This article is about stops of a pipe organ. For mixtures in chemistry and related fields, see mixture.

Overview

Mixtures are multi-rank organ stops most commonly of principal, or diapason, tone quality. The composition of pitches in a mixture changes, or "breaks" once or more than once over the compass of the keyboard.

The term "Mixture" does not include multi-rank solo stops, such as the Sesquialtera or the Cornet.

Mixtures are most often used to form the tonal "crown" of each of the organ's one or more Principal ensembles.

The composite effect of the interaction of the multiple pitches on a single note is used to provide new tone colors. It is the composite effect which is of importance: one should not readily be able to perceive the individual pitches within a mixture.

In a well-designed mixture, the unison and octave pitch line should be reinforced but never obscured.

Variables

Variables affecting a mixture's tone color and usage include:

1. Types of pitches included
   1. Unisons
   2. Quints
   3. Other pitches
2. Lowest pitch at C1 (tessitura)
3. Type and position of pitch breaks
   1. Octave breaks
   2. Fourth or fifth breaks
   3. Other breaks
4. Number of ranks per note
   1. Constant
   2. Variable
   3. Balance between number of unison pitched pipes and other pitches
5. Scaling of the pipes
   1. Diameter scale
   2. Halving ratio
   3. Parallel scaling of ranks
   4. Mixed scaling of ranks
6. Mouth widths
7. Voicing style

Mixture Breaks

The "breaks" of a mixture involve the dropping out of a higher pitch at a given point within the keyboard's compass, and/or the addition of a lower pitch on the succeeding note.

For example, here is the break structure of a typical Great Fourniture IV:

Note     Pitches on that note and above

C1                               1 1/3' - 1' - 2/3' - 1/2'
C13                         2' - 1 1/3' - 1' - 2/3'
C25                2 2/3' - 2' - 1 1/3' - 1'
C37           4' - 2 2/3' - 2' - 1 1/3'
C49      8' - 4' - 2 2/3' - 2'