[pianotech] Soundboard rise/fall theory

[David Nereson]

    By "treble break" I assume you mean the break between the 
bass and the tenor, not the break about an octave an a half 
above middle C.  The tenor break is where I find the most pitch 
variation.  I find it on the lowest tenor notes, sometimes 
extending up to about A4, usually not much farther.  The highest 
notes in the bass will vary some too, with the changing seasons, 
but not as much as the low tenor area.
    I've been telling clients for years that it's because of the 
soundboard absorbing and losing moisture and thus rising up and 
down (increasing and decreasing crown) because its edges are 
restrained, not because I've observed and measured it, but 
because that's what I've heard from other techs and books on 
pianos over the last 30 years.  I think that phenomenon probably 
does happen, but apparently the math shows it does not happen to 
a large enough degree to cause the pitch changes we observe.

>> This tells me that: 1. This can't possible have to do with 
>> soundboard rise/fall.  2. There is some sort of twist 
>> happening in the structure, with the pivot being at the 
>> treble strut. How else could flat notes be right next to 
>> sharp notes? <<

    You mean the low tenor is flat and the upper bass is sharp? 
And that it's vice versa roughly six months later?

    What would the pivot be?  What actually twists -- the whole 
wooden frame (of an upright)?  Then you'd think two diagonally 
opposing casters would come off the floor.  Or that the plate 
would crack from the strain, in an upright or a grand.  Or that 
the top and bottom panels would rub and not fit properly at 
certain times of the year.  By "structure" do you mean the 
wooden back (top beam, bottom beam, backposts); or in a grand, 
the rim, belly rail, and radiating beams?  Or are you including 
both the rim and the plate?  How much can the plate twist 
without cracking?
    Not challenging anything here, since I'm not a 
designer/rebuilder -- just trying to understand so I can tell 
clients what's really happening.
    There also was an article years ago about bridge elongation 
down in the low tenor, by Fandrich, I believe, but I never 
understood how the bridge could elongate when it's glued to the 
soundboard, unless they elongated equally (unlikely, with one 
being maple and the other spruce).
    Confused,   --David Nereson, RPT