The Aeolian Company manufactured a late-style Duo-Art reproducing
mechanism that has come to be known as the "fan-accordion system".
This name reflects the geometric similarity between a Japanese
hand-held folding fan and the two accordion pneumatic assemblies
found on the late Duo-Art expression box.

One fan-accordion assembly is used with the theme regulator and
the other is used with the accompaniment regulator.  Please look up
'ogi folding fan' to get an idea of what a Japanese folding fan looks
like.  Each of the two fan-accordion devices used in the late Duo-Art
expression box consisted of four conjoined (at one end only)
collapsible chambers.

Although it is not certain just when this fan-accordion system
was first released for sale, but thought to be early 1928, it was
incorporated in grand pianos as late as 1935 (e.g., Steinway L 279076),
and very likely until the end of production, around 1938.  The
fan-accordion system was also part of the reproducing piano mechanism
installed with Aeolian's not so well known Concertola, which was
offered in a single-roll remote version or a ten-roll remote carousel
(Ferris wheel) version.  The fan-accordion system may also have been
installed in Steinway uprights, but in very few of them, perhaps three
pianos, at most.

There are many aspects of the fan-accordion system which display a
marked departure from the physical and pneumatic implementation of
Aeolian's earlier (1920? - 1928?) Duo-Art products.  Only one unusual
aspect, the use of rather thick (.028") pneumatic cloth on the Theme
and Accompaniment regulators, will be addressed here.

In the fan-accordion system, the pneumatic cloth covering the Theme and
Accompaniment regulators was originally 0.028" thick.  The pneumatics
themselves (3.625" x 5.5" fan type) were significantly smaller (by
25.6% in area) than their earlier counterparts (3.25" x 8.25" box
type).

The question naturally arises as to why Aeolian used such a thick
cloth.  For a small wedge-shaped pneumatic chamber, a thinner motor
cloth (.011" thick) could easily have satisfied the intended purposes
of airtightness and supple flexibility.  For comparison, that thickness
(.011") of motor cloth was originally used to cover the regulator
pneumatics (theme and accompaniment) on the conventional Aeolian
Duo-Art expression box.

Being a firm believer that implementation changes were executed for
a valid reason, I studied the vacuum regulators in the context of an
automatic control system.  Obviously, the area of the 'movable board'
of the fan-accordion regulator was reduced.  The mass of each board was
correspondingly diminished relative to the 'conventional' earlier style
Duo-Art expression box.  Consequently, this would have shortened the
response time (transient response characteristics) of the regulator.

With this dimensional difference, the vacuum response of a regulator to
a change in the roll expression perforations would occur more rapidly
(speed up).  By itself, this modification would have been unacceptable
because the expression coding on previously released Duo-Art rolls
would not have been compatible with faster responding regulators.  With
such engineering changes, backward compatibility had to be assured.
Why Aeolian reduced the area of the theme and accompaniment regulators
in the first place is a topic for future discussion, and is not
considered here.

Something had to be done to concomitantly 'slow down' the
'fan-accordion' expression box.  Utilizing a thicker pneumatic cloth to
cover the two regulators solved the problem because the thicker cloth
exhibited additional internal friction (damping due to flexing thicker
cloth) which retarded the motion response in time.  The thicker cloth
(.028" vs. .011") compensated for the speed up factor associated with
the reduction in the areal size and mass of the movable board of each
pneumatic regulator.

In principle, one could have avoided all this adjusting of parameter
values by placing suitably positioned weights on or within the movable
board of the regulators.  The best place to do that would be at the
'open' end (opposite the hinge end) of the regulator, where the added
compensatory mass would have the greatest dynamic effect.  Unfortunately,
that was the location where the regulator spring(s) and the vacuum
control arm were already attached.

As a matter of simplicity and engineering judgment, it appears that
Aeolian chose to utilize the thick pneumatic cloth instead, which could
produce the equivalent effect of slowing down the sped-up system.
After all, the pneumatic had to be covered with airtight cloth anyway,
so utilizing a thicker cloth was simple, easy, consistent with previous
practice, and probably less expensive than re-engineering the structure
of the movable board.  The time-honored method for handling such
technical problems was trial and error, i.e., the empirical approach.

The dynamic behavior of the theme and accompaniment regulator
pneumatics is readily described by a third-order nonlinear differential
equation.  For the purpose of useful simplification, this 3rd order
equation can be approximated by a second-order linear differential
equation.  The simpler linear equation has a solution which can easily
be determined in closed form, a characteristic that is convenient for
design purposes.  This cannot be done for the original corresponding
nonlinear equation; numerical methods must be employed for that case,
which is more difficult and time consuming.  In any case, Aeolian would
not have had the capability to perform such calculations for the
nonlinear equation in 1928.

Although the mathematical system analysis is not presented here,
suffice it to say that the combination of reduced regulator mass (of
the movable board) and thicker pneumatic cloth rendered the performance
of the fan-accordion expression box the 'same' as the conventional
Duo-Art expression box.  This would guarantee that previously issued
Duo-Art rolls were backward compatible with the 'new' fan-accordion
system.

It is not known how many of the fan-accordion systems were manufactured
by the Aeolian Corporation.  My recommendation is to restore the
fan-accordion expression box using the same materials and techniques
that were used originally.  There may be a strong temptation to recover
the theme and accompaniment regulators with commonly available
pneumatic cloth (.011" thick) that is usually used for air motors.

If one succumbs to this temptation, you will likely find that the
Duo-Art rolls played on your fan-accordion system piano do not sound
quite right.  The reason is that the fan-accordion expression box would
be operating 'faster than intended'.  This applies to all those Aeolian
Duo-Art instruments that originally incorporated the fan-accordion
expression system, e.g., Duo-Art reproducing pianos and the special
Concertola-equipped pianos.

You may wish to become more familiar with the surprisingly complex and
subtle details of the theme and accompaniment expression coding for
Duo-Art reproducing piano rolls.  Fortunately, AMICA member Peter
Phillips, who resides in Australia, has written and published just the
document for you in the AMICA Bulletin, March/April 2012 issue, volume
49, number 2, pp. 84 ­ 86, entitled "Understanding Duo-Art Expression".

If you wish to obtain a better understanding of the utility of applying
differential equations to address this musical/pneumatic puzzle, feel
free to contact me.  But, be prepared to be exposed to and deal with
terms like root locus, characteristic equation, eigenvalues, damping
ratio, overshoot, critical damping, and hunting.

Bill Koenigsberg
Concord, Massachusetts
billkberg@comcast.net.geentroep [delete ".geentroep" to reply]

 [ Wouldn't a restriction (bleed) in the tube from the fan pneumatic
 [ 'slow down' the response time adequately?  -- Robbie