Building a Soundboard
When a piano key is hit by the pianist's finger, it moves a hammer, which in turn strikes metal strings. The strings vibrate, producing a sound. The sound is amplified by the tight skin of the drum. Since the soundboard is such an important part of the piano, it is made with special skill and care. Spruce wood is used because its grain is very close together and straight. These qualities help make the soundboard resilient and flexible. A workman carefully selects the different pieces of wood for each soundboard, making sure they are as similar as possible. This insures that the soundboard will be of excellent quality and will vibrate uniformly.
The pieces of spruce are glued together side by side to form one flat piece of wood. The glued pieces are clamped in a machine called a glue wheel or a windmill press, which can hold eight soundboards at the same time. After the soundboard is glued, workmen move it through a machine that shapes and sands it. The finished soundboard is eight millimeters thick in the center and five millimeters thick at the edges. A soundboard that has this shape vibrates as one unit, amplifying the sound most efficiently. Only Steinway soundboards are shaped this way.
Using another special press, a worker glues seventeen ribs made of sugar pine to the bottom of the soundboard. The ribs keep the soundboard curved slightly upward at the edges, which also helps to amplify the sound.
The bridge, which is made of hard rock maple, is glued to the top of the soundboard in another special press. The strings of the piano will be stretched across it and the vibrations of the strings will travel through the bridge to the soundboard. Whether the bridge consists of one long piece of wood or two separate pieces for treble and bass, its curved shape and its position on the soundboard determine how long a portion of each string can vibrate freely-the "speaking length" of the string. The length of the string, along with its thickness and how tightly it is stretched, determines how high or low the note will be. After the first and second belly the soundboard is then placed in the piano dry.
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