> Has anyone noticed how semilog progression bridge pianos are more > sensitive to seasonal pitch change than hockey stick bridge pianos, No, I haven't. >or > plain-capped bridges compared with laminated bridges? Yes, most definitely. >*Elongation* of > the bridge in the tenor area might affect tuning more than say upper > treble due to angle of the bridge and space between notes. > > Tom Cole Only if you assume the strings don't render through the bridge pins, when in fact they do. Low break% in the low tenor is largely responsible for the relatively large pitch changes with small string length changes. I still think a major factor in seasonal pitch instability is the bridge cap shrinking and swelling, moving the strings up and down a pair of oppositely angled bridge pins. For instance, a 0.25mm soundboard rise will make maybe 0.0022mm length difference at A-4 with a speaking length of 406mm, and a back scale of 100mm. This would bring the bearing angle from 0.42° to 0.59° which is, I suppose, within the realm of possible. Still, the length change won't account for the observed pitch changes we find all the time. A swelling bridge cap moving the string up the pins 0.2mm (0.0079", which is less than I have observed with controlled tests) will change the length around 0.025mm, or ten times as much as soundboard rise. By my measurements, the point of zero relative movement between the bridge cap and the bridge pin tends to be about 8mm under the cap surface, or just about at the glue joint between the bridge body and cap. An 8mm epoxy laminated cap changes dimension less than 0.001" (0.025mm) through humidity swings that change a solid maple cap 0.011" (0.279mm), so there's nothing awfully mysterious about laminated caps making for a more stable tuning. The next major factor behind the bridge cap/pin changes is, I think, dimensional changes in the pinblock. I consider soundboard rise/fall to be a far distant third on the list. Ron N
This PTG archive page provided courtesy of Moy Piano Service, LLC