Soundboard stiffening

[Phillip Ford]

>Phillip Ford wrote:
>>
>>      I keep reading posts by various people stating that the 
>>soundboard gets 'stiffer' as downbearing is applied.  But I have 
>>seen no experimental evidence to support this supposition.
>
>Well I agree the soundboard does not get stiffer in its self. That's 
>why I have using the phrase "apparent stiffness". When we pull the 
>tension up on a string the wire becomes more resistant to deflection 
>as if it were stiffer. But the steel in the wire is no more stiff 
>than before. What is happening is that the wire when in tension can 
>readily transfer any further loads to the plate and this greater 
>structure makes the wire resist deflection.

Yes, a component of the string tension resists a deflecting force, 
not the wire stiffness.  It would be misleading or inaccurate to say 
that increased tension has made the wire 'stiffer'.

>When the soundboard is pressed down by the down bearing it is no 
>more stiff than before but it is now connected to a portion of the 
>tension of the string plane and because it has crown it presses 
>outward and receives further support from the case.

I think this is where you part ways with some others on the list. 
There seems to be some disagreement about whether the rim is in fact 
supporting downbearing or trying to keep the soundboard from 
flattening out.  If it is, and the soundboard is exhibiting some arch 
behavior, then the soundboard would be stiffer than an equivalent 
flat board.  If it's not, but the crowned board is stiffer than a 
flat board, then some other mechanism is causing this stiffness 
increase, and I'm curious as to what it would be.  It it's not, and 
the crowned board is no stiffer than a flat board, then if the crown 
is necessary (as experience seems to indicate), that necessity is for 
some other reason than stiffness or resistance to downbearing.

>  When the vibrating strings meet the bride the impedance it reacts 
>to is that of the soundboard, a portion of the string tension in the 
>string plane and some of the mass and stiffness of the rim. Think of 
>it this way: the bridge is sandwiched between two springs, on top is 
>the down bearing and on the bottom is the crowned soundboard.
>
>John Hartman RPT
>

On the bottom could also be a flat board.  Just because a board is 
flat doesn't mean that it doesn't have any stiffness or resistance to 
downbearing.  The question is, does the crowned board having panel 
and ribs of the same dimensions have more stiffness?  I can see that 
the string plane is providing a restriction on board movement (at 
least to its upward movement).  If this is the essential element, 
then why aren't we talking about string plane load or string plane 
resistance rather than this hocus-pocus about board stiffening due to 
downbearing.

Phil Ford