Before presenting a possible solution to the Fan-Accordion System
Mystery [171206 MMDigest], some relevant background material is needed.
The so-called fan-accordion system is sometimes found in Aeolian
associated Duo-Art pianos made after about 1928, and always found in
Concertola-equipped (single roll and ten-roll Ferris wheel) Duo-Art
pianos.

As stated in the MMDigest 171206 posting, the mystery is "how does
the fan-accordion system perform as well as it does without making use
of Accompaniment accordion information to help control the closure of
the spill valve?"

In stark contrast with Duo-Art pianos made before about 1928, the
fan-accordion system incorporated many significant design and
implementation changes that exhibited more than just incremental
evolutionary modifications.  So many changes (some of them ostensibly
bewildering) were made, that some current day reproducing piano
rebuilders have questioned how the system could possibly have worked
properly in the first place.

To date, no original corporate literature documenting the specifics of
the fan-accordion system have 'turned up'.  This author suspects that
none will ever be found because Aeolian probably did not generate any
to begin with.  Consequently, we have to resort to archeological
forensic detective work to figure out the elusive details, using only
the remaining fan-accordion pianos as evidence.  It is to Aeolian's
credit that they achieved so much in the development, fabrication, and
subsequent sale of the fan-accordion systems in quality pianos (Steinway,
Weber, Steck, and one Concertola-equipped 6'2" Mason & Hamlin).

The flow of air within the pneumatic stack of a reproducing piano can
occur along several paths.  This air flow could be characterized as
intentional and some as unintentional.  One intentional path is followed
when a given note-playing valve re-admits air into a corresponding
striker pneumatic after it has been exhausted by the pump to play the
associated note.  Another unintentional path is by means of physical
openings, hidden cracks, or microscopic porous passageways in the body
of the stack structure itself.

A third path, which may be intentional or not, is followed when air
leaks through the inner (note not playing) and outer (note playing)
valve seat facings incorporated in each note-playing valve.  With this
as background, we focus on an important aspect of the construction of
many of the valves used in the fan-accordion system, viz., the travel
distance of the valve poppet.

A surprising change implemented by Aeolian in the fan-accordion system
was the introduction of note-playing valves whose nominal 'throw'
(poppet travel distance) was just 0.012".  This valve travel distance
was inordinately smaller (by a factor of 2.7) than the previous
conventional standard for note-playing valves of about 0.032".  Given
the broad range of temperature and humidity conditions encountered
around the country, one might justifiably wonder how such a small valve
travel (in a wooden container) could be maintained at that value over
long periods of time.

The choice of .032" for valve travel in standard Duo-Art designs (and
for Ampico, too) was a selection based on practical experience and
engineering compromise.  This Goldilocks-zone optimization problem was
strongly influenced by the fact that the valves (interior and exterior
components) were made mostly of wood.  Wood is a useful material, but
is notorious for expansion and contraction depending on the type of
wood utilized, local relative humidity, and the ambient temperature.

When note-playing valves made of wood exhibit too small a valve
throw, striker pneumatics can be starved for adequate air flow.  As
a consequence, unacceptable note-to-note amplitude variability can show
up across the keyboard.  When the valves exhibit too great a valve
throw, notes may drop out (fail to speak) and/or the air flow demand on
the pump may become excessive.

In order to realize a constant throw of just .012" for all valves, one
could not employ cost-effective wood for the body of a note-playing
valve.  Doing so would generate unacceptable variation in the throw of
each of the valve poppets over time.  Based on measurements from
existing untouched (factory fresh) fan-accordion systems, it appears
that Aeolian wanted a rather 'small' throw for its note-playing valves.

Aeolian Corporation addressed and solved the environmental variability
problem by fabricating their valve bodies almost entirely from metal.
Humidity or temperature-induced dimensional change of metals is
negligible compared to that of wood.  In fact, almost all of the valves
in a typical fan-accordion system are made of metal.

It may not be obvious why one would want to construct note-playing
valves whose throw was only 0.012".  There are several advantages
associated with reduced poppet travel for all the note-playing valves.
One is diminished audible impact noise during valve activation and
deactivation.

Another advantage is reduced influx of air into the pneumatic stack
during play, which produces a corresponding reduction of the load on
the main vacuum pump.  It also allows related vacuum regulators to
re-establish equilibrium conditions more rapidly.  The reduced influx
of air follows because each valve poppet does not have to travel as
far, thereby being able to complete its .012" travel in less time than
for a span of .032".

Incidentally, each of the note-playing valves in a Concertola-type
fan-accordion system is activated by a (potentially more powerful)
zephyr-skin pillow pouch instead of a conventional circular leather
pouch.  The valve poppet body itself consisted of only a miniature
wooden hockey puck covered on one side (outer valve seat) with ordinary
valve leather and on the other side (inner valve seat) with a tom-tom
covering of fine pouch leather.

Because of the reduced air influx, the (transient response) pneumatic
disturbance resulting from the momentary inflow of air into the stack
during the off-on valve transition was substantially reduced in
amplitude and time duration.  This means that the associated vacuum
regulator would be able to re-establish an equilibrium condition faster
than it would be able to do with a note-playing valve possessing off-on
travel of .032".

It appears that Aeolian was also able to reduce the size of the two
vacuum regulators (Theme and Accompaniment) that make up the bulk of
the fan-accordion expression box.  This modified geometry (reduced
moment of inertia) could also help speed up the entire vacuum
regulation process.

Standard Duo-Art expression box vacuum regulators are not known for
their rapid response to pneumatic disturbances produced by in-flowing
pulses of air associated with the playing of notes.  Normally, Duo-Art
piano rolls compensate for this circumstance by actively pre-setting
(in time) the Theme and Accompaniment vacuum regulators, thereby
allowing sufficient time for the needed (later) vacuum levels to become
established.

In addition, the piano roll controls the spill valve in the expression
box, based on the binary-coded information from both the Theme and
Accompaniment accordions which govern the vacuum levels in the
regulators.  The vacuum from the regulator with the lower pressure was
used to determine the position of the spill valve opening.

By reducing the throw of the note-playing valves in the fan-accordion
system, the regulators in its expression box can respond to
disturbances much more quickly.  Normally, the Theme regulator is set
to operate at a vacuum level that is slightly greater than (i.e., lower
pressure) that of the Accompaniment regulator.

It appears that Aeolian learned that they could utilize Theme-only
information from the Duo-Art piano roll to control the closure of the
spill valve and still realize the same (or nearly the same?) performance
that would be produced by a standard expression box system.  In effect,
the Accompaniment coding was redundant for spill valve control,
provided that the piano stack possessed reduced-throw note-playing
valves.

There is archeological support for the above statements.  At least one
piano (1936 Weber 88174 Duo-Art with a drawer) incorporates the
fan-accordion system using a stack that does not have reduced-throw
note-playing valves.  The stack is a modified-to-fit Ampico A/B
pneumatic stack.  The nominal throw of the wooden Ampico B valves is
about 0.032".  Curiously, this interesting composite fan-accordion
system does not exhibit Theme-only control of its spill valve.

As with pre-1928 Duo-Art pianos, this very late model system
incorporates both Theme and Accompaniment control of its spill valve.
That function is achieved entirely pneumatically, in contrast with the
conventional standard mechanical implementation.  A pneumatic sampling
device is used to determine which regulator (Theme or Accompaniment)
has the higher vacuum level (lower pressure), and that signal is used
to control the closure of the spill valve.

There are probably more of these unusual instruments 'out there'.
Because of the late date of manufacture (1936), Aeolian may have
exhausted its supply of pneumatic stacks with reduced-throw capability
in its note-playing valves.  It may also be that such Concertola-type
stacks were available, but not for that size piano.  Readers of this
document are invited to report if their late-date fan-accordion Duo-Art
piano (with conventional pneumatic stack) also utilizes the Theme and
Accompaniment pneumatic configuration for control of the spill valve.

Using the above scenario as a basis, it is likely that most (if not
all) of the fan-accordion systems with 'conventional' pneumatic stacks
(valve throw of .032") had to incorporate 'Theme and Accompaniment'
signals to adequately control the corresponding spill valve for
acceptable playback performance.  The author is not aware of any
fan-accordion systems with Concertola-type stacks that utilized 'Theme
and Accompaniment' signals to exercise control of the associated spill
valve.  The Concertola-type fan-accordion systems appear to be
characterized as 'Theme-only' for spill valve control.

It is important to mention that other significant changes are
characteristic of fan-accordion systems.  For example, the striker
pneumatics are smaller than in a conventional Duo-Art.  Because the
strikers are smaller, only two support decks are required in the
pneumatic stack, rather than the usual three.

In order to realize the same striking force with smaller pneumatics,
the zero-level vacuum must be greater, say about 9" of water instead of
the typical 6".  The two accordion assemblies are smaller (by at least
a factor of 2) than usual, and can be adjusted very easily with the
expression box still in the piano.

The cleverly designed Concertola-type stack is built in such a way that
there is no need for the conventional Duo-Art 'bleed rail' assembly.
This means that internal air leakage (within the stack) from
pneumatic-signal cross-talk between/among adjacent notes cannot occur.

Inadequate restoration of conventional bleed rail assemblies is a
common and seldom-diagnosed failing of standard Duo-Art systems.  The
fan-accordion system is very flexible in terms of being adjusted to
accommodate different operational conditions, e.g., room size and
humidity variations.

The observations and tentative conclusions described above came about
from work performed on the restoration of a Steinway Duo-Art with a
Drawer (DAWAD) from 1935 (L 279076).  The restoration is still a work
in progress, but the lessons learned to date are being shared so that
others might appreciate and preserve the integrity of their
fan-accordion instruments, and avoid rebuilding mistakes related to
misunderstanding the ingenuity of the Aeolian Corp.

The realization of the reduced-throw capability of Concertola-type
note-playing valves was derived from careful measurements of valve
throw in two original untouched fan-accordion systems (especially
the stacks).  This idea was substantially enhanced by reading and
re-reading these two articles published in the AMICA Bulletin:

1. Tockhwockh, The AMICA Bulletin, Vol. 46, No. 2, March/April 2009
2. Taylor, R. M. and Morgan, J., The AMICA Bulletin, Vol. 39, No. 6,
   Nov/Dec 2002

Bill Koenigsberg
Concord, Massachusetts