String instrument

A string instrument (or stringed instrument) is a musical instrument that produces sound by means of vibrating strings. In the Hornbostel-Sachs scheme of musical instrument classification, used in organology, they are called chordophones.

Types of string instruments

String instruments are usually categorized by the technique used to produce sound. In order for a string instrument to produce sound, its string or strings must vibrate. There are three common ways to initiate vibration.

Plucking

Instruments such as the guitar, kora and sitar are plucked, either by a finger or thumb, or by some other device such as a plectrum.

Bowing

Instruments like the cello and rebec are usually played by drawing a bow across the strings. All instruments in the viol and violin families fall into this category. Occasionally instruments which are normally bowed are plucked (this is known as pizzicato) instead, and instruments normally plucked are also bowed (for example, Jimmy Page of Led Zeppelin sometimes played the electric guitar this way using a violin bow, and more recently Jón Þór (Jónsi) Birgisson, singer and guitar player of the Icelandic band Sigur Rós has made use of the technique).

Striking

The third common method of sound production in stringed instruments is to strike the string with a hammer. By far the most well-known instrument to use this method is the piano, where the hammers are controlled by a mechanical action; another example is the hammered dulcimer, where the player herself wields the hammers. It should be noted that the piano is often considered a percussion instrument, since sound production through struck blows defines this instrument family; the proclamation that the piano is a percussion instrument has at times served as rhetoric for composers who relished sharp percussive effects.

A variant of the hammering method is found in the clavichord: a brass tangent touches the string and presses it to a hard surface, inducing vibration. This is a very inefficient method of sound production, thus clavichords have a very soft tone. The maneuver can also be executed with a finger on plucked and bowed instruments, where it gives equally soft results.

Other methods

The aeolian harp employs a very unusual method of sound production: the strings are excited by the movement of the air.

Some string instruments have keyboards attached which are manipulated by the player, meaning they do not have to pay attention to the strings directly. The most familiar example is the piano, where the keys control the felt hammers by means of a complex mechanical action. Other string instruments with a keyboard include the clavichord (where the strings are struck by tangents), and the harpsichord (where the strings are plucked by tiny plectra).

With these keyboard instruments too, the strings are occasionally plucked or bowed by hand. Composers such as Henry Cowell wrote music which asks for the player to reach inside the piano and pluck the strings directly, or to "bow" them with bow hair wrapped around the strings, or play them by rolling the bell of a brass instrument such as a trombone on the array of strings.

Other keyed string instruments, small enough to be held by a strolling player, include the plucked autoharp, the bowed nyckelharpa, and the hurdy gurdy, which is played by cranking a rosined wheel.

String length or scale length

This is the length of the string from nut to bridge on bowed or plucked instruments and ultimately determines the distance between different notes on the instrument. For example, a double bass with its low range needs a scale length of around 42 inches, whilst a violin scale is only about 13 inches. On the shorter scale of the violin, the left hand may easily reach a range of slightly more than two octaves without shifting position, while on the bass' longer scale, a single octave or a ninth is reachable in lower positions.

Contact points along the string

In bowed instruments, the bow is normally placed perpendicularly to the string, at a point half way between the end of the fingerboard and the bridge. However, different bow placements can be selected to change timbre. Application of the bow close to the bridge (known as sul ponticello) produces an intense, sometimes harsh sound, which acoustically emphasizes the upper harmonics. Bowing above the fingerboard ( sul tasto) produces a thin, "breathy" sound, emphasizing the fundamental frequency.

Similar timbral distinctions are also possible with plucked string instruments by selecting an appropriate plucking point, although the difference is perhaps more subtle.

In keyboard instruments, the contact point along the string (whether this be hammer, tangent, or plectrum) is a choice made by the instrument designer. Builders use a combination of experience and acoustic theory to establish the right set of contact points.

In harpsichords, often there are two sets of strings of equal length. These "choirs" usually differ in their plucking points. One choir has a "normal" plucking point, producing a canonical harpsichord sound; the other has a plucking point close to the bridge, producing a "nasal" sound rich in upper harmonics.

Production of multiple notes

A single string of a constant tension will only produce one note, so to obtain further notes string instruments employ two methods. Most instruments have more than one string - in the case of the harp or piano, for example, this is the only way in which extra notes are obtained. With instruments such as the violin or guitar the player may press down on the strings with their fingers or some other device in order to effectively shorten the length of it which vibrates. This is known as stopping the string. In such instruments, a fingerboard is often attached to the resonating box - it is between this and the player's finger that the string is stopped.

Sound amplification

Through resonance

A vibrating string on its own makes only a very quiet sound, so string instruments are usually constructed in such a way that this sound is amplified either by a hollow resonating chamber, a soundboard, or both. On the violin, for example, the taut strings pass over a bridge resting on a hollow box. The strings' vibrations are distributed via the bridge to all surfaces of the instrument, and thus amplified.

Achieving effective and beautiful resonance is something of an art, and the makers of string instruments often seek very high quality woods to this end, particularly spruce (chosen for its combination of lightness and strength) and maple (a very hard wood).

In the early 20th century, the Stroh violin used a diaphragm-type resonator and a metal horn to amplify the string sound, much like early mechanical gramophones. Its use declined beginning about 1920, as electronic amplification came into use.

Electric amplification

Electric string instruments use pick-ups to convert the string's vibrations into electrical waves which are amplified and then converted into sound by loudspeakers. Such instruments are often solid bodied, which makes them barely audible with the amplifier switched off. When the amplifier is switched on, they can be much louder than their acoustic counterparts. An advantage of this method is that it allows electronic manipulation of the sound; the guitar overdrive is a classic example.