Bridge (stringed instrument)

For other meanings, see Bridge (disambiguation).

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 that amplifies the sound. Most stringed instruments produce their sound through the application of energy to the string, 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 themselves requires amplification 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 indirectly on the larger surface beneath it. That larger, 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. 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.

The bridge may transfer vibration to the sound board or other amplifying surface in one of two ways. In the simplest arrangement, the tendency of the taut strings to seek a straight line creates a down force where the strings pass over the bridge at an angle. As the strings vibrate, their motion is transmitted as increases and decreases in the downward force applied to the bridge. The instrument's top plate is thus set into motion by the action of the string as it is transmitted through the bridge. This type of bridge configuration typically employs a tailpiece or other device that attaches the strings to the instrument aft-ward of the bridge itself. Violins, for example, have bridges that work in this fashion, with the strings passing over the bridge and then attaching to the tailpiece. Certain other instruments, such as steel-string guitars, have bridges that serve as the terminus for the strings. On this type of bridge, pins hold the string ends in position at the far edge of the bridge, which is firmly attached to the surface of the sound plate. As the strings are activated, rather than moving the bridge primarily in an up-and-down fashion, the changes in string tension pull and release on the bridge, causing it to move in a rocking motion. As tension on the strings increases, the rear of the bridge is rocked upward and forward, in essence, rotating on an axis running along the length of the bridege and perpendicular to the strings. As the stretched string rebounds, the bridge rocks downward and backward. Although this action is too subtle and rapid to be visible to the naked eye, these changes in the strings' tension as they vibrate cause a succession of sound plate motions that create a slight depression in front and slight bulge behind the bridge in one phase, followed by an opposite phase creating a bulge in front and corresponding depression behind the bridge. This sets the top into vigorous vibration, producing a wave-like motion and a clearly audible sound.

Bridges are designed to hold the strings at a suitable height above the fingerboard of the instrument, the part that holds the frets and that is pressed by the fingers in playing to shorten the length of the strings that are free to vibrate. 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 either determined during instrument manufacture in the case of nonadjustable bridges or may be alterable in the case of adjustable bridges.

In addition to supporting the strings and transmitting their vibrations, the bridge typically 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.