Robin,
Thank you once again for your continuing non-partial contributions to
the discussion. The fact that much of this RC / CC / RC&S discussion has
been actually going on for a very very long time indeed worth
remembering in all this. As Del said some years back... there is in the
end little that hasnt really been tried out several times throughout
history. Of course this very diversity in design philosophies has
resulted in the many different types of pianos that exist. One has to
simply admire those who have staked out their own course and succeeded.
Steinway has made their mark, Bosendorfer their own again, Bluthner,
Bechstein... etc etc. All these pianos have been hailed for their own
particular wondrous musical qualities... each has found their own place
on stages of all sizes and sorts, at schools, in the home. Such is the
way things should be. How boring it would be if everyone decided that
this or the other way was the only way to do things.
I agree entirely with you that those readers who are interested in the
subject matter take the time to read through some of the available
literature... once, twice, thrice... as many times as is needed to begin
to digest the material. One will find immediately that the structure
that the soundboard is, is simply not covered per se. That in order to
really describe the soundboard system, very much work remains to be done
with regard to measuring strength properties in the contexts that are
relevant instead of applying properties measured for other contexts in
an inappropriate manner.
As an example let me put forth the following little experiment I was
prompted to execute about a year ago when I tried to describe what I
suspected was a tension strain dynamic placed on the ribs when
compression in the opposing soundboard was strong enough. A suspicion
that was simply dismissed out of hand... yet......
A piece of soundboard material was dried as much as I could contrive in
a small home fashioned plastic hot box. Ribbing material, a generic
Norwegian Pine was glued and the whole thing was brought up to ambient
RH. The hotbox showed a RH reading of 16%, and the ambient room RH was
at 60 %.
The rib was measured carefully before gluing, and a jig was fashioned so
that it was tight enough to just barely slide down the ends of the
rib... these ends being << polished >> flat and smooth as I could make
them. I let the thing sit for a week having much else to do, and then
put it onto my workbench for examination. The now crowned test piece
was placed panel side down, and the rib was clamped down so as to press
the entire assembly flat. I then placed my jig on the ends of the rib
to see what would happen... and low and behold.... at about 2/3rds the
way down the jig got tight... and about 3/4 of the way it would not go
further without considerable force. The rib obviously had experienced
some absolute tension strain close to the panel. Something that those
here who rely only on strength properties numbers found in tables in
reference books said could not happen. Never mind that there actually
are no real reliable tension strain numbers for this particular kind of
application.... these were just inferred... assumed... etc.
I am quite sure a host of such things can be discovered by anyone who
takes a truly objective impartial approach to asking relevant questions.
Anyone can "prove" a pre-judged-iced point easily enough.... its more
difficult to task oneself to proving ones own perspectives false. Yet
it seems, as the philosophy of scientific method has repeatedly been
reminded by its own, that this is one of the most valuable approaches to
ascertaining whether or not any particular theory holds true.
Cheers
RicB
Personally, I think Terry is right when he makes the point that rib
crowning and compression crowning are just two extremes on a continuum
of possibilities. Although it may indeed be a trivial distinction, the
feathering at the ends of a rib on a compression crowned board, where
such exists, means, functionally at least, an element of similarity to
a crowned rib. As far as I can see, the only truly compression crowned
boards would be those with unfeathered ribs, which I have seen on just a
few pianos, one of which was a small Mehlin, if memory serves.
If one's measure of rib crowning is a rib which has a variable
thickness along the unfeathered area and is taller than wide, then
there are any number of older pianos out there with this
characteristic. Particularly noteworthy, are the pianos of Chickering
from about 1900 where, in their production they explored the use of
various radii, altered rib scaling, shorter or longer ribs, larger or
small average cross sections, larger or smaller treble sections, etc.
The late 123 has one rib, again if my memory is correct, which varies in
thickness by almost a quarter of an inch. Much of what is represented
here as novel, again with all due respect to those who think so, appears
to have been anticipated a century or so earlier through the efforts of
this firm and the present "new" school of design now clamorously present
as "modern" seems, in amazingly similar ways, to recapitulate the
technical evolution of this firm. This may, or, may not, be fortuitous
but it is certainly useful as a counterpoise to the "standard" approach
typified by Steinway.
The Boston school, as it were, of piano manufacturing, versus, say,
that of New York, as a whole displays a greater similarity in this
regard to English pianos while that of New York appears somewhat more
similar to those of German ones, although, of course, Mason & Hamlin, in
some aspects, at least in their independent production, is an exception
to this generalization. Broadwood's ribbing, at least on the four or
five mid 19th century pianos I have observed appears very similar to
that of Chickering, even though I did not measure the rib heights
looking for variations there. Nevertheless, I feel fairly comfortable
asserting that rib crowning has a long history, predating even 1900 and
would be more associated with English pianos in this regard; German
pianos may well have favored the thinner, wide rib used in compression
crowning.
It may well be that, in the United States, the collapse of the
piano industry in the third and fourth decade of the last century and
the subsequent unchallenged preeminence of the New York school has led
to the illustion that there has been no alternative to the crowning
methods used there, which I think is markedly incorrect. Rib crowning
has a long history and appears to have been well explored.
There is no doubt that Chickering pianos exhibit substantially
less cracking than those of Steinway. This appears to be consistent
with the claims of the RC advocates. It is interesting to note that
cracks in Steinways, in general, are significantly wider at the top
surface of the board, as most know, than they are at the bottom which
seems in keeping with the compression crowning view of tension on the
top of the board and compression on the bottom; cracks found on
Chickerings show dramatically less widening through the section of the
board.
Although I am not sure they qualify as RC&S boards, perhaps or
perhaps not, these boards certainly appear to be RC.
I have urged the view before that it is inappropriate to apply
simple elastic moduli indiscriminately as is done here when using them
to make a few modest calculations which are then generalized to the
conclusion that damaging levels of compression are inevitable, much
less, even achievable in ordinary soundboards, be they RC or CC. First
and foremost as the intrinsic condition of every soundboard is one of a
very complicated, idiosyncratic, triaxial stress distribution and not
one of uniaxial or plane stress, plane stress moduli are inappropriate
to the facts, yet these very moduli are used by those urging the
validity of compression damage.
What is the Fiber stress at proportional limit which is what is
quoted for disabling cross grain compression? It is simply the point at
which elastic action ceases and the material will be incapable of
achieving, when freed of compression across the grain, its previous
dimensions. This by no means is a point of "cellular crushing" or
destruction which is so often claimed here. The net result of such an
effect, should it occur, as Richard Breckne has just pointed out in a
recent post is, simply an increasing density and increasing strength
which, no doubt, increases the acoustic velocity somewhat, and, again,
may allow an actual improvement in the perceptual aspect of tone quality
or have other beneficial effects absent other structural failures in the
soundboard panel.
As I urged last year those interested in such things should take
the various manuals which, at one time, were suggested to contain the
material upon which such a claim of damage could be based and
familiarize themselves with them. One will find, should one do so, for
example, that the coefficient of variation were this simple property,
that of cross grain compression strenth, applicable, according to The
Encyclopedia of Wood is 28% which means that attempting to make any kind
of precise extrapolation from the results of such a calculation is
unreliable and inappropriate. As for an empircal indicator, I have
never seen, in thousands of pianos a significant change in the
circularity of nosebolt holes found here and there in soundboards,
which, were such a level of damaging "compression set" as is frequently
assumed here, underway, would be likely to develop a significant
eccentricity and become elliptical with a major axis parallel to the
grain.
In my opinion, crack formation in older, high quality soundboards
is, in most cases, the result of shear failure and stress concentration,
and not an indication of previous compression ridging, followed by
drying and the opening of a crack. Compression failure, per se, in my
opinion, is more likely to be found in more modern boards that have been
ribbed at too low a moisture content.
Loading of a crowned, or even uncrowned, soundboard along both
sides the bridges asymmetrically by the downbearing pressure, and their
interactions, inevitably introduces additional significant shear on
both sides of the bridges, in every case, which ranges with moisture
fluctuations and downbearing load. Stress relief over time results in
the very commonly found long crack or two or three or four, etc.
running sometimes as a chord between the ends of the bridge or,
sometimes, parallel to it a few inches away. Where the shear is
greater, for example boards with a tenor bridge, a characteristic field
of cracks may well develop. According to the selfsame Encyclopedia
indicated above shear parallel to the grain is virtually the weakest
property of wood. Combine the high shear levels in this area of the
board with the weakness of the material in this regard, high moisture
levels, the possiblity of detached ribs and throw in an additional
factor: the stress concentrating effect of nosebolt holes that are found
commonly in this area and you have an efficient mechanism for crack
formation. It is this shear field that accounts for the non-random
placement of cracks in boards and their association with nosebolt holes
as it is extremely common to find a crack originating in a nosebolt hole
and, sometimes, even the screw holes used for screws attaching the guide
rail . These are not random events.
A second mechanism for crack formation exists in the classic
interaction of the differences of radial and tangential shrinkage and
expansion along with the effects of grain angle which produces warping
in free boards. Even though the flitches are laid up with grain angles
aligned to minimize this, these efforts are only partially effective and
differential stresses, unique to each individual flitch are still
present in the completed panel which work their effects over time.
These effects are to separate the individual flitches along the joint.
I would argue that most cracks fall into either of these two categories
and are not, themselves, the results of "cellular crushing or
compression set", although a lot of newish pianos nowadays very plainly
show compression ridges.
I don't mean to imply that soundboards cannot undergo significant
functional failures; of course, they can and do, but these are not
failures of the wood material itself but, rather, structural failures
of aspects of the panel assembly: the most important of which, in my
opinion, is detachment of the soundboard from the ribs, bridges or rim.
Regards, Robin Hufford
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