Why do guitars go out of tune?

I would have to disagree with that. New strings stretch, but only to a certain degree. I put a guitar in storage and came back to find it not too far out of tune - and that was over 30 years later!

To my mind, the biggest factors (on their own or in combination with others) are, in no particular order:
Tremolo units
Poorly cut nuts
Too many wraps around the tuner post
Too few wraps around the tuner post
Poorly adjusted tuners
Changes in climate

Tension - there is a lot of it on the tuners
Temperature - strings stretch and contract when heated or cooled
Humidity - affects the wood of the guitar

It's already been said, just above, but.... if your question is why do they go out of tune as you're in the process of playing and have one in hand - then I think it is mainly due to the heat or temperature from your body, and your hands (sometimes we have sweaty palms too :roll: ) on the strings, and of course from string bends, if applicable. Both these things stretch the strings. As does that vibrato bar or tremolo arm, whatever you call it, it's listed up there too, by someone who knows. 8)

http://en.wikipedia.org/wiki/Vibrato

old strings have "memory", and once if start slipping out of tune, will continue to.
i got a used guitar, and a string would go half a step out of tune after about 20 seconds of playing, no matter how many times i tuned up. new strings and the problem was gone.

I've never heard of strings having memory. The only physical reasons I can think of for what you describing are: a) defective tuning machines, or b) not enough wraps around the tuning post.

I've had a few students with problem tuners (usually on Wal-Mart type guitars). But if new strings fixed it, I'd say they were badly put on in the first place.

yeah -- no memory effect here either, but one more reason that might explain it: weak or worn spot in the string. will sometimes continue to stretch until it snaps. had it happen to guitar strings, silly putty, fishing line ...

Tune your guitar up with an electronic tuner each and every time you play. It will most likely need it (even if just a touch up). This advice goes for every player-tunable instrument (that's everything but pianos, marimbas, etc.)

This will train your ears to hear a tuned up guitar, and eventually you will be able to hear when it is no longer in tune.

If taking the guitar to a gig, open the case and let it adjust to room temperature before tuning (if you have the time).

Normally my guitars stay in tune for a practice session (I don't have a whammy bar on them). When gigging, they stay in tune well, but do a touch up from time to time, depending on temperature changes, how hard I'm playing and whether I banged the headstock against the microphone (again).

Also, keep fairly fresh strings on the guitar. The frets wear the strings down and eventually that makes them go flat in the upper frets.

And when changing strings, a little pencil lead graphite in the grooves of the nut works wonders. It keeps the strings from binding in the nut and that allows them to return to their in-tune position after a bend. If your guitar has a string tree, a little original formula chap stick under the string tree does the same (this was recommended to me by a Kramer customer support rep).

I play sax, guitar, and wind synthesizer on stage. Funny thing, when the temperature goes up, my sax goes sharp, the guitar goes flat, and the synth stays the same. A re-tuning during a gig is not a defect of equipment, but simply the result of the laws of physics.

Insights and incites by Notes

I never realized that the amount of turns around the tuning peg effect how well the guitar stays in tune. Learn something new everyday.

here's what I use

E & A -- 2 times
D & G -- 3 times
B & E -- 4 to 5 times

I actually don't count, it just seems to turn out that way.

You're right, Notes - the laws of physics. In this case, coefficients of expansion (a fancy way of saying things don't react to temperature changes the same way). You'll notice different responses to temperature changes if you change mouthpieces, or string gauges, or anything else in the tone production chain.

But it's not string wear that makes old strings sound out of tune in higher frets - it's dirt. :)

The pitch a vibrating string makes is related to length, tension, and mass. Tension won't change when you fret a note - at least not enough to matter on most guitars. Length changes depending on where you fret. And mass won't change appreciably from moment to moment, but over time it adds up... the dirt from your fingers makes the string get heavier. So will tarnish/rust - the string is absorbing atoms of oxygen, sulfur, etc from the atmosphere.

As the string gets heavier, it gets lower in pitch. Easy to fix - we tweak the string tension. But the trouble is, the extra mass isn't evenly distributed - it's almost all where we touch the strings. So with a new string, fretting at the 12th fret produces a perfect octave - it's exactly half the open string length, and exactly half the mass, so it vibrates twice as fast.

But on a dirty string, fretting at the 12th still gives you half the length - but now it's less than half the mass, because the vibrating section doesn't include the dirty part. And having less than half the mass, the string will sound sharper than it should.

Denting a string will change the way it sounds too, but since the mass stays the same, the fundamental vibration won't change. You'll end up with an unequal distribution of mass around the string core, which will lead to changes in the distribution of the overtones (the displaced mass will favor some frequencies, and dampen others). So the overtones change, and the string produces a different tone - but all other things being equal, it'll still be on pitch. It's like playing a C on the 1st fret of the 2nd string and the same C on the 10th fret of the 4th string; both are Cs, but you can hear a difference. (The difference in overtones here has a different cause, but the effect is the same: different overtone distribution = different tone)

I've never heard of strings having memory. The only physical reasons I can think of for what you describing are: a) defective tuning machines, or b) not enough wraps around the tuning post.

I've had a few students with problem tuners (usually on Wal-Mart type guitars). But if new strings fixed it, I'd say they were badly put on in the first place.

if a set of strings is left in place for a long time, it tends to get structural weak spots where it's stretched over the nut or where it comes out of the tuning peg.
i had just gotten a used guitar that had probably set out in the storeroom for a couple months. the strings were hard and brittle feeling.
"memory" isn't my word, but i've heard the concept being referred to that way a couple times.

Jason, I can buy the structural weak spot. But what you described earlier doesn't fit that - you said the guitar kept returning to being a half step flat within 20 seconds, and that happened repeatedly. If that was caused by a weakness within the string, it would have snapped at the weak spot after two or three tunings - every time you increased the tension, the string would get thinner at that point, and that won't go on for very long.*

The most likely cause of the effect you'd described would be a loose tuner. The worm gear inside a tuning machine is held in place by friction against a gasket. If there's not enough friction, it'll slip exactly as you described. You can use the tiny screw at the end of the tuning key to increase the pressure on the gasket to fix that (if the tuner has those adjustment screws - some cheap Chinese/Indonesian ones don't)

But you fixed the problem by simply changing strings, so it wasn't a loose tuner. So the next most likely explanation is not enough wraps: the end of the string slipped farther back through the tuning post every time you increased the tension. This also won't go on forever... each time the tuning slips, the string is being pulled a little farther into the tuning peg hole, and as soon as the angle of the hole is shallow enough to overcome the friction, it'll slip completely through.

------ math geekery follows -------

*Just for fun, we can prove this with a little math. String tension, in pounds, can be found by T=(M x (2 x L x F)^2) / 386.4, where M is the mass of the string in pounds per linear inch, L is the string length in inches, and F is the frequency of the pitch produced. (386.4 is a constant to correct for units - the textbook forumua is usually given in meters, newtons, and kg). Let's say it's an acoustic guitar - L is probably 25.4". Let's say it's the high E string that's slipping - that's 329.6Hz.

Let's also say you're using a .013 plain E. I'll spare you the calculations, but the mass works out to about .0000369 pounds per linear inch. That makes the starting tension 26.773 pounds. Dropping it to Eb, which is 311.2Hz, happens with a drop in tension to 23.867 pounds.

If that drop in tension happens because the string is stretching on a weak spot, we're effectively lengthened the string. Two identical strings under the same tension will sound an E at 25.4" and an Eb at 26.8". So how much thinner does the string have to get to produce this effect?

The original volume of the string is pi x r squared x L, or about .013485 cubic inches. Doing a little more math, our diameter of the string to distribute the same mass evenly over 26.8" means our string is now .01265 in diameter - nearly 3% thinner overall. And that's only if the weak spot is the entire string; in the real world, one spot is considerably thinner, and the rest of the string's diameter hasn't changed.

If we go through just five iterations of this, our .013 string becomes an .011 string - and that's if the entire string is stretching evenly. Even if it IS stretching evenly, tuning it up to pitch eight times would exceed the ultimate breaking strength. If the stretch is concentrated on a single weak spot, it's going to get thinner at that spot even faster, and it's going to snap by the 2nd or 3rd time you bring it back to pitch.

Bottom line: it's plausible that a weak spot can make a string's pitch drop as you described. But if that caused the effect, it's not possible that you could keep tuning it up to pitch. My money's still on it being improperly strung in the first place :)

Everyone's missed what...in my experience...is the main culprit: zebra stripes! No kiddin'...the fret bars wipe at the strings and sort of scour transverse notches in the strings. This breaks the little ones and puts the fat ones hard to keep the right pitch. As your strings stretch...and you retune over and over again...the notches move off the frets and a fresh spot gets to grinding away. Next time you take a string off...run your fingers over it and you'll actually feel what I mean for yourself.

Hope this helps someone!

Cat

You're right, Notes - the laws of physics. In this case, coefficients of expansion (a fancy way of saying things don't react to temperature changes the same way). You'll notice different responses to temperature changes if you change mouthpieces, or string gauges, or anything else in the tone production chain.

But it's not string wear that makes old strings sound out of tune in higher frets - it's dirt. :)<...snip...>

Thanks for the detailed explanation. And I assumed it was those little notches - "learn something every day".

Here is South Florida, especially since I have a regular outdoor gig near a salt water lagoon (about 1/4 mile from the ocean), I've seen my fair share of rust.

No matter how much you wipe down the strings, and no matter how much contact enhancer you use on the jacks/plugs, metal is going to corrode.

Rust is the main reason why I modded a cheap LTD to bring to the gig instead of taking my Gibson ES-330 or my Epiphone Casino. I didn't want the salt air inside the hollow body. So I put Mean 90 pups and a Varitone on the LTD, it already had a nice neck.

Back on topic.

As mentioned by another poster, the D string wouldn't stay in tune very well. I tightened the screw at the end of the tuner and now it holds just fine.

and....

For many years before I learned to play guitar, I was the sax player in a guitar band. This is where I noticed that changing temperature had opposite effects on guitar and sax. As the guitars drifted in one direction due to temperature, the sax would drift the other way, and since it is much easier to re-tune the sax (only one note instead of 6 strings) I would find myself re-tuning the sax to keep in tune with the guitars.

Now I play in a duo with backing tracks that I create myself, http://www.s-cats.com . The tracks always are in tune, so when there are radical changes in temperature, I have to re-tune everything to A-440. This is the main reason why I bring the guitar in first, open the case, and let it adjust to room temperature while I set up the PA system and the rest of the gear.

Insights and incites by Notes

Cat, zebra stripes is what Notes was talking about - the frets wearing the string down. But it won't directly cause a string to be out of tune (although it will change the tone of the string)