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Sound on the organ is produced when the wind passes through the pipe. The process of getting the wind to that pipe follows this path: First, a motor blows air into a reservoir. This picture is the air reservoir from the Holtkamp at the Univ. of KY. Notice the weights that look like bricks on the top. These weights keep the air under pressure so that the sound does not waver. From the reservoir, the air moves into the wind chest, which is a box with rows of holes on the top. The pipes stand on top of the wind chest, one pipe to a hole. To make the pipe speak, the wind must move from the wind chest to the pipe. If the holes connecting the pipes to the wind chest remained open all the time, all the pipes would speak at once whenever the organ was turned on. There are two mechanisms that control two separate barriers to the flow of air from the wind chest to the pipe: stop action and key action.
The holes in the top of the wind chest have a movable barrier that is controlled by the drawknobs or tabs on the console. When a stop is turned off, the barrier blocks the holes. By pulling the stop out, or the "on" position, the barrier between the air supply and the holes moves so that the wind can get to the pipes. The organ has a separate stop action for each stop in the organ. The organist could remove the barrier to several sounds by turning their corresponding stops on. This would allow several pipes to speak at once when only one note is played.
There are different kinds of wind chests and the stop action for each kind works differently. To give a better illustration of stop action, here is Lynn Dobson's explanation of a slider chest from an interview.
In a mechanical action organ, the wind chest where the pipes stand is called a slider chest. And a slider chest means that it uses a long strip of wood called the slider. It has a hole for every note on the keyboard, or in other words, a hole for every pipe in the rank. If you pull the draw knob out, you actually physically move the slider from one side to the other. A series of holes line up so that when you play a note on the keyboard, air can pass from the wind chest below to the pipe above. If you push the stop off, then the slider moves a few inches over to the side and the air can't pass from the air chamber up to the pipe. So in this manner, if you pull on one draw knob, then you turned on one rank of pipes so you have one pipe playing from each note. If you drew two stops, there would be two pipes from each note and of course if, on this organ, you drew 30 stops, you could actually have 30 pipes playing from one note on the keyboard.
Of course, the organist needs control over each note individually. The stop action removes the barrier to all the pipes of one stop. These pipes cannot be allowed to speak all at once or there would be no music. The key action works with the stop action. After a stop is turned on, and the barrier to the holes is removed, the key action can control the wind flow to the pipe. When the key is depressed, it opens a valve beneath the pipe so that the wind can finally get through the hole in the wind chest. However, a stop must be on, removing one barrier to the pipe. If no stops are on, there will be no sound, even though the valve is opening when the key is pressed. Different styles of building over the last 400 years have resulted in two kinds of key action: Mechanical and Electropneumatic.
While the wind supply, stop action, and key action have many parts, most of the space in the organ is occupied by the pipes. To learn more, visit the organ pipes page.
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