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More about string tension and sound

Discussion in 'Setup & Repair [DB]' started by alexisbass, Dec 20, 2005.

  1. alexisbass


    Jun 11, 2003
    Dear friends... Here's another few challenging questions for our luthier experts about string tension:

    1) How does the string lenght affect the string tension? Pretending that we have the same exact bass with two different necks let's say one has a 41.5" string length and the other has a 43" string length. Which will be more "tense"?

    2) How does the neck projection, that is the angle at what the neck is glued to the body affect the tension of the strings? Which leads to ask, what is the ideal height of the bridge (measuring from the center of the bess to the very edge of the bridge between the D and A strings?)

    3) How does the position of the post affect the tension? If lower and closer to the F hole I'd say the bass feels looser... Am I right?

  2. For #1 see http://www.liutaiomottola.com/myth/perception.htm

    Of course, there is a thread where all of this was previously hashed out. There has been little new discovered since the middle 1600's ..... If the strings in question #1 are the same density/mass/gauge and tuned to the same pitch the 43 inch string will bear a higher tension. "Tense" might refer to the player, the moment, the conductor, the audience, or someone else, but not the string, if we are using terminology common to string physics. It's a matter of symantics, surely, but that is why there are scientific words and casual words.

    #2 The greater the angle of the neck projection, the greater the component of the tension that is directed downward on the bridge. The specific relation is trigonometric, but this angle does not affect the string tension in any way whatsoever, just the amount of static force transferred to the top plate by the bridge. The angle on the tailpiece side is similar. Neither one will affect the tension. The length, density or mass/volume, and the fundamental (open string) pitch are the only factors affecting string tension.

    #3 Regardless of how the string might feel, the position of the post will not affect the string tension in any way.

    I think we might want to use the word "flexible" or "flexibility" to discuss some of the qualities that "tension" is being used for. Certainly the post is a fulcrum and the further it is from the bridge foot the more "leverage" the "downward force on the bridge" (discussed in question #2) will have on the top plate and this may feel different, but if the top plate bends downward in response, loosening the string somewhat, then the string pitch will go flat a corresponding amount (meaning the tension has also dropped) and when you tune it up to compensate you will be back at the same string tension even if the wood of the top has "flexed" to a different position. Move it too far from the bridge foot and the top will sink at the bridge foot to a point of damage and deformation.

    Science is technical, luthierie is an art and a craft. Don't expect artists and craftsmen to have all the answers to scientific questions. Those best at applying science may not be scientists. They might be luthiers. The fellow who discovered these scientific / mathematical relations was both a mathematician and a musical instument string maker, Mersenne, who lived just a generation before Stradavari. But luthierie is much older and these relationships were understood practically before they were scientifically quantified.