A mixture is an organ stop where more than one pipe sounds for each note. On our organ there is a 3 rank mixture on the Swell division – so 3 pipes speak when each key is pressed. Each rank sounds at a different (and high!) pitch so that a blend of harmonics can be added to the chorus of pipes when stops are drawn.

Our mixture has pipes pitched at seventeenth (i.e. the bottom pipe sounds E above middle C), nineteenth (G above middle C) and twenty-second (C an octave above middle C). As you move up the keyboard, the pipes get smaller and shriller and the sound can get unpleasant (and eventually, if nothing was done, they would be pitched so high that the human ear cannot hear them).

The solution is to have each rank “break back” at different points in the range so that at the point where the rank “breaks” it is re-set with lower pitched pipes, and the pipes start longer and get smaller as they go up. Another trick, in blending the ranks to get the best “silvery” sound from the mixture, is to change the pitching of the rank.

On our mixture, we hit a real problem when it was being voiced. In the section about an octave above middle C, one of the ranks was much too dominant and produced a very shrill, and to my ears unpleasant sound. Mike and Simon eventually concluded that the best “fix” was to replace the offending pipes with slightly wider pipes tuned to the fifteenth, which had the effect of slightly lowering the pitch of that part of the mixture and giving a gentler and more pleasant sound. They used some of the small pipes from our old organ from a stop that was not useable as a complete stop, and then the pipes were cut down so that they sounded at a much higher pitch that used to be the case. Eventually, with skilful tuning and voicing (making minute alterations to the pipes) the whole of the Swell division now blends perfectly.