It is sometimes advertised that piano soundboards and violin parts act as resonators, amplifying the vibrations of the strings. This is not scientifically accurate. The air inside a violin or guitar and inside the case of a piano acts as a resonator, but the soundboard is an amplifier. Vibrations of the strings are transmitted through the bridge of a piano, guitar or violin to the wooden soundboard or top of the instrument. The soundboard vibrating at the same frequencies as the strings efficiently transmits the vibration into the surrounding air, due to its flat shape.
It has been claimed that spruce has a peculiar cellular structure, of “cells within cells”, that vibrate in sympathy with the strings of an instrument. Science can verify this structure, but microscopic cells cannot account for the way all the different pitches of the vibrating strings are amplified for listeners to hear. Listeners hear the vibrations through the air in the room, which is set in motion by the vibration of the soundboard. This is easily demonstrated by comparing an electric and acoustic guitar. The electric guitar has a solid body, with no soundboard. With an electric amplifier, the strings are barely audible. Put the same strings on an acoustic guitar, and you’ll hear plenty of sound. The real reason spruce works well as a soundboard is its high strength to weight ratio. It can be cut thin enough to vibrate easily, while retaining sufficient strength to be structurally sound.