The changes aren't just deflection changes. Expansion and contraction of the case contributes some too. David Love davidlovepianos at comcast.net www.davidlovepianos.com -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of RicB Sent: Saturday, March 10, 2007 3:41 PM To: pianotech at ptg.org Subject: Seasonal pitch change: Hi John Its true that the lower breaking % will cause a greater change in pitch for any deflection induced change in overall string length. But this still doesnt account for anywhere near enough as far as I can see. Take the following example string. 1400 mm long. 1.2 mm Ø, back length of 100 mm and front termination to pin of 200 mm with 132 lbs of undeflected tension. That puts us at 21 pitch. A very long and very low tension string ... yes ? Assume a 0.5 mm upwards deflection by the bridge... yeilding a 0.31 ¤ string bearing angle as a starting point. That brings tension up to 132.04 lbs, a pitch rise of 0.267 cents and a 0.0013 mm increase in overall string length. Now... if the board manages to push upwards at that point say 0.5 mm more you come up to 0053 mm in overall string length increase from the undeflected length, rises to string tension 132.16 lbs and a pitch rises to 1.068 cents over the undeflected pitch. Now on top of that lets say the bridge swells such that the string climbs up the pin 0.2 mm as has been tossed around. This yeilds an increase of 0.025 mm in string length assuming a 20 mm wide bridge, 10 degree offset angle and 20 degree bridge pin angle. Such an increase in string length by deflection will result in the tension rising to 132,9 lbs and a total combined pitch change of around 5.9 cents over the undeflected pitch, or about 5.6 cents over the nominally deflected string (0.5 mm). So, considering both these factors, a board rise of 0.5 mm, and a string climb up the bridge pin by 0.2 mm... we still dont see more then a 5.6 cent change in pitch for this very long very low tension string. Its significant... but not enough. Noteworthy is that a 60 mm string, 0.825 mm Ø, same back and front lengths at 160 lbs undeflected, a starting string deflection of 1 mm, similiar 0.5 mm soundboard rise and 0.2 mm string climb up the pin will expericence a combined pitch change of about 21 cents over the undeflected pitch, and around 17 cents over the design spec deflection of 1 mm. Seems to me that none of this can explain the pitch change patterns we see, excepting the left / right bit with unisons. Something else has to play a major part. As you, I am also tempted to think about the break in string tension at the doglegs, and endpoints. Cheers RicB Could it be because that's where the strings end? You have an abrupt loss of downward force on the bridge/soundboard, plus (if I understand what the Rons and Del are saying), those strings tend to have less tension due to poor scale design. I think Ron N was saying some days ago that his redesigns are not nearly as susceptible to pitch change as the original scale was. The strings adjacent the plate struts are more prone to change during tuning. Gotta be something to do with the lack of tension on those portions of the bridge at the ends of a section of strings. JF
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