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  #1  
Old 05-07-2018, 06:43 PM
rodmbds rodmbds is offline
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Default School me on G string intonation

Hope you can help my curiosity about this one. I do understand that string intonation in classical, steel or electric guitars exists so that the notes are closer to perfection especially in the high frets.

What I would like to know is actually "why" that happens, what happens to the string so it needs intonation.

I searched around and found it's because when strings are pressed they kinda go out of tune (sharp?) and etc. I couldn't get it.

Concerning the classical, I've usually seen a compensation on the G string only. I found some explanations saying that it's because the G is the "tubbiest" string. While I can accept that there's a difference and a reason why, in the end it didn't make any sense to me.

To my mind, if the thickest string needs intonation adjustment, why wouldn't the thinnest one need it as well?

Thanks a lot, I hope I could make myself clear in my doubts.
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Old 05-07-2018, 10:24 PM
Bax Burgess Bax Burgess is offline
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When I've seen saddles that are compensated only for the #3 string, on a saddle that is parallel to the frets, the compensation places the string to the back edge of the saddle. I don't see the worthwhileness of this, unless the saddle was at least 1/8th inch thick.

Last edited by Bax Burgess; 05-09-2018 at 09:13 AM. Reason: Bonehead misuse of "perpendicular"
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  #3  
Old 05-17-2018, 07:07 PM
charles Tauber charles Tauber is offline
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The shortest distance between two points is a straight line. A guitar string is supported at one end by the nut and at the other at the saddle. At tension, the string forms a (nearly) straight line over its end supports. When a string is depressed against a fret it is no longer a straight line. Instead it forms two, more or less, straight portions, one from the nut to the fretted location and one from the fretted location to the saddle. In order to do so, the string must stretch - it is stretched longer than the straight-line distance from nut to saddle.

The pitch a string produces is proportional to its tension and inversely proportional to its length. Stretching a string increases its tension, and, therefore, the pitch it sounds. To compensate for that relatively small increase in pitch, the string length is purposely increased, thereby reducing the pitch of the string. By increasing the string length by exactly the amount necessary to counter the increase in tension caused by fretting a string, one achieves the desired pitch.

Practically, the usual way in which a string is lengthened is to move the saddle further away from the nut. This is referred to as compensation at the saddle - or, simply, saddle compensation.

The pitch a string produces is also proportional to the bulk or mass of the string. Also, the thicker a string, all else being equal, the stiffer is that string. Deforming the string by pressing it against a fret increased the pitch more for stiffer strings than for less stiff strings. Thicker strings, generally, require more compensation than thinner ones.

To obtain the best intonation, the exact string length of each string should be set as required: each string will have its own amount of compensation. Often, on nylon string guitars, only the G string is set to a different length than the other strings. The un-wound G string has the largest diameter and requires more compensation than the other strings. (The wound bass strings have a smaller diameter core under the windings. The windings add mass, but not much stiffness.)

In a nut shell, there you have it.
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Old 05-17-2018, 07:36 PM
rodmbds rodmbds is offline
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Fantastic, that's exactly the kind of explanation I've been looking for. Though I've read about in some other places, you put it in a way I could definitely understand.

Can't thank you enough!
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Old 05-17-2018, 08:45 PM
charles Tauber charles Tauber is offline
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Glad to be of some help.
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Old 06-12-2018, 11:53 PM
lar lar is offline
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Charles,

For an open string, it doesn't matter how long the string is because the player will tune the guitar correctly in the open positions.

When fretting, the additional string tension forces the string slightly out of tune. In other words, the pitch of the string increased too much; Instead of a half-step, it maybe would increase a half-step + 10 cents for example. So to 'compensate' for this, the string is made longer. So does the extra length decrease the change in string tension when fretting? Is that what is going on?

I know this isn't practical, but would the perfect solution to have discontinuous frets? In other words, the 1st fret for each string would be at slightly different positions? (the fret for the G-string would be slightly closer to the nut than the other strings).

Thanks,
Larry
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Old 06-13-2018, 08:37 AM
lar lar is offline
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Quote:
Originally Posted by lar View Post
So does the extra length decrease the change in string tension when fretting? Is that what is going on?

Larry
After sleeping on this, I think I understand. When fretting, the string tension is increased, thus increasing the pitch. So, an extra length of string is used at the saddle to lower it back down to where it should be.

Does compensation work pretty well up and down the fretboard or do luthiers focus on correctly compensating a specific fret (say the 5th fret) knowing that the other frets will not be perfectly in tune?
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Old 06-15-2018, 08:41 AM
MC5C MC5C is offline
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The need for increased compensation as a percentage of the length of the fretted string is related to the increase in string height as the string height rises up to the saddle. The action height of the string at the first fret might be .010", at the fifth fret .040", at the twelfth fret .090". Because the string is a straight line and the fretboard is a very close to straight line, and the frets are spaced as they are (rule of 18), the increase in string height is very closely related to the fret position. The string at the higher fret is stretched farther, so gets more compensation relative to it's active length. It's not perfect but can be very close. The need for compensation changes with scale length, string stiffness, string mass to some extent, action height. It also changes with player technique - some players fret very vertically, some players push the string out of line quite a bit for some tricky fingerings. It's a compromise that compared to the out of tune related to equal temperament is fairly minor.

Edit: I did some simple math. For a 25" scale length steel string guitar, the compensation for the high E string would typically be around 1/8". For a note fretted at the first fret, the string length is 23.6" plus .125" and the compensation is .125/23.725 = .0053, or .53%. At the twelfth fret the string length is 12.5" plus .125" so the compensation is .125/12.625" = .0.099, or 1%. On a steel string guitar, the B string is quite a bit stiffer than the E string so gets more compensation. The G string, if unwound, is stiffer again but if wound is very close to the same stiffness as the high E, and so gets around the same compensation, all the way down to the low E string which gets the most compensation. Luthiers don't know what strings you will be playing, wound or unwound G's, preferred action height, so they place the saddle and do the compensation for the way most players might like the guitar, or the way they want the guitar, and players can have that changed if their preferences demand it.
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Last edited by MC5C; 06-15-2018 at 09:07 AM.
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Old 06-15-2018, 08:52 AM
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rick-slo rick-slo is offline
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Also look up inharmonicity - where the overtone series wavelengths are compressed more as the string core gets thicker. More of an issue
with steel strings compared to nylon strings but still a factor in nylon string guitars.
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Old 06-15-2018, 11:12 AM
MC5C MC5C is offline
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My understanding is that inharmonicity is more pronounced with plain nylon G strings. You wonder why anyone can play anything that sounds good on a guitar sometimes, then you realize that most of this minutia really doesn't matter very much...
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Old 06-15-2018, 11:39 AM
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Imperfection in an imperfect world, so flies under the radar most of the time.
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Old 06-15-2018, 01:59 PM
nickv6 nickv6 is offline
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Just to add a little more complication.., when a string is shortened by fretting, the stiffness of the string is increased as well, as stiffness is a product of diameter and length. To demonstrate what I mean think of this: take a 12inch plastic ruler and flex it...very easy. Now cut a couple of inches off it. Hold that 2 or 3 inches and try and flex it....impossible. same material, shorter length. Whilst the amounts are small, stiffness has a big affect on the harmonics generated which will be more out of tune the stiffer the string (in combination with the tension....which is another story...the string "knows" how long it's overall length is despite placing of bridge and nut).
Hope that adds to the controversy.
Nick
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Old 06-16-2018, 05:39 AM
MC5C MC5C is offline
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Nick, it definitely adds to the inharmonicity! Ever notice how the 6th string starts to sound a little dead if you play it above the 12th (or so) fret? That increase in stiffness makes the harmonics within the note go out of tune.
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