Function of the bridge
The bridge on a piano is one of the key components in the making of tone in a piano. The string goes all the way down from the pin to the striking point, where it is hit by the hammer. Those vibrations are passed down the bottom of the piano to the bridge. There are two bridges on a piano. One for the treble and one for the bass strings, and lower tenor.
When you play a note the vibrations of the string pass from the string to the bridge, and from the bridge to the soundboard.
What happens when you remove the dampers, by pressing the sustain pedal, is that the strings you haven’t played are also vibrating in harmonic resonance. Vibrations that are passed down through the bridge.
Bridge separations and cracks create errors
When a bridge separates, it usually happens on either side of the bridge. What happens in most cases is that the top layer, called the ‘bridge cap’ comes away from the bottom portion. Therefore vibrations aren’t carried by the direct portion of bridge, but pass through adjacent parts of the bridge. Therefore you do not get the fullness of tone, or the length of duration.
In this piano, the notes where the bridge has separated have a tubby dead sound, and those with the bridge connected, have a fuller tone, with longer decay.
Repairing a bridge
In order to fix this issue the bridge must be glued back again. In many cases when it is cracked, the bridge top or ‘cap’ will need replacing by a skilled restorer. The bridge shape, height, pin placement is measured and copied. Ensuring that the string remains at the right angle and height as before.
Before a piano is restored, or purchased it is a good idea to check for a damaged bridge first before doing more superficial tasks. A bridge is a key portion of the piano to maintain.