Jonathan King asked, "Has anyone ever tried to match the frequency of
two steam whistles to get the tremolo affect?"

The "Celeste" effect is beautiful, but to tune for it deliberately,
requires that the tone sources are stable enough in pitch to make it
worthwhile.

Steam calliope whistles are so unstable in pitch, due to temperature
and humidity fluctuations, that if you were to build two ranks of them,
no matter how you tuned them you would get plenty of "Celeste", maybe
much more than you bargained for!

Air-driven calliopes, which is what you see mostly today, would not
have this problem and could profitably be built with double ranks and
carefully tuned a few Hz apart.  Likewise the wooden-piped Caliola
types.

By the way, I was very skeptical of Celeste tuning applied to stopped
flutes, or to any organ pipe less bright than a diapason or string,
until I bought my first crank organ.  I was pleasantly surprised at how
effective this tuning is when applied to Bourdons.

A very slight mistuning is sometimes referred to as the "chorus"
effect, and naturally results from multiple ranks of pipes, whether
deliberate or not.  Lack of chorus was a major reason for the dull,
lifeless tone of early electronic organs.

Some large pipe organs always sound a little bit "Celesty" simply
because there is no way to keep the various divisions perfectly in
tune, when they occupy different elevations in a building.  Up to a
point, this really enhances the sound.

Mike Knudsen