summer sharpness in low tenor

[David Love]

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.  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.

 

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.  

 

An examination of the formula for frequency of a string as a function of
tension (or BP%), diameter, length (BP%) and looking at the differences in
the rate of change depending on tension levels should yield more clarity.   

 

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.  

 

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 David Nereson
Sent: Tuesday, July 15, 2008 12:09 AM
To: Pianotech List
Subject: summer sharpness in low tenor

 

    Most pianos go quite sharp in the summer because of higher humidity, and
the sharpness is most pronounced in the low tenor.  But what I've seen quite
often is that the lowest plain wire strings will be really sharp, but the
wound strings right next to them on the same bridge won't be anywhere near
as sharp.  How does this happen?

    Also, one would think that the middle of the soundboard would experience
the most swelling, which would in turn affect the mid-treble most, not the
low tenor strings down there in the corner of the soundboard.  What gives?

    --David Nereson, RPT

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