Two responses: In the words of Dick Nash (the owner of the famed
'de Hagenaar' 90-key Carl Frei Sr. organ) when asked, "Do you have
a way to turn that down," the answer is always, "Sure, stand back,
the further away you go, the softer it gets!"

On a more serious note, I was asked to build a 30-key organ by Paul
De Vries who lives in an apartment in Manhattan, New York.  Volume
was a definite concern, so several solutions were employed.  The first
was swell shutters, which obviously is a very effective way to control
volume as well as expression.  The other solution which I found worked
quite well is a sheet of polycarbonate (the stuff they make bullet
proof glass out of).  This material does not yellow, scratch or craze
the way Plexiglas does.

The polycarbonate panel slides into the back of the organ case when
not in use (e.g., outdoors) and slides inside the front of the organ
case when he is home inside the apartment.  The pipes are clearly
visible through the openings, but the windows are closed.

I was surprised to find out how well this solution works.  Indoors,
the sound is like a muted trumpet, very pleasant and soft.  Outdoors,
with the panel removed and the swell shutters open, the sound is full
and vibrant.  I hope that this is some value to enthusiasts who play
outdoors and inside as well.

Bob Yorburg