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MMD > Archives > December 2017 > 2017.12.06 > 04Prev  Next

Duo-Art Fan-Accordion System Mystery
By Bill Koenigsberg

The Duo-Art fan-accordion system for reproducing pianos was most likely
introduced to the American public by the Aeolian Corporation in 1928.
This estimated date of marketplace introduction is based on extant
fan-accordion Duo-Art pianos whose dates of manufacture are documented
in Pierce's Piano Atlas.  Compared with Aeolian's famous (conventional)
Duo-Art reproducing system, the fan-accordion equipment reflected
multiple significant engineering changes that some might view as
technically revolutionary rather than as-expected evolutionary.

The resultant fan-accordion system (end-to-end type) appears to have
undergone a substantial degree of pneumatic component modularization,
simplification, and miniaturization.  The pre-1928 Duo-Art expression
box is commonly encountered, and is large and somewhat ungainly looking.
This device was cleverly modified to appear sleeker while retaining the
original function (sound intensity adjustment) of Aeolian's classic
4-chamber box-cube accordion structure.

Among other system changes, the accordion modification was implemented
in a form reminiscent of a Japanese folding fan (sensu and ogi).  This
distinctive geometric characteristic gave rise to the present-day
descriptive name of the fan-accordion system.

One change was the smaller theme and accompaniment vacuum regulators,
whose two associated pneumatics were mounted end-to-end versus
side-by-side.  Another change was the introduction of rolling leather
curtain valves to replace the more conventional wooden knife valves.
Notably, the striker pneumatics in the piano stack were reduced in
size, allowing a reduction of the number of stack decks from 3 to 2.
In addition, the note-playing valves were completely redesigned; they
featured reduced travelling mass and a surprisingly small 'throw' of
only 0.012" vs. the more typical valve travel of 0.032".

Not only was the shape of the accordion structure different from
earlier implementations, but its overall size was markedly smaller
(miniaturization) than the conventional box-cube design.  Also, it
became significantly easier to adjust the spans of the fan-accordion
chamber openings without having to first remove the expression box
from the piano.

Most pre-1928 expression boxes had to be removed temporarily from
the piano to iteratively realize a proper state of sound-intensity
fine-tuning adjustment.  It would appear that the Aeolian Corporation
was sensitive to the needs of their on-the-road installation and
service personnel as well as the acoustic wishes of the listening

The most common fan-accordion expression box did not incorporate
the spill valve assembly that was formerly an integral part of many
earlier vintage Duo-Art expression boxes.  This departure from previous
construction could be viewed as a form of modularization.  This occurs
when complex composite structures are reconfigured into disjoint but
possibly simpler individual components (these may be easier to assemble
and/or install).

With the introduction of the fan-accordion expression box and its
associated pneumatic hardware, it would seem as if a 'new' group of
engineers and designers had gotten together at Aeolian for a technical
powwow.  Their assignment may have been to transform a previously
successful workhorse system into one whose components were physically
smaller, functionally simpler, easier to service, and possibly more
cost effective to manufacture.  If the fan-accordion system was
released in 1928, it might be reasonable to assume that the new design
team initially convened sometime in 1926.

Some (not all) of the design changes that accompanied the introduction
of the fan-accordion Duo-Art system are briefly described above.  In
each example, new and/or better ways were found to ensure that the
associated modifications would not detract from or interfere with the
overall system's ability to play previously issued Duo-Art piano rolls.
Surely, this must have been an overarching requirement for the task at

Curiously, there may be one design change (not mentioned yet) that, on
the surface, appears to violate the requirement that the fan-accordion
system be able to play previously-issued Duo-Art rolls with the same
fidelity as that exhibited by Duo-Art pianos of earlier manufacture.
This has to do with the disjoint spill valve assembly that is directly
connected to the Duo-Art pump.

To quote Mike Kitner in the 1974 Vestal Press Publication No. 5,
"Rebuilding the Duo-Art Reproducing Piano Mechanism":

  "The spill valve is linked mechanically to both sets of
  accordion pneumatics and serves to unload the pump during times
  when low vacuum is being provided by the [theme and accompaniment]

This statement specifically refers to the pre-1928 Duo-Art expression
box.  Because that spill valve is mechanically linked to both sets of
accordions simultaneously, the accordion demanding (from holes punched
in the piano roll) the greatest closure of the spill valve dominates.

Control of the spill valve associated with the (post -1928) fan
accordion expression box does not appear to function in the manner
described above for the pre-1928 conventional expression box.  The
'equivalent' statement for the fan-accordion expression box might

"The spill valve is linked pneumatically to one set of accordion
  pneumatics (the theme accordion only) and serves to unload the pump
  during times when low vacuum is being provided by the [theme and
  accompaniment] regulators."

Because the spill valve in the fan-accordion system is linked
(pneumatically) to and controlled by only one accordion (the theme
unit), sound intensity information associated with the other accordion
(the accompaniment unit) was effectively discarded and ignored.
Almost all (see below for exception) of the extant fan-accordion
systems exhibit this surprising structure.

This circumstance is not consistent with the previously "successful"
design and implementation of the Duo-Art expression system.  On the
other hand, because many design changes appeared simultaneously in
the fan-accordion system, a possible result was that the loss of
accompaniment accordion information (for spill valve control) no longer
mattered.  Aeolian may not have been looking for this purposeful
simplification, but realized the benefit anyway.  Nevertheless, so
many questions immediately come to mind.  See below.

(1) Is there some aspect of the much-modified fan-accordion system
(with so many independent but potentially interactive changes) that
allowed Aeolian to dispense with the former need to account for the
effect that the accompaniment accordion had on the closure of the spill

(2) Is it possible that the four accompaniment accordion channels
contain relatively little useful information, so that Aeolian
justifiably could choose to discard their use in determining the
closure of the fan-accordion spill valve altogether?  I am not aware
of musical complaints (though they may exist) associated with a lack
of fidelity from Duo-Art piano rolls played on restored fan-accordion

(3) Did Aeolian make a critical mistake in the design of the
fan-accordion system?

(4) The author is aware of only one fan-accordion system (1937 Weber No.
88174) where both the theme and accompaniment accordion intensity
channels were used to control the closure of the spill valve
(pneumatically duplicating the function of the pre-1928 expression
box).  Why would Aeolian have made such a system so late in the game,
when so many of the other fan-accordion systems utilized theme-only to
control the position of the spill valve?

(5) Were other Duo-Art pianos (that incorporated the fan-accordion
system) manufactured that made use of both the theme and accompaniment
accordions to control the closure of the spill valve?  Be aware that
most (if not all) of the Aeolian Duo-Art Concertola-equipped pianos
utilized the fan-accordion system in their implementation.  Those
Concertola installations that I have seen or heard about used
theme-only for control of the closure of the spill valve.

(6) Can someone in the MMD community shed some light on this mystery?
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?

For those who may not be familiar with Aeolian's fan-accordion system,
please refer to the excellent documentary articles (ample text and
great pictures) published in the AMICA Bulletin.  They were all written
by Tockhwockh.

  Vol. 46, No. 2 March/April 2009
  Vol. 45, No. 2 April/May 2008
  Vol. 45, No. 6 December 2008
  Vol. 44, No. 3 June/July 2007

Bill Koenigsberg
Concord, Massachusetts [delete ".geentroep" to reply]

(Message sent Wed 6 Dec 2017, 20:13:53 GMT, from time zone GMT-0800.)

Key Words in Subject:  Duo-Art, Fan-Accordion, Mystery, System

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