evaluating sdbd. crown & bridge downbearings in a new piano

[Richard Brekne]

Delwin D Fandrich wrote:

> ----- Original Message -----
> From: Richard Brekne <richardb@c2i.net>
> To: <pianotech@ptg.org>
> Sent: Friday, September 24, 1999 8:27 AM
> Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano

> The problem here is that both of these words have precise and fixed meanings
> within the wood technology community and within the engineering community.

I would aggree... Franks "viewing point" and subsequent reasoning creates
exactly this problem. But then if we are to understand what he is trying to
point out, we have to attempt to put ourselves into his shoes (grin,, however
uncomfortable they may feel ).

>
>
> Tension is "a force tending to stretch or elongate something."
>
> Compression is "the act or process of compressing," or "the state of being
> compressed."  Compressing means "to make an object more compact by or as if
> by pressing."  Compressed means, "pressed together or into less volume or
> space."
>
> We don't have the liberty of changing the definitions of these words.

Aggreed. But I dont think he really has done that... tho I find it easy enough
to understand how you come to that conclusion. It is indeed easy enough to
"read" his posts in that way.

>
> An object that is "less compressed" in not necessarily under tension.  It
> might be, but only if there are actually forces tending to stretch it or
> elongate it.  This will not be the case in the compression-crowned
> soundboard until the wood cells have become damged (due to excessive
> compression) to such an extent that they no longer have any resilience left
> and during periods of very low humidity the whole panel will actually be
> under tension.  Of course, by this time there will no longer be any
> effective crown and/or string downbearing either.  This is the mechanism by
> which cracks develop in these panels.  But that is another story...

As I read Franks post... he does not say directly that the panel is under
tension in the manner which you refer and rightfully object to. Oh.. he uses
those words true enough, but there is a qualifier a bit hidden in his words, and
that qualifier is to say that "less compressed" is the same as saying "more (in
the direction of) tense". Do you see what I am getting at ?.. Combine this with
his "reference point" of the soundboards condition when fully dried and unribbed
and his "logic" or reasoning holds water. And in doing so his point is aptly
enough illustrated. Tho I would tend to aggree with you that it perhaps leaves
out some potentially very negative aspects of the situation that are so well
shown by an engineers description. But you could reverse that. You gotta admit
Del, when you describe a CC board as being under "extreme compression" (as it no
doubt is from an engineering perspective) this sounds pretty bad. And bad it may
be. But this same engineering perspective does not illustrate the kind of
"equalibrium" picture Franks shows. The question then becomes more a matter of
whether this kind of "equalibrium", however it is described, is desirable or
not, which by the way is what interests me most at this point, as it seems
obvious that the RC panel is the stronger of the two configurations, at least
given a radically variable climatic condition, as you point out in the second
part of the above paragraph.

> Comparing an area having a high compression force acting on it with an area
> having a very slightly lower compression force acting on it within a given
> soundboard panel does not mean that the area having the slightly lower
> compression force is under tension.  It is still under compression and the
> wood cells are responding as they are known to respond when they are under
> compression.  The entire panel is under compression, it is just that some
> areas of this panel are under slightly -- very slightly -- more or less
> compression.

You get no argument from me here. But again.. I dont really get the feeling that
that is what Frank is really saying. (see Above paragraph)

>
> Technically speaking, tension and compression are quite different things.
> Pretty much opposites, in fact.
>

yepppo

>
> This may indeed be a question of semantics.  But in this case the terms in
> question do have precise definitions that are accepted by the technical
> community.  They are not really open to much interpretation and to attempt
> to redefine technical terms within a technical discussion simply introduces
> unneccessary confusion.  As it apparently has in this case.

I am not sure what unneccessary confusion is really, if one does not understand
a particular set of terms for a particular subject, yet is capable of makeing
some clear enough observations useing concepts more readily available, then I
suppose confusion is a neccessary part of communicating, and eventually leading
to some benifit. Course this takes a bit of patience.. grin.. but hey.. were all
pianor tuners .. patience is the name of our game. But on the whole, I think it
would be easiest if we all spoke the same language so to speak. And I think the
language of choice should be that of the engineers. In any case, the excercise
has taught me a bunch and I am grateful to both you and Ron, and to Frank for
setting the game afoot.

Richard Brekne
I.C.P.T.G.  N.P.T.F.
Bergen, Norway

>
>
> > I really dont see that Frank has said anything to the contrary of this.
> What he
> > is saying is that there is another way of viewing the situation, one that
> is left
> > out when one views the compression of the board only in terms of how much
> the
> > panel would have expanded with out ribs, versus how much the ribbing
> inhibits
> > this.
> >
> > If you substitute the terms "less compressed"(for the up side),
> "morecompressed"
> > (for the bottom side) with the terms "tense" and "compressed", then Franks
> > reasoning begins to become clear.Frank is saying that the top side of the
> board
> > is "tense" relative to the bottom side, and since this is simply another
> way of
> > saying it is less compressed (which everyone seems to be in aggreement
> with)
> > there is nothing "incorrect" about it.
>