Piano Tuning - How and Why Does a Piano Go Out of Tune?

Introduction

Why does a piano go out of tune? Generally speaking, pianos are thought of as ‘out of tune’ when any one or several of the strings of the piano becomes de-tuned from the corresponding strings. For example, each key on the piano usually has three strings “assigned” to it. Those three strings must be perfectly matched so that instead of hearing three sounds, we only hear one when the key is played. When one or two of those strings start to lose tension and change pitch ever so slightly, we hear phasing between the strings, and it starts to sound like a honky-tonk piano. This is technically referred to as an out-of-tune unison, and pianos frequently need unisons fixed (i.e. tuned). In this article, we're going to explain why that happens.

Piano Tuning - How and Why Does a Piano Go Out of Tune? Video Transcription

In our recording studio here at Merriam Music, we have a Shigeru Kawai SK-7 semi-concert grand piano. It has in fact recently gone out of tune, so it's a perfect example to talk about the three primary reasons these instruments do go out of tune.

Reason #1:

Why does it happen in the first place? The piano has been around for hundreds of years, haven't piano technicians and manufacturer's figured out a way around this problem? Well, one thing to keep in mind is that an acoustic piano is made out of thousands of parts, all interacting under thousands of pounds of tension. A piano string for example at full tension is incredibly tight. There's a huge amount of string tension required for a piano string to do it's job and anything wound that tight around a tuning pin that is fastened into a wooden pinblock is going to pull that pin slightly looser, very slowly over time.
The other thing that happens even slower than that is that the steel wire itself tends to stretch out and become softer and more pliable as it ages. So, you have the string very slowly stretching out over time, and a pin that because of the tension, actually starts to untwist itself slowly and not always evenly.

Reason #2:

The second thing that causes pianos to go out of tune can happen quite quickly relates to the soundboard. When we're talking about a piano's soundboard, we're talking about the flat piece of wood that you can see underneath the strings and the bridge that's glued to it. Piano tuners use beats of overtones to calibrate the slight flatting of 5ths that is needed to achieve equal temperament with an in tune piano. As the moisture level in the soundboard increases during periods of high relative humidity changes, the crown expands and pushes the bridge harder against the strings. The strings are stretched tighter and the piano’s pitch rises. Because this increase in crown is greater in the center of the soundboard than at the edges, the pitch rises more in the middle octaves than in the bass or treble registers.
So even though a soundboard looks flat, it's actually slightly bent which is what we call the crown of the soundboard, and soundboards are arched like this to provide the necessary resistance against the strings. Since soundboards are made of wood, and wood, of course, reacts to both temperature and humidity, the soundboard is subject to both shrinking and swelling. When the humidity goes down, that wood shrinks, which means it's actually going to pull itself flatter. The bridge that the strings are pulled over is going to move down as well and that will lower the tension of the strings.
Every time the humidity drops on a piano you will usually have a shift in the pitch of the piano because of this. Likewise, if the humidity level skyrockets and the wood is now absorbing moisture, the wood tends to expand. The curve then pushes up like on the bridge, causing the strings to become much tighter, in turn making the pitch go up.
Normally when humidity shits up, the pitch will go sharp, and a shift of humidity downward causes the pitch to go flat. New pianos especially can be quite uncooperative, and require some time to settle in, which is why sometimes a new piano needs an extra tuning in the first year of ownership.

Reason #3:

The last reason why a piano might go out of tune is of course due to use. If a piano's being played frequently and receiving maximum force when hitting the hammer up against the strings, the strings vibrate, injecting a lot of energy into the string.
The more the string is vibrating and getting stretched out by all of the mechanical energy being injected into it with the hammer, the more the string actually loosens up bit by bit over time. If you're playing the instrument a lot versus not, a piano that gets played more is going go out of tune faster than a piano that's not.

Conclusion:

Thanks for reading, we hope this article has been helpful. Now you know exactly what it is for a piano to be out of tune, and what causes it to go out of tune.

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