An acoustic guitar is one not dependent on an external device to be heard but uses a soundboard which is a wooden piece mounted on the front of the guitar's body. The acoustic guitar is quieter than other instruments commonly found in bands and orchestras so when playing within such groups it is often externally amplified. Many acoustic guitars available today feature a variety of pickups which enable the player to amplify and modify the raw guitar sound.
There are several notable subcategories within the acoustic guitar group: classical and flamenco guitars; steel string guitars, which include the flat top or "folk" guitar; twelve string guitars and the arch top guitar. The acoustic guitar group also includes unamplified guitars designed to play in different registers such as the acoustic bass guitar which has a similar tuning to that of the electric bass guitar.
- Renaissance and Baroque guitars
- These are the gracile ancestors of the modern classical guitar. They are substantially smaller and more delicate than the classical guitar, and generate a much quieter sound. The strings are paired in courses as in a modern 12 string guitar, but they only have four or five courses of strings rather than six. They were more often used as rhythm instruments in ensembles than as solo instruments, and can often be seen in that role in early music performances. (Gaspar Sanz' Instrucción de Música sobre la Guitarra Española of 1674 constitutes the majority of the surviving solo corpus for the era.) Renaissance and Baroque guitars are easily distinguished because the Renaissance guitar is very plain and the Baroque guitar is very ornate, with inlays all over the neck and body, and a paper-cutout inverted "wedding cake" inside the hole.
- Classical guitars
- These are typically strung with nylon strings, played in a seated position and are used to play a diversity of musical styles including classical music. The classical guitar is designed to allow for the execution of solo polyphonic arrangements of music in much the same manner as the pianoforte can. This is the major point of difference in design intent between the classical instrument and other designs of guitar. Flamenco guitars are very similar in construction, but are associated with a more percussive tone. In Mexico, the popular mariachi band includes a range of guitars, from the tiny requinto to the guitarron, a guitar larger than a cello, which is tuned in the bass register. In Colombia, the traditional quartet includes a range of instruments too, from the small bandola (sometimes known as the Deleuze-Guattari, for use when traveling or in confined rooms or spaces), to the slightly larger tiple, to the full sized classical guitar. The requinto also appears in other Latin-American countries as a complementary member of the guitar family, with its smaller size and scale, permitting more projection for the playing of single-lined melodies. Modern dimensions of the classical instrument were established by Antonio Torres Jurado (1817-1892). Classical guitars are sometimes referred to as classic guitars.
- The modern Ten-string guitar
- To put it succinctly, the addition to the guitar of four strings tuned a specific way (C, A♯, G♯, F♯) is to this instrument as the invention of the various pedals is to the piano. To appreciate the analogy, three concepts first need to be understood. These are that: there exists a phenomenon called resonance; a string has certain resonant frequencies corresponding to its overtones or harmonics; the guitar's fingerboard is normally tempered so that, of the pitches produced by the left hand on the fingerboard, really only the octaves of the open strings, and to a large extent their fifths, are 'in tune' with the strings' resonant frequencies. As a consequence of the above, the fact of the matter is that the (normal) guitar as an instrument has an inherent inconsistency between those sounds produced on the fingerboard that are effectively enforced and sustained by so-called sympathetic resonance (that is E, B, A, and D) and those other eight tones of the chromatic octave, each of which lacks the same support from a sympathetic string, in other words, from a string whose strongest resonant frequencies (that is, fundamental, octaves or fifths) include a resonant frequency that corresponds to the pitch produced on the fingerboard. This inconsistency has been corrected by the introduction of the modern 10-string guitar.
Conceived in 1963 by Narciso Yepes, the concept of the modern 10-string guitar follows a strict musical and scientific logic. Yepes discovered that, by adding four strings tuned to four very specific pitches, string resonance would be available for all 12 tones of the chromatic octave, without introducing any redundant resonances (more of E, B, A, and D) that would serve only to perpetuate, or worse, augment the inconsistency. In other words, the invention of Yepes was the addition of the strings C, A♯, G♯, F♯, which vibrate when pitches corresponding to their resonant frequencies are initiated on adjacent strings. Thus (to consider only one of these tuned resonators, the 10th string, or F♯): if F♯ is played (on the 6th string), the 10th string will resonate in unison. Likewise, it will produce the higher octaves f♯, or f♯' when these pitches are played on higher strings. In addition, this same F♯-string will resonate when its fifth is played, producing the tones of c♯ or c♯'. In the same manner each of the twelve tones of the chromatic octave is now supported by a string resonator, and (should the performer so wish) any tone can be sustained beyond the moment when the left-hand finger has let go of (or shifted position on) the string upon which the sound was initiated. The analogy to the pedals of the piano should now be evident. It should also be noted that (correctly understood) the idea of the modern 10-string guitar is, in a sense, exactly contrary to what is in the popular opinion considered to be the 'obvious' reasons for adding extra strings, that is to say extended bass range as open strings and/or simplification of left-hand technique through the avoidance of barrés and stretches. While the modern 10-string guitar does offer the extended bass range (scordatura of the 7th, or lowest, string is possible down to Helmholtz A1), it is contrary to other so-called 'multi-string' guitars, including ones that arbitrarily also have ten strings, in the sense that these make no attempt to resolve the resonance issue and, indeed, even augment it when additional B, A, or D strings (and their resonances) are introduced. Nevertheless, for better or worse, the popular opinion seems to prefer (as far as methods of stringing and tuning go) the 19th century concept of a 10-string guitar (extra basses for the sake of extra basses) rather than the nuanced interpretative possibilities that the modern instrument has to offer.
- Portuguese guitar
- The Portuguese guitar is a 12 string guitar used in Portugal for the traditional Fado song. Its true origins are somewhat uncertain but there is a general agreement that it goes back to the medieval period. It is often mistakenly thought to be based on the so-called "English guitar" – a common error as there is no such thing. For some time the best instruments of this and other types were made in England, hence the confusion. "English guitar" refers to a quality standard, not really an instrument type. This particular instrument is most likely a merge of medieval "cistre" or "citar" and the Arabic lute.
- Flat-top (steel-string) guitars
- Similar to the classical guitar, however, within the varied sizes of the steel-stringed guitar the body size is usually significantly larger than a classical guitar and it has a narrower, reinforced neck and stronger structural design. This allows the instrument to withstand the additional tension of steel strings. The steel strings produce a brighter tone, and according to many players, a louder sound. The acoustic guitar is used in many kinds of music including folk, country, bluegrass,pop, jazz and blues.
- Archtop guitars
- These are steel string instruments which feature a violin-inspired f-hole design in which the top (and often the back) of the instrument are carved in a curved rather than a flat shape. Lloyd Loar of the Gibson Guitar Corporation invented this variation of guitar after designing a style of mandolin of the same type. The typical Archtop is a deep, hollow body guitar whose form is much like that of a mandolin or violin family instrument and may be acoustic or electric. Some solid body electric guitars are also considered archtop guitars although usually 'Archtop guitar' refers to the hollow body form. Archtop guitars were immediately adopted upon their release by both jazz and country musicians and have remained particularly popular in jazz music, usually with flatwound strings. The electric semi-hollow body archtop guitar has a distinct sound among electric guitars and is consequently appropriate for many styles of pop music. Many electric archtop guitars intended for use in rock and roll have a Tremolo Arm.
- Resonator, resophonic or Dobro guitars
- Similar to the flat top guitar in appearance, the sound of the resonator guitar is produced by a metal resonator mounted in the middle of the top. The physical principle of the guitar is therefore similar to the banjo. The original purpose of the resonator was to amplify the sound of the guitar. This purpose has been largely superseded by electrical amplification, but the resonator guitar is still played because of its distinctive sound. Resonator guitars may have either one resonator cone or three resonator cones. Three-cone resonators have two cones on the left above one another and one cone immediately to the right. The method of transmitting sound resonance to the cone is either a "biscuit" bridge, made of a small piece of hardwood, or a "spider" bridge, made of metal and larger in size. Three-cone resonators always use a specialized metal spider bridge.The type of resonator guitar with a neck with a square cross-section -- called "square neck" -- is usually played face up, on the lap of the seated player, and often with a metal or glass slide. The round neck resonator guitars are normally played in the same fashion as other guitars, although slides are also often used, especially in blues.
- 12 string guitars
- The twelve string guitar usually has steel strings and is widely used in folk music, blues and rock and roll. Rather than having only six strings, the 12-string guitar has six courses made up of two strings each, like a mandolin or lute. The highest two courses are tuned in unison, while the others are tuned in octaves. The 12-string guitar is also made in electric forms.
- Russian guitars
- These are seven string acoustic guitars which were the norm for Russian guitarists throughout the 19th and well into the 20th centuries. The guitar is traditionally tuned to an open G major tuning.
- Acoustic bass guitars
- Have steel strings or gut strings and often the same tuning as an electric bass guitar.
- Tenor guitars
- There is very sketchy background information about tenor guitars on the Internet. A number of classical guitarists call the Niibori prime guitar a "Tenor Guitar" on the grounds that it sits in pitch between the alto and the bass. Elsewherethe name is taken for a 4-string guitar with a scale length of 23" (585 mm) – about the same as a Terz Guitar. The tenor guitar is tuned in fifths, C G D A, as is the tenor banjo and the cello. It is generally accepted that the tenor guitar was created to allow a tenor banjo player to follow the fashion as it evolved from Dixieland Jazz towards the more progressive Jazz that featured guitar. It allows a tenor banjo player to provide a guitar-based rhythm section with little to learn. A small minority of players close tuned the instrument to D G B E to produce a deep instrument that could be played with the 4-note chord shapes found on the top 4 strings of the guitar or ukulele. The deep pitch warrants the wide-spaced chords that the banjo tuning permits, and the close tuned tenor does not have the same full, clear sound.
- Harp guitars
- Harp Guitars are difficult to classify as there are many variations within this type of guitar. They are typically rare and uncommon in the popular music scene. Most consist of a regular guitar, plus additional 'harp' strings strung above the six normal strings. The instrument is usually acoustic and the harp strings are usually tuned to lower notes than the guitar strings, for an added bass range. Normally there is neither fingerboard nor frets behind the harp strings. Some harp guitars also feature much higher pitch strings strung below the traditional guitar strings. The number of harp strings varies greatly, depending on the type of guitar and also the player's personal preference (as they have often been made to the player's specification).  The Pikasso guitar; 4 necks, 2 sound holes, 42 strings] and also the Oracle Harp Sympitar; 24 strings (with 12 sympathetic strings protruding through the neck) are modern examples.
- Extended-range guitars
- For well over a century guitars featuring seven, eight, nine, ten or more strings have been used by a minority of guitarists as a means of increasing the range of pitch available to the player. Usually, it is bass strings that are added. Classical guitars with an extended range are useful for playing lute repertoire, some of which was written for lutes with more than six courses.
- Guitar battente
- The battente is smaller than a classical guitar, usually played with four or five metal strings. It is mainly used in Calabria (a region in southern Italy) to accompany the voice.
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