Robert Linnstaedt wrote in MMD 980123:

> ....I should expect a Celeste to be found on larger instruments.
> Unless the on-pitch rank ever plays alone, the original scheme may
> have been to tune the two to straddle the pitch so the net effect
> would sound in tune with the rest of the organ.  :-)...

Celeste voices are of French origin.  The great organ builder
Cavaille-Coll started to build these registers, to create an orchestral
effect in his organs.  Gavioli seems to be the first one to build
celeste registers in fairground organs.  Later almost all French,
Belgian and Dutch builders followed.

The German organ builders never made use of celeste voices until Mr.
Carl Frei in Waldkirch started to build fairground organs there after
WW2.  Organs with celeste voices should always be tuned in the equally
tempered scale, or else the tuning will be a mess.

The principle of celeste tuning is the formation of "subtraction tones"
(I don't know if that is correct English); in Dutch "verschiltonen".  A
tone of, for example, 400 Hz will, together with another tone of 404
Hz, produce four waves per second.  That is why it is extremely
difficult to sample celeste voices to a MIDI-instrument: one octave
higher the waves will have doubled, with a disastrous effect!

Also, straddling the pitches of the two celeste ranks will therefore not
give the desired effect.  Just tune one rank a bit higher (or lower)
than the standard pitch, and you will have the desired effect.  How
much to detune is a question of personal taste, of course.  Lovers of
Carl Frei street organs like more detuning, on the border of the
unbearable.   (-:

Hans van Oost, Holland

 [ The English-speaking physicist would say "beat frequencies";
 [ organ-builder Frederic Keller (of the Carl Frei heritage) calls it
 [ "schwebende Stimmen", or "wavering voice".  He suggests to tune the
 [ primary rank with the other ranks, and then tune the Celeste rank
 [ slightly higher, such that the beat frequency is constant.  Thus
 [ the detuned rank is not aligned with the equally tempered scale;
 [ it becomes progressively sharper as the note gets lower.   -- Robbie