OK you've convinced me. Ergo, if the piano is scaled such that all strings are at the same BP%, pitch changes that accompany swings in humidity should be much more uniform. 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 Ron Nossaman Sent: Tuesday, July 15, 2008 7:51 AM To: Pianotech List Subject: Re: summer sharpness in low tenor > The degree to which a string will change pitch with a given change in > length (that's what happens when the case, bridge, board expand or > contract) will be a function of tension (or as Ron N has corrected me > breakpoint %--though I have some questions about that). The low tenor > is generally the lowest tension (and BP%) and so changes the most. > Hockey stick type bridges are where you often see a huge drop off in > tension at the bottom of the tenor bridge and it is on those pianos > where the pitch changes are the greatest. Pianos with more uniform > tension through the bottom of the tenor bridge will not change in the > same way. Using the same formulas I use in my scaling spreadsheet (Sanderson's, mostly), I find that different wire sizes on a given length give the same pitch change with a given change in wire movement. Take 0.002" out of the overall wire length, and they both react the same. A wrapped string with the same overall length and same speaking length and pitch, will have a bp% around 50%, where the plain string is around 25%, will change pitch something near half as much as the plain string. The correlation is in the core bp%, more than the tension. >The wound strings tend to be much higher tension (though not > necessarily higher BP% and therein lies my question about which it is) > and therefore change less. I don't think I've seen a low tenor scale with both plain and wrapped strings on the same bridge where the bp% is similar between the two. Is that even possible? You'd have to go to a core significantly bigger than the adjacent plain string to even get close. > At any rate, it has little to do with differences in the degree to which > the soundboard crown changes in different parts of the piano. Yes, very little. > For example, take two strings of equal length producing equal frequency > (the dependent variable) but with different diameters (gauges-they will > have different amounts of tension and they will also have different BP%) > and then change the length equal amounts and you should see a difference > in the change in frequency between the two. If someone has the formulas > handy (too early for me to go looking) maybe they can post it. That's not exactly correct. Change the gage size of a plain wire unison in your scaling spreadsheet and watch the tension and bp%. The tension will change, but the bp% won't. For a given pitch the only way to change bp% is by changing speaking length. Ron N
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