Organ stop
From Wikipedia, the free encyclopedia
An organ stop (or just stop) is a component of the pipe organ. The term can mean one of several things:
- The control on an organ console that selects a particular sound
- The mechanism that controls the access of the pressurized air (wind) to the pipes
- The set of pipes that produces a particular sound
- The sound itself
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[edit] Mechanics
Organ pipes are organized by pitch and timbre into sets. A set of organ pipes is called a rank. In pipe organs, each key on a manual or pedalboard is connected to each pipe in the ranks associated with that particular division. When the key is pressed, wind is allowed to flow up from the windchest into the pipes.
In order to control which pipes will sound, devices called stops were developed. The mechanism used to operate the stops vary widely between various organs, but the principle is the same: the stop control at the console or keydesk allows the organist to select which ranks of pipes will sound when a key is pressed. When the organist desires the rank to sound, he operates the control at the console, allowing wind to flow to the pipes. Likewise, the organist can deny wind to the pipes by operating the same control in the opposite direction.
Organ consoles are connected to the pipes by varying means. Those organs that have a console where the stop knobs are directly connected to the pipes are referred to as having mechanical action. Those that are not directly connected often rely on electronics or pneumatics to control the stop mechanism. Many organs have a system of combination action, allowing the organist to quickly operate several stops at once.
In electronic organs that do not have pipes, the stops may be operated by tabs instead of knobs, which are typically located above the manuals. In theater organs and their electronic equivalents, the stop tabs are traditionally arranged in a crescent moon formation surrounding the manuals. Regardless of the physical configuration of the stop controls, turning on a stop on an electronic organ merely adds the "rank" in question through various means. The means used have changed over the years since the introduction of the electric organ in the first part of the twentieth century.
[edit] Pitch and length
[edit] Unison stops
The pitch produced by a pipe is a function of its length. An organ stop may be tuned to sound (or speak at) the pitch normally associated with the key that is pressed (the written pitch or unison pitch), or it may speak at a fixed interval above or below this pitch. The pitch of a pipe rank is denoted by numbers on the stop knob. A stop tuned to unison pitch is known as an 8′ stop ("eight foot"). This refers to the approximate speaking length of the longest open flue pipe of that stop, sounding C (the C two octaves below middle C). A 4′ stop (so called because its C pipe is approximately four feet long) speaks an octave above an 8′ stop. Therefore, a 2′ stop speaks two octaves above an 8′ and a 1′ is three octaves above an 8′. Consequently, a 16′ stop speaks one octave below an 8′ and a 32′ stop speaks two octaves below an 8′.
[edit] Mutations
Ranks that are not at one of these pitches are called mutation stops (or just mutations). Because they sound at intervals other than an octave above or below the unison sound, they are rarely used on their own. They are rather combined with unison stops to create new tone colors.
The pitch level of a mutation is denoted by the length of the C pipe in fractions of a foot. For example, a stop at 2 2/3′ pitch (called a "Nazard" or a "Twelfth") sounds at an interval of a twelfth (one octave plus a fifth) above unison pitch. (In some historical organs a stop at 2 2/3′ pitch is labeled as 3′; this is purely a representation of historical convention and does not indicate that the stop is any different from one labeled 2 2/3′.) Likewise, a "Tierce", a stop at 1 3/5′ pitch, sounds at an interval of a seventeenth (two octaves plus a third) above unison pitch.
These two mutations are by far the most common ones found on organs. Other common mutations include:
- the "Larigot" at 1 1/3′ sounding at an interval of a nineteenth (two octaves and a fifth) above unison pitch
- the "Quint" at 5 1/3′ sounding at an interval of a fifth above unison pitch (usually found in a manual division as part of the 16′ chorus)
- the "Quint" at 10 2/3′ sounding a fifth above a 16′ stop (usually found in the pedal as part of the 32′ chorus
[edit] Mixtures
Certain stops called mixtures contain multiple ranks of pipes sounding at consecutive octaves and fifths (and in some cases, thirds) above unison pitch. The number of ranks in a mixture is denoted by a Roman numeral in the stop name: a stop labeled "Mixture V" on a 61-note keyboard would contain five pipes for every note, with a total of 305 pipes. For every key pressed, five different pipes sound, all controlled by the same stop. This is only theoretical, however, as mixtures are designed to break every so often along the compass. This means that the highest-pitched rank of pipes in the mixture drops back an octave after a certain point. In this case, the stop knob will usually denote the extremes in the number of ranks (e.g. Mixture III-V).
The multiple ranks in a mixture are intended to reinforce certain partials of the fundamental, like mutations; however, mixtures are generally used to color the foundation sound as opposed to use in "solo" combinations. Mixtures are the survival of the old Blockwerk in which there were no individual stops, but merely the unvarying full ensemble. Thus "stops" were originally just that, a means of stopping off some ranks of the Blockwerk to enable the use of single ranks of pipes.
[edit] Further reading
- Stevens Irwin Dictionary of Pipe Organ Stops
- George Ashdown Audsley Organ Stops and Their Artistic Registration
