A correctly adjusted Duo-Art expression box ostensibly provides
32 different dynamic levels: 16 accompaniment and 16 theme.  This
assumes that the accompaniment and theme expression curves correctly
interlace throughout.  Such accurate interlacing is possible and
I believe intended whether equal or different 'strength' original
regulator springs are fitted.

I am excluding any modifying effects of the sustaining and/or soft
pedals, and the inherent droop (not to be exaggerated) in the system
as note numbers increase for any given dynamic, all of which were taken
into account or used by the roll editors.

It should be noted that by definition a theme can never actually be
the same as an accompaniment level.  A theme snakebite has no effect
if a theme dynamic lower than the accompaniment is signalled: the
accompaniment suction will momentarily pull up the level in the theme
windways in such an instance.  (Some have suggested that snakebites
without dynamics were used to dump accompaniment levels quickly for
sfp sforzando/piano events; more likely they were left in simply to
allow Duo-Art rolls to be played with proper manual expression.)

So ostensibly 32 different levels may be signalled, but consider
the expression box spill valve: if this were only to prevent pump
overload, a simple spring-adjustable valve like that fitted to Ampicos,
Hupfeld pumps and indeed other Aeolian instruments would have been
cheaper, more effective, more reliable and easier to adjust.

It's clear that Votey adopted a spill operated by both the
accompaniment or theme accordions and closing fully at power 10 for
a definite reason, namely that during the intervals when there are no
snakebites the theme dynamic perforations may be used to modify the
accompaniment level.  Actually they do anyway, whether the editors
made conscious use of the facility or not.

Measurements show that by closing the spill, the theme dynamics raise
the accompaniment level by a maximum of nearly 1 level at any power
from A0 to A9.

At A0, 10 supplementary levels up to nearly A1 are possible; at A1,
9 are possible up to nearly A2, and so on, 65 supplementary levels up
to A9 and 6 from A10 to A15.  So a total of 71 different accompaniment
dynamics may be signalled between A0 and A15 when no theme is required.
Note that the greatest number of supplementary levels are available at
the lowest dynamics where they are most needed for subtlety, unlike in
the Ampico.

Now, some of these supplementary accompaniment levels may actually
be the same as interlaced theme levels, but never at the same instant,
as an intentional theme level can never be lower than an accompaniment
level because its accordion closes the spill too.  So add the 16 theme
levels and a total of 87 different dynamic signals are possible.

It's fairly clear that, especially with the later "broad-brush plus
additional details" approach, the editors didn't use this facility
consciously, but in Reginald Reynolds (original style) UK editing,
overhanging theme dynamic perforations regularly contribute to small
differences in the accompaniment level.  And sometimes clumps of
theme dynamics without any snakebites appear on rolls which modify
accompaniment levels quite markedly.  And anyway in all rolls the
theme dynamic stream causes the accompaniment level to rise and fall
continuously.

It's notable that on Duo-Arts which don't have this type of spill (some
in which it has been disabled and some of the newer book-accordion
designs) the rolls sound diffident and flat, and it's clear that the
Duo-Art developers thought its function significant as they attempted
in the later expression box designs several ways to emulate it with
cross bleeds etc.

The Duo-Art really is much cleverer than most people assume!

Patrick Handscombe
Wivenhoe, Essex, UK