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A Tibia Clausa is a large-scale, stopped wood flute pipe, usually with a leathered lip. Tibia Clausas provides the basic foundation tone of the organ with few overtones or harmonics. The Tibia Clausa is arguably the most important rank of pipes in a theatre pipe organ, with some organs having as many as 5. The stop shares similarities with the Bourdon and the Gedackt found in some church pipe organs. In the 18th and 19th centuries, Tibia Clausa was sometimes used as an alternate name for Doppelflüte. Most tibias are made from wood, as by Wurlitzer etc., although examples of metal tibias may be found made by the John Compton Organ Company.
The Tibia Clausa, or Tibia, is generally found at 16′, 8′, 4′ and 2′ pitches as a unified rank. The mutation ranks Tibia Quint 5⅓′, Twelfth 2⅔′ and Tierce 13⁄5′ are also drawn from this unified rank of 97 pipes. In some larger organs, a second Tibia rank may be present, extended to 1′ instead of 16′, allowing a 1⅓′ Nineteenth mutation and a 1′ Piccolo to be drawn from this rank. A few of the largest theatre organs, and some church organs, may have a separate 32′ Tibia Clausa rank of 12 pipes. In smaller organs, a Bourdon or Stopped Diapason may be substituted for a Tibia Clausa at 16′ pitch.
The Tibia may be voiced on wind pressures from 10″ to 25″ WP (water pressure). The Tibia is generally used as a chorus stop, with or without tremulant; it is not normally used as a solo stop due to its relatively dull tone.
Other variants of the Tibia Clausa include: Tibia Bass, Tibia Flute, Tibia Major, Tibia Plena (open tibia) and Tibia Rex.
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