How weather affects pianos

[Wimblees@aol.com]

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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. 



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