Intonation compensation is the adjustment of the speaking length of a string on a fretted instrument to compensate for the strings mass and distance from the frets. This adjustment is often referred to, simply, as intonation.
Fretted instruments, like our beloved guitars, have fixed note locations…the frets themselves. Not only does this give cello players the ammo to look down on us, it also presents us with a set of compromises.
If you play a guitar with frets in it you are going to have to just accept the fact that, because of these fixed note positions, it will play in-tune to a certain point, the frets themselves are a compromise giving you the ability to play pretty well tuned in every key but not perfectly in any one…those are the facts, deal with them!
Before we get into the tricky subject of intonation/compensation adjustment i want to make this clear:
INTONATION COMPENSATION IS A BROAD COMPROMISE TO MAKE THE GUITAR PLAY AS “IN-TUNE” AS IT CAN GIVEN IT’S CONSTRUCTION…IT WILL NEVER BE PERFECT. IF YOU DON’T LIKE IT THEN GIVE UP GUITAR!
Right, now that’s out of the way lets move on!
When you apply pressure to a string so it makes contact with a fret you are “stretching” the string slightly. This stretching effect actually pulls the string slightly sharp. As you move up the fretboard the distance between fret and string increases, this means the further you move up the board the sharper you are going to pull notes. Going back to the cellists for a moment….those guys don’t have frets so they can compensate by fingering the note exactly where its perfectly intonated…but they play a cello so there’s always a trade-off ha ha! Now that’s all well and good but how the f**k do we play in tune?…well that’s where string length compensation comes in. The 12th fret on a guitar should, in theory, represent the exact half way point between nut and saddle (speaking length), however this cannot be the case or we run into the pulling notes sharp trap, so we slightly increases the the scale length of certain strings to counteract this pulling sharp of notes…this is compensation! You can already see that this is never going to be perfect, once again it is a compromise to make up for a fretted instruments even-tempered tuning issues.
If you look at the bridge on a correctly compensated guitar you will see the saddle(s) are slanted with more compensation at the bass side*. This is due to the fact that the increased mass and action height of the bass side forces us to compensate these strings more as they pull sharper than the treble strings. Many factors affect the compensation requirement of each string. String height, mass, scale length and even humidity all play their part in this merry little game of tail chasing!
*Most classical/nylon strung guitars have no offset bridge compensation as the string height and core tension across the strings is almost the same!
So how do we check our intonation?
Play your open low E string or the 12th fret low E harmonic…..now play the FRETTED E at the 12th fret (1 octave above open)….are they in tune? If the note at the 12th is sharp then the string needs compensating (moving further away from the nut to increase scale length) If the note is flat the string is over-compensated and the scale length must be reduced…simple right?
WRONG! Because so many different factors affect intonation it can be very frustrating getting it “right”…again it can be a compromise to get everything ALMOST in tune rather than one key PERFECTLY in tune. One of the problems involved in setting intonation is what i refer to as secondary compensation or attack/decay compensation.
We already know that when we push a string down we stretch it and change its shape and pitch but we also need to think how we strike a string. If we use a heavy pick and really dig into the string then the string will vibrate with an exaggerated elliptical pattern….ie more “flap”. This extra “flap” also stretches the string slightly pulling it sharp. As we strike a string hard and let it ring it’s elliptical vibration pattern slowly decreases as the string loses energy and so the note heard flattens slightly. This is attack and decay. If you setup your guitar testing intonation by lightly tickling your strings then play aggressively after you set it you will pull notes sharp and if you setup your guitar by pulling the strings and letting them ping back with massive attack then play gently when set your notes will sound flat….this is the headache of intonation!
When i setup guitars for people i ask them to play for me, not for free entertainment but to assess their individual attack and decay habits. I will watch and listen very carefully to work out how they ATTACK notes and how they let them DECAY. For example a slap-style bassist is going to have bucket loads of attack in his/her playing and very little decay so when setting their intonation i will COMPENSATE for this by allowing the 12th fret note to hover very slightly flat. When i do this it means their momentary pops and slaps will SHOULD be pretty much in tune. For a very gentle player who lets notes sustain and maybe uses a soft pick i will allow the 12th fret note to go slightly sharp so as to allow their ringing notes to decay in tune.So you see again..COMPROMISE!!! It can take a few goes to get it as spot-on as possible but it is something that has to be done. Most people can’t hear the tiny differences in tuning between different notes on the guitar as “perfect aural intonation” is very rare, but for those of us who do posses this gift/curse it is a whole world of pain. These rare humans either give up on guitar all together or invest in a a Buzz Feiten tuning system….but i’m not going into that here!