Why?

[Ron Nossaman]

>I am wondering who can provide a definitive answer to the question of why most
>pianos seem to go out of tune the most at the treble break.  I have heard
>several versions of answers, and would like to hear from the best.  I am new to
>the List, and inquiring minds just have to know!    Clark Sprague  Greenwich,
>Ohio
>



Well, I don't know about "definitive", but here are some thoughts. The
tuning change at the bass/tenor break happens (mostly) for a couple of
reasons. First, the bridges are in somewhat different areas of the
soundboard, which react  a little differently to humidity changes. The
biggest change seems to come from the tenor bridge growing and shrinking in
length with these humidity swings. In the low tenor, the bridge is closer to
being parallel to the strings than anywhere else in the scale, so a minute
change in it's length will affect the string tensions much more than in the
bass where the strings are more nearly perpendicular to the bridge. That's
why the bass tuning doesn't move nearly as much as the low tenor.
 
At the treble break (killer octave), it's the soundboard and rib assembly
that is the problem, rather than the bridge. The soundboard often isn't
stiff enough in this area. This seems to be much more of a problem in a
compression crowned board, where the dried panel is glued to flat ribs and
panel expansion forms the crown, than in a rib crowned board where the
shaped ribs form the crown. The bearing angles are increasing through this
area, as the speaking lengths are getting much shorter (the length
progression should be logarithmic) so any change in crown height brought on
by humidity swings will make a much bigger difference in string tensions in
this area than it will in other areas of the scale. Everything seems to be
working against tuning stability in this area in a traditionally designed,
compression crowned soundboard.

I hope this makes sense instead of just making it worse.
 Ron