Soundboard crown

[Phillip Ford]

>>In 1700 wood was the only logical material from which to make soundboards.
>>And it remained the only logical material until fairly recently. While the
>>piano soundboard started out as basically an overloaded harpsichord
>>soundboard, it had evolved considerably by the late 1800s. The motivation
>>for this evolution was the increasing demand for power and the resultant
>>string loads placed on them. In response it became thicker and, by virtue
>>of increasingly stiff and massive rib systems, stiffer. Using construction
>>techniques that seemed reasonable to piano makers of the day, one viable
>>way to achieve an adequately stiff soundboard system without overloading it
>>with excess mass was to form a positive wood spring and then compress it
>>with another spring--the string set that was stretched across the bridge
>>with some amount of tension and deflection. In so doing the stiffness of
>>the soundboard system was increased considerably with no increase in mass.
>>
>>...
>>It would be an interesting experiment, but lacking that we are not left
>>without clues. We can come close to this in real life by examining a
>>selection of otherwise nicely rebuilt pianos having old soundboards that
>>have ended up little or no discernable crown after stringing. I've
>>encountered examples of both: freshly rebuilt pianos with zero crown and no
>>measurable string deflection and freshly rebuilt pianos with zero
>>crown--i.e., some distorted but with the bridge line at the same elevation
>>as the outside edges--but with some string bearing. Yes, they both work,
>>but it becomes a judgment call as to how well they work. In general the
>>respective pianos have exhibited the tonal characteristics described above.
>>
>>Del

I keep reading about this increase in soundboard stiffness and I 
don't understand what it means.  Are you distinguishing between 
preloading and stiffening?  By preloading I mean that if you put more 
load on the board you get an increase in preload.  Put on 100 lbs and 
you have 100 lbs of preload.  Put on 200 lbs and you have 200 lbs of 
preload.  By stiffening I mean that as you deflect the board it takes 
increasing increments of load to achieve the same increment of 
deflection (like a spring with an increasing spring rate as it is 
compressed).  Put on 100 lbs and you get a deflection of x.  Put on 
another 100 lbs and you get an additional deflection less than x.  Is 
this what you mean?  Do you have some soundboard deflection vs. load 
data to substantiate this?

It seems at odds with your belief that all the downbearing is 
supported by rib bending and none of it by panel loading.  Beams 
don't get stiffer as they deflect.  If it takes 100 lbs to deflect a 
beam 0.1 inch then another 100 lbs will deflect it another 0.1 inch. 
If the soundboard is getting stiffer as it is pushed down then 
something must be reacting that load other than just rib bending.

If it's not getting stiffer, if instead its preload is increasing, 
then the same effect could be achieved on a flat board.  And there 
must be another explanation as to why a crowned and preloaded board 
seems to give better performance than other arrangements.

Phil Ford

-- 
Phillip Ford
Piano Service and Restoration
1777 Yosemite Ave - 130
San Francisco, CA  94124