User:Cor anglais 16/Organ actions
also need a key action article: balanced, suspended, tubular-pneumatic, electro-pneumatic, electric
A windchest is a component of a pipe organ on which the pipes sit. As the organist plays the instrument, the keys, stops, and windchest work together as a mechanism (called an 'action') to direct pressurized air (called 'wind') into the pipes, thus creating sound. Windchest design has varied considerably over the course of organ building history and across geographical boundaries.
The most common action is the 'slider chest'. In a slider chest, each rank of pipes is arranged on the top of the chest, which contains wind. The pipes for each individual note sit together on their own 'key channel', a pathway carved out of the windchest that admits wind to all the pipes for that note. The wind is admitted into the key channel via the 'pallet', a small strip of wood and leather that is connected to the keys either via trackers (in instruments with mechanical action) or via electric circuits (in instruments with electric action). The pallet is held in place under the pipes by a spring. When a key is pressed, the corresponding pallet opens, admitting wind into the key channel. Between the pipes and the key channel are the 'sliders'. A slider is a strip of material, such as wood, with one hole for each pipe drilled through it. The slider can be moved via the stop action. When the organist activates the stop, the slider moves so that the holes are directly under the pipes, thus admitting wind to that rank.
Therefore, in order for a pipe to sound in a slider chest, two events must occur:
- A stop must be activated so that the corresponding slider is in position to admit wind to that rank.
- A key must be pressed, opening the corresponding pallet, so that wind is admitted to the key channel.
The slider chest is favored by builders of mechanical action organs for its mechanical simplicity and the resulting lightness of the key action. A well-built slider chest results in a very responsive touch at the keys. It also aids in blending and intonation between pipes of the same note, as all the pipes controlled by a single key are fed from the same key channel. Slider chests are used for everything from very small portative and positive organs to large organs of over 100 stops such as the Cavaillé-Coll organ in Saint-Sulpice.
The 'spring chest' was developed in the early sixteenth century and was used in some Renaissance organs, particularly in Italy and Spain. It is identical to the slider chest in the key action; i.e., the keys operate pallets that admit wind into key channels. Instead of sliders, each stop knob operates a stop lever bar which runs alongside each rank of pipes. The bars sit on top of a second set of pallets and springs, one pallet per pipe per rank, called groove valves. When a stop knob is pulled, the bar moves down, opening the groove valves and admitting wind to the pipes of that rank. When the stop knob is pushed in, the groove valve springs push the valves and the bar back up. Because the groove valve springs are constantly pushing the groove valves up, the stop knobs must be latched in place once they are drawn; otherwise they would be pulled back into the off position.
Organs with spring chests were not susceptible to tuning issues caused by warping sliders.
Cone valve chest
The 'cone valve chest' became popular in nineteenth-century Germany when E.F. Walcker refined its design. Its concept is similar to that of the slider chest, except that the mechanisms for admitting wind to each note and activating each stop are reversed. In a cone valve chest, the windchest is divided into 'stop channels' instead of key channels. Each stop channel admits wind to all the pipes in a rank when the corresponding stop is activated. Each key is connected to a set of cone valves, one for each rank. The cone valves admit or deny wind to all the pipes of each rank for a particular note. All the cone valves move when the corresponding key is pressed.
Therefore, in order for a pipe to sound in a cone valve chest, two events must occur:
- A stop must be activated so that the the stop channel is filled with wind.
- A key must be pressed, opening the cone valves under all the pipes for that note, so that wind is admitted to any pipe for which the corresponding stop channel is winded.
The cone valve chest works well in organs installed in harsh climates, as sliders can warp and malfunction when exposed to extreme changes in temperature and humidity. It aids in the blending of pipes in the same rank, as all the pipes of each rank are fed from the same stop channel. However, different ranks do not blend as immediately with each other because they sit on different stop channels. Furthermore, because each key controls one cone valve for every rank in the division, organs with cone valve chests tend to have very heavy key actions. In a large organ with perhaps fifteen ranks in a division, this means that the organist may be moving upwards of sixty cone valves at any time in a four-part texture. Many organs with cone valve chests make use of pneumatic assist devices such as the Barker lever to reduce the perceived weight of the keys.
The 'pitman chest' is the most common electrically-operated windchest action built in America. Ernest Skinner first used it in organ design. It uses electromagnets to control the flow of wind through various compartments of the windchest.
Unlike the slider chest, the pitman chest has both key and stop channels. Furthermore, in a pitman chest the entire chest, including the key and stop channels, is a closed system filled with wind before a key is pressed. Pressing a key activates an electromagnet that raises a small disc (the 'armature') that allows only the wind in the key channel to escape. The other end of the key channel leads to the foot of the pipe, from which it is separated by a pallet attached to the sides of the key channel by a flexible leather membrane (the 'pouch'). The pallet is held in place under the pipe by the wind pressure in the chest. When the key channel is exhausted, the pallet moves down, allowing the wind in the chest to flow into the pipe.
The stop channels run perpendicular to the key channels, intersecting them between the armature and the pallet. At the intersection between the stop channels and the key channels are the 'pitmans', small pieces of metal. There is one pitman per stop per key. Before a stop is pulled, its stop channel is filled with wind and the position of the pitman blocks the flow of wind through the key channel. When a stop is pulled, the wind exits the stop channel, pulling the pitman out of the key channel and allowing wind to flow through the key channel.
Pitman chests often incorporate smaller chambers within in the key channel that allow for a greater amount of wind to be released from the key channel when the key is pressed than the armature would allow. This in turn allows the pipe to speak more promptly.