(no subject)

[Ron Nossaman]

>Please help a newbee understand.
>
>I am trying to picture where the stiffness is in soundboards. If you
>consider the typical compression type that has ribs and board pressed into a
>caul, then the ribs are pulling the board toward the flat side or the
>position that the ribs want to come to rest. Is this creating stiffness in
>the soundboard?

Hi Gene, (a hot subject in it's own rite here lately)
Soundboard assembly stiffness comes primarily from two sources; rib 
stiffness, and panel compression. Ribs act as a linear spring in that if a 
given rib deflects 3 millimeters under a 5 pound center load, then it will 
deflect 10mm under a 10 pound center load. Panel compression acts as a 
progressive rate spring, meaning that the more it is compressed, the 
greater the resistance to compression.

In a rib crowned board, the curve machined into the rib helps to form and 
support crown. The ribs then supply a POSITIVE linear spring rate 
determined by their dimensions and the material from which they are made. 
Panel compression supplies a bit more crown support and a progressive 
spring rate determined by panel composition, thickness, and the degree of 
compression it is under. The progressive spring rate means that the 
assembly will deflect less than twice as far with twice the load.

In a compression crowned board, the ribs are supplying a NEGATIVE linear 
spring rate, and the crown and string bearing is carried almost entirely by 
the panel compression, which still supplies the progressive spring rate. 
Because of this, you have far better control of stiffness and better 
longevity with a rib crowned board.


>The reason I ask is that recently I had a chance to be instructed on how to
>handle a new soundboard out of the box without ribs attached. It quickly
>bacame obvious how fragile they are in this state. Any mishandling could
>easily cause a fracture. Then I think about the grain orientation and how
>the bridges are aligned to this with exception of the extreme treble and
>maybe some of the bass. Along the grain is the weakest part of the board -
>am I wrong? So where is the stiffness.

The panel is quite stiff along the grain, especially with the addition of 
the long bridge. Stiffness control across the grain is the problem.


>Also curious, is board stiffness something that can be calculated?

To a reasonable degree, yes. Designing a soundboard with a new rib scale 
requires that very thing, taking into account the string scale, bridge 
placement, etc.


>Is it a factor in rib design?

Of course.


>Is it used to calculate how bearing is supported?

Absolutely. Case in point: Suppose you have a string scale you intend to 
use in this piano. You also have decided on a bearing schedule you would 
like to end up with. Since the soundboard load is a result of the bearing 
angles and tensions you have pre-determined, the soundboard needs to be 
designed to support it. You can design a soundboard assembly with 
relatively low crown, stiff ribs, and a panel under high compression that 
(in an arbitrary spot for illustration) deflects 2mm under load and leaves 
a crown of 4mm. Or you can design a higher crown and lighter ribs, with a 
panel under much less compression that deflects 6mm under load, leaving a 
4mm crown at your target bearing. So you need to have some idea what the 
soundboard is going to do under load before you get to the "what I should 
have done" part. Now, since both of these mythological boards are now 
supporting nominally identical loads with nominally identical measurable 
crown left over. Which is "stiffer"?


>Or
>is board stiffness related only to musical qualities?

That too. Stiffer, and volume goes down as sustain increases. More 
flexible, and volume goes up as sustain shortens. There's a lot more detail 
than that, naturally, but that's the gist of it. The killer octave (5-6+) 
in compression crowned boards is the result of a soundboard assembly that 
is too flexible in that area. Loud attack, short sustain, and a lot of 
distortion.


Ron N