The Vibration of the Piano String
Vibrations of String
The strings of a piano are the source of the sound. Strings are stretched tightly over the cast-iron frame of the piano. The hammers of the inner mechanism strike the strings, causing them to vibrate. When the strings vibrate, they move in periodic waves. Each wave has a different sound based on the rhythm of its vibrations. So when the strings begin to oscillate, the recognizable sound of the piano is produced.
The strings of a piano are made of steel wire, and all are of different lengths and diameters. When the length of a string is doubled, its pitch lowers by one octave (according to the physics of sound). The strings are not all the same diameter; this is because the strings creating lower sounds, which would have longer string lengths than higher sounds, would have to be very, very long in comparison with the short strings. They would be so long they would stretch far beyond the length of a grand piano - if all of the strings were the same diameter, then the very lowest strings would stretch more than 30 feet. So, the stings are of different diameters to provide a feasible option for housing all of the sounds desired in a piano.
The highest treble strings have the thinnest diameters, usually around .7 mm thick. Then, as the strings gradually increase in thickness and length, the pitch gradually decreases. Near the middle, the diameter is 1.4 mm. However, soon after middle C, the thickness of the wire makes it too stiff to vibrate properly. Now, these wires are made of a regular steel wire which is wound with a copper wire. This copper wire adds mass, which helps oscillations, without affecting the stiffness of the wire. Fortunately, this also reduces the need to greatly increase the wire as well.
The main vibrating portion of a string is the speaking length. The speaking length is determined on one end of the piano by the capo d'astro bar and on the other by the bridge that is attached to the soundboard. The strings are strung on top of one another, called "cross-strung." The bass strings cross over the treble and tenor strings, allowing for the bass strings to be longer in length.
The soundboard amplifies the sound produced by the vibrations of the strings. The soundboard is a large, wooden plate that radiates a large volume of sound over a wide range of frequencies. When the hammer strikes the string, it moves back and forth. In doing so, it displaces air - this is how sound waves are created. However, since the strings are very small and do not create a very loud sound, they need an intermediary to transmit their sound to us. Attaching the strings to a bridge allows the sound vibrations to go into the wooden resonator. This displaces much more air than the tiny string, creating a loud sound. A crown is located at the top of the soundboard. The crown keeps the strings set firmly on the bridge by pushing up against the strings' downward pressure.
Shorter strings are less resonant than longer strings. Unison strings - two or more strings tuned to the same pitch - are what help to achieve a louder sound from the shorter treble strings. Treble pitches have three unison strings; tenor pitches have two; bass strings have only one. So, there are 88 notes and as many as 236 strings.Definitions
Cast-iron frame: The metal plate found in the belly of the piano that is responsible for withstanding the high string tension.
Hammer: The hammer is the focus of the action - the action works to throw the hammer against the strings. The hammer has a felt head that strikes the strings in order to produce the sound.
Damper: The felt-padded block that rests on the strings, damping the string vibrations. When the key is depressed, the damper is raised, allowing the string to vibrate.