The Mechanism of a Classical Piano
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           The action of a piano is fairly complex and hard to immediately understand, so for the sake of your comprehension I will simplify where i can. Lets start at the key. When a key is depressed, the end of the key travels approximately 1 cm to the front rail. Typically manufacturers aim for a touchweight of 50 grams, meaning the force required to depress a key is 50 grams. The key, acting as a lever, activates a series of smaller levers to output a travel distance of approximately 5 cm at the head of the hammer. Usually this ratio, called the action ratio, is kept between 5 and 7 to 1. The hammer then strikes three strings of the same note, producing a full pure sound. The reason that untuned pianos sound so terrible is because each key is playing 3 off tune notes.

           The main improvement upon the harpsichord, the piano's predecessor, was the backcheck (Number 15 in the schematic at the bottom). The backcheck allowed for more musical expression through the ability to vary the volume produced by the instrument. When a key is struck lightly, the backcheck moves slowly and does not affect the hammer, allowing it to rebound freely. When the key is struck quickly, the backcheck stops the hammer sharply right as it reaches the string, producing a sharper (louder) sound. The gif below shows a soft strike followed by a firm strike.

                                                                 gif - action closeup
          Most pianos have three pedals. Some upright pianos only have two pedals due to space constraints, but typically pianos have three pedals. The right pedal, the most frequently used and perhaps most important, is the sustain pedal. The sustain pedal simply lifts the damper (Number 1 and 2 in the schematic) allowing the string to resonate instead of being deadened after the hammer strikes. This makes other unplayed strings resonate softly, adding richer harmonics to the sound. The middle pedal, called the Sostenuto pedal, is the least used and perhaps most confusing pedal. This pedal is also a sustain pedal, but is very conditional. The sostenuto will only sustain keys that are held down at the time of the pedal being pressed, allowing for following notes to be staccato and unaffected. This is done by lifting the damper from only the depressed keys. The sostenuto is usually used when sustaining bass notes while playing a higher tune staccato, which would not be possible with just the sustain pedal. On upright pianos or lower-end pianos, the middle pedal instead sustains the entire bass register, allowing for a similar but less genuine effect. Finally, the left pedal is known as the Una Corda pedal, meaning "one string". Originally pianos were designed with two strings per hammer, and this pedal shifted the entire action to the right slightly, causing each hammer to strike only one string instead of two. This produces a much softer sound, as well as reducing the volume. In modern pianos, the action still shifts right but hits two out of three strings instead of one. The sound is still softer and quieter, but less so. In upright and lower-end pianos, this pedal simply moves the hammers half way to the string. The effect is similar, and most ears don't hear the difference.

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