The Sound post and bass bar

The treble foot of the bridge (the one under the E string) is quite near the soundpost, which is a small post connecting the relatively flexible belly plate of the violin to the much stiffer back plate. This post prevents the belly from collapsing under the vertical component of the tension in the strings, and it also couples the vibrations of the plates. This connection to the stiffer back plate restricts the motion of the treble foot considerably. The bass foot of the bridge is much easier to move up and down. (Press gently with your fingers and you can feel this difference.) As a result, when a string is driven from side to side by the action of the bow, the bridge tends to pivot about the treble foot, highlighted in yellow in the sketch at right. The bass foot moves up and down a little, moving part of the belly with it. The position of the soundpost (the pivot for the motion of the bridge) is critical to the sound of the instrument. Makers will sometimes move it slightly to change the response of the instrument. Small changes can have a noticeable effect. Under the belly on the bass foot side of the bridge is the bass bar. It extends beyond the f-holes and thus transmits the motion of the bridge over a large area of the belly.

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