Re: Intonation facts and fiction


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Posted by Bob 'Skippy' Blechinger on November 20, 2000 at 23:59:24:

In Reply to: Intonation facts and fiction posted by Pat Harrington on November 20, 2000 at 01:04:50:


:
: I just read a posting on another website by a very famous bassist
: saying that "pulling the bridges back will NOT affect tuning"
: This statement is not only ignorant but absolutely DEAD WRONG!!!
: A bass guitar (or ANY other stringed instrument for that matter)
: works on a MATHEMATICAL equation of incremental string lengths.
: (This was discovered by Plato, by the way) For instance, the
: Fender basses use a 34" scale. If you cut that in half, you have
: 17 inches, which will give you the 12th fret, the OCTAVE...
: As you go further and further up the neck, the divisions get
: incrementally smaller (as do the distance between frets) This
: technically is a CALCULUS FUNCTION, which determines the accelerated
: rate of change, (divisions of smaller and smaller divisions.
: Not only do we have this mathematical function over the normal
: notes (divisions of string length), but the string also has
: an overtone series ("harmonics" for the layman), just like a
: trumpet, guitar, whatever...SO.........If you wanna goof with
: your bridge saddles, and yank them all over the place, go right
: ahead, but your bass WILL NOT play in tune, NO WAY, NO HOW....
: The Fender type bridge has OVER 1 1/2 inches of adjustability in
: the travel of the saddle, this is nearly 5% of the total scale
: length...It may not sound so bad on the first couple of frets,
: but as you climb up the neck, it will get EXPONENTIALLY worse...
: Many factors affect intonation, especially string guage and
: the height of each saddle...If you drastically RAISE your bridges,
: you will marginally go out of intonation (remember the Pythagorean
: Theorem?). SO...if you want to your bass to play in tune, check
: your tuning with a GOOD tuner...Tune the open string, check the
: tuning at the 12th fret, and check the harmonic at the 12th fret
: as well...Leo Fender was no dummy...The 34 inch scale should be
: almost EXACTLY that...give or take like a 32nd or so of an inch
: to adjust for the differing diameters of the strings themselves..
: People can all kinds of weird OPINIONS about all kinds of crazy
: shit, but MATHEMATICS DO NOT LIE!!!!... Keep 'em in tune gang....

:
: Cheers,
: Pat H

Pat,
Fantastic post! A couple little clarifications here, though...

1) The declining distance between the frets is a logarithmic function. If you measure the distance between the nut and first frets and compare it to the overall scale length, then measure the scale from the first and second frets and compare it to the remaining scale length (between the first fret and the bridge), and so on, you'll find the same percentage of scale length between each successive pair of frets.

2) By "GOOD tuner", we should mention that it's best to use something like a strobe tuner.

3) The intonation is adjusted more to compensate for the *stretch* of the strings when they're pressed down than the diameter of the strings themselves.

Here's some info about setting intonation from a guitar repairman named Larry Vigneault, from his site at:

http://www.ma.ultranet.com/~larryv/guitarsetup.htm

It's more specific to guitars, but everything works just the same for bass, too.

-Bob

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Adjusting the intonation

If you have an adjustable bridge, the final step is to adjust the intonation. Intonation refers to whether or not a note plays sharp or flat from it's intended sound. When you depress a string, you actually stretch it a bit beyond it's unfretted position. This generates a slight sharpness in pitch which is compensated for by adding a slight excess of string length. To check intonation on a given string:

Play the harmonic at the 12th fret, listen closely to the resultant pitch
Now play the same note by fretting the note at the 12th fret. The two notes should match exactly if the intonation is correct.
If the fretted note sounds sharp, then adjust the bridge saddle so that it moves back away from the fingerboard.
If the fretted note sounds flat, then adjust the bridge saddle so that it moves towards the fretboard.

The adjustment is really dependent upon your skill, and the accuracy of your ear in determining the pitch between the two notes. Always make the adjustment to the saddle in small increments so you can fine tune the intonation. If you're unable to get the intonation accurate, bring it into a qualified repair-person to determine if you have other problems.

One problem I've seen a number of times is where a guitars intonation is set properly, but notes fretted between the 1st and 5th fret sound sharp regardless. This is almost always due to the fact that the nut slots are cut too high. The additional distance required to push the string to the fret is causing the note to be sharp. Bring the guitar in and have the nut regulated properly to resolve this problem.

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