evaluating sdbd. crown & bridge downbearings in a new piano

[Delwin D Fandrich]

----- 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


> 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.

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

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.

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...

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.

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

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.

Regards,

Del