What is a soundboard

The soundboard is an essential part of every classical stringed instrument.
It is substantially a spruce diaphragm , the function of which is to transfer the vibrations produced by the strings and conveyed by the bridge, to the air. A vibration that propagates in air (the transmission medium) is a sound wave (the sound perceived by our ears ).

The ribs are spruce bars, shaped in accordance to particular specifications, that are glued on the back side of the soundboard. They support and bend (keep in tension) the soundboard in the piano as well as in the other stringed instruments.