Why does a piano go out of tune?
There are actually three different reasons why a piano goes out of tune: environmental, structural, and technical.
The environmental reason
A piano is made out of wood, steel and cast iron, with some felt thrown in for good measure. 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.” Underneath the strings is the soundboard, with is the source of most of our tuning problems. (It’s the same on an upright piano, it’s just that you can’t see the strings)
What you can’t see with the naked eye is that 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 amplifies the sound of the piano. If there were no crown on a soundboard, the tone of the piano would be very dull.
When you look inside the grand piano, you will see the strings crossing over a long piece of wood with little pins sticking out of it, called the bridge, just like the strings of a violin go over a 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 vibrations of the strings. That is basically how a piano creates sound.
We all know that wood absorbs and releases moisture. On very humid day, wood absorbs moisture and expands. When the soundboard absorbs moisture, it also wants to expand. But the soundboard cannot expand outwards because it is restricted by the sides of the piano. So the only way the soundboard can expand is for it to rise up in the middle.
The bridge is attached to the soundboard, and the strings cross over the bridge. So when the soundboard expands, and the crown of the soundboard increases, it pushes up the bridge. When the bridge is pushed up, the strings are stretched tighter. When the strings are stretched tighter, the tension of the strings increases, which in turn, raises the pitch of the strings. But since the strings are of different lengths and thicknesses, they do not all raise their pitches the same amount. And that is when we hear a piano as being out of tune. This effect on the soundboard is reversed when there is less humidity.
The greater the change in humidity, the greater the effect is on the soundboard. The good news in Hawaii is that the humidity and temperature is very stable, with less than a 20% change from one season to another. But even a 5% change in temperature and/or humidity will cause the piano to go out of tune by an equal amount. Since the humidity changes constantly, the piano never really stays in tune.
The structural reason.
There are three major structural components on a piano that will cause a piano to go out of tune: The soundboard, the bridges and the pin block. Fortunately, none of them are real a problem here in Hawaii. In climates where there are wild changes in humidity and temperature, like in the Midwest or the Northeast, where in the summer temperatures can reach over 100, and the humidity is often over 90%, and in the winter the temperature drops below zero and the humidly can reach in the low 20% range, those changes raises havoc with the wood in a piano. The wood in the soundboard, bridges and pin block will crack, which will cause loose tuning pins and bridge pins. And the cracks in the soundboard will vibrate at certain frequencies.
But very seldom do we have any of those problems here in Hawaii, unless the piano came from an area of the country where the weather effected it. But there is one culprit that does create havoc on pianos. And those are termites. Termites love the seasoned and kiln dried wood found inside the piano. Despite taking precautions, somehow these little critters will find their way inside a piano without you know even knowing it.
I’ve seen termites eat the rim of the piano clean through from the inside to the outside. I’ve seen termites eat the bridge to where the pins are falling out. And one pin block I took out of piano had so many termites in it, it sounded like maracas when I shook it,
It is best to have your piano technician inspect your piano on a regular basis to make sure there are no termites setting up housekeeping. If termites do invade your piano, if it is the only piece of furniture that is infected, it can be taken to a termite treatment company for a weekend “get-away”. However, it might be best to consider tenting the whole house.
When termites do eat a substantial part of the pin block, so that the pins don’t hold a tuning, there is a spot treatment that can be done if the affected area is on just a few pins. However, if the invested area is too large, it is best to replace the pin block. This is a major, expensive operation, and is only recommended for high quality grand pianos.
The technical reason
The third reason a piano goes out of tune is technical, meaning, how well a piano tuner does his or her job in the first place. While a piano tuner won’t cause the whole piano to go out of tune, if just one note, or even more specifically, just one string goes out of tune, it will make the whole piano sound sour.
As you probably know, in the upper two thirds of the piano there are three strings for every note. (There are two strings for every note in most of the bass, and the lower octave or so only has one string). All three of those strings have to be exactly in tune with each other, called unisons. Unisons are the most important interval in a piano. When one or two of those strings are out of tune, it will make that note sound bad. And when there are several unisons out of tune on a piano, it makes the whole piano sound out of tune. To make sure unisons stay in tune over a long period of time, the tuner uses two techniques called “setting the pin” and “setting the string”.
The pitch of a string is adjusted by turning the tuning pin. The tool used to turn the pin is called the tuning hammer. The pin is set very tightly in hard wood called the pin block. This pin is over 2 inches long, but all you see is the top third of it. As strong as the pin seems, the force to turn the pin creates a torque, which means while the top of the pin turns, the bottom portion, that which is in the pin block, does not. When the tuner stops turning the tuning pin to where one string is in tune with the other strings, when the hammer is taken off the pin, the bottom portion of the pin will turn the top back, which will cause the string to go out of tune again. To prevent this, the tuner has to “set the pin”, which is done by literally wiggling the pin with the tuning hammer. This equalizes the tension on the pin, and helps stabilize the tuning.
The other technique a piano tuner uses to keep the piano in tune is called “setting the string”. Each string is in contact at nine different contact points on the piano. In comparison, there are only four contact points on a string on a violin: the tuning peg, the nut, the bridge and the hitch pin. A piano string has to move around the tuning pin, over the under string felt, under the pressure bar, under the V-bar or through the agraff, by the front bridge pin, across the bridge, by the rear bridge pins, over the duplex bar, and finally around the hitch pin.
While a string might look very smooth, there are very small “barbs” on the string that will prevent it from moving smoothly past any of those contact points. As the string is tuned, it has to be moved, or “rendered”, past each one of those contact points. When a string is hung up on one of those little barbs, when the pianist plays that note, especially when the note is played at a fortissimo, the string will move past the barb, and the string will go out of tune. To keep that from happening, the piano tuner has to “set the string”, which is done by the piano tuner giving not just each note, but each string that he has just tuned, a sharp, staccato blow. This will help the string render over all the contact points.
Different piano tuners use different methods of setting the pin and the string, but mostly it’s done by constantly giving each note those sharp blows and constantly and wiggling the tuning hammer, often called jerking the pins. But no matter how hard a piano tuner tries, it’s still possible for a unison to go out. That is why even when piano was tuned just before a performance, a unison could still go out of tune during a concert.
Of course, setting the strings and pins won’t make the whole piano in tune unless the temperament is set properly. But that is discussed in another blog post, setting the temperament.