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Home Page The correct placement of your banjo bridge:
Despite the logic that says a banjo's bridge should be exactly the same distance from the 12th fret as is the nut, you cannot accurately set its position by measurement--it must be done so as to make the instrument play in tune.
First: A little explanation:
The pitch of a string is not only determined by its length, but by its mass and its tension. When you put a string on your banjo, its mass has already been determined by its gauge. When you tune it to a predetermined pitch, its tension will be a function of its length. In theory, the 12th fret of a stringed instrument is exactly half the distance from the inner edge of the nut to the inner (facing) edge of the bridge. This halfway point doubles a note's frequency, producing a note an octave above the open string. Similarly, halving the remaining distance to the bridge would produce another octave (if there were 24 frets), literally ad infinitum.
Unfortunately, though, when you press a string down to the fret, you have to bend the string slightly to get it there, increasing its tension, which causes the note to sound sharper than it should. To compensate for this, the bridge needs to be slightly further away than arithmetic would tell you (i.e., by slightly increasing the string length, you lower its pitch slightly to compensate for the raising of the pitch through increased tension.
Next complication: Despite its increased mass, the 2nd string is typically at greater tension than the first (though with some lighter gauges and tunings, the reverse is true!). This means that it will sound even sharper when fretted than the first string, requiring the bridge to be even further away. This effect is repeated with the 3rd string, too. The 4th string is a different matter, as it is usually a wound string, which effectively increases its mass but not its tension; pulling on the end increases tension on its core, but not on the windings. Typically its tension is similar to that of the first or second string, meaning its compensation should be lessened.
You can see how this works by looking at a guitar or a banjo sporting a Moon bridge. In the case of an acoustic guitar, you'll note that the saddle on the bridge slants in such a way that the thicker strings are longer than the thinner ones. Look carefully at the saddle of most modern guitars, and you'll see that the first string is the shortest, touching the front edge of the saddle, while the saddle is usually shaped so the second string contacts its back edge, to increase its length the most. The 3rd-6th strings are usually wound, so the saddle brings the 3rd string back to its front edge, and gradually moves each of the lower strings progressively towards its rear edge. On an electric guitar, each string's length can be individually adjusted to set the intonation.
With a straight banjo bridge, there is little one can do other than to slant the bridge to progressively increase string length--but then the 4th string is out of tune. If you put the bridge nearly straight across, the 4th string may sound right, but the 2nd will usually be sharp, and the 3rd becomes dismally sharp. "Compensated" bridges, such as the Moon Bridge, curve or are staggered, so that the shortest string is the 1st string, and the strings get progressively longer until you get to the (wound) 4th string, which comes forward again.
Second: OK, OK, get on with it...How, then, do you set a banjo bridge in the right place?
The best way is to use the harmonics ("chimes"). If you touch the string directly over the 12th fret wire (or nearly so--the more the bridge needs to be moved, the further off from the 12th fret the node will be!) without pushing it down at all, pick the string, and then quickly remove your finger, your finger will have stopped the string from vibrating its full length (the "fundamental"), but it does allow the half-length, alternating wave to oscillate on either side of your finger (i.e., when one half goes up, the other half goes down, and vice versa. The node where they pivot back and forth is where your finger is, so you do not interfere with the vibration) . Now, clearly, that note will be exactly an octave higher than the open string. If you then fret the same string at the 12th fret, you should hear exactly the same note as the harmonic (if you have a good ear, otherwise, simply use an electronic tuner). Most likely, you will not hear the same note, which tells you that the bridge is in the wrong place.
To get it right, check the 12 fret harmonic and the 12th-fretted notes on the first string. Move the bridge to correct the fretted note (i.e., if the fretted note is higher than the harmonic, make the string longer by moving the bridge toward the tailpiece. If the fretted note is lower, shorten the string by moving the bridge toward the neck). When the first string is correct and produces exactly the same note fretted as it does when "chimed," check the 2nd string the same way: But now we can't move the whole bridge, as that would lose what we did for the first string. Instead, pivot the bridge under the first string to adjust the 2nd string. Obviously, this also sets the 3rd string (which should end up about right, anyway, for reasons discussed above).
Fine tune as necessary; remember it is always a compromise. If you rarely play the 3rd string above the octave, get the 1st and 2nd absolutely right. If you frequently play the 4th string up there--buy a compensated bridge, because you'll never be happy with a straight bridge.
That's all there is to it. Remember, though, that if you change string gauge (and therefore string tension), your bridge will need to be in a different place. Likewise, if you change bridge height or adjust your neck, or do anything else that affects the action, your bridge will need to be repositioned. This is why I tell folks not to bother marking the banjo head, because no matter where you mark it, it's going to be wrong. Instead, I encourage my students and customers simply to learn how to set the bridge each time they change strings.
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