While this article deals specifically with standard single scale instruments in mind, you may use the same process on multi-scale instruments. Multi-scale instruments are designed to address some of the intonation problems outlined below, but still are subject to the same physics and must have their intonation set in the same manner.
Instrument intonation is a mysterious thing if you’re not familiar with the physical properties of your strings. It sounds silly, since all you need to know before playing is that it vibrates and makes a sound that changes when fretted. And it really is that simple to play! It’s also why a lot of guitarists & bassists start out without any knowledge of how or why to set your intonation.
It’s a critical piece of knowledge for both players and luthiers. You’d fail to win anyone’s trust as a luthier without understanding why, or how to intonate a guitar. And you probably wouldn’t get too much pleasure listening to a guitarist whose notes are all detuned for the same reason.
If you’re just looking for step-by-step intonation instructions, skip down to the How To chapter.
If you’d like to know exactly what intonation is and how it affects your instrument’s sound, scroll onward..
What is Intonation?
There are two main factors that affect your tuning: scale length & tension. In your daily guitar tune ups, you’ll simply be adjusting the open string tension until the string’s vibration frequency perfectly matches a defined note pitch.
When you get a perfect match, you move onto the next string. And you assume that all of the notes you’ll fret on that string will be in tune as well.
If your intonation is off, that will not be the case..
You may notice the notes begin fall out of tune as you travel upward on a perfectly tuned guitar’s neck. The reason for this has multiple layers, but it starts with string properties. It ends with the string oscillating too little (flat) or too much (sharp). And it’s addressed by adjusting string lengths.
To better understand the theory of scale length and fret placement, you will have to learn about the mathematic contributions of Pythagoras and Vincenzo Galilei. You may read more about them and the math behind your fretboard in these two PDFs: 1 | 2
Identifying Poor Intonation
When your fretted notes are sharp, the string’s vibration frequency is too fast. Your [fretted] scale length is too short.
When your fretted notes are flat, the string’s vibration frequency is too slow. Your [fretted] scale length is too long.
We need to compensate, and that’s exactly what your saddles are for. If you pick up a new guitar from the shelf and look at its bridge, you’ll see the saddles are all different lengths. They’re essentially creating slightly different scale lengths for each string.
As suggested by Luthier’s Mercantile, the first step to understanding how intonation works is to dispel the notion of scale length being equal to string length. It is not.
Scale length is in relation to calculated fret placement. When Fender tells you that a Stratocaster has a 25.5″ scale, they’re saying that the fret distances were calculated using 25.5″ as the base distance.
In a perfect world, your strings’ physical properties wouldn’t affect vibrational frequency. If that were the case, the open string length would indeed be 25.5″.
But because each string on your instrument has different properties affecting vibrational frequency when fretted, we need to adjust the scale length accordingly.
Your Stratocaster’s frets are still calculated at 25.5″ but your open string length will vary. This ensures that fretted notes are dividing the string’s length upon those frets to achieve an expected frequency. The expected frequencies are in relation to your open string tuning, which is based upon a pitch standard (440Hz).
Here’s a few of those physical properties that affect your string’s vibrational frequency:
- Action – The distance required to fret the string (more commonly known as bridge action, string height).
- Force – The force used to fret a note – higher frets allow the string to be depressed more, which increases tension and pulls the string length behind the fret, which in turn increases pitch.
- Material – harder metal, like steel, increases the string’s stiffness and decreases the frequency oscillation. The opposite is true for nickel.
- Gauge – the diameter also plays a role in increasing or decreasing the number of oscillations made at a given length. Thicker decreases frequency.
- Scale Length – that’s right, your intonation will require more compensation at the bridge if you’re using a shorter scale length.
- Tuning – if you’re using a dropped tuning that slackens your strings more, you’ll need to compensate more at the bridge for that as well. Like the scale length, your string’s vibrational qualities are affected more significantly as tension is lowered.
Bringing a guitar or bass to a state of flawlessly accurate pitch across the board is actually not possible. Fretted instruments are inherently un-tunable for the reasons listed above.
To achieve truly perfect intonation on a guitar, you would need the following:
- A fixed string gauge and material
- Fixed action and fret size
- Fixed fretting pressure and tuning
Even air pressure and humidity can affect the vibrational potential of a string.
One of the ways this is being addressed with modern instruments is with compensated construction. This is where multi-scale guitars come into the picture – a subject for another day.
However, we can still get close enough to perfect pitch with a single scale instrument. Close enough that our ears won’t notice or care about the few missing or extra oscillations here and there.
How do You Know When to Re-Set Your Intonation?
Depending on the severity of your intonation problem, you may not actually notice until you listen very closely or check with an electric tuner (outlined below). There are, however, several instances in which you should definitely intonate your guitar without need for diagnosis:
- Changing strings.
- Strings have been played for a while and worn in.
- Adjusting the action and/or truss-rod.
- Performing a setup, bridge change, or nut adjustment.
- When purchasing or building a brand new guitar.
How to Intonate a Guitar/Bass
If you know how to tune your guitar, you know how to set your intonation. All you need is a chromatic tuner and a hex key (or whatever tool is needed to adjust your saddles).
If you’re performing a setup, make sure you’ve run through all steps in the operation in proper sequence first. Follow the instructions in the perfect setup article. Note that intonation is the last setting made.
- Tune your guitar/bass strings up to pitch.
- Fret the 12th fret on your bass string (low E in standard tuning).
- Tune the 12th fret note to the same note as the open string was tuned to. If your intonation is out, you will notice the 12th fret note is off.
- Once the 12th fret note has been tuned to perfect pitch, play a perfect harmonic octave (by placing your finger above the 12th fret on the string without fretting it). Make note if it’s sharp or flat.
- If the harmonic octave is sharp, use your hex key to move the string’s saddle backward to lengthen the string. If it’s flat, do the opposite.
- Repeat steps 3 to 5. After every adjustment, you must re-tune the string on the 12th fret before checking the harmonic note’s tuning again. If the strings are new, make sure to pull and bend them to stretch them out before tuning them back up.
- Once your harmonic octave and 12th fret note are both perfectly tuned to the same note, move on to the next string and begin the process again.
After you’ve finished, your instrument is as close to perfect intonation as it can possibly be!