Violins (as well as violas, cellos and basses) have a sound post to provide support between the top and bottom wooden plates of the instrument. Shaped like a thin cylinder, the sound post is placed through the right-hand F-hole. In addition to helping to keep the wooden plates apart, the sound post plays a very important role in the overall sound of the violin; each violin has a specific spot in which the sound post should be placed to provide the best possible sound. Good quality sound posts are often made of spruce.
Before a sound post is placed, a luthier must have the proper tools to reach the area inside the violin where the sound post rests. A sound post tool allows the luthier to slide the sound post through the F-hole into the interior of the violin and into place. This is a very delicate process, as the tool should not touch the violin's wood. In addition, luthiers use a small set of pliers to adjust the sound post once it is in place. Some luthiers also employ a small gauge that allows them to measure the sound post's angle of inclination, as adjustments can be made based on this measurement.
The sound post's placement is very important, as it transmits vibrations between the top and bottom wooden plates of the violin; it also keeps the top plate from sinking towards the bottom plate. The sound post rests under the right side of the bridge, slightly behind the bridge's right leg -- the default distance for the sound post is 1/8-inch behind the right leg, but this will vary slightly on each violin. The violin's tone and volume can vary noticeably based on the sound post's placement, so the luthier tests the location until finding the best placement for optimal sound . In addition to finding the right location for the sound post, the luthier must also ensure that it is perfectly vertical once inside the violin's body.
The construction of the sound post has become an extremely refined process; in addition to being made of spruce wood, there are other requirements to attain the best possible sound. The standard diameter of a sound post is 1/4-inch, and the spruce wood for the sound post should be selected so that its grain lines run vertically along the length of the post. The post's height is determined by measuring the space between the top and bottom wooden plates, and the post must have angles cut into its top and bottom to properly conform to the bends in the top and bottom wooden plates of the instrument.
The strings of the violin are first loosened or removed completely; the luthier then measures for the proper height of the post and then manufactures it to specification. After the sound post is made, the luthier inserts it through the F-hole and places it behind the bridge; this placement is usually an approximation, as pliers are then used to move the sound post to its optimal position. After the post is in place, the luthier measures to make sure that it is perfectly vertical. After the post is secured, the strings are tightened to allow for sound testing, after which further adjustments can be made to optimize the violin's sound.
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