A string can vibrate in several patterns at once
A plucked guitar string does not move as a single rigid unit. Its motion can combine a basic pattern, in which the whole string bows back and forth, with patterns that include one or more stationary points called nodes. Between those nodes, sections of the string vibrate in opposite directions. These patterns are the string’s modes of vibration.
The lowest mode produces the fundamental frequency—the pitch we usually hear as the note. Higher modes produce frequencies at whole-number multiples of that fundamental. They are called harmonics. Because the string can vibrate in several modes at once, the sound contains a blend of the fundamental and its harmonics rather than just one frequency.
The pluck determines the blend
When a pick or finger pulls and releases the string, it gives the string a particular shape and motion. That initial disturbance sets some vibration modes going more strongly than others. Picking near the bridge, where the string is held close to the guitar’s body, tends to emphasize higher harmonics; picking farther from the bridge tends to give the lower modes more prominence. The pitch remains essentially the same, but the balance of frequencies changes, making the sound seem sharper, rounder, or brighter.
The string’s motion also passes energy into the guitar. The bridge transfers vibrations to the soundboard, the broad top plate of the instrument, which moves air and helps the note carry. The body does not create the string’s harmonic pattern, but it affects which parts of the sound are strengthened or damped.
That is why a guitarist can alter a note’s character without changing the fret or the string: the note’s fundamental sets its basic pitch, while the mix of harmonics—and how the instrument responds to them—gives it much of its recognizable tone.
