Soundboard grain angle

[Phillip L Ford]

On Mon, 3 Dec 2001 19:59:06   
 Delwin D Fandrich wrote:
>
>----- Original Message -----
>From: "Phillip L Ford" <fordpiano@lycos.com>
>To: <pianotech@ptg.org>
>Sent: December 04, 2001 12:07 AM
>Subject: Soundboard grain angle (was Negative bearing)
>
>
>>
>> Why does the grain have a positive effect in the highest part of the
>treble?
>>
>> Phil
>>
>
>Because the soundboard assembly wants (needs) a relatively large amount of
>stiffness there to avoid dumping energy into the soundboard overly fast. The
>proximity of the bridge to the bellyrail/soundboard liner uses the
>with-the-grain stiffness to help raise the mechanical impedance of the
>system enough to maintain a reasonable amount of sustain. Thinning the
>soundboard in this region detracts from this effect--one reason why post
>1930s Steinway pianos often have sustain problems through the sixth and
>seventh octaves. Thinning the board down to approximately 5 mm considerably
>reduces the soundboard stiffness as felt by the bridge through that region.
>It also reduces the ability of the soundboard panel to hold the compression
>needed to support crown.
>
>Del
>
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


---
Phillip Ford
Piano Service & Restoration
1777 Yosemite Ave - 215
San Francisco, CA  94124