of the volume antenna. If this is not the case, you should be able to adjust the PITCH and VOLUME controls until it is. You can adjust the WAVEFORM and BRIGHTNESS controls to obtain the sound quality that you prefer.

How does it work?

The theremin has inside it three radio-frequency oscillators, of which two form the heart of the pitch circuitry and the third drives the volume circuitry. The pitch circuit uses a fixed-frequency oscillator operating at 260 kHz and a variable-frequency oscillator having a range of 257-260 kHz. The frequency of the variable-frequency oscillator depends on the capacitance of the pitch antenna circuit. As you bring your hand near to the pitch antenna, you raise the capacitance in the circuit, which lowers the frequency of the oscillator. Conversely, as you move your hand away from the antenna, the capacitance goes down and the frequency rises. The outputs of the two oscillators go to a mixing circuit, where they are superposed. Since the two signals are close in frequency, they produce beats whose frequency is the difference between their frequencies. Thus, as you lower the frequency of the variable-frequency oscillator, you raise the beat frequency, and as you raise its frequency, you lower the beat frequency. Depending on your hand position and the adjustment of the instrument, the beat frequency can be anywhere from around 65 Hz to about 3 kHz (260 kHz minus 257 kHz). These are audio frequencies, and they correspond to a range that spans from about two octaves below middle C to about three-and-a-half octaves above middle C. The mixing circuit extracts this beat frequency and sends it to a voltage-controlled amplifier. The volume control uses a fixed 450-kHz oscillator connected to an LC circuit, part of which is the volume antenna. When your hand is far from the volume antenna, this LC circuit is in resonance, so the voltage across the inductors in the circuit is a maximum (see demonstration 72.63 -- LRC circuit: phase differences, resonance). A detector circuit connected to one of the inductors converts this high-frequency AC voltage to a DC voltage and feeds it to the control line of a voltage-controlled amplifier, which amplifies the beat signal and sends it to the output jack of the instrument. When you bring your hand close to the volume antenna, you take the oscillator out of resonance, thus lowering the voltage across the inductor and, in turn, lowering the DC voltage controlling the amplifier, thus lowering the amplitude of the output signal. In the original theremins, the output of the volume oscillator provided the current that heated the filament in the vacuum tube that performed the amplification of the audio output. When the oscillator was in resonance, the current was a maximum, and so was the volume. When the player took the oscillator out of resonance by bringing his hand near the volume antenna, the filament current dropped, which reduced the gain of the amplifier tube and thus lowered the volume. The BRIGHTNESS and WAVEFORM potentiometers vary the biases on the input of the voltage-controlled amplifier, so as to distort the output waveform. This changes the harmonic content and the relative strengths of the various harmonics in the output waveform, thus changing the timbre.

You can find a graphical and mathematical explanation of beats on the page for demonstration 44.51 -- beats with tuning forks.

You can also find more information on the theremin at the following links: Wikipedia, Thereminvox, Oddmusic and Theremin World.