Staying in Tune

Why does a piano go out of tune?  Here's one short answer:  a piano holds strings that are under tension, and any material under tension tends to react in a way to diminish that tension.

This is not to say that the strings are the only material in a piano reacting to tension.  Certainly when the strings are new they go through a period of "stretching out," but modern string manufacturing has ensured that this stretching comes almost to a halt within a couple of years.  The tension of the strings, however, is brought to bear on many different parts of the piano, and it is the reaction of all these parts to the tension that contributes to the instability of a tuning.

String tension is possible only if the strings are secured at each end, so these end points carry the brunt of the string tension.  One end is the hitchpin, which is usually a small steel pin driven into the cast-iron plate.  This end is pretty stable.  The other end is the tuning pin, which is a large steel pin driven into the wooden pinblock.  This end is much less stable because the wood reacts rather strongly to the tension.  The tension is enough to slowly pull the pin right out of the wood, a phenomenon seen on many smaller uprights.  Before that point is reached, though, the hole that the tuning pin sits in can become sufficiently distorted and enlarged that the pin can rotate, either slowly or suddenly, releasing the string tension.

The string also passes under, over, and around various bearing points, all of which react to the tension.  An important bearing point is the wooden bridge, which miraculously carries tension in three directions:  each string is bearing downward on the bridge, bearing to the right against one bridge pin, and bearing to the left against another bridge pin.  Bridges are known to crack, split, and otherwise fall apart as a result.  But even if a bridge doesn't fall apart, the pins still creep and the wood compresses, releasing some of the tension.

The bridge, in turn, bears down upon the soundboard, a giant raft of wood that must support all this tension.  This brings us to another short answer to why pianos go out of tune:  a piano is made mostly of wood, and wood expands and contracts with changes in humidity.  As the humidity changes, and the soundboard absorbs and releases moisture, the board flexes up and down, and the bridge and strings with it.  The bridge also rocks and tilts back and forth, and indeed all the wooden parts including the pinblock expand and contract.  All this movement changes the tension of the strings.

The soundboard is often constructed under its own tension, pushing upward against the bridge to forestall its eventual collapse.  When the humidity is high and the wood expands, it crushes itself against its joints and constraints, and then, when dry, it shrinks and cracks.  The glue lets go.  It is no surprise that almost all pianos have cracked soundboards.

Even the metal bearing points give way to the string tension.  If you examine the agraffes and pressure bars and duplex scales and aliquots and capos, you'll see where the strings have slowly and steadily pressed grooves into the brass and steel and iron.

So it is a wonder that a piano stays in tune at all.  Then comes the tuner, and another short answer to why pianos go out of tune:  the tuner has to temporarily destabilize the string to add or subtract tension, and may not be successful in restoring the string's stability.  The instability introduced by the tuner comes in two forms.  Theoretically, the tuning pin simply sits in its hole in the pinblock, but in reality the tuning pin floats in a bed of highly compressed wood, with the greatest compression where the string pulls against the pin.  The pin, the wood, and the string have arrived at a temporary stasis, which the tuner disrupts when he or she turns the pin.  The tuner must then try to set the pin back in a position where it won't wander under the new tension.  I can tell you that this is the trickiest part of tuning.

The second instability comes because of all the string bearing points.  Theoretically, any change made to the string's tension applies evenly to the entire string, but in reality the friction of the string against its bearings prevents the even distribution of the tension.  The tuner must ensure that there is no extra tension hidden in some section of the string that will suddenly release itself in a glorious detuning as soon as the string is hit with the hammer.  This is why tuners strike the notes so vigorously when they tune, to work out the uneven distribution of tension.  Some instability is inevitably left behind, to be eventually worked out to the player's dismay.