From Wimblees@aol.com
We all have to deal with customers asking why pianos go out of tune.
Sometimes customers do not understand why a piano goes out of tune, or blames
you for doing a b ad jib, when there is a drastic change in the weather. That
is why I've written an article, that, I think, explains in layman's terms why
a piano goes out of tune. I've copied the article below. I hand a customer
the article when they ask questions. Comments are welcomed. (You're welcome
to copy the article and use it).
Wim
Why does a piano go out of tune?
By Willem Blees RPT
Have you ever noticed how a piano seemed to go out of tune, but then shortly
thereafter, it sounded better again? Blame it on the changes in the weather.
It's the changes in temperature and humidity that cause a piano to go out of
tune. This article explains exactly what goes on inside a piano that causes
this to happen.
A piano is made out of wood, steel and cast iron. On a grand piano, when you
lift up the lid, you can see the strings, which are made out of steel. The
strings are attached to the cast iron plate, which is that large gold "harp".
If you look through the strings, you will see wood. That is called the
soundboard. This is the soundboard that everybody worries about cracking.
(That is a topic for discussion some other time).
It is this soundboard that creates all the tuning problems. Although you
can't see it with the naked eye, the soundboard is crowned, which means it is
higher in the middle than at the edges. To give you an idea of how much of a
crown there is, if the arc of the soundboard was part of a ball, that ball
would be about 65 feet in diameter. It is this crown that produces the sound
of the piano. If there were no crown on a soundboard, the tone of the piano
would be very dull.
We all know that wood absorbs and releases moisture. During the moist summer
months, wood absorbs moisture and expands. This is very evident in the summer
when you have trouble opening windows and doors. When the soundboard absorbs
moisture, it also wants to expand. And, like a door, which expands against
the frame, which is also expanding, the soundboard expands against the sides
of the case. But because the soundboard cannot expand outwards, and because
it is crowned, the only way the soundboard can expand is for the middle to go
up.
When you look inside the grand piano, you will see that the strings cross
over a long piece of wood with little pins sticking out of it. This long
piece of wood is called the bridge. The bridge transfers the vibrations of
the strings, which are set in motion by the hammers, to the soundboard, which
in turn amplifies the sound of the strings. (That is basically how a piano
creates sound). What you cannot see, however, is that the strings actually
go up and over this bridge. In other words, if you were a flea, you would
need to go up hill to get to the bridge and down hill once you have crossed
over the bridge.
Since the bridge is attached to the soundboard, and the strings cross over
the bridge, when the soundboard expands and the crown of the soundboard
increases, bridge will be pushed up. When the bridge is pushed up, the
tension of the strings is increased. And just as a rubber band will produce a
higher pitch when it is stretched tighter, the strings will increase in pitch
when they are stretched tighter by the pushed up bridge. And whenever the
tension of strings is changed, we perceive that a piano is out of tune.
This effect on the soundboard is reversed when the humidity is very low, as
in the winter. Even though the outside humidity during the winter months is
about the same as during the summer, when moist cold air is heated, it looses
its moisture content. It is not unusual for a house to have a relative
humidity of less than 25% during the winter months.
The greater the change in temperature, or humidity, the greater the effect on
the soundboard. Since the temperature and humidity changes constantly, the
piano never really stays in tune. But since most homes now have air
conditioning, and the humidity and temperature are controlled more, the
changes are not as dramatic as they used to be. Still, every time the
temperature or the humidity changes, the piano goes out of tune.
So the next time your piano goes out of tune shortly after the piano was
tuned, think back about the changes in temperature and humidity. Most likely
it was the fault of the weather. But when you do hear the piano out of tune,
the piano tuner should be called, because there is nothing worse than
listening to an out of tune piano.