Why do pianos go out of tune?

There are several different ways a piano goes out of tune.  All of them have something to do with the strings.  The strings in a piano are made of wire, and I’ll use the two words string and wire interchangeably.  Let’s take a closer look at the strings in the photo below.  This view is in the area of the piano where the strings are shortest, so you can see both ends in the picture.  

When a string is installed on a piano, the correct gauge of wire is selected to match the design of the piano. Different notes use different gauges.  The wire is looped around the hitch pin.  (Some wires have individual loops on single hitch pins and others make u-bends around the hitch pins as shown here, but that doesn’t matter for our purposes.)  Next the wire is set on the bridge between the bridge pins.  The part of the piano string that vibrates and makes sound is called the speaking length of the string. It extends from the bridge to the termination, and goes over and under some additional bearing points.   (Again, we don’t need to get into the various configurations of string terminations, here.) Then the wires are inserted through small holes in the tuning pins. The wires are tightly wrapped around the tuning pins.  The tuning pins are held by friction in the pinblock, which you can’t see, because it’s behind the cast iron plate.  

Now, we can talk about various ways the wire might have a change in tension and go out of tune.

The first and most dramatic reason pianos go out of tune starts with the fact that when the wires are installed, they are straight or gently curved.  After the wires are installed, as you can see from the picture, there are several tight bends in the wires… at the hitch pins and tuning pins as well as other bends across the bearing points that you can’t see very well from this angle.  Under tension, the wire develops these bends more tightly over days and weeks.  The strings go dramatically flat.  That’s why, when your piano technician replaces a broken string, they will need to come back and re-tune the string several times before it’s as stable as its neighbors.  

Another thing is happening due to the tension over time (hopefully more slowly).  The tuning pins make tiny slips.  The amount of slippage varies depending on the condition of the piano.  The tension in the part of the string we’re calling the bearing is reduced.  A skilled tuner will leave the piano string with a little extra tension in the bearing area to forestall this effect for as long as possible.  If the tension in the speaking length stays constant, so does the tuning, even after the tuning pin has slipped a little bit.  

At the other end of the speaking length, at the bridge pins, we have another dynamic:  humidity changes.  That’s because the bridge and the soundboard are made of wood and are attached together. Wood absorbs and releases moisture as the relative humidity changes.  When the wood is more moist (usually in the summer months), it expands and pushes up on the strings.  The tension in the wires increases and the strings go sharp. The strings in the middle of the lower half of the piano generally move more than the ones in the other regions. 

That’s all for now.  Next time I will build on this to discuss another question that many of you ask:  How often should I have my piano tuned?

Thanks for spending time with me, and don’t hesitate to ask more questions.  I love hearing from you.

Amy