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A theatre organ is basically a pipe organ designed for entertainment (initially, it was also designed to provide orchestral-style accompaniments to silent films), as opposed to a church organ, which is a pipe organ designed to accompany and enhance worship (and, in most cases, for to permit the performance of at least part of the classical organ recital repertoire). The two types of instrument have much, at least superficially, in common, but in view of their widely disparate functions and environments, they obviously differ in many respects. Most theatre organs were installed in cinemas, where the acoustical environment is very different from the rolling echoes of a large church or cathedral. This also has an effect on the design of instruments.
Both types of instruments have pipes, and it is here that the differences start to appear. Theatre organs are built on what is known as the "unit" system, whereas in traditional classical organs there is one (or more) pipe for each note of every individual stop. The technicalities of the unit system need not concern us at this stage, but it may in essence be regarded as an Organisation and Methods person's approach to organ building. A relatively small number of pipes each do a lot of work, whereas in the traditional method, there are large numbers of pipes, many of which may be silent for nearly all the time. This means that a theatre organ of medium size (8-10 ranks of pipes) will possess some 650-700 pipes, whereas a modest-sized church organ, with two manuals and 24 speaking stops will contain 1500 or more pipes.
Theatre organs draw their stops from ranks of pipes, each of a distinctive tone colour, say, a Tibia or a Tuba rank. Unlike the church organ, where a given set of pipes can be played from only one keyboard (manual or pedal) and only at one pitch, the theatre organ's ranks can be played from whichever keyboard and at whatever pitch deemed desirable by the designer. Thus, the 8-ft. Tuba on the Accompaniment manual and the 4-ft. Tuba (usually called Clarion) on the Solo manual are both drawn from the same rank of pipes.
Each rank has the number of pipes necessary to sound 61 notes at each pitch at which it can be tapped (there are certain exceptions, but these need not concern us). This practice is frowned upon in the classical organ world, as, particularly in contrapuntal music, it can give rise to what are known as "missing" notes, and choruses of tone do not build up in the accepted classical way. In the repertory of the theatre organ, contrapuntal music is not often encountered, though, and chorus effects are handled differently, so these problems do not arise.
This explains why theatre organs are generally described as possessing "x" ranks of pipes, rather than "x" speaking stops, as a dozen or more stops can often be derived across the whole instrument from a single rank of pipes.
A theatre organ does not only contain less pipes than its classical counterpart. Its tonal make-up is very different. Classical organs are based on Diapason (sometimes called "Principal" tone), whereas theatre organs were originally based on string and orchestral tones, but later became more based on Tibia or heavy flute tone (like most entertainment electronic organs of today). All but the smallest theatre organs do possess at least one Diapason rank, but in their make-up it has a secondary rôle.
The pressure of air entering the pipes is greater in theatre organs than in most church organs, typically by a factor of three or four times. There are several reasons for this: greater volume, greater smoothness of tone (remember, the dry acoustics of a plushly furnished cinema will make over-bright voicing sound very shrill) and greater promptness of speech, essential for "rhythm" playing. Both theatre and church organ contain wood and metal, open and stopped, flue and reed pipes (these terms will be explained later). Theatre organs often contain also diaphones, which are a special type of bass pipes.
In most church organs, only some of the pipes are enclosed in "swell-boxes" (in some none of the pipes are enclosed). In the theatre organ, all of the pipes and other assorted sound-producing hardware will be enclosed in "chambers". In all but the smallest instruments, where the whole organ is in one chamber, at least two chambers will be provided (in some of the very large theatre organs in America there can be five or more chambers). Of these two chambers, one will be the "main" chamber, in which one could expect to find the strings, diapason, flute and possibly the odd quiet reed, the other will be the "solo" chamber, housing the tuba, trumpet, tibia, "colour" reeds, etc., plus most of the percussions and effects. Thus the whole organ is under expression control. The purpose of having two or more chambers is so that sounds from each can be "mixed" to achieve the precise balance, blend and volume required.
One of the distinguishing features of a theatre organ is the quantity of miscellaneous hardware in the chambers, known technically as percussions and effects. Percussions fall into two categories, "tonal", on which one can play a tune (e.g. chimes, xylophone) and "non-tonal", on which one can't (e.g. drums, cymbals, castanets). The effects were originally aimed to add immediacy to silent film accompaniments, but now are more often used in novelty items, and include virtually any of the sounds of man and nature. They can range from a Baby Cry to a Crockery Smash, with assorted birds, train and other whistles, fire alarms, klaxons, sirens, etc. thrown in for good measure.
The chambers will also generally house the relay switches (in some modern reconstructions replaced by micro-chips) which act as a "telephone exchange" between the pipes and the console, where the organist sits. In larger organs, there may be an upright piano, connected electrically to the organ, or even a grand piano on the stage.
Near to, but separate from, the chambers, will be the kinetic fan blower to generate the air pressure needed to sound the pipes and operate parts of the organ mechanism. The blower for an eight-rank organ would have a motor of between 5 and 7˝ horse-power. This motor would often drive a generator to provide steady low-voltage current or around 12-18 Volts d.c. to power the organ's electric mechanisms. In more recent times, generators have in many cases been replaced by transformer-rectifiers, but when these instruments were originally built, mains electricity was not as constant as it is now, and fluctuations in voltage could have caused unreliability in performance and even damage to the finely-adjusted mechanisms.
By now, you will begin to realise why it has been said that a theatre organ resembles an iceberg, in that the part you don't see is about ten times larger tha the part you do see. All the components we have discussed so far are located out of sight of the audience.
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