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The Sound Post
One of the major books on violin making was written by A. Heron-Allen, and published in England in 1885. It has a great, highly detailed section on the sound post and how to adjust it. This is one of the best treatises on the operation of a violin’s sound post I have yet seen. His tips on where to move the sound post to change the sound of the violin do work!
Phil Williams
Voyager Recordings & Publications
The Sound Post - A. Heron-Allen, Ward Lock & Co., London, 1885.
The sound post is a little round stick of fine, even-grained pine, varying, of course, in length with the distance from each other of the back and belly of the fiddle, both of which it must just firmly touch. It must not be long enough to force the back and belly apart ever so slightly, and must not be so short as to fall down when the instrument receives a jerk, or when the strings are let down. The violin, as I have said before, must be so constructed as to be able to sustain the pressure of the strings without giving way. If the sound post, in addition to its most important duties (set out below), has to bear a part in sustaining the belly, or if originally it is cut too long, the result will be a feeble tone; for the sound post will in this case check the vibrations of the belly, instead of communicating them to the back. It has a diameter of 1/4 inch, and its fibres must form a right angle with, that is, must be set across the fibres of the belly. Its diameter must be, to a certain extent, adapted to the fiddle in which it is set, for if it is too slight the tone will be thin, and if it is too thick te tone will be rendered dull. Its exact position depends entirely upon the quality and peculiarities of the fiddle, and must be carefully regulated by an experienced workman, bu it is almost invariably within 1/4/ inch behind the right foot of the bridge. A high-built instrument will require the sound post nearer the bridge than a flatter model. It is the more important to trust this work only to a skilled hand, for the only access to it being through the right-hand f hole, an inexperienced operator is very apt to destroy the appearance of this f hole, not to mention injuries to the internal surfaces of the fiddle. A poor violin is often improved by placing the post nearer the bridge, but a violin thus arranged requires very careful playing to render the tone even. When, though even, the tone is rough and harsh, the post must be moved back little; if the high strings are weak and the lower ones harsh, the post must be moved a little outwards towards the f hole; if the low notes are weak and the high ones shrill, it must be moved very slightly toward the centre.
It must, however, be borne in mind, that every time the post is moved the equilibrium of the instrument is deranged, and it will take some time to get accustomed to the change, and as these changes are naturally deleterious to the fiddle, they must be made as seldom as possible. As I have said before, the mass of air contained in a fiddle ought to yield a certain note; if the post is too short, a lower note will be produced, and the upper notes of the violin will suffer; if it is too long, the contrary will happen. In fact, if your sound post is too short, it will have the same effect as if the back and belly had been worked too thin, and vice versa. It sometimes, however, occurs, that when a sound post has been made of unseasoned wood, it contracts after being set in a fiddle; this, if suspected, should be ascertained by relaxing all the strings, when, if the sound p;ost has shrunk, it will fall, and a new one must be put in, fulfilling the proper conditions.