The Source of the Sound
The source of the sound on the piano is the strings, which are stretched tightly over the cast-iron frame. When struck by the hammers, the strings vibrate, producing their recognizable sound. The piano “strings” are actually made of high-tensile steel wire. The strings come in a variety of lengths and diameters, and can be either plain or wound.
According to the physics of sound, when the length of a string is doubled, its pitch is lowered by an octave. If all the strings in the piano were the same diameter, the bass strings would need to be over 30 feet long! The solution to this problem is to gradually increase the diameter, or gauge, of each string. The high treble strings are the thinnest – usually about 0.7 mm thick. The gauge and length of the strings gradually increase as the pitch gets lower. The string gauge reaches about 1.4 mm in the area around middle C. After a certain point, increasing the gauge of the strings can cause the strings to become stiff, adversely affecting the sound. At this point (an octave or so below middle C), the strings change from plain steel wire to a steel wire core wound with copper wire. The copper winding increases the mass of the string, without noticeably changing its stiffness. This allows the string to vibrate at a lower pitch without having to dramatically increase its length. In the extreme bass, the strings may have a double winding of copper.
The shorter treble strings are less resonant than the longer ones – in fact, the top octave of the piano does not even use dampers. In order to achieve a louder sound from the shorter strings, unison strings are employed. Unison strings are two or more strings that are tuned to the same pitch. The treble pitches have three unison strings, the tenor range uses two unison strings, and the bass strings use only one string. The end result is that for 88 notes, there can be as many as 236 strings. Each string has a tension of 160-200 pounds, resulting in a total string tension of 35,000 pounds! This high tension must be supported by a strong cast-iron frame and solid back frame.
The speaking length of the string is the main vibrating portion. On the end of the piano closest to the keys, the speaking length is determined by either the agraffes or the capo d'astro bar. On the opposite end of the string the speaking length is determined by one of the bridges that are glued to the soundboard. The strings are cross-strung (or overstrung) - the bass strings cross over top the tenor and treble strings (see photo on right). This allows for slightly longer bass strings, and also lets the bridges be more centrally located on the soundboard.