Physics behind intonation

Discussion in 'Hardware, Setup & Repair [BG]' started by Jazz Ad, Oct 16, 2004.

  1. Jazz Ad

    Jazz Ad Mi la ré sol Supporting Member

    I know what intonation is and how to adjust it, I understand the phenomenon, but I've yet to read a clear explanations of why we have intonation issues and need to adjust the length of a string to get rid of it (or at least attenuate it).
    I'd really appreciate if somebody could put words and possibly formulas on it.
    I understand it must have to do with the mass of the string being ignored in the 2^1/12 ratio used in the suite of frets, but that's about it.

    So... :help:
  2. john turner

    john turner You don't want to do that. Trust me. Staff Member

    Mar 14, 2000
    atlanta ga
    it's because the string is not infinitely flexible and the nodes aren't infinitely stable. if it were, then all the harmonic series would line up perfectly and there would be no issues, but because it isn't there's a slight discrepancy at each of the terminating node points (nut and bridge) which is exacerbated by the fact that there is not an infinitely immovable termination of the string at those points. the tiny amount of discrepency there is exagerated along the length of the string.
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yes, exactly. This dovetails perfectly with what a physics buddy was just telling me about the length of the string above the nut, and how it influences stuff that happens along the playing surface. But that's another post. In general (but not always), the heavier strings will require a slightly longer scale. The string itself has a lot to do with it, 'cause the various brands are wound differently and so on, so one man's 35" isn't the same as another one's. Most bridges are adjustable plus or minus a very small amount, it's not like there should be major differences in the string length. And even in the "fixed bridges", like the bone bridges on the fretless F basses and so on, they get "close enough" to where small differences are difficult to hear. Most of the time when changing string brands, people recommend double checking the setup, 'cause they're sufficiently different where it "could" matter. In practice, I usually stick to the same brand of strings once I've found the one that best suits the bass.
  4. I can help with this part of the question. The formula's will have to be left for somepone with more edumacation than me!

    Imperfections in the mating surfaces of the string at the contact points of the nut and saddles will do all sorts of things to a strings vibration. Even break angle - according to Trev Wilkinson - can create problems with "witness points". For instance, a slip of a nut file while cutting a slot for a string can round the front edge off enough to effectively move that edge back from being aligned with the other strings. That would necessitate an adjustment on the other end. Given that there could be several of these imperfect relationships affecting a single string - the cumulative affect would be significant.
  5. Tim__x


    Aug 13, 2002
    Alberta, Canada
    It is also needed to correct problems caused by the extra tensioning of the string when you press down on it. That's why lower action some times solves people's problem intonating B strings.
  6. Rumblin' Man

    Rumblin' Man Banned

    Apr 27, 2000
    Route 66
    I've suspected that the string length above the nut may do that. Could you post your buddy's thoughts?