evaluating sdbd. crown, etc.

[Richard Brekne]

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Since I am a fan (at least to some degree) of Ed McMorrows book on pianos, I
thought I might inject what he has to say on the subject matter. I will take his
opening paragraphs only for now.

Crown
    There is much discusssion amoung piano technicians about soundboard crown.
Crown is the curve of the soundboard towards the strings. I should be a gentle
belly-shape. Achieving this curve is what technicians refer to as "bellying the
board." Crown has two main purposes: to make the soundboard stiffer (hence more
acoustically responsive) without adding any weight; and to function as an
expansion joint for the wood as it reacts to humidity changes.

    One could ask: "Since a high stiffness-to-weight ration is desireable, and
crown increases stiffness, why not put as much crown as possible into the
soundboard, since that will make it stiffer without additional weight?" The
piano may sound good for five minuets, five weeks, or if you're lucky, five
years, before the natural elasticity of the wood is ruined by this exaggerated
tension. The piano maker must not crown the soundboard beyond the limitys of the
wood's elasticity. A soft spruce soundboard cannot take much internal load
without collapsing in a short period of time; if an extreme curve is forced into
the board, the elastic limits of the wood will be exceeded.

    The most important factor in achieving tone quality and long-term stability
seems to be the elapsed time in the bellying process. If the soundboard panel is
dried too quickly, it will lack longevity once crowned, due to the reduction in
the elasticity of the wood. If the newly crowned board is fitted to the rim and
plate too soon, it may continue swelling to the point of internally crushing the
wood. If the installation fo the final plate and bridge elevations are rushed as
well, one risks haveing too much downbearing on an internally crushed
soundboard. This will inhibit the sone significantly and in a few years could
result in loss of crown, which will cause the piano's tone to become even more
lifeless.

    The best piano makers take enough time in assembly for all the parts to
reach relative equilibruim. If a piano maker has already invested time and money
into makeing an finishing a plate and rime, it is hard to resist the cash-flow
pressure to pop that board in, string it up, slike the action in and ship out
the piano. But building a fine piano means playing a waiting game.

from Ed McMorrows "The Educated Piano" side 10


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

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