With over 10,000 separate parts, the action is a mechanical marvel that causes a hammer to strike the proper string when a key is depressed. In the vertical piano, this means converting downward pressure on a key into forward motion of the hammer toward the strings, with the hammer traveling five times as far and approximately four times as fast as the front end of the key. A piano action could be fairly simple if striking a hammer against a string was all that was required. However, the hammer must instantly rebound, otherwise, the note would be muffled by the hammer interfering with its vibration. You can't repeat that same note quickly, unless the hammer returns to its starting position immediately after you release the key. In a modern piano the hammer is in contact with the string less than 1/200 of a second. The mechanism that accomplishes all of the above is a complicated system of levers and pivots which must be assembled with painstaking accuracy so that the action is responsive to a player's touch. Materials used for the best actions are still largely wood, felt, woolen bushing cloth and leather. Just consider the skill required to assemble thousands of small moving parts (many so small that a variance of a thousandth of an inch affects their performance) so that they work smoothly and silently for 50 years or more without lubrication or constant adjustment.
Part of the action, the damper is a delicately balanced device that lets the performer stop a string from vibrating. More than 900 precisely made parts are required for a set of dampers.
The Sounding Board
The sounding board reflects and amplifies the tone caused when a hammer hits a string. It is probably the most important part of the piano. Without it, no music would be produced. Like a hi-fi speaker, a sounding board is a vibrating diaphragm. It must vibrate at exactly the same rate as the strings, and to work properly, must be under constant tension. This is accomplished by having the center of the board arched, or crowned, with the strings pressed down on the board where they cross the bridges. So that it vibrates freely, a sounding board is usually only 1/4 to 3/8 of an inch thick. Ribs help strengthen the board and preserve the crown. Woods used in a sounding board obviously must be carefully selected and seasoned. Also, the heavy and massive cast iron plate that covers the sounding board never touches any part of the sounding board.
The Piano Strings (Wires)
The pitch or sound of each note is determined by the number of vibrations per second of each string's "speaking length" (the part between the wooden bridge at the bottom of the sounding board and where the string bears on the ironplate). The number of vibrations given off by a string can be controlled three ways:
By tension on the string. (The lower the tension - the lower the pitch).
By length of the string. (The longer the length - the fewer vibrations and the lower the pitch).
By diameter of the string. (Wrapping additional wire around a string lowers the pitch)
These three factors are called the scale. Piano manufacturers obtain the 88 different notes by varying the scale.
To lower the pitch a single octave, you must double the string length and so on, redoubling all the way down the scale. However, if you started with a two-inch speaking length string, kept string diameter and string length constant, when worked down all 88 notes, you would end up with a 22-foot upright piano. This is why strings in the bass section are wound with wire. In the extreme bass, another wire is used around the first wrapping because a single winding heavy enough for the proper diameter couldn't be wrapped tightly enough around the center wire.
When a hammer strikes a string, not just a single sound, but a combination of sounds is produced - each overlapping and blending in a way only a trained ear can distinguish. The pitch of the string, call the fundamental tone, sounds first. The lowest pitch a string makes is produced by the vibration of the entire speaking length. Next the strings produce a number of higher tones called harmonics. The first harmonic is produced when the string divides itself in two and each half vibrates separately. Then, almost simultaneously, the string divides itself again. This time it breaks into three partial sections each vibrating at three times the original rate a producing a still higher tone.
Ideally, the singing length of the piano strings should coincide with the length of the sound wave of the note being played. This works well in the top of the piano since the the sound waves are very short, and it is easy to duplicate their length in the piano.