Mixture (music)
A mixture is an organ stop, usually of principal tone quality, that contains multiple ranks of pipes. It is designed to be drawn with a combination of stops that forms a complete chorus (for example, principals of 8′, 4′, and 2′ pitches). The mixture sounds the upper harmonics of each note of the keyboard. The individual pitches in the mixture are not perceived by the listener; rather, they reinforce the fundamental pitches of the chorus, adding volume and brilliance to the sound. Historically, the mixture descends from the medieval Blockwerk concept, an organ in which there were no stops and all the ranks sounded simultaneously.
Mixture stops are typically labeled with the number of ranks of pipes that they have, ie. how many pipes sound when a single key is pressed. For example, a Mixture II contains two ranks (sounding two notes), a Mixture V contains five ranks, and so forth.
The mixture is sometimes designed so that the number of ranks per note increases as one ascends the compass of the keyboard. A mixture exhibiting this trait is referred to as progressive, and is labeled with two numbers (eg. Plein Jeu III-VII).
Sometimes a mixture is labeled with multiple numbers, and in that case it will be more exact. That is the intervals above root that it will sound. For example: Mixture 15.19.22.26 means when a note is pressed, the intervals of a 15th, a 19th, a 22nd and a 29th will sound (or 2' + 1 1⁄3' + 1' + 2⁄3').
Mixture stops might include only unison and fifth pitch levels, though they commonly include thirds and less commonly include other harmonics such as sevenths and ninths. The pedal division normally contains lower-pitched mixtures that reinforce the 16' fundamental.
The frequency at which the various ranks in a mixture drop back by an interval (or break) as one ascends the compass determines to a great extent how the mixture sounds. If a mixture does not break, it is referred to as a harmonic mixture. Aristide Cavaillé-Coll built many examples of mixture stops labelled "Plein jeux harmonique".
The composition of pitches in a mixture will usually change (or break back) several times across the compass of the keyboard, often on subsequent C's, as in the example below. The "breaks" of a mixture involve the dropping out of a higher pitch at a given point within the keyboard's compass, and the addition of a lower pitch. This feature causes the mixture to sound relatively higher pitches in the lower parts of the keyboard, and relatively lower pitches towards the top end. It provides for definition in the bass and additional fundamental in the treble. Another reason for breaks, particularly in very high mixtures, is that builders find it impractical to make pipes smaller (higher) than the top of a 2′ rank, the C that's about 2/3″ long. When that pitch is reached, it is usual practice to repeat the previous octave of pipes at the lower pitch level. Since mixtures commonly begin at pitches higher than 2′, octave repeats would be required as the pitches ascend.
...
Wikipedia