intonation help

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greggome
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intonation help

Post by greggome » August 3rd, 2013, 3:08 pm

Okay this has been diving me crazy for quite awhile. I have a few fender type electric guitars that appear to be out of intonation. I have attempted numerous times to adjust them as follows: eg. I play the high E string on the 12 th fret -> then play the harmonic for the 12 th fret if sharp I turn the screw at the bridge clockwise. It takes a few tries to get them fretted note and the harmonic to agree with each other. Now I check my E open string tuning which needs to be retuned. Now I go back and recheck the 12th fretted E vs the harmonic E and its out of whack again. I can go back and forth with this all night and the guitar never shows an E open string in tune with matching 12th fret harmonic and fretted 12 fret E. Since I'm having this problem with more than one guitar I feel I have got to be missing something. Shouldn't an open E string when adjusted properly also show on a tuner with an E when fretted on the 12th and an E 12th harmonic? I have replaced pick ups , pots etc. But this has me stumped. Any help would be appreciated.

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Re: intonation help

Post by NoteBoat » August 4th, 2013, 8:37 am

Sounds to me like you need new strings.

Here's my guess as to what's happening...

The pitch of a vibrating string is determined by three factors: the length of the string, the tension on the string, and the mass of the string. When we change the tension with the tuner we get a different pitch; when we change the length by fretting a note we get a different pitch, and when we put a heavier string under the same tension we get a different pitch (for example your E and B strings are different gauges - the heavier one sounds lower).

As we play, the strings get dirty - they build up oil and other junk from your fingers. This adds mass to the string, which makes it sound lower. So we tweak the tuner to raise the pitch up to where it's supposed to be.

But your strings aren't getting heavier evenly - more junk is building up where we fret, and less junk where we don't touch them (for example, over your pickups).

When you play a harmonic, it will ALWAYS be in tune with the open string - the 12th fret harmonic vibrates exactly twice as fast, so it produces the octave... which should match the pitch of the fretted 12th fret - because that's exactly half the string length. But with a dirty string, when you fret at the 12th fret you'll get half the length, but LESS than half the mass - the cleaner half of the length is vibrating, while the heavier dirty half isn't. Since the part of the string that's vibrating has less mass than needed to be in tune, the fretted note will sound sharp. No matter what you do, you won't get it to match the harmonic, because physics is working against you, and physics always wins.

A guitar's intonation should ONLY be adjusted when you've just put on new strings. If they're not new, physics is getting in the way again. And once you've got the intonation set right with new strings, whenever you notice your intonation is off - well, that's your guitar's way of telling you it's time for a string change.
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Diceman
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Re: intonation help

Post by Diceman » August 4th, 2013, 9:07 am

In my experience , new strings need to be played a little before you intonate or the results will be less than perfect . Yes , new strings but not BRAND new strings ! The newly installed string always seems a little loose and then calms down with a little break-in . Anyone else notice this?
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Re: intonation help

Post by imalone » August 5th, 2013, 2:02 pm

NoteBoat wrote: When you play a harmonic, it will ALWAYS be in tune with the open string
I have a tele that wants a word with you... (I think it's likely due to oldish strings and maybe a difference in the tuner note tracking between the open and 2nd harmonic, having tried plugging it into anything more accurate yet.)

greggome
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Re: intonation help

Post by greggome » August 5th, 2013, 3:05 pm

I have tried this in the past with new strings as well. I also have 3 different electronic tuners which I've used. I have gotten suggestions to forget the harmonic and just work with open string vs 12th fretted note as getting the 3 to match up on most guitars is close to impossible so they say. so I guess I'll give that a try.

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Re: intonation help

Post by NoteBoat » August 5th, 2013, 6:54 pm

You know, I've been thinking through the physics of this today... and I think if your strings are REALLY dirty, the harmonic might not match - but if that's the case, it also won't have much sustain. (I'm thinking the two halves of the strings will vibrate at different rates, and destructive interference will quickly kill the sound). Maybe there's a physicist or engineer here who can shed some light on that aspect :)
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imalone
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Re: intonation help

Post by imalone » August 6th, 2013, 12:51 pm

<- is a physicist, but it'd take some work to figure out the details of this (there are people who spend their careers looking at the physics of instruments).
It's not quite destructive interference, it's more that energy leaches into different modes and they decohere, but yes if they're close together then that would happen faster (it might not be the main process of loss though). If I had to come up with a guess about the weird harmonic thing it would be loss of elasticity in the older strings means the effective length is different for the two pitches. Funnily these particular strings are old (a few months) but not very dirty (because the guitar hasn't been played in that time). Of course the second guess is that I put it together myself and something (nut, bridge, neck joint) isn't quite right.

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Re: intonation help

Post by NoteBoat » August 6th, 2013, 6:48 pm

Not sure I follow the elasticity premise... I can see how the stiffening of a string would shorten the speaking length, but it seems to me that the speaking length is identical for both the fundamental and the overtones; if part of the string isn't vibrating for one, it won't for the other either.
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Re: intonation help

Post by notes_norton » August 7th, 2013, 8:56 am

A couple of possibles I can add.

Strings not only get dirty, but the frets wear into them. Take an old set off and you can see the stripes in the strings.

Also, does your guitar have a whammy bar?

When I check the intonation on my Parker, I wiggle the wiggle stick first to get it from being "stuck" - whether it's the nut or the bridge, there is friction there. In fact, I even do this when I tune it up (it's my only guitar with a whammy).

I also put pencil graphite in the nut slots when changing strings.

All this helps so that when you are tuning one string, the other doesn't slip a little in a place where friction is holding it (nut, bridge, string trees).

I really don't know how much of an effect it would have on adjusting intonation, but I figure it certainly cannot hurt.

And yes, I always adjust intonation with new strings.

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Re: intonation help

Post by imalone » August 9th, 2013, 5:02 pm

NoteBoat wrote:Not sure I follow the elasticity premise... I can see how the stiffening of a string would shorten the speaking length, but it seems to me that the speaking length is identical for both the fundamental and the overtones; if part of the string isn't vibrating for one, it won't for the other either.
The wave equation depends on a couple of assumptions, mainly small deformation and no stiffness, once you start to break them the behaviour is no longer linear and different wavelengths get different conditions, for example the shorter wavelengths of higher frequencies are going to be affected more by that, on the other hand you might expect that over the whole length of the string they're affected less by what's happening at the ends than the longer wavelength waves.

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