When doing some research a few weeks back into getting violin's restored, I came across this brief, interesting article. If you aren't a musician I suppose it might be boring...but I found it informative and thought I'd share it.
By Richard Biggs
Set-up is a term to describe the outfitting of a stringed instrument through the proper cut of the bridge and the correct placement of the sound post. Every violin has the potential to sound lovely and a good set-up will bring it to its best ability. When people are unhappy with the sound of the violin it may be because of an incorrect set-up. I have helped several people by preventing them from buying a more expensive violin by setting it up properly. People were shocked with the sound it made. To learn about why this happens, let's explore what your instrument does when you play and what makes the bridge and sound post so integral to its sound.
Every part of the stringed instrument vibrates and produces sound as the musician plays. The strings produce most of the overtones you hear, while the body of the instrument provides a considerable part of the rest. When the musician bows, the bridge transmits the sound waves to the body, which acts as an amplifier. The body moves and vibrates as the strings are bowed. The bridge rocks and moves the top of the instrument up and down. The sound post acts as a plunger and transfers this movement to the back of the violin.
The bridge does a lot more than hold up the strings. It may look like a simple wooden piece, but in fact it's very complex. Take a close look at your bridge and you will notice there are cuts on the bridge where the strings lie. Each little cut causes the bridge to go into a different action. This action sets all the parts of the violin into motion and creates the complex acoustical sound of the violin that we mentioned above. Bridges are purchased by the luthier as ‘blanks.' It's up to him or her to customize the bridge to the particular instrument. One size does not fit all. The feet of the bridge must be in full contact with the belly of the violin, which is very difficult as many older violins have dents on their bellies caused from the bridge's movement over time. This cannot be changed so the luthier must be creative when fitting the feet to the belly.
If your bridge collapses immediately put it back up by loosening the strings and sliding the bridge back up. You can prevent a bridge from collapsing by checking to make sure it is never leaning forward. In fact, it should look as though it's leaning slightly backward. I always explain the position of the bridge to my customers in case their bridges fall. Many times I will make a template to show the exact location.
The Sound Post
The sound post is a wooden dowel set inside the instrument. The French call the sound post the soul of the instruments, L'ame, because it's the most important piece of the entire violin. If you peer through the F holes of your instrument you will see it on the treble side of the violin (by the E string) just back of the right foot of the bridge, and about in line with the right leg of the bridge. The sound post has both an acoustical and a supporting role. Along with the base bar on the left side, the sound post keeps the top of the instrument from collapsing. When the strings are bowed the sound post transmits vibrations from the top of the instrument to the back.
Fitting a sound post is one of the most precise and difficult tasks a luthier faces. Serious damage can be done to an instrument when fitting a sound post and only those luthiers with proper training and experience should attempt it. It must be fit exactly and perfectly to the taper of the inside of the instrument, neither too loose nor too tight. Too tight and it will sound shrill, too loose and it will sound dumpy and may buzz. A slight adjustment can change the entire sound of the instrument.
If your sound post collapses, immediately loosen the strings to take the pressure off the top of the instrument, and get it to your luthier as soon as possible. He or she should also tell you why your sound post has fallen. This needs to be determined so it doesn't happen again. Sound posts usually fall due to improper cutting. In this case, a new sound post must be cut and customized for your instrument.
It is your greatest responsibility as a musician to look after your instrument. With good care your stringed instrument will reward you over and over again.