No, there are other factors besides the plate and soundboard like all the other wood parts of the piano: soundboard, bridges, rim, pinblock, bracing, etc. All are affected by changes in the moisture content due to changes in humidity. The percentage contribution from each is more difficult to determine. The plate doesn’t change with changes in humidity because it has no moisture content to fluctuate. It may change, however, because the wood parts that it is fasted to change and the plate does flex in various directions. The greatest pitch change on a piano will depend on the BP% in the scale and is often the low tenor where it drops precipitously. The placement of struts may also have an impact because the plate may flex differently near the struts than not. Temperature doesn’t much effect the wood (though it may make a difference in terms of available moisture in the air) but it does influence the metal parts, strings in particular. Take a hammer shank and rub it on a string until it heats up some. Notice that the string goes flat but as it cools it goes sharp again. Similarly, start tuning a piano in a room where the temperature is quite cool and if the heat comes on while you are tuning and the room changes temperature significantly the area you began with will have gone flat (that’s why church tuning can be so much fun). Things like broken strings impact plate flexing and tension as has been pointed out by Ron Nossaman. If the bearing downward on the bridge were the issue then when a string breaks on a piano with zero bearing (and there are many) the strings around the broken one would not go out of tune since there is no change in the downbearing. Not surprisingly, however, broken strings cause surrounding strings to go out of tune by roughly the same amount regardless of the amount of downbearing. It’s the plate. There are many factors that influence tuning stability and it’s difficult without a completely controlled situation and experiments to determine the exact input of each factor on any given piano. Fortunately, for us, the many factors help contribute to our job security. David Love www.davidlovepianos.com If it isn't the soundboard going up and down, and I have trouble accepting the concept of the string riding up on the bridge having an effect on tuning, there is only one other factor that could be the culprit, and that is the plate expanding or contracting. If you think about it, where is the greatest change in pitch? I find the biggest change is at the first couple of notes on the treble bridge. On most pianos, that is the longest distance from one end of the plate to the other. If the plate expands from top to bottom, (or front to back on a grand), it wouldn't take much to change the pitch even a few cents. The reason the bass strings, and especially the lowest few, don't change much is because of the big bass strut, which gives the plate more stability. So perhaps the reason the piano goes out of tune with humidity and temperature has almost nothing to do with the soundboard/bridge moving up and down, but is primarily because the plate is expanding and shrinking. Someone else mentioned something along this line, but it seems have been lost in the conversations. Has anyone done an experiment or taken measurements of this? Wim -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://ptg.org/pipermail/pianotech.php/attachments/20100403/aea84729/attachment.htm>
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