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Dumb Questions about intonation and bridges

Discussion in 'Luthier's Corner' started by ii7-V7, Jan 14, 2006.


  1. ii7-V7

    ii7-V7

    Aug 4, 2002
    Baltimore, MD
    I'm having to seriously reconsider my design based on the fact that I can't find an aftermarket Bridge that will fit with what I want to do. All this thinking about bridges got me wondering about intonation.

    I'm sure someone will come up with some response to make me feel like this was a stupid question, but I can't figure this out.

    Why is the distance from the nut to the 12th fret not the exact same as the distance between the 12th fret and the saddle? For example: My SX fretless has been setup by me. The neck is pretty darn flat. There is a very small degree of relief. The intonation is set. So, I measure the distance from the nut to the 12th fret marker on the E and G strings. Both are 43cm. exactly. From the 12th fret marker to the string break at the saddles (as best as I can measure it) on the G string is 43.1 cm...pretty close...but on the E string its 44.1 cm. I don't get why? Is it string diameter...and if so isn't this going to effect intonation up the neck? Or does it have to do with equal temperment?

    And if I'm going to create a handmade bridge I'd like to be able to estimate with some degree of accuracy the measurements from the 12th fretmarker to the saddle. Is there any formula for this taking into account scale length and string diameter?

    Also, regarding my bridge...I've decided that I will create an Ebony bridge in the style of an Archtop guitar. My decision now is how to affix it to the bass. I could try to use barrels and thumb wheels to make it adjustable, but I'm not sure how to create the reversed threads in the wood. I'm also not sold on the coupling, if the bridge itself isn't actually sitting on the face of the bass. The other idea that I had was to simply bolt it to the face with removeable shims to adjust the height.

    Feedback?
     
  2. The calculation for the fret placement doesn't assume that the tension on the string is changing when it is fretted. It would be pretty hard to do this because as soon as someone changed the amount of relief in the neck and/or the action height, or any number of things, it wouldn't work any more. When a note is fretted, the string is deflected and the tension increases. The result is that the note is sharp compared to what it should be. The solution is that if you move the bridge back, you increase the length of the string slightly and move the note back from sharp towards where it should be. The problem is that the tension change is different at different places on the fretboard, so the intonation is usually set so that the 12th fret is in tune. It's sort of a middle of the road solution, instead of having some notes really out, you try to average it out.
     
  3. ii7-V7

    ii7-V7

    Aug 4, 2002
    Baltimore, MD
    You know not long after I posted this I pretty much figured out my own answer. If you could have a prescribed relief, string guage, and action at the 12th fret then you could probably figure out exactly where your saddles needed to be. However, as soon as you change one of those aspects all bets are off. I'm not sure of exactly how you would do the math however....never was my strong suit.
     
  4. If you new the exact geometry that the nut/fretboard/saddle were going to be in, it wouldn't be very hard to figure out the ideal fret positions and just fret it like that. Moving the saddle couldn't fix it if it were fretted using the 12 root of 2 method.

    There is no exact placement for the saddle when intonating a standard stringed, fretted instrument that will make the string in tune at every fret. It's a matter of setting the offsets so that the maximum offset is minimized, generally in the typical playing area of the neck.
     
  5. ii7-V7

    ii7-V7

    Aug 4, 2002
    Baltimore, MD
    Well, this will be a fretless bass. None the less, I'd like to get the intonation as close as possible.

    Any thoughts on the style of Bridge and affixing it to the bass?
     
  6. Fretless... even better. You can just worry about the intonation with your fingering. I guess that you could still intonate it like a fretted bass, as I'm sure most people would (could be wrong though).

    I've always liked the idea of a carved wooden bridge with an acoustic style saddle for fretless.

    For your original question, it is possible to determine what the saddle positions should roughly be given certain diameter string. You'd have to know what the string height would be at the 12th fret, but it can be calculated. If I was doing a bridge like I described above I'd just calculate it for the two outside strings and then make a straight line between them. As long as the strings were changing pretty uniformly it should be pretty close to what the actual offset should be.
     
  7. ii7-V7

    ii7-V7

    Aug 4, 2002
    Baltimore, MD
    Thanks,

    Do you know precisely how to calculate it? Is there a formula? I''m not a math guru...but I can follow a formula.
     
  8. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    You can solve this very easily: you can choose an adjustable bridge. Whereof there are plenty, just pick one that fits your wallet. Yes, they do the same job, and muóst look very much the same.

    If you want a fixed bridge, you would need to be more meticulous in the placement, but the design would be quite simple.
    IIRC, Pilotjones posted some spreadsheet calculation template on the issue. But it was a year or two ago... Try a search any way: pilotjones and string tension would be a suitable entry.
     
  9. ii7-V7

    ii7-V7

    Aug 4, 2002
    Baltimore, MD
    Well, I can't use an aftermarket Bridge... none of them fit my design which is to have a archtop style with a separate tailpiece. But, I appreciate the advice of looking up Pilotjones post.

    Chad