A bridge is a device for supporting the strings on a stringed instrument and transmitting the vibration of those strings to some other structural component of the instrument in order to transfer the sound to the surrounding air.
Most stringed instruments produce their sound through the application of energy to the strings, which sets them into vibratory motion. The strings alone, however, produce only a faint sound because they displace only a small volume of air as they vibrate. Consequently, the sound of the strings alone requires impedance matching to the surrounding air by transmitting their vibrations to a larger surface area capable of displacing larger volumes of air (and thus producing louder sounds). This calls for an arrangement that allows for the strings to vibrate freely, but also conducts those vibrations efficiently to the larger surface. A bridge is the customary means by which this is accomplished.
Typically, the bridge is placed perpendicular to the strings and larger surface (which are roughly parallel to one another) with the tension of the strings pressing down on the bridge and thus on the larger surface beneath it. That larger, more acoustically responsive surface may be coupled to a sound chamber — an enclosure such as the body of a guitar or violin — that assists in sound amplification. Depending on the type of stringed instrument, the resonant surface the bridge rests on may be made of wood, as the top plate of a guitar or violin; of calfskin or plastic, as the head of a banjo; of metal, as on certain types of resophonic fretted instruments; or of virtually any material that vibrates sympathetically with the strings.
Bridges may consist of a single piece of material, most commonly wood, that fits between the strings and the resonant surface. Alternatively, a bridge may consist of multiple parts. One common form is a bridge that incorporates a separate bearing surface on which the strings rest, termed a saddle. This is often of a material harder than the bridge itself, such as bone, ivory, high-density plastic, or metal. A classical guitar uses a saddle which sits loosely in the hardwood bridge and is kept there by string tension. The saddle has shallow grooves in it, at least for the treble strings, to prevent them moving around during hard playing.
Yet another type of multipart bridge is common on instruments whose sound plate is curved rather than flat. Instruments of this type, such as arch-top guitars and mandolins, often have a bridge comprising a base and a separate saddle that can be adjusted for height. On classical and flat-top guitars the bridge is glued to the top. Where it held on to the top by string tension, as in archtop guitars, it is known as a floating bridge, and requires a separate tailpiece.
The bridge must transfer vibration to the sound board or other amplifying surface. As the strings are set in motion, it does this by bending to and fro along the string direction at twice the rate of the string vibration. This causes the sounding board to vibrate at the same frequency as the string producing a wave-like motion and an audible sound.
Bridges are designed to hold the strings at a suitable height above the fingerboard of the instrument. The ideal bridge height is one that creates sufficient angularity in the string to create enough down force to drive the top but places the strings sufficiently close to the fingerboard to make noting the strings easy. Bridge height may be fixed or alterable.
In addition to supporting the strings and transmitting their vibrations, the bridge also controls the spacing of the strings from one another. This is accomplished by shallow grooves cut in the bridge or its saddle. The strings sit in those grooves and thus are held in their proper lateral position. The nut, situated at the opposite end of the instrument from the bridge or tailpiece (typically where the head holding the tuning pegs joins the fingerboard), serves a similar string-spacing function at the strings' other end.
Electric guitar bridges 
Bridges for electric guitars can be divided into two main groups, "vibrato" and "non-vibrato" (also called hard-tail). Vibrato bridges have an arm (called the vibrato or whammy bar) that extends from below the string anchoring point. It acts as a lever that the player can push or pull to change the strings tension and, as a result, the pitch. This means that this type of bridge produces a vibrato rather than what some call to be tremolo, but the term 'tremolo' is deeply entrenched in popular usage and some use the word tremolo over vibrato. Non-vibrato bridges supply an anchoring point for the strings but provide no active control over string tension or pitch. A small group of tremolo bridges has an extended tail (also called longtail). These guitars have more reverb in their sound, because of the string resonance behind the bridge. The Fender Jaguar is a famous example of such a guitar.
All bridges have advantages, depending on the playing style, but, in general, a non-tremolo bridge is thought to provide better tuning stability and a solid contact between the guitar body and the strings.
Vibrato bridges 
Generally, the more contact the bridge has with the body (i.e. the lower the position), the better the sound transfer will be into the body. A warmer sound with increased sustain is the result.
Vibrato bridges usually need to be suspended in some way, which reduces contact. Most tremolo designs today use a group of springs in the guitar body, which oppose the tension of the strings. Some players feel that the vibration of the springs affects resonance in a way that makes the guitar sound better; others disagree. Naturally, it all depends on personal preference and the musical style of the individual player.
Since many playing styles make use of a tremolo obligatory, most solid-body guitars today are manufactured with one of two kinds of tremolos:
Non-Locking Tremolo 
Non-locking (or vintage) tremolos are the bridges found on all guitars manufactured prior to the advent of the Floyd Rose locking tremolo in the late 1970s and many (typically cheaper) guitars manufactured thereafter. For many playing styles, vintage tremolos are a good choice because they are easy to use and maintain and have very few parts. Some people feel that they can also provide a better degree of sound transfer, especially with tailpiece type tremolos such as the Bigsby. However, "Synchronized Tremolo" type found on the Fender Stratocaster is balanced against a set of screws in much the same manner as a locking tremolo and offers little improvement in the way of sounds transfer. Given that this type of tremolo is installed on solid body guitars the degree to which sound transfer affects the sound that the instrument produces is minimal. Also, keeping a guitar with a non-locking tremolo in tune can be difficult. The most common types of non-locking tremolos are the "Synchronized Tremolo" type and an almost endless stream of copies. The Bigsby vibrato tailpiece is another option, although that type has largely fallen out of favour.
Locking Tremolo 
Another type of tremolo is the locking tremolo. These bridges have a sort of vise in each saddle which clamps the string into place (usually with an Allen key). The end of the string has a clamp, which holds the string in place, stopping it from slipping. This is especially useful for songs that require tapping or heavy/hard playing styles. Once set up for tuning stability, locking tremolos are very good. However, their fulcrum points offer extremely minute contact with the body, which may disturb sound transfer.
Non-Tremolo bridges 
It is generally thought that non-tremolo bridges offer better transfer of string vibration into the body. This is due to direct contact of the bridge to the guitar's body.
These bridges are bolted directly to the guitar body and, assuming good quality, limit longitudinal string movement completely. Therefore tuning stability is assured.
The improved transfer of string vibration into the body has an effect on the sound, so guitars with this type of bridge have different characteristics than those with tremolos even when it is removed. There are no springs in the body or a cavity to accommodate them, which also affects resonance.
See also 
- A.B.Wood (Admiralty Research Laboratory), A Textbook of Sound, Publ Bell, 3rd ed. 1955. No ISBN found.
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