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Intonation Problems

One of the main objectives of every good guitar player and luthier is to have an instrument in tune throughout the entire scale. Some variables that can affect your intonation are different string gauges, scales lengths, set-ups, and playing techniques.
Tuning problems related to intonation problems are best described by saying that the notes of some chords sounds in tune while others are not, and upon retuning to bring one chord in to tune only throws the others out,,, and on and on. This can happen even when the instruments individual strings are in perfect tune. To understand this you must realize that differences in the strings mass, tension, length and fretting stretch all effect the notes pitch. Of course there are other factors that affect the instruments ability to play in tune which will be covered later in this series.
Note: Tune your instrument to pitch and fret each string naturally at the 12th fret. This note is one octave higher than the open note and should be in the same tune, not flat or sharp. If they are noticably flat or sharp, then the strings length and position of the saddle or shape of its crown are needed.You will notice when looking closely at the crown of a compensated saddle that its crown (highest peak) doesnt run down the center but tends to waver slightly before and after. This changes the strings length ever so slightly which allows for greater fine tuning of the strings length.

Causes of Poor Intonation :


Incorrect positioning of the bridge or saddle:

This causes the strings to be either too long causing intonation to be flat or too short causing it to play sharp.On an acoustic guitar this can be caused by the saddles crown or the actual position of the saddles slot. (electric guitars to be covered later)

High Action:

Basically means that the strings are too high from the neck which will cause the string to be stretched further before contacting the fret which sharpens the note.High action at the nut will cause chords played at the 1st to 3rd position to be way out of tune.

Sloppy or loose saddle :

The bridge saddle slot should fit snuggly or the saddle can lean or move forward causing the strings length to shorten and be sharp.

Worn or grooved saddle:

Wear and tear can change the shape of the saddles crown which in turn changes the strings length. Resurfacing or replacement is necessary to correct this.

Frets:

Frets that are worn flat or badly grooved will also throw off intonation as the strings length is changed. Frets must be leveled and dressed to remove grooves or replaced if necessary. The height of the fret will also have an effect. If the fret wire is too tall it will play incredibly sharp when the string is fretted hard and your fingertip bottoms out on the board. This takes us to "technique" (listed below). As you fret with different pressure notice the pitch changes (use tuner to prove this) making you the problem !

Production defect:

Sometimes when an instrument is made there can be a mathematical error. If the bridge itself has not been postioned on the top correctly, it has to be fixed. In some instances it has to be physically removed.

Fret Layout / Spacing:

It is not a popular problem but still is found on occasion on mainly cheaper imported instruments most likely to be an issue on a fretboard that was slotted by hand. The majority of factories now use sophisticated machines that carefully, correctly, and consistently cut fingerboard slots.

Technique:

Not a defect but rather a "style" issue. Some guitarists use a powerful fretting technique which causes excessive pressure on the strings when fretting, and if the instrument has tall frets, this messes with the intonation. Some players (esp. beginners) tend to bend a string sharp when fretting.