The best way to understand the bridge is to think of how a piano generates sound. You, the pianist, hit the key, which in turn causes a hammer to strike a string and make it vibrate. If it were just the string itself vibrating, the piano would generate a fairly weak sound. But, as we’ve seen with the soundboard, it’s all about amplifying that sound you make.
The bridge is a pathway for the vibration of the strings down into the soundboard, where the real amplification happens. So the bridge’s job is to transmit as much of the vibration as possible in as faithful a way as possible into the soundboard. It’s like a pipeline for sound waves. Piano strings are secured to a “bridge” at one end, and either to an Agraffe or a pressure bar at the other end.
The bridge is glued to the soundboard, the strings rest on top of the bridge which transmits the energy of the vibrating string to the soundboard which in turn amplifies its sound. They are made of hardwood with steel pins on which the strings are secured.
But because the bridge is a complex design it is made of composite materials. Look at the cap for example. This runs along the top of the bridge and is sometimes made of a different type of wood. On top of this there is often a graphite coating on which the strings rest.
This allows for the vibrations to move more uniformly down into the soundboard.Sometimes the pins can work loose or the bridge can become dislodged from the sound board
Also, the bridge can crack or split. With cracks in the bridges and pin-block, the piano will stop holding it’s tuning and will go out of tune more and more often until it’s not serviceable. Once the bridges are relocated onto the new soundboard they are lubricated, notched, pinned, and varnished.