Strictly speaking, all guitars that do not rely on amplification to produce sound are acoustic. It is only in the last fifty years though that the term has become necessary, since the invention of the electric guitar. Generally speaking, acoustic implies steel-strung as distinct from nylon-strung instruments, which themselves are commonly known as classical or Spanish guitars.
In fact, the steel-strung guitar is a relatively recent development. The Nineteenth and early Twentieth Centuries saw an explosion of distinctively American folk styles, that today would be described loosely as hillbilly music. The popularity of loud instruments such as banjos and fiddles meant that the Spanish guitar, with its gut strings, struggled to be heard.
The steel-strung guitar was created as a response to this problem. But steel strings exert a greater pull on the body and neck, and the features common to most modern acoustic and electric guitars were in turn developed in response to this issue by the two leading makers of the time - Martin and Gibson.
Classical guitars are reinforced by transverse bracing - pieces of wood glued across the underside of the
SoundboardThe resonating surface inside a piano or other stringed keyboard instrument, which resonates in much the same way as the top of an acoustic guitar or violin. The finest grand pianos generally have a slightly curved soundboard, while most others are flat.soundboard
or top. This provides enough strength to match the tension of nylon or gut strings, but a better solution was required for steel strings. A thicker top would of course
SustainA general term with various specific meanings in music/audio production: 1) In general terms, how well an instrument's sound persists once played. 2) The third part of the ADSR envelope used by many synthesisers, Sustain ('S') determines to what volume a note decays after it is triggered, but before the key is released. (Decay 'D' controls how fast this happens) 3) The piano pedal which removes the dampers from the strings, so that notes will continue to sound even after the keys have been released. This is implemented in MIDI as a specific message with 2 possible values: 127 (pedal down) and 1 (pedal up).hold
up to higher tension, but would also result in a loss of
Volume1) In audio and music, the loudness or amplitude of a signal. 2) In computing, a fixed amount of storage space, addressed as a single entity ('C:', 'D:' etc). A physical drive may contain more than one volume, but a single volume may also span more than one drive!volume
. Martins system of intersecting diagonal braces, known as the X-brace system, solved this problem while retaining a relatively thin, resonant top.
The neck also required reinforcement - classical guitar necks are usually made of a single piece of wood, but this is not sufficient to withstand the tension of steel strings, and even very slight neck warping is enough to ruin the playability of an instrument. The resolution of this problem arrived in the form of a steel rod placed in a channel under the
FingerboardThe part of a stringed instrument against which the strings are pressed when playing. Usually called the fretboard on fretted instruments.fretboard
. Almost all acoustic and electric steel-strung guitars and basses made in the past century employ this device, which is known as a truss rod - some even have two.
While Martin were essentially responsible for the design of the modern flat-top acoustic guitar, Gibson borrowed from violin makers to create the archtop acoustic guitar, which has had a significant influence on the development of large-bodied semi-acoustic electric guitars, popular with jazz players to this day.