Connecting the Strings and Soundboard
The bridge plays a crucial role in the sound of the piano. It is the job of the bridge to connect the source of the sound (the strings) to the amplifier of the sound (the soundboard). There are two bridges attached to the top of the soundboard. The treble and tenor strings pass over the long bridge, and the bass strings pass over the short bridge . The use of two bridges allows the strings to be cross-strung (or overstrung) – which positions the bridges on the central and most resonant portion of the soundboard.
The strings are held over the bridge at an extremely high tension, so the bridge must be made of a very hard wood in order to keep the strings from cutting in. Today, bridges are usually made from several strips of beech or maple which are laminated on top of each other. The top of the bridge is capped with a thicker, solid strip of maple. The bridges are glued directly to the top of the soundboard, and the bottom edge of each bridge is designed to precisely fit the crown of the soundboard. Metal bridge pins are driven into the top of the bridge, and they serve to evenly space and align the strings as they pass over the bridge. There are over 220 strings on the grand piano - at two bridge pins per string, there are over 440 bridge pins driven into the bridges. Notches cut into the top of the bridge make its front end slightly higher than the path of the strings. This ensures a tight connection and optimal transmission of sound vibrations between the strings and the soundboard. The bridges define the exact speaking length of the strings on the far end of the piano (the agraffes and capo d'astro define the speaking length on the ends near the tuning pins).