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Open strings a few cents off from harmonics?
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=37211		 Page 1 of 1
Author:  Ken Jones [ Wed Jul 25, 2012 12:52 pm ]
Post subject:  Open strings a few cents off from harmonics?
Hey all. On one of my older guitars, the A is three cents sharper than it's 12th fret harmonic, the D is 5 cents flatter than it's harmonic, and the G is three cents sharp. The other strings are dead nuts on. What could be causing this? TIA.

Ken
Author:  Rodger Knox [ Wed Jul 25, 2012 1:01 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Are you saying the open string is different than the 12th fret harmonic? That's not possible.
If you're comparing the fretted 12th and the harmonic, then your intonation is a little off. If the fretted note is sharp, the saddle needs to move back a bit, and vice versa.
Author:  Ken Jones [ Wed Jul 25, 2012 1:13 pm ]
Post subject:  Open strings a few cents off from harmonics?
No, I understand intonation. As stated in the title, the open strings are off from their respective harmonics. And yes, it is possible, because it's happening.
Author:  Rodger Knox [ Wed Jul 25, 2012 2:13 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Check it again. The first partial is exactly an octave above the fundamental, except where the laws of physics don't apply.
Author:  Sondre [ Wed Jul 25, 2012 2:52 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
The pitch drops as the string vibrations decay, and it does so (in my experience) at a higher rate for the fundamental than for the partials. Did you allow the string to reach its median pitch before reading the tuner?

Or maybe the "wrong" notes couple with other resonant modes in the guitar, so that their frequencies are affected.

I suppose weird things also can happen if the strings are worn/dirty, because the mass/stiffness of the string is not evenly distributed across its length.

Sondre
Author:  murrmac [ Wed Jul 25, 2012 3:31 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Ken Jones wrote:
Hey all. On one of my older guitars, the A is three cents sharper than it's 12th fret harmonic, the D is 5 cents flatter than it's harmonic, and the G is three cents sharp. The other strings are dead nuts on. What could be causing this? TIA.

Ken


You say "older guitar" . Older guitar with older strings, or older guitar with brand new strings ?

Also, are you using a Peterson tuner ?
Author:  Ken Jones [ Wed Jul 25, 2012 3:51 pm ]
Post subject:  Open strings a few cents off from harmonics?
Thanks for the feedback. I also thought it might be worn strings, so I changed them. Same result. I'm using the recently updated Petersen IPhone app, which included a major stability fix, as well as the ability to adjust the resolution from tenths to whole cents. I find it to be a huge improvement over previous versions. I wonder if they sacrificed accuracy for the sake of stability, and it's the tuner that could be off. Seems unlikely since half the strings are dead on. More likely, as mentioned, it could be some resonant frequencies creating a false reading. I did try to muffle the top so as to isolate the individual string. Interesting. Any other possibilities?
Author:  Trevor Gore [ Wed Jul 25, 2012 6:34 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Most tuners measure the strongest partial present in the mix and that is often the second, because players tend to pluck at 1/4 string length. Also, most strings will show some level of inharmonicity if you're pedantic enough about measuring it.

On some strings you might be measuring the same partial twice and thus getting the same result. On others you might be measuring the first partial and then the second and so seeing what you see.

Generally, the only way to know the difference is to use a "tool" that shows you, i.e. as well as measuring frequency, it also produces a spectrogram of sorts where you can see the relative magnitude of the partials. I have a version of G-tune that does that quite well, but there are others.
Author:  Alan Carruth [ Thu Jul 26, 2012 2:20 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Actually, I'd expect the octave partial to be a little sharp as compared with the fundamental of the string, and the higher partials to be sharper yet. This is due to the fact that real strings have some stiffness: when you hold one out it doesn't droop like cooked spaghetti. The stiffness adds to the restoring force of tension, and the more the string bends the more the stiffness comes into play. Thus the higher partials are thrown progressively sharp, at least on a rigid mount.

On a guitar things get more complicated, mostly because the bridge moves. Any strong resonance that causes a lot of motion of the top at the bridge location spoofs the string. Depending on the exact frequency relationship between what the string 'wants' to do, and the top resonance, any partial can be thrown either sharp or flat relative to the desired pitch. The rule is that whichever resonance is higher in pitch gets nudged upward, and the the lower one gets nudged down.

Thus, for example, when I did my 'string' experiments a few years ago, I checked out a Thomastic 'A' string both on a rigid rig and on a guitar. On the rig, if the fundamental of the string was tuned accurately to A=110 Hz, the partials drifted progressively a little higher as you went up. The guitar, however, had it's 'main air' pitch a little lower than A=110, which tended to shift the fundamental of the string upward. In order to get it up to standard pitch it had to carry just a little less tension than it 'should' have. As a result, when the fundamental was in tune, the second, third and fourth partials were actually a little _flat_: they didn't feel the effects of the strong top mode an octave or more down from their own pitches.

So:"the A is three cents sharper than it's 12th fret harmonic, the D is 5 cents flatter than it's harmonic, and the G is three cents sharp."

It's likely that the 'main air' pitch on your guitar is somewhere just below A=110, the 'main top' pitch is probably a little below G=196, and there's some top or air mode around the second partial of the D string, just below 300 Hz, throwing that partial flat. It would be possible to hunt these down if you wanted to.
Author:  Rodger Knox [ Fri Jul 27, 2012 11:56 am ]
Post subject:  Re: Open strings a few cents off from harmonics?
I should know by now that the simple answer isn't always correct :o
Sorry about that oops_sign .
Author:  L. Presnall [ Fri Jul 27, 2012 3:51 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Just from a curiosity standpoint, have you capoed the 1st fret and checked the harmonic at the 13th? This may expose a problem at the nut, which refiling or refining the slots' break point may resolve. Or not. Sometimes these intonation issues can only be explained as being caused by quark-sized spirit dwarfs that are part and parcel of the nut, frets, and saddle... :shock:
Author:  Billy T [ Sat Jul 28, 2012 3:33 am ]
Post subject:  Re: Open strings a few cents off from harmonics?
L. Presnall wrote:
Sometimes these intonation issues can only be explained as being caused by quark-sized spirit dwarfs... :shock:
You've been having a lot of problems with QSSD's lately .... Lar? Eat Drink
Author:  John Coloccia [ Sat Jul 28, 2012 11:11 am ]
Post subject:  Re: Open strings a few cents off from harmonics?
When the string vibrates it has to stretch, increasing the tension and driving it sharp. I don't think it's any more complicated than that. Hit the string lightly and try again. They'll never be perfect because guitar strings aren't "ideal" strings, but it should be dang close.
Author:  L. Presnall [ Sat Jul 28, 2012 1:08 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Billy T wrote:
L. Presnall wrote:
Sometimes these intonation issues can only be explained as being caused by quark-sized spirit dwarfs... :shock:
You've been having a lot of problems with QSSD's lately .... Lar? Eat Drink


Hey Billy!

Right now they are apparently living in my spine...I'm laid up with a back outage. The shop is so close yet sooo far! How u doin'?
Author:  Ken Jones [ Sat Jul 28, 2012 3:51 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
Quote:
quark-sized spirit dwarfs that are part and parcel of the nut, frets, and saddle...
laughing6-hehe
Author:  Billy T [ Sat Jul 28, 2012 8:55 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
L. Presnall wrote:
Hey Billy! .....How u doin'?


Great! I was just working on detecting QSSD's with my linear particle accelerator patterned after the CERN version...I made mine from birch plywood though!!! They have great plans for one at Stewmac! laughing6-hehe

Sorry to hear about quarks in your spine! Hope you're up and around soon Bud!
Author:  L. Presnall [ Sun Jul 29, 2012 12:23 pm ]
Post subject:  Re: Open strings a few cents off from harmonics?
It's comin' Billy! Today's the second time I've been downstairs in a week. Wife was at church and I braved the stairs to get to the coffe maker! Figure I can die like a man, but not like a man without his coffee!
Author:  philosofriend [ Mon Jul 30, 2012 11:56 am ]
Post subject:  Re: Open strings a few cents off from harmonics?
Real strings can have their harmonics a little off from whole number multiples of the fundamental frequency. The main reason is the stiffness of the string. Some piano strings and very few bass guitar strings are made with the wrapping not extending all the way to the bearing (the saddle). Generally the overtones tend to be sharp. This in fact is the reason why pianos are tuned progressively sharper as you go higher. This makes the high notes in tune with the overtones of the low notes. This is also the true explanation of the Buzz Feiten tuning system. A good book, if these kind of things interest you, is Fundamentals of Musical Acoustics by Arthur Benade. When string makers experiment with materials and with changing the ratio of core string thickness to wrapping thickness, they are affecting how well the overtones will be in tune.
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