The All-Important Soundboard
If you stretch a strand of wire, fabric or even gut, to a tight tension, and then pluck it, it will make a sound. Children often explore this by stretching a string between two cans and then trying to talk over the "wire’, or even by just stretching and plucking the rubber band that was wrapped around the Sunday paper.
But the sound created by a wire stretched across thin air, no matter how thick or tight the wire, is pretty thin and soft. Once you stretch the wire across an amplifying body, like a slab of wood, the sound can grow exponentially in richness and volume. This is the theory behind all soundboards – the wooden top of a violin, a guitar, and the large wooden diaphragm spanning every piano.
As in the violin and guitar, the strings don’t actually touch the soundboard, but are suspended just above it, laying across a one or two bridges, themselves made of wood or some other efficient sound-conducting material. When the strings are set into motion, the vibrations pass down through the bridge into the larger wooden slab, the soundboard, which then itself vibrates across the whole breadth of wood grains, producing a deep, rich tone. Piano manufacturers scour the earth for the most close-grained, sound-resonant woods they can find, of which spruce has been the favorite.
Here’s a photo of a full grand soundboard, with the short bass bridge and longer snaking treble bridge already glued into place on top, before it gets installed or strung in the piano:
In order for the soundboard to hold it’s slightly bowed shape (for better resonance and resistance to the pressure of the strings,) angled ribs are glued on to its back:
On a grand piano, you can see these ribs by looking up from underneath the piano. The strings then cross over the bridges, themselves made of a very resonant wood, and the strings’ vibrations are conducted down into the soundboard. As you can see in this photo, the strings arre then wrapped around "hitch pins" on the iron plate to hold them at the back end. On the front end, as we have seen, they are coiled around the tuning pins.
The bridges are just slightly higher than the plane of the string running front to back, meaning that they lift the strings a tiny bit higher than perfectly parallel to the soundboard. This insures the massive downward pressure of the strings. The true contact between the strings and the bridge is further secured by forcing the strings to jag to a slight angle, accomplished by a set of angled "bridge pins" for each note, here seen across the treble bridge…
And the bass bridge:
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