When Yoko strikes one of the 88 keys, a number of things happen to produce that beautiful piano sound. First, the key itself acts as a lever, connected by other levers to lift the hammer and damper. The key, the other levers, the hammer, and the damper are known together as the “action.”
The hammer, which is a small wooden core surrounded by a pad of highly compressed felt, strikes the strings. Most of the notes on the piano are actually produced by three strings together, and the hammer strikes all three. Some of the lower notes have only one or two strings. The strings for higher notes are made of steel, and those for lower notes are made of steel wrapped in copper.
The sound produced by the strings when the hammer strikes is not very loud on its own. The sound you hear from the piano is the sound the strings make reverberating through the soundboard, a large, thin board made of spruce. This functions much like the soundboard on a violin, guitar, and a variety of other string instruments – the strings are pressed down onto a bridge, which sends the reverberations through the soundboard, helping the sound to resonate and grow.
To a lesser extent, the sound also resonates through the piano’s frame, the metal plate on which the strings are mounted. The combined force of the strings exerts literal tons of pressure on the frame of the piano, which is why the frame is traditionally a strong plate of cast iron.
Soft felt dampers rest on the strings when they are not in use. When Yoko presses a key, the damper for those strings is raised, and when she releases it, the dampers falls back onto the string, stopping the vibrations. Although it’s less common in jazz than some other genres, the most commonly-used pedal on piano can be used to left all of the dampers at once, allowing notes to be sustained after they are played and allowing the unplayed strings to vibrate gently with those around them.