Why Isn’t My Acoustic Guitar Intonating Well, and What Can I Do About It?

People often bring an acoustic guitar into the shop complaining of poor intonation. Maybe it plays less and less in tune as they move up the neck, or their B string is always sharp, or every winter things go flat. Of course, there are several real tuning limitations that are inherent to guitars: they’re designed to play many notes at a time, thus increasing the conflict to our ear; there’s no way to subtly vary the pitch of a given note as there is on a violin or trombone; the physical phenomenon of pressing down a string increases tension on it rather than just shortening; and don’t get us started on equal temperament. But today we aren’t going to talk too much about those limitations. We are going to talk about how to keep things reasonably well intonated so that your average guitar player’s ear isn’t too bothered.

First things first—see what you have. Tune your guitar with a digital chromatic tuner. Check the open string by sounding the harmonic at the 12th fret, as this is more easily “heard” by the tuner. Once you’re satisfied that the open string is in tune, try fretting it and see how things look. Many people just check the intonation at the 12th fret, but I’d encourage you to check the intonation at the fifth and seventh frets as well. For example, if a D string is perfect when played open, you should be able to get a good G at the fifth, an A at the seventh, and an octave at the 12th.

If you notice something is outside of the reasonable range of intonation, and if it’s off enough to bother the ear, here are a few questions to ask yourself.

Is it strung correctly?

The ball ends of your strings should be tucked into the pin holes in your bridge, and they should firmly abut the bridge plate. You shouldn’t be able to see the thicker wound part of the string running over the top of your saddle—this indicates bridge-plate wear, and makes the string unstable. At the tuning machines, you should only have a couple of winds around each post, with the final wind(s) pointing down toward the headstock.

Do I have the right gauge strings?

If the strings you use are very light, they can be easy enough to bend that just the act of pressing them down toward the fingerboard is enough to bend them out of tune. In this case, heavier strings, and the increased tension they add, will keep things more stable.

Is the environment stable?

Bear in mind that guitars are made of organic material, and as such, changes in humidity and temperature can slightly shrink or swell a guitar, just enough to wangle it out of intonation. Does it always read a bit sharp at the 12th on a humid day? Good to know! While we can affect many things, we can’t control the way certain cuts of wood respond to the weather.

Is the action off the nut and saddle reasonable?

If the strings take off from the nut way too high, which will be clear if it feels like you have to press down really hard to fret in first position, you’ll find that the notes on the first few frets are slightly out of tune. Likewise, if the saddle is too tall for comfortable action, just pressing the strings down can distort their pitch.

Is the neck relief acceptable?

A neck with too much relief can also mean that fretting distorts the string.

Is the bridge glued down?

If the bridge is pulling up or creeping forward, the scale length could be slightly shortened, making it impossible to intonate accurately.

Are the bridge and saddle located correctly?

Did this guitar come from the manufacturer with a really good, accurate placement of the bridge and saddle? If the bridge was put down just a bit too far forward or backward, allowing for relief in the neck and a host of other variables, it will always be a bit frustrating. If you suspect this is the case on your guitar, take it to the repair shop and have your luthier measure it. Sometimes, the answer is filling the existing saddle slot with a matched strip of wood, and routing a new one in the right spot.

Are the saddle slot and the saddle itself compensated to account for different string gauges?

At its most basic, compensation means that the saddle is placed at an angle to make a slightly longer-sounding length for the lower strings and a shorter one for the high strings. The angle is dependent on a few things, including the scale length and recommended string gauge. Each string may have further, more specific, needs to intonate well. The biggest culprit is the B string, which is quite a thick plain-steel string, and on a compensated saddle, it will get a bit more length compared to its neighbors. You might see a saddle with variable takeoff points, where the top ridge looks like a zigzag, or lightning shape. Additionally, the shape of the saddle’s crown can be altered a bit. Creating a compensated saddle is a simple way to address many intonation issues.

This article originally appeared in the October 2018 issue of Acoustic Guitar magazine.

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