[pianotech] soundboard rise/fall theory

[David Nereson]

----- Original Message ----- 
From: "David Nereson" <da88ve at gmail.com>
To: <joegarrett at earthlink.net>; <pianotech at ptg.org>
Sent: Friday, April 02, 2010 9:20 PM
Subject: Re: [pianotech] Soundboard rise/fall theory


>    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
>