Proper intonation gives an instrument the ability to play in tune up and down the neck. An instrument that is not properly intonated may be perfectly tuned but certain chords and areas up the neck may produce sharp or flat notes.
The position of the bridge on the top affects the strings overall length and therefore affects the mandolin's intonation. Moving a bridge closer to the fingerboard obviously shortens the string, making notes sharper. Likewise a bridge placed too far from the end of the fingerboard would produce flat notes.
The position of the bridge on the top of the mandolin affects it's intonation as does the overall string height (action).
Compensation or adjustment of the strings length is necessary to properly intonate any instrument.
Checking Your Intonation
The twelfth fret is the center point of your scale and one octave higher than the same string played open.
After tuning your mandolin, fret each string at the 12th fret and check it's tuning. Ideally, it will be neither flat or sharp. Realistically, having a majority of those notes in perfect tune is more likely.
Things That Affect Intonation
- High action
- Improperly cut nut/saddle slots
- Bridge position
- Worn frets
- Improperly slotted fingerboard (hand slotted/vintage)
- String gauge
Adjusting an instrument's intonation is the last item of business when performing a set up. Before adjusting intonation, all other aspects of playability should be corrected. Changes to neck relief, string height, even the nut's slot can affect intonation.
What I normally recommend is that you tune just the outer E and G strings to pitch first, leaving the A and D string's slack to facilitate easier movement of the bridge
Once these strings have been tuned to pitch, fret them at the 12th fret and check the tuning.
If the note at the 12th is Sharp, move the bridge further from the fingerboard.
If the note at the 12th is Flat, move the bridge closer to the fingerboard.
Now re-tune the E and G strings and re-check the tuning at the 12th fret. Repeat until the 12th fret note is in tune. You may need to make several small adjustments before getting the positioning just right.
Now that we have the basic positioning of the bridge we can bring all 8 strings up to pitch and re-check the tuning at the 12th.
Make note of the results. This is where one may find themselves chasing a rabbit! Let's say the E, A and G strings are in tune but the D strings are slight sharp. Moving the bridge further away may bring the D strings to tune but it will also cause the E, A and G to be sharp now.
This is a common compromise when adjusting intonation without modifying the bridge and quite frankly...on a majority of instrument's it is good enough.
Obviously not all ears are created equal! And for players with perfect pitch have my sympathies. Welcome to the world of the equal tempered scale and all of it's short comings.
Most bridges will sit on the top slightly slanted, the treble side closer to the fingerboard than the bass side. This is known as compensation. Bass strings are a larger diameter and as a result begin noting sharper as you move up the neck, as a result, they are lengthened to reduce sharpness.
Compensated mandolin bridge
After adjusting the bridge using the simple method the remaining sharp (or flat) notes are adjusted by reshaping the crown of the bridge. This affects the strings overall length, thereby changing it's tuning.