Danny Boone posted here:
>I have a question concerning reasons Kent gave for a piano going
>out-of-tune. He writes: "...the soundboard swells with water
>absorbed... increasing the...crown, and forces the bridge to press
>harder against the strings, INCREASING THE SIDEWAYS DEFLECTION OF THE
>STRINGS, and increasing the tension on the strings..."
>Perhaps I am missing something here, but I cannot figure out how a
>rising bridge can cause an increase in string deflection.
I replied to Danny in private email:
By "sideways deflection" I meant to denote the direction that is
perpendicular to the plane of the piano strings. The description is
difficult, both because piano technicians use the term sidebearing to
denote the zig-zag of the string across the bridge caused by the
bridge pins, and also because vertical and grand harps are rotated 90
degrees from each other, which means that "up," "down," "sideways,"
"front," and "back," are all ambiguous terms when speaking about
pianos in general.
Danny replied:
>I guess it is the word "sideways" which threw me, not the word
>"deflection." When I read that sentence in your article, the picture
>which immediately came into my mind was strings moving sideways,
>toward or away from the sides of the piano. The only way I could
>visualize this to be possible was for the strings to crawl up the
>bridge pins, going up one pin toward the left and up the other to the
>right. This, of course, would cause an increase in tension, but I
>could not understand how it could happen because of a rise in the
>soundboard.
>
>I understand string deflection, and the fact that it does increase
>when the board swells, thus increasing tension. But I wouldn't
>describe it as sideways deflection unless I was standing at the end of
>a vertical piano. Standing at the keyboard, the movement of the board
>in a vertical is directly toward or away from you, and in a grand it
>is up or down.
>
>It's funny how a mental image can pop into one's mind and cause one to
>read something totally different than what another person has written.
>I think this is a case of misinterpretation on my part. However, if
>it was that easy for me to misunderstand, I think the word "sideways"
>would be even more likely to be misunderstood by customers.
Danny is right, of course. How about the following revised version?
"A piano has a few hundred pieces of heavy steel wire called
strings that are strung very tightly, that is, with a great amount of
tension, across a heavy cast-iron frame called the plate. There is a
thin, arched piece of wood parallel to the strings called the
soundboard. A long block of hardwood bridges the gap between the two
and pushes against the strings. When the humidity is high, the
soundboard swells with water absorbed right out of the air, increasing
the arch, called crown, and forces the bridge to press harder against
the strings, increasing the tension on the strings much like when one
pulls back on a bow and arrow. The increased tension forces the pitch
of the piano higher. When the humidity is low, the process reverses;
the soundboard loses water and shrinks, the soundboard crown
decreases, the bridge presses less against the strings, the tension on
the strings decreases, and the pitch of the piano goes down."
Kent Swafford
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