There is more than what meets the eye on your average guitar. The truth is that there are far more invisible parts then there are visible and the bridge plate ranks amongst the highest in importance in regards to tone production.
The bridge plate is one of the most important parts of the guitar. This is the plate of quarter-sawn wood underneath the bridge which the string ball ends rest on. Yup, the ball ends are not held in place by the pins even though it may look like it from the top. The ball ends pass through the holes in the bridge (typically 3/8″ thick), then the top (1/10″ thick), and then through the bridge plate. The bridge plate is a polyhedron that fills in the space behind the joint in the X braces. Its grain is often orthogonal (intersecting or lying at right angles) to the orientation of the grain in the top.
You can see this clearly in the featured photo above of Dan Richter’s D FS guitar arriving soon. In this case the bridge plate is also angled; this will be a multi-scale guitar.
This channel of energy from the string into the bridge plate is very important and is the key player in our “pleasing” tone generated by the instrument. The role of a good bridge plate is most easily understood once you realize how the “pleasing” properties of the soundboard are exploited. The neck of the guitar is designed in such a way to absorb as little kinetic energy as possible from the vibrating strings. If you could imagine a neck made of a material that vibrates freely in your hand you can gauge how much “tone” we could loose in just the neck alone. When the string is plucked by the player, the energy is sucked out by the nut, the neck, the tuning pegs, the headstock, the saddle, the bridge, and the top. There is no focusing of the energy anywhere. Good guitar necks shed this energy down the neck and into the bridge. Luthiers want as much energy as possible to be tapped by the saddle and bridge. The way this is done is brilliant.
The vibrating string is directed downward into the top at a given break angle through the bridge. The bridge pin is then fitted properly to channel any micro-vibrations or gaps in the energy chain between the nut and bridge plate. In other words, a properly fitted bridge pin with at least 4mm of room left after the ball end is essentially a tonal “goalie” stopping trailing vibrations and guiding them back into the sound board. By varying the densities of materials we can change the performance and responsiveness of this tonal “goalie.” The bridge pin is slotted as to leave room for the wire, but no room for the ball end. So when the string is at pitch the ball end is pulling up on the bridge plate. The ball end is resting against the bridge plate and the pin is simply keeping the ball end from coming back into the hole while channeling all the micro vibrations that are lost in the energy chain back into the sound board.
The bridge plate, invisible as it is, plays the most important role out of all the invisible features of the guitar and plays a huge role in sound transfer and tone production.
(source: Storyteller Guitar by Doug Larson)