Soundboard crown

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Erwinspiano@aol.com=20
  To: pianotech@ptg.org=20
  Sent: August 16, 2003 9:29 AM
  Subject: Re: Soundboard crown



  >>Phil-- My thought was that any wood will shrink over time especially =
quartered wood because of it's expansion contraction properties and =
actually that could take some time. However We've all seen old stwy =
board with no cracks ,compression ridging, crown or bearing for that =
matter that make my statement  questionable. Or perhaps whatever little =
compression the wood had originally has equalized or was never great =
enough to cause compression set or damage.
     Do you think that would be enough tension to exceed the tension =
strength of the board?  I wonder.  I also wonder what causes cracks in =
boards to occur.  Do you think it's from tension exceeding the nominal =
allowable strength of the wood? =20

Wood does not shrink on its own--at least not so you would notice--only =
as a result of changing environmental conditions, i.e., with changes in =
the MC of the wood. Kept in an temperature/humidity stable environment =
what would cause it to shrink?

Soundboards crack when the wood is placed in tension and that tension =
exceeds the tensile strength of the wood and, in a piano soundboard this =
may not be--probably is not--the same tensile strength the wood started =
out with. This condition occurs when the environment around the piano is =
quite dry drawing moisture from the soundboard panel causing it to =
shrink. Initially, a compression-crowned soundboard panel is installed =
at such a low MC that it is nearly inconceivable how it could ever come =
under tension. But, over time compression set modifies the wood cells, =
actually causing them to deform. With each humid environmental cycle the =
amount of compression-set increases. Eventually, the modified--i.e., =
compressed--condition, or shape, of the wood cells becomes the norm. =
This is now the soundboard panel's new equilibrium condition. At this =
stage whatever crown the soundboard assembly may have had has =
essentially dissipated. There is no longer the requisite compression to =
maintain the stress interface between the soundboard panel and the ribs =
to force crown into the system. At this stage there will probably also =
be at least some visible evidence of compression damage on the panel's =
surface (though it may not be recognized as such).=20

This is a gradual and unpredictable cycle, generally occurring over a =
period of years. With each year's humid season the maximum level of =
compression within the panel is reduced as compression-set takes its =
toll. With each year's dry season the level of compression within the =
panel decreases, eventually reaching neutrality and then coming under =
tension. By this time the wood fibers have become damaged to such an =
extent that the tensile strength of the wood panel has also been =
reduced. As each weather cycle comes and goes the potential for cracks =
increases. Depending on the specific wood used this can take one or two =
years, ten or twenty years or however many years.

Wood is a variable material. Compression set occurs primarily in the =
earlywood portion of the growth ring and no one (to my knowledge) has =
figured out how to predict its effect (i.e., the amount of compression =
set, not whether or not it will occur) in any given piece of wood by =
examining it visually other than to be reasonably certain it will occur =
first in the widest rings.

Why some soundboards never seem develop cracks is only a partial =
mystery. Pianos that originally received rib-crowned soundboards have =
less tendency to develop cracks. But even pianos that were originally =
"rib-crowned" may have been glued up at very low wood MC's. Wolfenden =
(in "The Art of Pianoforte Construction")described essentially a =
rib-crowning process but still wanted his boards quite dry at the time =
of ribbing--a combination. Still, if these boards do crack the crack is =
generally not as acoustically significant. We will even run across the =
occasional pure compression-crowned soundboard that seems to have =
resisted cracking over many years. We can only guess, but this is =
probably a combination of exceptional wood, a somewhat wet board at =
glue-up (no, their process controls were not perfect), and/or a =
relatively stable environment after manufacture. For example, we find =
less visible soundboard damage on pianos that have been located in =
Portland, Oregon throughout their entire history than on those that have =
come out from New York or Michigan. So, if a board of particularly =
resilient and tough wood is ribbed at a relatively high moisture content =
and the piano spends much or all of its life in a relatively stable =
microclimate it will survive nicely for some considerable time.=20

   =20
  I also wonder how a flat board that is pushed into reverse crown by =
the downbearing will behave with changing humidity.  With decreasing =
humidity the board wants to shrink.  It seems to me that this would mean =
that the board would want to move up, back toward the flat position.  =
But the ribs are resisting this.  So a moment would be set up between =
the ribs and board that would want to push the board down.

>>> If it's reversed the ribs to my mind would now be acting as  a =
spring in tension and could behave the way you describe. =20
        The reverse would be true with increasing humidity.  The board =
wants to grow, which seems to me would mean moving downward.  But the =
moments between ribs and board would be tending to push the board up.  =
So perhaps this configuration is actually more stable than the =
conventional configuration.

... I dunno. It could be


I assume you're referring to the (so far, unsubstantiated) phenomena of =
"oil-canning." I don't see how this is expected to happen. The same =
stress interface between the ribs and panel that originally created the =
crown will still try to force the soundboard assembly up into a crowned =
configuration. It would be more likely to develop increasing reverse =
crown during the dry times. The tensile strength of wood is not all that =
great, especially after it has been damaged by compression stress.




    With a conventional board having crown and with a downbearing load I =
think the board is always in compression.  However, with this reverse =
crown board I can imagine that there is some humidity level where the =
expansion of the board induces a compression load in the board which =
just balances the tension load from the downbearing so the board is in =
an essentially unloaded state.  As Del was conjecturing in another post, =
the acoustical behavior of a board in this state may be very different =
from a conventional board.
  >>>I agree
I am uncertain what is meant by "a conventional board." Assuming this =
refers to a compression-crowned soundboard system, the key words in the =
above are then "having crown." However, a strong argument can be made =
that this is no longer--if it ever was--the conventional soundboard =
system. There are a lot more pianos out there with hybrid crowning =
systems--if not pure rib-crowned systems--than I think we realize. The =
hybrid systems were certainly thriving during the early 1900s. True, =
most of the soundboards found in pianos in which we regularly replace =
soundboards were all or mostly compression-crowned, but this does not =
necessarily make them "conventional."=20

As well, it is quite possible to have a string downbearing load without =
having positive crown.=20

Del

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