Making Stringed Instruments
In order for a string instrument to produce sound, its string
or strings must vibrate. There are three common ways of bringing
this about.
Instruments such as the guitar and kora are plucked, either by a finger or thumb,
or by some other device such as a plectrum. Instruments like the cello and rebec are
usually played by drawing a bow across the strings. However, instruments
normally bowed are occasionally plucked (this is known as pizzicato), and instruments normally plucked are sometimes
bowed (Jimmy Page of Led
Zeppelin sometimes played the electric
guitar this way, for example).
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 manoeuvre can also be executed with a finger
on plucked and bowed instruments, where it gives equally soft results.
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 she does 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.
Choosing the contact point 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.
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 so as
this sound in 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)
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.
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