What's intonation and why should I care - Dec 2000
According to Every Musician's Handbook, intonation is, "the quality of a performer's execution of accurate pitch; playing in tune." Pitch is the particular sound created by the vibration of a specific number of air-pressure waves occurring each second. Your instrument has a particular ability to accurately create pressure waves at desired frequencies. Your intonation depends on your instrument's.
String length, thickness & tension interrelate with the pitch that a string produces. The distance between the nut & saddle/bridge (scale length) determines the functional string length; players' choices dictate string thickness; tuning conventions define strings' pitches. Therefore, string tension is a function of length, thickness & pitch.
On guitars, scale length is about 25.5". With a light-gauge acoustic A-string that's 0.042" thick, 30 pounds of tension allows the string to vibrate at pitch. At the same thickness and tension, a shorter string produces a higher pitch (half the length produces a pitch an octave higher). Musicians shorten strings to intonate by pressing strings to the fingerboards of their instruments. While the idea is simple, playing in tune is not.
Each stringed instrument has a specific scale length that allows optimum intonation. Changing the nut-bridge distance changes intonation.
Because string thickness influences vibrational behavior, many fretted instruments compensate for string thickness with saddles that lay at an angle relative to the nut & have notches for thicker strings. Improper bridge compensation & arbitrary string-gauge choice damage intonation.
If strings are high from the fingerboard, players depress them farther; string tension increases; pitch gets higher. Similarly, pressing strings beyond the frets of fretted instruments to touch the fingerboard makes the pitches sharp. Aggressive left-hand technique and high action can cause imperfect up-the-neck intonation.
Fret location determines the vibrating string length on fretted instruments. Therefore, fret placement must be perfect for proper intonation.
What Can I Do?
Nut/bridge distance: verify proper nut-bridge distance, especially as it relates to fret location.
Bridge compensation: consider fitting fretted instruments with compensated bridges & verify that compensation matches string selection.
String gauge: check if bridge/nut adjustment is needed to accommodate string-gauge changes.
High action & over-pressure: correct high action & use playing technique that presses strings as lightly as possible (fingerboards last longer too).
Fret placement: verify that each fret creates a tone that's a half-step from the next (e.g., A-A#).