A year or two ago I used to tune for a university that had Boston GP156s
in practice rooms. In the winter they dropped 20 cents at A4 and rose 20
cents at A4 in the summer. I set up a simple measurement system to see
what was actually moving. These are the things I measured at A4 from the
winter to the summer or vice versa (can't remember now):
- Lateral movement of the bridge
- Vertical movement of the soundboard directly in front of the left
bridge pin
- Vertical movement of the soundboard directly behind the left bridge
pin (to see if the bridge tilted
- Vertical movement of the bridge measured from the middle of all six
bridge pins
- Vertical movement of the front bridge pins
I wanted to see if the vertical movement of the soundboard was great
enough to account for a 20-cent change in pitch.
My conclusion: The soundboard did NOT move up or down enough to account
for that pitch change.
Other factors I did not include were total length of each of the three
strings, downbearing and the effect of the wooden pinblock.
Ron contacted me when about my results, which started a conversation
about why pianos go sharp and flat. I got distracted by work and we
never finished our conversation.
I will revisit my data soon.
I also set up an experiment under a grand piano to see how much upward
pressure I would have to apply on the soundboard to account for a 10- to
20-cent pitch change. My set up was crude, but simple. I put a hydraulic
jack on a bathroom scale under A4, then set a dial indicator on top of
the bridge to measure the upward movement of the SB/bridge assembly. I
don't remember all the numbers, but I do remember that I stopped at 150
pounds upward pressure because I was afraid I was going to break
something. It would be fun to repeat that experiment to also measure
string grooves before and after. I can't remember how much of a pitch
raise I eventually reached, but I remember that my other experiment did
not show as much soundboard movement as I had to generate to reach
approximately 20 cents change in pitch.
What have you all done to test this soundboard-only-causing-pitch-change
theory?
Just curious who's out there setting up ways to verify all the things we
believe.
Good discussion,
John Parham
Original Message------
I believe Ron stated that the rise in pitch was due to a swelling of the bridge cap pushing the strings up the pins thus increasing the length of the wire segment across the bridge due to the slant of the pins. The fall in pitch is due to the shrinking of the bridge cap and decrease in the string segment length. The low tenor has such a marked fluctuation due to the lower tension or breaking percentage.
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Regards,
Jon Page
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