Note by Note
If you read From Key to Ear, you know what happens in the piano when Yoko strikes a key. Have you wondered what happens when a piano hammer strikes a string? Three factors affect how the strings (also called piano wire) associated with a particular key produce the note, and a series of balances and compromises between these factors create the differences in pitch and the evenness in sound.
Length: The longer the string, the lower and more loudly resonant the note it produces. This accounts in part for the grand piano’s distinctive shape. When the length of a string is doubled, the pitch is lowered by one octave. Imagine how strange a piano would look if length were the only variable in the strings – considering that a standard piano spans just over seven octaves! If the highest strings were the length they are in a concert grand, the lowest strings would be over thirty feet long.
Thickness, or gauge: The thicker the string, the lower the note it produces. This is how piano-makers avoid the absurd 30-foot-long problem: the strings vary in thickness considerably. The thinnest, highest strings in a piano are about 0.7 mm thick. Once they start approaching 2 mm, pure steel strings start to become stiff, and produce a dull sound. So, starting at around an octave below middle C, strings have a steel core but are wrapped in copper wire to achieve the correct thickness.
Number: Since shorter strings produce softer sounds, the highest notes on a piano do not have dampers to stop their resonance. However, even without dampers, the highest strings just do not transmit sound to the soundboard nearly as well as their bass counterparts. Because of this, the majority of notes on a piano are produced by more than one string. Bass notes have only one string per note. Tenor notes have two strings each and treble notes have three strings each. Each string is tuned to the same note and struck simultaneously by one hammer, which is why they are called unison strings.
The amount of tension the strings are put under, combined with the amount they are pounded on by felt hammers and the way they are stretched during tuning, means that they need to be very strong and well made. In fact, the high-tensile high-carbon steel wires are among the most demanding applications of steel, and only a handful of companies manufacture them.