Soundboard crown

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: August 18, 2003 1:00 AM
  Subject: Re: Soundboard crown


  Hi Del=20
  I guess what I was getting at was what range of RH would we need to =
confine a panel to for it avoid tension caused cracking. Actually, as =
long as this is up. Maybe a rough comparison for safe operating RH's for =
both CC and RC boards.=20

  RicB=20



I can't give you a cookie recipe, no. There are too many variables. Only =
the principles and generalities I've written about and taught in the =
past. Once again:

Within the continental U.S. (the only geographical region I've studied =
to any extent) the climate is such that within the so-called "average" =
home the wood moisture content averages approximately 4% and 5% (the =
high southwest desert--eastern California, Nevada, SE Oregon, SW Idaho, =
Utah, New Mexico and eastern Colorado and Wyoming--during the summer) to =
a maximum of 12 % to 13% (the western seaboard from southern California =
to Washington and along the eastern seaboard from southeast Texas along =
the Gulf of Mexico, across Florida and up the coast during the summer). =
Heating and air conditioning systems can alter either of these extremes =
considerably.=20

For example, I once looked at an Unnamed (to protect the guilty) piano =
in Sparks, Nevada (just up from Reno) with "a soundboard problem." It =
was January and the outside temperature was about 10=BA F. Relative =
humidity was about 25%. The home was heated with a forced-air gas =
furnace. There was no moisture control of any kind. Still, the =
soundboard had popped away from the ribs in kind of an upside-down "V" =
and, at the joint, was standing about 10 to 12 mm proud of the surface. =
I have always wondered about the MC of that board when it was ribbed.

A compression-crowned soundboard made of excellent spruce and bellied =
and installed properly, then, by design will have zero crown during the =
summer throughout the region mentioned above. And it will have little, =
if any, during the winter in the upper Midwestern states. Neither, in a =
perfect world and despite the extremely low MC, will there be any =
tension on the panel.=20

During the summer, on the other hand, the soundboard panel will be =
highly compressed if it resides anywhere along either coast. I don't =
remember the numbers off hand but taking a wood sample through the range =
between 4% MC and 13% MC will cause it to expand by approximately 1%, =
give or take some. (To find out how much exactly, dry a sample of spruce =
down to 4% MC and record the dimension perpendicular-to-grain. Then take =
it up to 13% and record that dimension across the same span. The =
difference is how much a typical compression-crowned soundboard panel =
will want to grow and shrink.)=20

On paper this is within the strength capability of the average piece of =
spruce wood. But not all spruce is average. Some is above average, some =
below. As well, and this is the problematic part, any time the MC is =
significantly above 4%--enough above to force crown into the =
system--there is going to be some compression-set going on; the rate and =
amount being determined by how much above 4% the MC goes, by how long =
it's up there and by the mechanical characteristics of the wood itself. =
As compression set alters the shape of the wood fibers the panel =
physically shrinks. This is a cyclical process with each high MC swing =
causing a bit more fiber deformation than the last. Now as the MC drops =
low and the wood fibers shrink the panel would also like to shrink. =
Since the natural state of the wood fibers is now just a bit deformed =
the panel would like to shrink to some physical size smaller than it was =
when originally ribbed. Unfortunately, the ribs prevent this and the =
panel comes under perpendicular-to-grain tension. Alas, the wood fibers =
have also been made weaker by virtue of their being deformed by =
compression (i.e., they've been physically crushed) and their tensile =
strength has been reduced. Cracks readily develop.=20

This is why it is impossible to give safe range of relative humidity for =
the compression-crowned soundboard system. It is the MC swing that is =
the killer. If, once the piano is built, it is immediately taken to an =
environment that puts the wood at 13% and held there permanently there =
would eventually be some moderate compression damage (due to the =
inevitable compression-set) but cracks would never develop because the =
wood would never come under tension. And I suppose if you could keep the =
MC in the 4% to 6% range you'd be pretty safe. There would not be enough =
compression to cause any damage at all, even over the long term. Of =
course, the soundboard wouldn't have any crown either, but that's =
another issue.

I mentioned earlier (in another post) that even pianos with compression =
crowned soundboards that had been located in the Pacific Northwest =
frequently exhibited little or no soundboard damage even after some =
decades of use. I should have added that, in spite of their pristine =
appearance, rarely are these still acoustically functional soundboards. =
Time and compression-set have taken there toll even here. The may look =
good (and be appropriately rebuilt) but their soundboards are =
acoustically shot.=20

(I received another phone call just last week from a disenchanted =
Steinway owner who had his piano rebuilt just last year by a reputable =
and competent regional rebuilder who declined to replace the soundboard =
because "it looked really good." The tone, while perhaps acceptable to =
some, is not at all what the owner wanted. Sustain is still short, =
especially through the upper tenor and treble, and the piano is, in his =
words, "lifeless." The work was well done, the board does, indeed, look =
good, the action works beautifully, the hammers are nicely voiced. And =
the money is spent and the owner is not at all a happy camper.)

By contrast, a soundboard panel intended for a rib-crowned soundboard =
assembly will be ribbed at approximately 6.5% MC. At least our are. At =
any moisture content below this it will shrink and be under tension, =
above this it will expand and be under compression. But the amount of =
tension developed is slight and, well within the strength limits of =
undamaged wood cells. And the MC swing from 6.5% to 13% is considerably =
less than it is from 4% to 13% and the level of compression in the =
rib-crowned board at 13% MC will also be less. Hence the rate of =
compression-set will also be lower. And at either extreme there will =
still be system crown (even if the panel itself is in tension). And =
there will continue to be system crown even if cracks should eventually =
develop.

Now in both cases I must point out that the MC swings mentioned are the =
extremes. In most homes they won't go this low or this high except, =
perhaps, under special circumstances and that only occasionally. As =
well, coating the soundboard with a good quality lacquer or varnish will =
also retard the passage of water vapor into and out of the soundboard =
panel.=20

With a rib-crowned soundboard system exposure to any of the typical =
environmental extremes found in the continental US should be tolerable. =
With a compression-crowned soundboard system it is not the cracks that =
are the problem. It is the loss of stiffness due to the continual =
deformation of the wood fibers that is the problem. If the piano is =
going to be kept in an environment that works to provide the soundboard =
with crown and stiffness, damage is going to be taking place. The only =
way to prevent that damage from taking place is to keep the piano in an =
environment that keeps the board in an uncompressed condition. Beyond =
this, the best we can do with a solid wood panel is to design the thing =
to minimize the destructive effects of piano's environment while =
optimizing its acoustical potential.=20

Del

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