Soundboard grain angle

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip L Ford" <fordpiano@lycos.com>
To: <pianotech@ptg.org>
Sent: December 04, 2001 5:18 PM
Subject: Re: Soundboard grain angle


> >
> This is an interesting and seemingly inefficient way to gain this
stiffness.  As I see it there are two ways for the board (neglecting the
ribs) to resist the motion of the bridge:
> 1.  The bending stiffness of the board itself.
> 2.  The compression in the board from it acting as an arch.
> The board is stiffer along the grain than across.  Let's say for a moment
that the board had no cross grain stiffness.  Then the resistance of the
board would come from the board acting as a beam along the grain.  Since the
grain is running along the bridge we have a very long (and therefore
flexible) beam.  The same situation applies to the arch.  For every action
there is a reaction.  A compressive load in the panel along the grain at the
belly rail in the high treble will be reacted at the corner of the rim at
the low bass -a long and therefore more flexible arch.
> Now in practice we do have cross grain stiffness and also shearing
stiffness.  So some of the bending stiffness in the board will come from the
cross grain bending.  The compression load in the arch also doesn't have to
be reacted at the bass end of the board but can be reacted by shear in the
panel over to the rim.  So in the real world you have a workable amount of
stiffness.  But, if what you want is stiffness at the top end, it seems to
me it would be more efficient to run the grain about 90 degrees to the
direction it currently runs so that the stiff direction of the board is
across the short span rather than across the long span.  I've seen old
pianos that have the grain oriented in this fashion.
> Why do you think everyone seems to have settled on the current
orientation?
>
> Phil
> ---------------------------------------

But, we can't neglect the ribs in the real world piano. Except, of course,
that the top rib, sometimes the top couple of ribs, have virtually no effect
on the performance of the soundboard. In some pianos we could probably take
them out and not hear much difference in tone--power or sustain. And we
might as well forget the function of the arch in the real world piano. Even
a highly crowned board has so little once it's loaded by string down force
any effect the arch might contribute is lost in the compliance of the wood.

Through the treble, then, that leaves us with the bending stiffness of the
board itself. Most of that comes from with-grain stiffness and some lesser
amount from cross-grain stiffness. You are correct, of course, the treble
would be much more efficient if the grain angle were 90º to the bellyrail.
Less so in the low bass, however, where the with-grain stiffness would even
further lock down the bass bridge. Unless we are floating the bass and then
it won't really matter much. I don't know where the convention of a grain
angle of 45º (+/- a bit) came from. Nor do I know what the ideal might be.
In my own practice I usually end up with soundboard grain angles of between
about 60º and 75º, depending on the length of the piano. It's something I'm
still thinking about and experimenting with, but this seems a good
compromise and I've been standardized with this long enough now to be
comfortable with it.

Obviously, when we start changing soundboard grain angles we also have to
change rib configurations. I am also doing this. I have been refining the
radial-dispersion rib configuration over the years and am now fairly
comfortable with it as well. Though I've no illusion either of these are the
final answer.

Del