As stated, theater pipe organs require huge amounts of air to operate properly. Theater organs generally run on 5 to 10 times greater air pressure than that of church or classical organs. In their original theater installations, a large quantity of high-pressure wind was necessary in order for the organ to be heard in the large and sometimes cavernous auditoriums. In the case of the Organ Stop Wurlitzer, the quantity of air required is truly staggering.
Our blower room contains four Spencer-brand turbines. Only three of the four are utilized at any one time. The fourth blower is on standby to ensure no downtime in case of mechanical failure in any of the other three. They have a combined power of 60hp. and are capable of producing air at the rate of over 14,000 cubic feet per minute!
The turbines are designed to produce two separate pressures of raw or “static” wind. The main static line is 20″ in diameter and feeds the majority of the pipe organ with “lower” pressure wind. A separate 10″ diameter line provides the “high” pressure required by some of the more powerful pipes in the organ. Both of these lines branch throughout the organ, like arteries, feeding the wind trunks (the wooden boxes upon which the regulators sit).
There are other considerations necessary for a successful blower room. The concrete floor of this room is totally separate from the foundation of the main building, minimizing the transmission of rumble generated by this massive machinery. The blower room walls are almost totally independent from those of the main building, further ensuring the isolation of noise.
The temperature of the wind generated is a major consideration, especially in the Arizona desert. Excessive heat plays havoc with the tuning stability of the instrument. The turbines generate significant air friction, which in turn generates heat. It is therefore necessary to pre-cool the air going into the blowers to keep the temperature within the pipe chambers stable. A 5-ton air conditioning unit is in operation whenever the organ is running, even if the outside air temperature is 20F!
One further consideration is the source of the air for the blowers. It is essential that an extremely clean source of air is provided, therefore the organ’s wind system is totally closed, relying entirely on recirculated air. Make-up air is pulled from each of the organ’s chambers and is returned to the blower room through a 36″ diameter underground plenum. Many stages of air filtration ensure grease and dust-free air, so vital to the organ’s sensitive pipework.
The numerous rectangular wooden boxes visible in the sub-chamber are known as “regulators”. The regulator takes the raw, static wind produced by the blowers, and regulates it to a precise pressure for the pipework it feeds. One regulator may serve one set of pipes (known as a rank) or a “division” of several ranks. Generally speaking, the larger regulators serve the larger divisions, and the smaller ones are quite often dedicated to a single rank.
Tremulants, like the regulators, are located in the sub-chamber. Patrons often compare their appearance to that of a fireplace bellows. The tremulant is the single most important device for creating the heavy vibrato sound so uniquely characteristic of the theater organ.
Each tremulant works in conjunction with a regulator. While the regulator is always seeking to stabilize the wind pressure, the tremulant is engaged to quickly release air from the regulator at short, set intervals. This creates a momentary drop in pressure. This up-and-down oscillation of air pressure causes the pitch of the pipework to change, creating a pleasant vibrato, much the way a talented vocalist controls their voice.
Proper adjustment of the tremulants is perhaps the most important aspect of an excellent-sounding instrument. It is also the most subjective process involved in the tonal finishing of an instrument. There are numerous ways of controlling the depth, shape, and speed of a tremulated system. The end result must be well-suited to the pipes it serves, but also must work in harmony with the other tremulated systems in the organ. Think of it in terms of all the voices of a choir working together to form one cohesive sound.