The sound post in a violin family instrument is often referred to as the “soul of the instrument”. As I explained in an earlier blog, the sound post transmits vibrations from the table to the back in order to initiate the pumping action of the back arching. This assists the table in getting the air inside the instrument to vibrate to the frequency of the pitch being played at that instant.
There are many factors to consider when choosing the wood one uses for the sound post. Age, diameter, length, grain pattern and hardness of the wood itself are all important. All of these characteristics must be taken into consideration and applied to the perceived characteristics of the instrument being worked on. Then, its position in relation to the bridge and bass bar has to be determined. Whenever possible, I try to utilize spruce that has a similar distance between the grain lines as one finds in the spruce of the table. But more importantly, I try to match the hardness or softness of the sound post material with that of the table.
Another variable one must consider is the thickness of the plates. If the thicknesses are too thin in relation to the design of the arch and density of the wood, then one should utilize a slightly thicker diameter post and fit it somewhat closer the the back of the bridge foot, and possibly one or two millimeters closer to the bass bar in order to more properly support the arching.
Conversely, when the plates are determined to be somewhat or too thick and or too hard for the design of the arch, one can place the top of the sound post further away from the bridge foot in order to allow the top to flex more to initiate the pumping action than it can if the post is too close to the bridge foot.
There are also numerous variables that can be explored with the angle of the sound post – front to back and side to side. These movements can be used to fine-tune an adjustment for a particular instrument. The preferred starting position is for the post to be fit and set to be perpendicular in all directions. From this neutral position, if the treble needs more support, the top of the sound post can be moved slightly toward the outside of the treble bridge foot. Conversely, if the bass needs more support, the top of post is moved slightly toward the bass side of the bridge. If the instrument needs more smoothness in the sound, the bottom of the post should be moved slightly from the vertical toward the end block. Conversely if the instrument needs more edge, the bottom of the post is moved slightly toward the neck block. Sometimes the bottom of the post can also be moved toward the outside to stiffen up and support the arch and to quicken the response. These are all 3very subtle movements.
Keep in mind that all of these changes have to reflect not only the construction of the instrument itself, but also the skill and/or approach of the person who owns and performs on the instrument-different styles of playing often require different approaches to adjustment to get the best results.
In the next segment, we will talk about how different string choice and tensions can affect adjustment.