The sound post is a ‘dowel’ of quarter sawn spruce that is positioned inside all of the violin family instruments. It is known in French as the âme (soul) because it is the graphic equaliser that transfers sound and is the key to how well an instrument resonates depending on its position inside the instrument.
Once it has been positioned in the relatively correct place it can be moved within fractions of a millimetre to alter and regulate the response of the instrument. The smallest movement of the sound post can have a major influence on how well or poorly the instrument sounds. Even a Stradivari violin can sound harsh and wild if the sound post is too tight or completely dead if too loose or far from the bridge in the wrong direction.
The relative position of the sound post determines the intensity of the sound, the balance between the upper and lower registers, and how well the harmonics of the instrument work with each other because of the vibrational modes between the two plates. Of course there are other influences in the construction of the instrument that determine the sound and where the sweet spot of the sound post would be. An example of this could easily be experienced when you decide to use a different set of strings and then find that the fantastic reviews are seemingly rubbish! Nope, in such a case it is just the response of the instrument with different strings that needs regulating so that the two relatives ‘meet’ in a position that causes a pleasing resonance.
It is ‘loosely’ held in place between the top and back plates of the instrument, about the diameter of itself behind the treble side of the bridge foot. It is supposed to be cut exactly the shape of the instrument’s inner arching shape so that the contact area of each end is a flush fit. As a general rule of thumb if the fit of the sound post is snug it will help to produce a clean and pleasing sound. If it does not fit well on either end or both it will most probably produce an intensity that is not exactly pleasing to listen to because the ‘concentrated’ transfer of sound often amplifies in such a way that there is too much for the plates to handle; same principle between being stepped on with a regular shoe heel or a stiletto.
Additionally if the sound post is cut with too much angle on either end there is a chance that it may cause a sound post crack because of the pressure on it causing it to skewer through the quarter sawn grain of the plates. This is of course detrimental to the instrument and particularly expensive to repair depending on the type of instrument. In the case of cheaper mass produced instruments we have found that simple glue work and a cloth lining is sufficient although opening, doing the work and then preparing to close and closing the instrument comes at a price either way. Better a well a cut post in the first place.