Why does the sound post matter?
If you don’t like the weather, wait 10 minutes.
That can work if you’re hiking, but when you have three concerts on the weekend, the constant changes in temperature and humidity can drive you, and your instrument, crazy! Is it time to “adjust” the sound post?
The sound post plays an important role transferring the vibrations from the top plate to the back plate. The sound post is fit to both the back and top plates, and is precisely placed in a position relative to the bridge and the bass bar. Because the top plates and back plates are curved, the sound post will fit properly, with all points touching, in only a small area. To get the optimal sound, the sound post will have to fit properly, and be the proper length so the post puts enough pressure on the top and back plates to stay upright and support the softer spruce top.
When do we replace a sound post? New instruments require new sound posts in the course of the first year, and often again over the next three to five years. The top plate flexes upward in the new instruments with the tension from the strings. As the distance between the top and back plates increases, the sound post becomes too short relative to the growing space between the two plates. As a result, the sound becomes less focused, dull, and the top-plate will sag as the bridge pushes the plate down to the short post. The new post will initially sound brighter, but the ringing tones will return. The process repeats until the plate settles in. If the original, short sound post is left in place, the top plate will actually “sink” and reshape above the sound post and, ultimately, can result in a very damaging and devaluing crack. Similarly, if you move to an area with a more humid or, drier climate like Utah, the top plate may expand or contract. Have the sound post checked to make sure it is not correspondingly too short or too long.
When do we adjust a sound post? If a sound post is too long, we trim down the sound post to prevent the pressure from cracking the top plate. Repairing a crack over the sound post requires a patch on the inside of the top plate. This is time consuming, expensive, and devaluing to the instrument. We do not typically have two sound posts in the northeast despite the fluctuations in humidity between summer and winter. But, the pressure on the top plate will change as the top adjusts between those two extremes. In the winter, the sound may be harsh, or glassy. In the summer, the sound is often described as dull or muddy. A trained luthier can move the sound post within the area it will still fit to compensate for changes in humidity.
The precision of a good sound post set is, like much of violin lutherie, extremely exacting. Learning to carve a sound post can be a long process for most incipient luthiers and it is extremely common for us to see the results of a poorly set post. Sagging tops, soundpost cracks, and weak or unbalanced tone are all signs of a post incorrectly set, or not maintained.
How often do we recommend adjusting a sound post? As often as twice per year. Like the weather, an instrument’s sound will fluctuate on a daily or weekly basis within a range. As a player, you want to get to know the “moods” of your instrument. You can control those moods to some extent with humidifiers in the winter and dehumidifiers or air conditioning in the summer. If your instrument’s sound goes out of its normal range, first check for open seams, the placement of the bridge, and other factors that impact sound. If everything is in place, a sound post adjustment may indeed make sense.