Intonation

Acoustic Guitar

Intonation refers to the ability of an instrument to play in tune in all positions along its fingerboard. It is the desired wish of every guitarist and the goal of every luthier to achieve this. The intonation on a guitar will be influenced by several factors; string gauge, scale lengths, playing techniques and action height to name just a few. However, since we are dealing with the equal tempered scale, certain compromises have to be made.

Compensation
A string length is the length from its fixed points at the nut and the bridge. Generally speaking, on all guitars the fixed point at the guitars bridge differs from string to string. This is known as string compensation and it is necessary to allow for different string mass and the stretching effect on the string as it is depressed.

Checking intonation
If you suspect that the intonation on your guitar not set correctly, try the following test:

  • Lightly touch a string over the 12th fret and play the harmonic

  • Play the same note at the 12th fret

If the notes do not correspond or sound in tune, an adjustment is necessary.
Other factors that influence intonation can be string height, string tension and string length.

Equal Tempered scale
Equal Tempered scale is the tuning of an instrument by dividing the octave into 12 semitones all divided by the same ratio. The ratio 1.0594631:1 is used as the basis for calculating semitone intervals. Since the intervals must be equalised to fit within the octave, an acceptable compromise is made which allows one to play all intervals in all keys with the same relative accuracy.
Suggested additional reading on Equal Temperance: Science and Music by Sir James Jeans, Cambridge University Press


Electric Guitar Intonation

A simple test by ear can reveal if the intonation is set correctly. This check will be made easier if the guitar has been fitted with new strings.

  • Check the harmonic at the 12th fret by lightly touching the string and sounding the note.
  • Now play the note at 12th fret, if the latter sounds sharp or flat compared to the harmonic an adjustment is needed.
  • A more accurate assessment can be made by using an electronic guitar tuner.

How to adjust
If the played note is sharper compared to the harmonic, the string length will need to be lengthened. Locate the string length adjustment screw on the bridge, rotate clockwise to lengthen. Repeat this process until the harmonic note and the played note correspond.

If the played note is flat compared to the harmonic, the string length will need to be shortened. Locate the string length adjustment screw on the bridge, rotate anticlockwise to shorten (could differ depending on manufacturer). Repeat this process until the harmonic note and the played note correspond.

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