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The soundpost is a simple transmitter, hundreds of years before radio it was broadcasting sound. How can a short, round, wood post make much of a difference to my string instrument?

Lets review how the soundpost works!

The player initiates the sound by vibrating the string either through arco or pizzicato (bowed or plucked) methods. The string transfers the vibration, directing it through the bridge. The bridge extends the vibration to the instrument’s top. The top, like a pond of once still water, expands the vibrations across and pulsates like a speaker cone. This function moves the air, creating sound. The soundpost routes the vibrating action to the back, amplifying the sound inside the entire instrument body.
This same process occurs in an electric bass, try it: Sit on a wooden chair or stool and play the bass without an amplifier. Press the instrument to the chair or stool; you will feel the chair become an amplifier or transmitter of the sound, similar to the soundpost.

The acoustic bass (just as in any other string instrument) is designed to have the soundpost in a specific location inside the instrument’s body. It should stand one soundpost distance away from the center of the treble foot of the bridge. It is a misconception that there is a “sweet spot”. The top is graduated by differing thickness dimensions specifically so the sound is maximized. If you move the post around due to the arch of the top or back, you make the fit tighter or looser. An optimum fit is what you should seek. To optimize the fit, my longtime friend Louis DiLeone created the “soundpost indicator”. This device measures the fit of the soundpost to the thousandth of an inch, allowing the luthier to determine a very precise fit.
soundpost2
Performers are advised to have two soundposts, one for winter and one for summer, due to the extremes in humidity which expand or contract the wooden instrument. You can observe this during the high humidity of the summer, the strings will sound sharp. This is due to the wood absorbing the moisture in the air and swelling. You should also understand that wood fibers expand in girth not length. So as your top and back expand, the space between them gets smaller. The post standing between them also grows fatter, but not shorter while the cubic feet internal to the bass body declines. The pressure builds against the top and back which can cause the strings to tighten, raising the pitch, and at worst the top will split! Look out for bulging at the front or back of the instrument where the soundpost is located, if this occurs, an immediate trip to your luthier is advised!

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