Pianos are made of the finest materials, not only for appearances but for excellent sound production. The long fibers of maple wood are strong and supple for construction of the rim, but long fibers of spruce are needed for the strength of the braces. Wood is also needed for making patterns of other parts. Metal is used for a variety of parts, including the cast iron plate. Sand is needed for casting molds. The character of the sand is modified by using additives and binders such as bentonite (a type of clay) and coal dust. Molten iron for the casting is made of pig iron with some steel and scrap iron to add strength. Strings are made of high tensile steel wire that is manufactured at specialized piano string mills.
Pianos are designed by specially trained and educated engineers called scale engineers. Scale engineers choose the materials, create the designs and specifications, and develop the interactions of the parts of the piano. Perhaps the most important aspect of design relates to the structural strength of the piano. About 160-200 lb per sq in (11.2-14 kg per sq cm) of tension is exerted on each of the 220 or more strings in the piano. The piano must perform well, but it also must remain stable over time as changing conditions affect the many materials in the piano differently.
The cast iron plate must support the tension of the stringing scale, covering the sound-board very little; it must have maximum mass for strength, but minimum mass for sound quality. Its shape is unique to the design of the piano because it conforms to the string layout, the placement of the bridges on the soundboard, and the paths of the strings. Because the material is brittle, it must be supported in places where the strings apply tension. Holes are designed in the curved side to prevent the plate from cracking due to thermal stress after it is poured and cooled, and this design allows sound to rise from the soundboard too. The scale engineer first sketches a proposed plate, draws it to scale, and makes a wood pattern; this design is later used for manufacture.
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