Bridge (instrument)

A bridge is a device that supports the strings on a stringed musical instrument and transmits the vibration of those strings to some other structural component of the instrument—typically a soundboard, such as the top of a guitar or violin—which transfers 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 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 provides resonance that helps amplify the sound. Depending on the type of stringed instrument, the resonant surface the bridge rests on may be made of:

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 saddle 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, which prevents them moving around during hard playing.

...
Wikipedia