Sound Post and Bass Bar
The sound post, or "soul post", fits precisely between the back and top, just to the tailward side of the treble bridge foot. It helps support the top under string pressure, and has a variable effect the instrument's tone, depending on its position and the tension of its fit. Part of adjusting the tone of the instrument is moving the sound post by small amounts, both laterally and along the long axis of the instrument, using a tool called a sound post setter. Since the sound post is not glued and is held in place by string tension and by being gently wedged between the top and back, it may fall over if all the strings are slackened at once.
Running under the opposite side of the bridge is the bass bar. While the shape and mass of the bass bar affect tone, it is fixed in position, and not so adjustable as the sound post. It is fitted perfectly to the inside of the instrument at a slight angle to the centre joint. On many German trade instruments it used to be common fashion not to fit a bass bar but to leave a section of the front uncarved and shape that to resemble one. During the baroque era bass bars were much much shorter and thinner. Often modern players have asked luthiers to change a baroque bar to a modern one in an old instrument. Occasionally the luthier will refuse to change this feature feeling that it may compromise the instrument.
Read more about this topic: Violin Construction And Mechanics
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