The Structure of the Piano
Design of the Strings Enriches the Sound

A piano keyboard has 88 keys. The number of strings depends on the model, but is usually around 230. For the tenor and treble notes, three strings are strung for each key, and for bass notes, the number of strings per note decreases from three, to two, and then to one as you approach the lowest bass notes.
In addition, the strings become shorter in length going from low-pitched notes to high. The thickness of the string changes in steps, and the higher the pitch of the note, the thinner the string. Strings for bass notes are wound with copper wire, while strings for tenor and treble notes use bare wire and are not wound.

Three strings are strung for each key

Three strings are strung for each key

The three strings for middle pitch and high pitch notes are not only intended to increase the volume during play, but also enrich the quality of the sound.
Even though the three strings that correspond to the same note are hit by one hammer, the point at which the hammer makes contact and the positions of supports vary between the strings, so the three strings do not oscillate in exactly the same way, bringing life to the reverberation of the strings after they have been hit and a rich, full quality to the sound.

Let's compare the sound when variation in the vibrations of the strings have been deliberately introduced.
With one of the strings tuned to A at 440 hertz, example one has the other two strings tuned 1.5 cents higher and lower, example two has them each at an interval of 1.0 cents, and the third example has them each differing by 0.5 cents. Example four has the three strings tuned to the same 440 hertz frequency.
One cent is equal to the difference in frequency for one hundredth of a semitone interval on the equal temperament scale.

Example 1 : String 1(440 Hz), String 2(+1.5 cent), String 3(-1.5 cent)
Example 2 : String 1(440 Hz), String 2(+1.0 cent), String 3(-1.0 cent)
Example 3 : String 1(440 Hz), String 2(+0.5 cent), String 3(-0.5 cent)
Example 4 : String 1(440 Hz), String 2(+0.0 cent), String 3(-0.0 cent)

The examples above use an artificial piano sound with exaggerated pitch interval, in order to make it easier to identify the difference in the reverberation of the strings after they have been struck.
Quantifiable differences in the piano sound can be identified depending on how the vibrations of the three strings vary.
Professional piano tuners are able to discern even more subtle distinctions in tone by ear, in order to tune the instrument to produce the richest quality of sound.

The way individual strings are designed also enriches the tone. A bridge supports one end of the string. For tenor to bass strings the other end is supported by an agraffe, and for treble notes, by a part called a bearing. The segment of the string between these supports is called the "speaking length." For the A note in the exact center of the compass, the string will vibrate at a frequency of 440 hertz, i.e., 440 times per second. In the treble section of the compass, resonating segments at the front and back of the string are called the front duplex and back duplex respectively. These segments vibrate sympathetically with the speaking length, and increase the attractiveness of the sound. With no resonating segments at all, the tone is less rich.

Support Structure for Strings in the Treble Range

Support Structure for Strings in the Treble Range