Adjusting the Nut for Intonation Correction

[Jim.S]

Originally Posted by rick-slo

Jim, you misunderstood my point. I do hear fretted notes going sharp near the nut and I do think nut compensation is a good thing to do to help reduce that.

Oops, sorry about that

Originally Posted by rick-slo

The point was, if you nut compensate a guitar you do not have to then also move the bridge. The frets are already in the right place.

Then it seems you have missed mine and Howard's point. It can easily be demonstrated that for the most accurate intonation across all frets then both compensating the nut and well as the bridge is needed, this is not opinion it is just the way things work. Again if you don't understand this fact and you would like to, then buy the book as I can't seem to explain it any better than that.

Jim

Life is good when you are amongst the wood.

[rick-slo]

Originally Posted by Jim.S

Oops, sorry about that



Then it seems you have missed mine and Howard's point. It can easily be demonstrated that for the most accurate intonation across all frets then both compensating the nut and well as the bridge is needed, this is not opinion it is just the way things work. Again if you don't understand this fact and you would like to, then buy the book as I can't seem to explain it any better than that.

Jim

Life is good when you are amongst the wood.

Jim, you missed interpreted what I was saying again. I know about compensating the bridge already. Once more try - if miss-read again, yikes.

A guitar that is already set up with a compensated bridge. However fretted notes near the nut play sharp. Everything else about the intonation is fine. Bridge compensation is fine as is.

OK, that is the guitar.

You decide to compensate the nut to stop the guitar's playing sharp near the but.

OK, that's the change you made.

You do not now need to also move the bridge because you compensated the but.

Period, end of point being made.:-)

[rick-slo]

Originally Posted by Jim.S

Oops, sorry about that



Then it seems you have missed mine and Howard's point. It can easily be demonstrated that for the most accurate intonation across all frets then both compensating the nut and well as the bridge is needed, this is not opinion it is just the way things work. Again if you don't understand this fact and you would like to, then buy the book as I can't seem to explain it any better than that.

Jim

Life is good when you are amongst the wood.

Jim, you missed interpreted what I was saying again. I know about compensating the bridge already. Once more try - if miss-read again, yikes.

A guitar that is already set up with a compensated bridge. However fretted notes near the nut play sharp. Everything else about the intonation is fine. Bridge compensation is fine as is.

OK, that is the guitar.

You decide to compensate the nut to stop the guitar's playing sharp near the nut.

OK, that's the change you made.

You do not now need to also move the bridge because you compensated the nut.

Period, end of point being made.:-)

[Alan Carruth]

Wow. It's late, and I've had a long day, so I hope I don't step on anybody here.

If you set up a nut, fret board, and bridge on a solid beam, with the nut and saddle in the 'correct' theoretical positions, and plot out the intonation, what you'll find is that the notes fretted on the first fret are sharp by a small amount, and the pitches get progressively sharper from there. How much sharper will depend on the string and the action height. A string that is 'slack' relative to the load it can take, such as a plain G string, will go much sharper as you go up. A string that stretches a lot, like a nylon high E, won't go very sharp at all. If you chart it out, every string will start out at a bit different height from the base line of 'perfect' intonation, and the slope of each line will be different.

Moving the nut forward shifts the whole line down. If you move the nut toward the first fret by enough to make that string play in tune, it will still go sharp as you go up the neck, but the vales will all be lower than they were with an uncompensated nut.

Moving the saddle back changes the slope of the line. It hardly flats the note at the first fret at all, but has a greater effect on every fret as you go up. If you move the saddle back far enough to get the 'correct' pitch at the 12th fret, the string will still be sharp in the low fret positions, and will be flat above the 12th fret.

In theory, then what you want to do is shift the saddle back far enough so that the line doesn't slope upward as you go toward the higher frets, and shift the nut forward enough to drop the whole line down to zero; perfect intonation. 'In theory, there is no difference between theory and practice, but in practice there is'.

Note that the string doesn't know whether you're fretting with your finger or a capo: the correct pitch is the correct pitch, and you have to shift both the nut and the saddle to get it.

The 12th fret 'harmonic' is probably not the best thing to use. For one thing, it's always likely to be a bit sharp due to string stiffness. I like to compare the open string pitch with the fretted pitch at the 12th fret, using a good tuner. It's harder to do than comparing the 12th fret note and the 'harmonic' by ear, but when your ears are as bad as mine, it's more reliable. Are you sure your ears are that good?

Each string will be different. You could move all the frets a different amount under each string, making them crooked, but it's a lot easier to just make the nut and saddle crooked.

If you set up a test beam and start out by, say, compensating the saddle to get the correct 12th fret intonation, and then move the nut to fix the first fret, you'll find you need less compensation on the saddle after you've fixed the first fret. You move the saddle, to get that right, and need to change the nut. After a few iterations you'll be close.

The fact that top is moving (as it must to make sound) will alter some of the pitches; the ones near resonant frequencies of the top and/or air. That's going to vary from one guitar to another. You might need to shift frets to fix that.

There is probably no way you can get any guitar to be 'perfect' on every note. You can get 'arbitrarily close' most of the time by getting the right settings on the nut and saddle. These might be significantly different for different strings.

This ain't rocket science!

[rick-slo]

Hi Alan, I don't know if you last post was directed at all to what I have been posting but you did make the comment about the guitar not knowing the difference between a capo and a finger, which is an example I used.

Making the pitch correct at the first fret with nut compensation would have the same effect on fretting intonation on the rest of the frets as putting a capo on the first fret and tuning the guitar to making the pitch correct on the capoed fret (first fret). With the capo the first fret is the new -0- point on the scale and the frets are spaced correctly accordingly up the neck. Move the bridge and they are not in the correct place. When you use a capo on the first fret, or the third fret, or the fifth fret, etc.,it does not throw off the intonation of the higher frets more than they would be otherwise.

Compensating for sharp frets near the nut by recessing the bridge further out would be pretty ineffective and would throw the upper frets way off. So I am thinking most builders don't even try to do it. Address the problem closer to the source, with the nut.

[Jim.S]

Originally Posted by Alan Carruth

The fact that top is moving (as it must to make sound) will alter some of the pitches; the ones near resonant frequencies of the top and/or air. That's going to vary from one guitar to another. You might need to shift frets to fix that.

Good addition Alan or shift the frequencies for top,back and/or air away from a problem note, easier than shifting a fret.

Jim

[Plumley]

Any chance one of you geniuses could just estimate for me the effect on intonation at the 12th fret, if moving the nut forward to compensate for about 2-3 cents sharp there (at first fret) would have? See, my Rainsong acoustic is already 3 cents sharp at the 12th.

So I was hoping moving the nut forward might get me lucky enough to have the 12th fret now play in tune instead of sharp in addition to fixing the sharpness while fretting the first position...

Thanks for your time!

[Alan Carruth]

In theory moving the nut up closer to the first fret drops the pitches of all the fretted notes by the same amount in terms of cents: in effect you're moving all the frets back by the same amount. Thus a three cent correction at the first fret should also correct the 12th fret note by three cents. In practice it may not be quite that, for various reasons, but it is likely to help.

[Monsoon1]

Originally Posted by Alan Carruth

Wow. It's late, and I've had a long day, so I hope I don't step on anybody here.

If you set up a nut, fret board, and bridge on a solid beam, with the nut and saddle in the 'correct' theoretical positions, and plot out the intonation, what you'll find is that the notes fretted on the first fret are sharp by a small amount, and the pitches get progressively sharper from there. How much sharper will depend on the string and the action height. A string that is 'slack' relative to the load it can take, such as a plain G string, will go much sharper as you go up. A string that stretches a lot, like a nylon high E, won't go very sharp at all. If you chart it out, every string will start out at a bit different height from the base line of 'perfect' intonation, and the slope of each line will be different.

Moving the nut forward shifts the whole line down. If you move the nut toward the first fret by enough to make that string play in tune, it will still go sharp as you go up the neck, but the vales will all be lower than they were with an uncompensated nut.

Moving the saddle back changes the slope of the line. It hardly flats the note at the first fret at all, but has a greater effect on every fret as you go up. If you move the saddle back far enough to get the 'correct' pitch at the 12th fret, the string will still be sharp in the low fret positions, and will be flat above the 12th fret.

In theory, then what you want to do is shift the saddle back far enough so that the line doesn't slope upward as you go toward the higher frets, and shift the nut forward enough to drop the whole line down to zero; perfect intonation. 'In theory, there is no difference between theory and practice, but in practice there is'.

Note that the string doesn't know whether you're fretting with your finger or a capo: the correct pitch is the correct pitch, and you have to shift both the nut and the saddle to get it.

The 12th fret 'harmonic' is probably not the best thing to use. For one thing, it's always likely to be a bit sharp due to string stiffness. I like to compare the open string pitch with the fretted pitch at the 12th fret, using a good tuner. It's harder to do than comparing the 12th fret note and the 'harmonic' by ear, but when your ears are as bad as mine, it's more reliable. Are you sure your ears are that good?

Each string will be different. You could move all the frets a different amount under each string, making them crooked, but it's a lot easier to just make the nut and saddle crooked.

If you set up a test beam and start out by, say, compensating the saddle to get the correct 12th fret intonation, and then move the nut to fix the first fret, you'll find you need less compensation on the saddle after you've fixed the first fret. You move the saddle, to get that right, and need to change the nut. After a few iterations you'll be close.

The fact that top is moving (as it must to make sound) will alter some of the pitches; the ones near resonant frequencies of the top and/or air. That's going to vary from one guitar to another. You might need to shift frets to fix that.

There is probably no way you can get any guitar to be 'perfect' on every note. You can get 'arbitrarily close' most of the time by getting the right settings on the nut and saddle. These might be significantly different for different strings.

This ain't rocket science!

Finally, one of these walkthroughs that I understand. Thanks, Alan.

[murrmac123]