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Why do we have Fret Buzz?
In order to eliminate fret buzz you must first understand how fret buzz is caused. And to do that you need to understand something about how the guitar's neck is constructed, and the effect of the string tension on the neck. Although it is true that as much a couple of thousandths of an inch in the wrong place can cause fret buzz, realize also that with limited tools and equipment absolute precision is neither necessary nor attainable.
The Purpose of the Neck Angle is to Prevent Fret Buzz
The purpose of the neck angle is to situate the neck such that the plane of the strings will create a stair step down from fret to fret as you move up the neck on the fingerboard while at the same time putting the strings at the correct height at the bridge, an important factor on a classical guitar. This stair stepping of the frets puts the next fret lower than the fret before it all the way up the fingerboard. This is stair stepping is necessary because when a string is plucked it travels up and down, both toward and away from the fingerboard at increasing amplitude from the point where it is fretted to center of the effective string height. Therefore clearance must increase as you move up the fingerboard.
The above drawing demonstrates how the stair case effect enables the strings to clear the frets since the distance from the string to the fret increases slightly from fret to fret up the fingerboard. The difference in height from fret to fret is small, therefore, if a fret is even slightly higher than the plane, fret buzz may occur. If a high fret is causing your fret buzz, the fret leveling kit can quickly and easily eliminate the problem. Watch a video demonstration of how fix fret buzz with the fret leveling kit.
Some types of strings travel more than others and may require more clearance than others. The bass strings on a classical guitar travel quite a bit and therefore need more clearance than the treble strings.
Incorrect neck angle may cause frets to buzz and be difficult to easily repair. The strings may have to be raised at the bridge in order to artificially create this angle. The effectiveness of raising the strings in this way is limited and may result in the strings that are too high making the instrument difficult to play. And if the neck angle is very small, or in the wrong direction this method will not work at all as raising the strings will still result in them being somewhat parallel to the fingerboard.
The angle the neck of the guitar makes with the flat plane of the top is critical to the guitars performance. This angle is also different depending on the type of guitar. For instance, on classical guitars the nut is typically 2.5 - 3mms higher than the plane of the top. On acoustic steel string guitars the angle is approximately -1 degree which puts the nut a bit lower than the flat plane of the top but the far greater string tension pulls the neck slightly forward creating enough angle for the strings to clear the frets.
The fact that most guitars have a domed top to some degree, or have the sides ramped in the upper bout to better accept the fingerboard with the neck angle, or some may even have a combination of the two, while others the upper bout is ramped significantly to add clearance in the upper frets, making it difficult, if not impossible to measure this angle after the guitar is constructed since the flat plane no longer exists.
The Effect of String Tension on the Neck
The tension of the strings on the neck of a guitar is great enough to pull the neck forward slightly. The nylon strings of a classical guitar do not generate nearly the tension of steel strings used on acoustic and electric guitars but the effect is the same. The result of the tension pulling the neck forward is to bow the neck slightly. In other words the neck does not remain flat but will have a slight concave curve from the nut to where it meets the body of the guitar. This curve can be corrected to some degree with a truss rod or similar device built into the neck as in steel string guitars, but typically the neck on a classical guitar will have some slight curve to it. Classical guitars do not typically have a truss rod in them for a number of reasons and if the wood for the neck is carefully selected and the neck properly constructed it will not need one.
Fret Buzz - Finding Possible Causes
Tracking down the cause of fret buzz begins with checking the guitar's neck. The first thing to look for is the correct neck angle. If you are not familiar with what this should look like have a professional sight down the neck. A pro will be able to tell not only if the angle is correct for your type of guitar but if the neck has warped or twisted. Improper neck angle as well as warping or twisting can be the cause of your fret buzz. This type of problem is not easily repaired and is best done by a qualified professional.
If the neck checks out next you must determine if your string height is appropriate for the type of guitar and strings you are using. Too low a string height will also cause fret buzz. Raising the string height will involve raising the saddle height or possibly raising both the nut and the saddle. Typical string heights for a classical guitar are given in the section below.
If your string height is within reason then you need to start checking for high frets. Any high frets will have to be leveled and can be easily leveled with the Buzz-Off® Fret Leveling Kit. Watch a video demonstrating how to use the fret leveling kit to find and level high frets.
Eliminating Fret Buzz and Optimizing the Guitar's Set Up
Leveling your frets as demonstrated in the video may not only eliminate fret buzz but will allow you to achieve an optimal set up for your guitar. Once the frets are all in the same approximate plane, and only then, will you be able to set your string height to the optimal height for the type guitar and strings you are using. Trying to lower your action without first checking the neck angle and leveling your frets will only result in frustration.
For classical guitars the string height at the 12th fret on the bass side should be between .140" - .150" and is typically about .020" lower on the treble side. The string height at the first fret on the bass side should be about .040" and typically .030" on the treble side. A lower action, even if attainable, may result in undesirable effects and possibly degrade some of the techniques particular to classical guitar and is therefore not recommended.
Sometimes frets behind the fretted note will buzz creating a back buzz, a slightly different sound but equally as annoying. The string movement behind the fretted note is caused by sympathetic vibrations in the string. This kind of buzz could be a sign that the nut is too low or there is not enough neck angle. But it is caused once again by the frets not stepping away enough from the fret in front. After first checking the nut and the neck angle it may be possible to correct this by lowering the high fret.