Soundboard Deflection and Pitch Change / was Downbearing

[John Delacour]

At 9:18 am +0200 17/8/06, Ric Brekne wrote:

>... I've been under the impression that soundboard deflection 
>changes substantially enough due to climatic changes to account for 
>at least some significant amount of the seasonal pitch changes and 
>tuning instability.  At this point... I'm not so sure about that at 
>all any more. In fact it <<looks>> like the contribution to pitch 
>change from soundboard deflection is nearly insignificant... almost 
>negligible !

Many 19th century straight-strung grands had the grain of the belly 
parallel to the lock-rail.  I presume the general adoption of the 
board with the grain roughly parallel to the long bridge was due to 
the greater stability it gave under variations in humidity.  The 
extent to which wood swells _across_the_grain_ with increased 
humidity is quite astonishing, and this happens to some degree to 
every wooden part of the piano, so that the change in pitch with 
varying humidity is due to the sum of all sorts and directions of 
movement in the structure, not simply to any minute change in the 
angle over the bridge.

At 8:36 pm +0200 16/8/06, Ric Brekne wrote:

>How do you go about equalizing the downward force the string places 
>on the bridges front and back edges ? Or do you do this at all ?

Why would you want to?  Ideally, in my view, the top of the bridge 
should be parallel, when the piano is up to pitch and well settled, 
with the length of string beyond the bridge so that the only angle in 
the string is where the speaking length begins, at the front pin. 
That is to say that the downward pressure at this point is the 
greatest.

JD