----- 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 >
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