How to fix intonation ?

[Alan Carruth]

Trevor Gore discusses the whole question in detail in his book on guitar design. The take away is that 'perfect' intonation is an unrealizable aspiration on an acoustic guitar, but you can get arbitrarily close with good fret work and a compensated nut and saddle. Even if you did get it 'perfect' it would still be 12-tone Equal Temperament, with intervals that are noticeably inharmonious. In fact, there is no scheme of temperament that totally avoids that problem: it's mathematically impossible.

[Monsoon1]

Originally Posted by bausin

There was a long thread over at frets.net a couple years ago.

http://fretsnet.ning.com/forum/topics/nut-compensation

Wow, thats a good writeup, thanks.

[Monsoon1]

Originally Posted by Alan Carruth

Trevor Gore discusses the whole question in detail in his book on guitar design. The take away is that 'perfect' intonation is an unrealizable aspiration on an acoustic guitar, but you can get arbitrarily close with good fret work and a compensated nut and saddle. Even if you did get it 'perfect' it would still be 12-tone Equal Temperament, with intervals that are noticeably inharmonious. In fact, there is no scheme of temperament that totally avoids that problem: it's mathematically impossible.

Alan, would that be the case with 12-tone equal temperament on any instrument, even an electronic keyboard?

[charles Tauber]

Originally Posted by Monsoon1

Alan, would that be the case with 12-tone equal temperament on any instrument, even an electronic keyboard?

Not Alan, but yes. The nature of the beast.

[ChalkLitIScream]

How would tweaking string spacing att he nut affect intonation? You do change the scale lemgth, even if its a minute amount by moving string spacing at the nut. The frets stay in place, while the scale length changes

[D. Shelton]

Originally Posted by Alan Carruth

Trevor Gore discusses the whole question in detail in his book on guitar design. The take away is that 'perfect' intonation is an unrealizable aspiration on an acoustic guitar, but you can get arbitrarily close with good fret work and a compensated nut and saddle. Even if you did get it 'perfect' it would still be 12-tone Equal Temperament, with intervals that are noticeably inharmonious. In fact, there is no scheme of temperament that totally avoids that problem: it's mathematically impossible.

Arbitrarily close is good enough for me, and I've known that "perfection" is a fantasy since I got the Norman 20 years ago and asked about it. Doing the work shouldn't be a problem; knowing what to do is where I need some learning. The Norman is a good guitar, but it's not a Lowden or Taylor , and those two, especially after they had the BFTS done to them, spoiled me. I could get another pricey guitar , or even have the Norman BFTS modded, but for some weird reason, I'm looking into tweaking it myself. Thanks for chiming in, Alan.

[charles Tauber]

Originally Posted by ChalkLitIScream

How would tweaking string spacing att he nut affect intonation?

It doesn't.

You do change the scale lemgth, even if its a minute amount by moving string spacing at the nut. The frets stay in place, while the scale length changes

The scale length is the theoretical vibrating string length that is used to calculate the positions of the frets: it never changes, once the fret slots are cut. If nut and saddle are placed in accordance with that theoretical length, the instrument doesn't achieve the desired pitches of equal temperament: it has poor intonation. To improve the intonation, the positions of the nut and/or saddle - usually only the saddle - are repositioned from their theoretical placements. That yields the actual vibrating string length. Movement of the nut, closer to or further from, the first fret is referred to as nut compensation: movement of the saddle away from the nut, making the vibrating string length longer, is referred to as saddle compensation. In essence, one is shifting the breaking points of the strings relative to the fixed fret layout.

For all practical purposes, moving a string laterally in the nut does not bring its breaking point over the nut closer to the first fret - or saddle - and will not make any practical difference to the intonation.

[John Arnold]

You do change the scale length, even if its a minute amount by moving string spacing at the nut. The frets stay in place, while the scale length changes

It is so minute as to be totally inconsequential. Example, if you move the string 1/16" sideways at the nut, the change in string length is about 0.00007 inches. That is about 1/30 the thickness of a human hair.
Even then, the change in scale is not a problem, because it is reflected in a change in the fret spacing.....since the string is crossing the frets at the new angle all the way down.

I have heard similar questions about changing the scale after a neck reset. While there are things to be concerned about, this is not one of them.

[D. Shelton]

Originally Posted by John Arnold

'Out of tune' does not specify sharp or flat. It makes a huge difference. It's like giving directions, but not specifying which way you should turn at the traffic light.

It's impossible for me to address that in an accurate, useful way. If I make an open E-form E chord sound good down on the 13th and 14th frets, things are weird back on the other end. Open E good up in 1st positions, weird down on 13/14. Everything in between. Differences more accentuated on this guitar (Norman B-20) than on the pricey ones I used to have.

[murrmac123]

Originally Posted by D. Shelton

It's impossible for me to address that in an accurate, useful way. If I make an open E-form E chord sound good down on the 13th and 14th frets, things are weird back on the other end. Open E good up in 1st positions, weird down on 13/14. Everything in between. Differences more accentuated on this guitar (Norman B-20) than on the pricey ones I used to have.

Do you possess an electronic tuner ?

[MC5C]

Compensation is easy. You start with a perfect fretboard, put the bridge in a place where the string is in tune at the 12th fret, and put the nut in a place where the string is in tune at the second fret. Actually doing it perfectly is impossible, and you look for good enough. The reason you need compensation is the string stretches and goes sharp as you fret it, so you you compensate by flattening the note by moving the bridge farther away. At the nut, you compensate by moving the nut closer to the fretboard so the space between the nut and the first fret is slightly shorter than theoretical, to sharpen the open note relative to the fretted notes near the nut.

The big problem seems to come with fixed bridge instruments, flat top guitars mostly, where the bridge can flat put just be installed in the wrong place. That can happen with high end guitars (Martin put out a ton of guitars with the bridge installed wrong at one point) and with cheap guitars, or if a player decides to use really different gauge strings than the builder intended (like an unwound third string on a guitar designed for a wound string). All I do is compensate the nut by around 0.015" (half the width of a fret slot plus a tiny bit of cleanup), adjust the nut for minimal string clearance at the first fret to minimize the need for nut compensation in the first place, and locate the bridge saddle so the 12th fret notes plays in tune. Most people are pretty happy with a plain slanted saddle with some extra compensation for the second string (and third if unwound) and some people want every single string to be individually compensated, which I do on electric and archtop guitars (because it's easy to do), but not on flat tops unless I hear a problem (sometimes the slot is at the wrong angle for the string gauges in use so you have to grind on the saddle to move the break point a tad). At the end of the day the guitar suffers enough from inharmonicity when played up the neck and equal temperament all over that getting intonation close is good enough.

[charles Tauber]

Originally Posted by D. Shelton

It's impossible for me to address that in an accurate, useful way.

The ability to do so is the starting point: if you can't identify the problem, you won't be able to fix it.

Start by obtaining an electronic tuner that is calibrated in "cents" - 100ths of a semitone. One inexpensive option is an application for a phone, such as IStorboSoft by Peterson (less than $15).

Install a new set of strings of your choice.

Tune the open bass E string using the tuner. Check the pitch of 12th fret fretted note using the tuner. Write down how many cents sharp or flat it is. Check the pitch of the first fretted note (F), writing down how many cents sharp or flat it is. Repeat for the second and third frets.

Repeat the above for the remaining strings, giving you a "chart" of what is sharp and what is flat.

Once you've done that, come back and we can discuss the results, what they mean and what action is required, if any.

[Monsoon1]

Originally Posted by John Arnold

It is so minute as to be totally inconsequential. Example, if you move the string 1/16" sideways at the nut, the change in string length is about 0.00007 inches. That is about 1/30 the thickness of a human hair.
Even then, the change in scale is not a problem, because it is reflected in a change in the fret spacing.....since the string is crossing the frets at the new angle all the way down.

I have heard similar questions about changing the scale after a neck reset. While there are things to be concerned about, this is not one of them.

But changing the nut isn't the same as moving the string sideways.
Changing the nut by 1/16" changes the scale by 1/16" as well.

[Rodger Knox]

Originally Posted by Monsoon1

But changing the nut isn't the same as moving the string sideways.
Changing the nut by 1/16" changes the scale by 1/16" as well.

Nothing changes the scale length once the fret slots are cut. The scale length is a number used to determine the distances between frets, and has nothing to do with compensation. The vibrating length of the string is what changes with compensation, and along with string mass and tension determines the pitch produced.

[Alan Carruth]

We've got a terminology problem. IMO 'scale length' should refer to the theoretical length that was used to calculate the fret positions. The actual vibrating length of the string can be a bit different, depending on how the nut and saddle are compensated. It is usually different for each string, and may not be the same as the scale length for any of them.

As has been pointed out, the strings stretch when you press them down to fret them. This makes them go sharp. How much the pitch changes depends on a lot of things, but mostly the properties of the string, how tight it is to begin with, and where and how much you press it down. Since you may not press the string down the same way every time there's some variation right there.

Pressing the string down on the first fret makes it go a little bit sharp, and that effect becomes greater as you go up the fretboard. If you chart it out, you get a rising line as you go up. Moving the saddle back to increase the vibrating length of the string can drop the pitch. The effect of a given offset on the pitch becomes larger as you go up: it doesn't change the pitch to speak of at the first fret, but can have a lot of effect at the 12th and up.Thus saddle compensation changes the slope of the 'sharpness' line. If you move the saddle enough, you can get no sharpness at the 12th fret. However, the string will still sound sharp at the first fret, and will actually sound flat above the 12th.

To get the first fret to play in tune you could move the fret back toward the nut, but that would leave you with the second fret sounding sharp. so, OK, move that back, and then the third and the fourth.... Whet the heck; why not just move the nut forward a bit and tune the string down a hair. It turns out that works very well. If you look at that 'sharpness' plot, shifting the nut forward moves the whole line down by the same amount, but doesn't change the slope.

So; the way to go about it is to shift the nut up enough to get the first or second fret in tune, and then shift the saddle back to correct the slope down to level. Then, at least in theory, all the notes should be in tune.

In practice, that won't work nearly as well on an acoustic as it does on a solid body electric. The top on an acoustic guitar moves, more at some pitches than others. This fools the string into thinking it's longer or shorter than the nut to saddle distance for some notes, and it makes the wrong pitch. The best remedy for that is to shift the top vibration frequencies a little, but that changes the sound of the guitar.