Every note sounded on a piano is the result of a string, or set of two or three strings, vibrating at a specific frequency (rate of traveling back and forth) determined by the length, diameter, tension and density of the wire. A shorter, lighter string, under more tension, vibrates faster, and produces a higher-pitched sound.1
The strings on a modern piano are made of hard, tough steel wire that can nick the blade of regular wire cutters. A piano technician needs wire cutters with a compound-leverage joint and tungsten-carbide jaws. Each note, from the treble (highest in pitch, at the right-hand end of the keyboard) down toward the bass (low, at left-hand end of the keyboard), is produced by three strings vibrating at exactly the same speed, sounding together when struck by their hammer. At some point in the bass, and this is not standardized from one make of piano to another, the number of strings per note changes from three to two, then for the lowest notes, one string per note. Strings tuned to the same note are called unisons. If unison strings are not all at the proper tension, they will produce different pitches, and the piano will sound "out of tune"; tuning the piano involves adjusting string tensions so they match again.
Strings lengths and diameters increase from treble to bass. Several notes are strung with the same thickness of wire, but cut to different lengths, and tuned to different tensions to produce the desired pitches. In the bass range of the piano, the strings are wound with other wire to make them thicker so they will vibrate more slowly. On modern pianos the winding wire is almost always copper. The copper winding wire and the steel core wire are both so heavy that the lowest string is about a quarter of an inch in diameter.
Each string is tuned (as in any stringed instrument) by the turning of its tuning pin. A piano's tuning pins have square sides above the round shanks that are driven into the wrest-plank or pin-block. The tuner's wrench has a long handle for leverage, and a square socket to fit over the pin's top half. The other end of the string is anchored by a hitch-pin which does not move.
1 "High" and "low" pitch are not to be confused with "high" and "low" volume, which is loudness. High pitch is like a mouse's squeak; low pitch is like thunder rumbling.
When a piano key is pressed, a hammer flies up and strikes the strings tuned to produce the corresponding note, then falls away from the strings quickly so as not to stop their vibration. The mechanical action allowing the hammer to drop instantly away from the strings is called the escapement. If a hammer remained in contact with the strings, it would produce a "clunk" sound instead of a sustained musical tone. Modern piano hammers are made of wood covered with thick, tightly compacted felt. The size of the hammers increases steadily from treble to bass. If a piano is played so much that the felt becomes extremely tightly compacted from striking the strings, the piano may produce an unpleasant, harsh tone. The tuner can voice the hammers by loosening the felt fibers a bit with special needles so the tone becomes mellower.
The vibration of a piano's strings alone would be too quiet to be heard; their sound must be amplified. Piano strings, like those of a violin or a guitar, press down on a bridge which conducts their vibration to the large, thin piece of wood called the soundboard. Wooden ribs glued across the board, underneath, help spread the strings' vibration throughout its mass. While a crack in a violin body is a very serious matter, a crack in a piano's soundboard can be repaired easily, without losing any of the piano's tone quality, and without "major surgery". Often, cracks in a piano's soundboard are of no musical consequence, and should be left alone. Maintaining proper humidity during the winter heating season helps to prevent cracks from occurring.
The function of a damper is to stop the vibration of a string when the sound has continued long enough. As long as the player's finger depresses the key, the damper belonging to that key's strings remains lifted, and the strings are free to vibrate. When the key is released, the damper falls back against the strings, pressing soft felt against them to absorb the vibration. The highest strings on a piano usually do not need dampers, because the energy of their vibration is released so quickly, they stop sounding in a short time.
The pedal on the right is for the purpose of lifting all the dampers away from the strings at once, allowing the player to sustain a series of notes whose sound continues even after each key has been released. Furthermore, because strings can vibrate in sympathy with other strings whose vibrations are mathematically related to their own, lifting all the dampers allows strings to vibrate which have not been struck, but which are in harmonic relationship with those which have been. This gives a fuller, richer sound. One does not want to use the damper pedal indiscriminately, or the result is something like using too much water in water color paints; the colors run into each other and become blurred and muddy. There are sounds which do not blend well and should not be sustained together.
The pedal on the left is for producing a softer tone. On a grand piano, it shifts all the keys and their hammers to the right, just far enough so two things happen; the hammers strike fewer strings (two of a set of three, one of a set of two. The bass strings are so large they still are struck;) and the part of the hammer's surface that has become firmly packed from repeated contact with the strings is moved over so a softer, less-used part of the surface strikes the strings. On a vertical piano, the soft pedal moves the hammers closer to the strings so they strike with less momentum.
The middle pedal on a modern piano (a comparatively recent American device) can be for lifting only the bass dampers, or on other pianos, for sustaining whatever note or notes were played at the moment the pedal was pressed. (neither of these pedals has any historical relationship to European classical music.) On vertical pianos, the middle pedal sometimes activates a muting effect, placing cloth between the hammers and the strings for an extra soft sound. This is a very old device that was used on pianos in Beethoven's time.
The highest combined string tension on a large modern concert grand piano is around thirty tons, although most pianos have far less string tension than that. To withstand the tension of the strings, a piano must have a tremendously stable frame. A modern piano's strings are supported by an iron plate, cast in a single piece and bolted to a heavy wooden frame. Part of the tone of a modern piano is the resonance of this metal plate. Because metal is less subject to changes in humidity than wood is, a piano with an iron plate holding its strings stays in tune through changes in weather that would untune a wooden-framed instrument.
-material of wire; harder or softer
-material of winding wire on bass strings
-wire diameter per note
-number of strings per note in various parts of range
-parallel string layout throughout range (straight-strung)
-layout with bass strings crossing above tenor strings (over-strung)
-material with which hammer core is covered: felt, leather, or combination
-hardness of striking surface; voicing
-size and weight of hammer per string
-point at which string is struck by hammer; how far from bridge
-qualities of the wood from which it is made
-direction of wood grain relative to direction of strings
-ribs under soundboard: number, shape in cross-section, size, spacing
-bridge placement on soundboard; size, relationship to center, edge of board
-size of key; length, width of naturals, sharps
-material of key covering: ivory, ebony, bone, plastic
-depth of key-dip (distance key moves down to activate hammer)
-squared or rounded edges to key-tops
-leverage ratio from key to hammer
-weight of action parts being moved by key; some keys are counterbalanced with lead weights
-single-escapement action (key must be completely released in order to repeat)
-double-escapement action (key will repeat after partial release)
-fewer moving parts give sense of more direct control from finger to hammer
-more complex mechanism; sense of more remote-control playing