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The Wind Supply

All organ pipes require air to make them sound, or "speak". Once air has been drawn from the atmosphere and compressed it is known as "wind". Air is compressed by means of an electric fan, known as a "blower". The blower is large and often noisy, and for this reason is usually located some distance away from the rest of the organ. It is normally driven by a three-phase electric motor rotating at some 1800 r.p.m. Small organs sometimes have a single-phase motor of five horse-power or less. Large organs may have blowers of up to twenty-five or even fifty horse-power. Attached to the blower motor, and driven by it through a shaft or by belts is a dynamo, the "generator" which provides low-voltage electric current (12-18 volts d.c.) to operate the organ's internal electrical mechanisms. These days, many generators have been replaced by transformer-rectifier units. There is usually a bank of resistances or some similar device so that when the blower is switched on it builds up speed gradually, and not overload the power supply circuit. It can thus take up to thirty seconds for the blower to achieve full speed and for the organ to be ready for playing.

The blower may draw air from outside the building, from the auditorium, or from the organ chambers. Ideally, the air drawn in should be come from the chambers, as it will thus be at the same temperature as the chambers. A difference in temperature will cause the organ to go rapidly out of tune. However, if air is drawn from the chambers, it is important not to play for too long with the shutters fully closed, as the cycling air will start to increase in temperature as a result of repeatedly passing through the blower. In some cases, a filter is placed over the blower's air intake to remove any dust or other solid impurities from the air, as these can cause problems once they enter the organ's mechanism. A speck of dust can easily cause a note to sound when it should not by preventing an armature from creating a proper seal; the resulting drone is called a "cipher".

Wind leaves the blower through a large wind-trunk, usually not less than about a foot in diameter, and is conveyed to the organ chambers, where it enters large wooden boxes with folding leather sides, the tops of which are anchored to the floor by powerful coil springs. These are known as "regulators", and their purpose is to store volumes of wind at constant specified pressures. Wind pressures are measured in inches of water, 10 in. pressure meaning that the wind is capable of supporting a column of water ten inches high. The regulators contain valves to admit wind immediately to replace wind as it is used. They also act as stabilisers, so that wind can be drawn out at a constant pressure as the organ is played. Their function in this regard is similar to a large capacitor in an electrical circuit. Organs contain several regulators, which often each store wind at different pressures. Wind leaves the regulators through smaller trunks, usually between three and six inches in diameter, through which it is conveyed to the chest, and thence into the pipes.

An outlet trunk from the end of the chest takes wind to a tremulant, which, when activated, allows puffs of wind to escape at regular intervals. This causes the wind pressure in the chest to fluctuate in a controlled fashion, giving a vibrato effect to the pipes' sound. Both the speed and intensity of the "beat" of the tremulant can be controlled by slides to suit organists' preferences. Alterations to their settings can drastically change the overall sound of an organ from a vibrant, breathing sound into a distressed bleating unless great care is used. It can take several hours on a large organ to set a series of tremulants so that when the ranks are used in combination the tremulants interact to produce the optimum vibrato. Recordings made in the 1930s and 1940s of Australian theatre organs reveal tremulant settings which fell more or less midway between the faster, shallow, beat popular in England and the slower, deeper, beat favoured in America.

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