This is an introduction to the physics behind how strings produce their various pitches.
The pitch of a string is a function of three factors:
- the diameter of the string (thicker strings = lower tones; thinner strings = higher tones)
- the tension of the strings (lower tension = lower tones; higher tension = higher tones)
- the length of the string (longer strings = lower tones; shorter = higher tones).
Diameter: Another way to look at string diameter is weight or mass — thicker strings have more mass than thinner strings, and will naturally produce lower tones. Guitar strings graduate in thickness from about .08/1000 of an inch (for the higher pitched strings) to about 0.60/1000 of an inch (for the lower-pitched strings). A complete set of strings will typically be comprised of relatively thin, unwound strings for the higher-pitches, and wound strings (a solid core wound with some kind of metal, often bronze) for the lower-pitches.
Tension: How tightly the strings are stretched is controlled by the tuning mechanism (geared machines or friction-style tuning pegs, similar to those still used on the viol family of stringed instruments, on older guitars). By adjusting the tuners, the tension of each string is altered, and this allows the guitar to be “tuned” – each string adjusted to sound a specific pitch.
Length: So on any stringed instrument, the diameter of each mounted string is a given, and the tension of each tuned string is a given. This leaves only the third factor to manipulate to actually change the pitch of a string to play it: by altering it’s length.
Altering a guitar string’s length is of course accomplished by pressing the string down against a fret, one of the series of thin metal strips inlaid into the fingerboard, at precise locations, perpendicular to the strings.
When the string makes contact with a fret, the string can only vibrate from the bridge saddle to that fret. This in effect shortens the string, and the pitch of that string is therefore higher by the prescribed amount. The precise placement of the frets allow for discrete half-steps up and down the string (analogous to each key of a piano played one by one, ascending or descending in pitch).
For more info on how fret placement is calculated, click here.
Non-pitch-related elements that determine
a guitar’s timbre (or tonal quality)
Each part of a guitar serves a particular function, and particular woods and other materials used to make each part of the instrument are selected for the properties they have which make them especially useful for a given function.
The soundboard is often a softwood (spruce and cedar being favorites because of their tight, consistent grain and flexibility), while the back and sides are often made of rosewood, mahogany, maple, or other hardwoods, which give the instrument stability, durability, and help project the sound made by the vibrating softwood top.
The bridge and neck are also of hardwood (again, for density and strength), and the best fingerboards are ebony, an extremely hard and tight – grained wood which resists the abrasive nature of guitar strings, or of rosewood, also a very hard and durable wood.
On inexpensive instruments, the nut and saddle may be made of plastic, while on finer instruments bone is used. Bone is denser and harder than plastic, and better transfers the vibrations of the strings to the guitar, making for better tone and sustain. Synthetic materials such as Tusq which possess high sound – conductive properties are also being used extensively these days.
