All you ever wanted to know about sound post adjustments, but were afraid to ask:
The long term effect of sound post adjustments is misunderstood by many musicians. The movement of a post can definitely change the sound. Moving it closer to the treble f hole usually makes the sound brighter, sometimes harder, the opposite direction makes the violin sound warmer, darker sometimes a little less powerful. It is not really the position of the post itself, which makes the sound brighter but the different pressure it exerts on top and back. We made a lot of experiments making two posts with slightly different lengths from the same long dowel. Then we inserted them aiming for the same pressure. That resulted in two different positions of the post. However the sound felt identical. Of course when moving the longer post to the position the shorter post had held the sound was quite different.
So far this is not a surprise to most of you. But there is a catch. We all know that changing humidity has a big influence on the sound of instruments. Generally dry wood is stiffer than wood in high humidity. Stiffer wood makes the sound brighter. The stiffer the wood the quicker it wants to get back to resting position after being forced to vibrate. The quicker vibrations cannot force the played note to be higher, but the higher harmonics of that note get more enforced and that is what makes the sound brighter or in some cases tighter.
In addition to this phenomenon comes the fact that wood shrinks, when the air is less humid. You know that from the cracks, which can develop in the upper and lower bout near the edges when it is very dry. Dryness will not create cracks in the top where it is arched. Since the top is glued to the ribs it cannot shrink side ways but it flattens out. Top and back arching are now a little flatter than when it was humid. The post does not shrink lengthwise. The result of the flatter arches is that the post now exerts quite a bit more pressure on top and back. That as mentioned before will also make the instrument sound brighter.
Depending on the initial stiffness of the wood some instruments, which are naturally too tight, sound better when it is humid, others are better when the weather is very dry. If your instrument has a better sound when it is humid, you can alleviate the problem in dry weather by inserting a humidifier. (Dampit etc.) works. Case humidifiers only might prevent the pegs from shrinking and slipping down. The little two-inch tubes filled with water were originally made for things like cigar boxes. Once the case is closed there is no air movement within the case. For everyday use there is no way that enough humidified air will go through the f holes to the inside of the violin body where it is really needed to prevent the plates from shrinking. If you play the instrument a couple of times a week, all the humidity, which may have slowly accumulated in the case, disappears when you open the case. If you go on vacation for two weeks a good case humidifier may help a little, though a dampit in each f hole is much more efficient for the inside of the violin body. There is not much you can do if the instrument sounds better in dry climate. Inserting small silicone bags for dryness into the f holes is not an option of course.
Post movements away from and closer to the bridge are more permanent in their effect. The further you move the post away from the bridge the more leverage you have for the bridge to move the top and the instrument may respond easier. But if you move the post too far from the bridge you will loose power.
In our experience the difference in thickness of the post has more effect on the sound. A slightly thicker post makes the sound darker, too thick will make the sound muffled, a thinner post creates a brighter sound and too thin a post will create a shrill sound. The most appropriate thickness depends entirely on the individual instrument.
A very effective way to manipulate the sound is to experiment with different strings. Don’t go out and buy lots of different strings. If you are familiar enough with a particular dealer ask him or her if he/she has some used g strings to try out and compare that to your other strings on the violin. It is easier to make a comparison to the neighboring d string as you play, rather than going by memory and play one set against another.
Finally I would like to address another misconception: If there is a particular note on the violin, which does not sound good or is too weak etc. you will not be able to improve the strength of this note in relation to the others by moving the sound post. If you get more power with an adjustment the whole area gets louder but the individual note is still weaker than the notes above and below.
Don’t underestimate the power of suggestion, which can make you feel that the problem was resolved, but a few days later it miraculously comes back. It reminds me of the woman which was sawn apart by a magician right in front of me and was revived again just a few minutes later.