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


> So which board is best ??? beats me.. again I am just trying to follow
along,
> look at the two opposing views and attempt to understand what each view is
really
> "saying". Personally, I doubt that there is any data to support the claim
that
> either is from a holistic point of view any better, tho from the point of
load
> bearing it seems clear that the rib crown is stronger, especially in the
long
> term. This however might be a "negligable" if climatic conditions for the
> finished piano are kept at an optimum.

Well, no.  It's not really negligible.  Even under "optimum" climate
conditions the compressive stress within a compression-crowned soundboard
panel (whether measured on the top and on the bottom of the panel) generally
exceeds the fiber stress proportional limit for spruce.  It must to form and
maintain crown in the ribs.  And that compression stress will remain high
until it is dissipated by the permanent deformation of the wood cells and
the panel has lost much of its ability to force crown into the ribs (and
support the string load).  By which time, of course, the acoustic damage has
been done.

Fortunately, there is a certain amount of latitude here.  Soundboard
structure are not always as critical as we sometimes make it out to be.  So,
while the performance of partially collapsed soundboard may not be optimum,
it may not be all that bad either.  Tone degradation -- as a symptom of
soundboard compression damage -- will show up first as a decrease in sustain
time through the 4th, 5th and 6th octaves.  If the sustain time through this
region is adequate (to the owner or pianist) I wouldn't worry about it much.
But, if the owner is complaining about a short, percussive sound through the
upper tenor and lower treble sections, don't be surprised if you are unable
to "voice" the piano into submission.

Regards,

Del