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What's the deal if, in setting intonation, your 12th fret harmonic strobes different from your open note? I have an A string that dead on at A440 open comes up -.03Cents off at the harmonic...is this a nut problem? TIA.

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You got me . As far as I know the harmonic should be twice he frequency of the open. Make sure your string is stopped at the leading edge of your nut and maybe check the batteries in your tuner.

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Hi John,

The tuner's plugged into the wall, so good there...you may remember my thread a while back about nuts...I suspect the problem is there...I'll check it out...

Thanks,
Larry

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Question! Is the 12th truly at the half way node? A miss set saddle position will cause the 12th's and the other frets relitive position to open to be either short or long thereby causing it to be either flat or sharp in relation to the open. This is most likely a scale lenght at the 12th issue.

Is this happening on all string or just one?MichaelP38796.4722569444

Comparing the open string note to the 12th fret note is one way we check for intonation issues. They should be the same (or in this case, the number should be doubled)and if it isn't, it's a bridge issue.
If you want, you can eliminate the nut as a question by putting a capo on the first fret, and measuring the 13th fret.
If your 12th fret (or 13th) note is sharp, the saddle is too close to the nut and you need more string length- move saddle back. if the note is flat, then the saddle is too far away from the nut and you will need to move it forward. Small adjustments are what I'm talking about here, if you read up on it you'll find a lot out there to help you.

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Mark Swanson
Swanson Guitars
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Mark, he's talking about the 12th-fret harmonic compared to the open string.

Larry, as John H. mentioned, it could be that the string doesn't have a crisp break-off point at the nut or saddle. Or, it could just be a bad string; the windings may be inconsistant, or a poorly made core might have made it past quality control. It happens!

I will bet that something in the scale is off either the nut break is back from the edge of the nut, the fret position is off or the overall scale lenght of the string or saddle position is off. Something has the 12th and maybe other frets out of position in relation to the as built scale lenght. If the the saddle is (for dicussion sake) 1/16 long of ideal position then the 12 fret will be a 1/32" short of the half scale node if the fretboard was cut for scale 1/16 shorter than the as built length and vise-versa if built short. Something in the fret to scale length ratio is off. You did not say how much off tone it was. If a small amount then it may be adjusted in the itonation of the saddle. If more than can be adjusted in the saddle then something in the scale length is out of wack

Thanks Carlton, I didn't understand at first. Sorry!

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Swanson Guitars
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[QUOTE=MichaelP] Something has the 12th and maybe other frets out of position in relation to the as built scale lenght.[/QUOTE]
Okay, I could very well be missing something here, but his fret positions shouldn't have any effect on his half-string (12th fret area) harmonic, should they? If I'm understanding the original question correctly, his harmonic would still be off, compared to the open string, even if there were no frets on the fingerboard.

Michael, not on all strings...just 1 or 2...primarily the A....

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I am brain dead this morning your right if the 12th harmonic sounds then he is tapping at the node DA!

And Carlton, that's the thing!   The frets aren't even involved other than the open air position over the 12th...the harmonic is off with the OPEN string...fretted I can understand....

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replace the stings and check again. make sure all break points are clean and sharp

Sounds like the strings if no fret is involved. Changes in winding density or core diameter along the length of the string. By definition the mid point harmonic must be the same as the open string, that of course assuming the string is homogenious.

Colin

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I agree if the nut were out of wack, it seems that would affect both the open and harmonic. Only thing left is the string.John How38796.5541898148

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Cool! I'd prefer it be the easiest thing for a change!

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HI Larry

I have one more thing to throw into the mix, it could be a string behaving erractically, so I would change that first, if it remains, it could potentially be a flat spot on the fret, this can be caused either by a flat spot due to dressing or a dent caused by hammering the frets, when you fret to generate the harmonic it is not quite contacting the centre of the fret.

I hope I have understood the problem you describe correctly.

Russ

Here's an off-the wall answer: what are the pitchs of the 'main air' and 'main top' resonant modes?

In theory the octave 'harmonic' should be twice the frequency of the string's fundamental. In practice that's seldom exactly the case, although it's usually pretty close. There are several things that can alter the relationship, even on a 'good' new string.

All of that vibrating string theory starts from four premises: that the string itself is perfectly flexible, that it's uniform, which implies that it's perfectly cylindrical and so on, that the tension remains constant as it vibrates, and that the ends don't move. A nice new string will be pretty uniform, but one out of four doesn't cut it here.

Basically, the more the string bends the more effect the stiffness has, sharpening the pitch. The octave harmonic involves more bending than the fundamental, and each harmonic as you go up has even more. Thus the higher partials go progressively sharp. This effect has been measured on real guitars, BTW; it's not just theory. A thin string that is close to it's breaking point, like the high E, tends to have a 'truer' harmonic series than a thicker, slacker string. Wound strings should only have the core stiffness to mess them up: 'should'. That's one reason they make wound strings. Still, this sharping of the upper partials is a big problem for makers of small pianos, even though they use mostly wound strings.

Strings get tighter as they are displaced from the 'rest' position, so the tension in the string changes twice per cycle of vibration. Again, there are a lot of variables involved in this. As with the string stiffness, strings that are slack relative to their breaking tension change tension more than tight ones. Steel strings change tension more than nylon, unless they're really slack. The tension change is proportional to the _square_ of the displacement, so you might find that, if tension change is part of the problem, plucking the string less hard will decrease the effect.

I suspect that most of it is due to the fact that the ends of real guitar strings can't be absolutely stationary. If they were, the top would not move and you wouldn't hear any sound! Movement of the top can shift the pitches of the strings, and the top is most likely to move a lot at the pitches of the 'main top' and 'main air' resonances. The match has to be just about 'dead on' to effect the pitch of the string.

The main air resonance often comes in around G on the low E string, but can be as high as the open A. The 'main top' resonance is generally about an octave higher. Thus either of them could be effecting your A string, either the fundamental or the octave, depending. This effect is usually not large, but it might very well show up on a strobe.

One interesting trick; the amount the pitch 'bends' depends on how strongly the string and top 'couple'. The string can usually move the top pretty easily, and couple strongly, in the 'vertical' direction; that is, when the string is vibrating perpendicular to the plane of the top. When the string vibrates parallel to the top the bridge generally moves much less, so the top has less effect on the string pitch in one direction than the other. Since you almost always pluck the string so that it moves in both directions to some extent you may hear the string 'beating' with itself; getting louder and softer at more or less the same rate that the strobe goes in and out of phase. This can help you figure out which resonance is contributing to the problem.   

[QUOTE=CarltonM] [QUOTE=MichaelP] Something has the 12th and maybe other frets out of position in relation to the as built scale lenght.[/QUOTE]
Okay, I could very well be missing something here, but his fret positions shouldn't have any effect on his half-string (12th fret area) harmonic, should they? If I'm understanding the original question correctly, his harmonic would still be off, compared to the open string, even if there were no frets on the fingerboard. [/QUOTE]

Isn't it possible that fret positions do matter IF the twelfth fret is a little off AND he is using that fret to locate the center of the string being tested?

(just my inexperienced thought)

Ron

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[QUOTE=old man] [QUOTE=CarltonM] [QUOTE=MichaelP] Something has the 12th and maybe other frets out of position in relation to the as built scale lenght.[/QUOTE]
Okay, I could very well be missing something here, but his fret positions shouldn't have any effect on his half-string (12th fret area) harmonic, should they? If I'm understanding the original question correctly, his harmonic would still be off, compared to the open string, even if there were no frets on the fingerboard. [/QUOTE]

Isn't it possible that fret positions do matter IF the twelfth fret is a little off AND he is using that fret to locate the center of the string being tested?

(just my inexperienced thought)

Ron[/QUOTE]

No you'll only get the harmonic at the node. The fret board could have no frets and it would make no difference. Forget all the pseudo-science and the rest, it's a duff string. It happens a lot but we only really notice it when we are setting up the guitar. If I went round the ten or so guitars I've got around the place I expect I'd find a few strings that did this. A good reason to use high quality strings.

For this effect, it makes no difference where the string breaks over the nut or saddle, the open string and the mid node harmonic should be the same if the string is even all the way down. If the open and harmonic are different it means the string isn't consistent. If the break positions are wrong it will move the position of the node away from the 12th fret, but the open and harmonic will still be the same. (Think 25.4 and 24.9 scale lengths)

Colin

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What Al said.

I would generally expect the half-string overtone (12th fret harmonic) to be slightly sharp of double the fundamental frequency, because of string stiffness. This is normal string behavior, and not a problem.

Now, if yours is coming up flat rather than sharp, then there may be a problem with that string, or resonances are coming into play, as Al suggests.

I always find it helpful to remember that when you pluck a string, you're not really plucking a string, you're plucking the whole guitar. The parts do not act independently of each other.Todd Rose38797.2792592593

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Thanks Colin

You are right of course, makes sense to me, so even if the fret was off the harmonic would adjust itself back to the mid point.

Larry

Ignore my theory

   The 12th fret harmonic is the physical mid point of the string's register and the placement and compensation and placement of the saddle and nut are adjustments that actually align the note register with the harmonic blanket. Your 12th fret harmonic can be achieved at points that are off of your 12th fret's location if the saddle is misplaced in relationship to the nut and the intended scale length.

   When we intonate, we're just sliding the note locations under the harmonic locations. That's why when we hit our 12th fret note it needs to be aligned with our 12th fret harmonic. If it's not, we compensate to align them. Since the harmonics are activated without the introduction of an interruption or termination of the string along its vibrational path but by the indication of a node of the total by the touching at the precise point of the harmonic's location, it is always consistent that the particular harmonic intervals fall along the string according to and relative to its overall length.

   Regardless of nut and saddle placement, the 12th fret or mid harmonic will always read at twice the frequency of the open note since it is simply the vibration of the two halves of the total node. If not, I would suspect an improper contact surface or exit at either end that may be causing an inaudible buzz that would easily cause the strobe to read additional frequencies being generated and combined with those of the note or harmonic and the subesequent misalignment of the two registers. Buzzing against the sides of a nut slot can cause this phenomenon to occur as well as inconsistent windings in the sring in question.

   The same imperfections and inconsistencies in the string are present and implied during the activation of the open note and the 12th fret harmonic since one is the whole and the other is the combination of the two halves vibrating. stiffness and deflection are not issues since they are the same for the full note or mid harmonic. We can really make this so unclear that it all becomes muddy if we try, but it ends up doing little good.

   I'm sure you're not digging in to get the pitch like you would at an old fashioned flatpicking competition, but are simply lightly pulling the note and harmonic. The phenomenon of note variation according the direction of the string's activation is a very real thing, but it really only occurs when we get pretty aggressive while flatpicking, in my opinion.

   Just my opinion and some things I've found through experience over the years.

Regards,
Kevin Gallagher/Omega GuitarsKevin Gallagher38797.3871180556

I learned a lot about real-world string behavior when I was doing some piano restoration and learning to tune pianos. Al still knows a lot more about it than I do, though.

To restate part of what Al said in perhaps simpler terms... the reason the octave harmonic and all other higher partials sound slightly sharp of their theoretical pitch is because they are produced by the oscillation of shorter segments of the string. The shorter the length of string, the more its frequency is affected by the string's stiffness. This has nothing to do with frets, nuts, saddles, or intonation (although it might influence how you choose to set the intonation), it's just the way strings behave on any instrument.

To date, I haven't, personally, paid much attention to this with regard to guitars. Nor have I used a strobe tuner to check intonation on a guitar. I have a good ear, and I use it for tuning instruments and checking intonation. I've never noticed the sharpness of an octave harmonic on a guitar. If measured, it may well turn out be extremely slight, I don't know. Higher partials would certainly (as Al says) measure significantly sharp, though - there's no way of getting around that this is what strings do.

All of this may be perfectly useless blather to many of you. So be it. I find it very interesting, at the least, and sometimes useful to know that harmonics (on real strings, not theoretical ones) are NOT perfect. And they change depending on string gauge, scale length, alternate tunings, etc... If we rely too much on them (for tuning a guitar or whatever), we can be led astray. On the other hand, there are times, esp when tuning a piano, when we have to pay very close attention to what they are doing.

When tuning a guitar, I'm mostly just making little adjustments here and there until all the chords sound good. The result is equal temperament, give or take a little depending on the instrument.

If I used a strobe when setting intonation, I'm inclined to think I'd want the fretted octave (12th fret) to sound as a perfect octave, no matter what the corresponding harmonic sounds like. But I'm very interested to hear how others approach this.

Al, or anyone, please correct me if I've said anything flat out wrong.Todd Rose38797.6200462963

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First: I agree that the most likely problems are a bad string, or a bad setup. Since both of those had already been mentioned, I brought up other possiblilties.

The actual amount that the octave partial is sharped by string stiffness is really small. Ditto for the pitch 'bending' by coupling with a top resonance, usually. However, strobe tuners can be quite precise: you could be seeing something that you couldn't hear.

The whole point of spending time on that post was to bring up some uncommon phenomena for consideration. These things don't usually give you trouble, but when they do they can be mystifying if you don't understand the dynamics of the system. I thought I'd give you the benefit of the hours I've spent tracking down some of those 'flaky' problems: it's not _always_ the 'usual suspect'.