The Wurlitzer 4600 Series Electrostatic Organ
The Wurlitzer electrostatic organs of the 1950s are some of the most unusual organ-like electronic musical instruments in existence. Please read the Hammond article first, however, as much of the basic info that will make this article more understandable is included there.
They are sometimes referred to as Wurlitzer reed organs, however this term is not really accurate in the usual sense. A reed organ is an instrument which uses air-blown reeds for its sound production. An accordion is a reed organ in a portable form, for example. The musical tones that you hear from a reed organ or an accordion are the actual acoustical tones of the reeds. In the Wurlitzer electrostatic organ, however, despite its having reeds, you do not hear the sound of reeds, because the reeds are not used as sources of acoustic sound, but instead are used as electrostatic generators of audio alternating current waveforms. These AC audio frequencies are then further processed and amplified, ultimately being converted to sound via conventional loudspeakers, just as the audio waveforms generated in Hammonds and other similar instruments are converted to sound via speaker systems.
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Figure 1, left. A wurlitzer model 4600 elec-trostatic console organ. From all external ap-pearances, it looks like a typical electronic organ designed for general playing. Stops across the top are grouped according to function. Extreme left, 5 pedal stops. Next Swell or upper manual stops, next three tabs con-trolling vibrato, Great or lower manual stops, then the power on-off switch and in the lower right cheek block, three stops controlling the percussion feature.
On the player panel to the right of the picture you can listen to several sound clips of music played on a Wurlitzer 4600 series instrument.
In reed organs
and accordions, the reeds remain at rest and only vibrate when actually called
upon via the keyboards and associated valves to produce particular tones or
notes as required by the music. In the Wurlitzer electrostatic organ, all
of the reeds in the instrument vibrate all the time as long as the instrument
is turned on. In the typical 4600 series instrument, there are 85 reeds, covering
seven full octaves plus an additional top C. The frequency range of the reeds
is from low CCC of 32.7 Hz. to 4186.0 Hz.
The reeds in the electrostatic Wurlitzer are
free reeds made of brass. As such they are similar to those found in accordions
and reed organs, and indeed they sound the same. They are called free reeds
because they are fastened to suitable frames called shallots and vibrate over
very close tolerance openings in the shallots. The openings in the shallots
are slightly larger than the reeds, and therefore the reeds vibrate into these
openings during part of each cycle of vibration, but, except for the ends
where they are fastened to the shallots, they do not touch, or are free of,
the shallots for their entire vibrating length.
Tolerances on these reeds are very close, especially
in the case of the smallest reeds which produce the highest generated tones
of the instrument. Here, the clearance between the reed and the opening in
the shallot is not more than 0.001 inch. Manufacturing, adjusting and tuning
these small reeds requires the precision of watchmaking. The larger reeds
which are responsible for the lowest frequencies are still close tolerance
devices, the clearance between the reed and the opening is about 0.01 inch.
Needless to say, eighty-five brass reeds all
vibrating simultaneously produce a lot of very unpleasant noise. The tones
of free reeds are in general not the nicest of musical sounds by any means.
You need only listen to an accordion to confirm this widely-shared opinion.
There really is a reason for the multitude of accordion jokes around, such
as this one: What is the definition of a gentleman? A man who knows how
to play the accordion, but refrains from doing so.
If you have eighty-five accordion reeds all
sounding continuously, it is necessary to employ some rather elaborate soundproofing,
and indeed in the typical Wurlitzer electrostatic organ, there are no fewer
than five different layers of material between the reeds and the outside world.
The reeds are arranged in octave groupings in a series of seven steel pans
on the reed unit. Each pan is lined with 1/8" thick rubber. Already this
accounts for two layers of material.
Surrounding the reed unit is a special three-layer
enclosure which consists of a steel-frame rectangular box with, from inside
to outside, first a layer of Masonite, then a layer of 1 inch thick felt,
and finally a layer of 5/8" Homasote. Thus, five layers of material;
rubber, steel plate, Masonite, felt, and Homasote all combine to insulate
us from the sound of the reeds, and such elaborate soundproofing is necessary
to make these instruments playable, for the continuous racket of the reeds
would otherwise render these electrostatic organs useless.
In order to keep all of these reeds vibrating,
it is necessary to develop an air pressure differential across them, which
function is accomplished by a centrifugal compressor which is an integral
part of the reed system. All of the reeds are connected to a central low-pressure
region which is routed to the compressor intake. The compressor discharge
feeds channels which go to the interiors of the steel pans that surround the
seven groups of reeds. The pans are held in place by suitable bolts and seat
on gaskets on the main body of the reed unit, thus making the system hermetically
sealed when all pans are properly bolted in place. Cleanliness is absolutely
essential to the proper functioning of free reeds, and especially when they
are used in an electrostatic generating system. Therefore, when all pans are
in place, the interior of the reed unit is isolated from the outside air.
The air within the reed unit is continually recirculated through the system
by the compressor, a 3600 RPM centrifugal blower unit with a squirrel-cage
type impeller which is powered by a two-pole induction motor.
In the earlier ES instruments, the compressor
impeller was behind the reed unit and necessitated a right-angle gear
unit as part of its drive. In later versions, this was eliminated by redesigning
the reed unit so that the centrifugal compressor was on the top of the
unit and in a direct line with the motor. To hear a sample of this instrument,
click on any of the play buttons on the player panel above to the right
of the picture
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