An internal sound post helps transmit sound to the back of the violin and serves as structural support.
The body of the violin acts as a "sound box" to couple the vibration of strings to the surrounding air, making it audible. The construction of this sound box, and especially the arching of the top and back, have a profound effect on the overall sound quality of the instrument. The sound-producing system of the violin body includes the top and back (and to some degree the sides, or ribs), the bass bar that is glued to the underside of the top, and the bridge and sound post. In addition to the resonant modes of the body structure, the enclosed air volume exhibits Helmholtz resonance modes.