# A Question for the Physics majors

bug music
August 25th, 2004, 11:46 PM
Someone else asked me this, and I've noticed it too, so I know it's not just me. Here goes.

Why is it that when you let your guitar sit for a couple of days and go to tune it, that the tuning seem to be sharp ? Wouldn't it stand to reason that the strings would strech slightly over time and cause it to go flat ?

I know that the guy I was jamming with, who asked me this question, just likes to throw out those "blow your mind questions". But I really wish I had a scientific answer to throw back at him, something about "the difference in temperature coefficient of the wooden neck against a steel truss rod" or some such nonsense to shut him up. But Damn, he's right my tuning does go sharp over time. Hmmm...

GuitarJonz
August 26th, 2004, 01:31 AM
Dunno where you live, but here in the Northeast, I have noticed that in the summer, when things get humid here, my guitars always go sharp between playings. This is especially noticeable when I have stored them in my basement, where it is more humid, and more noticeable with acoustic instruments. I think it's the humidity, as that never happens once I turn the heat on and the humidity drops in the house.

bo
August 26th, 2004, 10:20 AM
I think it's because when you're playing the strings and instrument are warm (due to lotsa body contact and perhaps stage lights) and in tune. After playing they cool off and contract, thus going sharp. Not to hijack yer thread, but why do both my guitars with Bigsbys go sharp when I play them :lol:

Michael R
August 26th, 2004, 03:25 PM
Not a physics major but would think heat/humidity would expand the wood and make it sharp while the opposite would 'flat' it, thats the way it seems to work around here for me.

"the difference in temperature coefficient of the wooden neck against a steel truss rod"

Just tell him to take the dot product of the coefficients of thermal expansion and moisture expansion for maple converted to polar coordinates divided by acos(2piR^RTg) rhere R^RTg == neck radius raised to the power of relative humidity * current average temperature in the greenwich meantime zone... of course I'm kidding, but something like that to keep him busy for a few min 8)

bug music
August 26th, 2004, 06:24 PM
I thought it may have something to do with the temp./ humidity, but I too am in the Northeast and will have to wait til winter to know for sure. Thanks for the replies.

e-merlin
August 26th, 2004, 07:10 PM
I think it would have more to do with heat generated by the mechanical action of strumming and vibration. You tune the guitar after it's been played a while and the strings are warm. When not played, the strings cool off and shrink.

Here's a related question. After you've played a while does the guitar go back in tune?

lenny
August 26th, 2004, 07:15 PM
i live in the norteast and this spring i noticed the same thing happening to my guitar. i am fairly sure it is because the increased humidity makes the neck swell, and the strings go sharp.
:lol:
i am expecting it to go flat in the fall as it dries out

Michael R
August 27th, 2004, 09:32 AM
i live in the norteast and this spring i noticed the same thing happening to my guitar. i am fairly sure it is because the increased humidity makes the neck swell, and the strings go sharp.
:lol:
i am expecting it to go flat in the fall as it dries out

My MIM tele is like a humidity sensor, it changes with the weather (humidity) so I don't have to wait till fall. The other week when it was real humid it kept getting sharper as the humidity increased, I didn't use it for a few days but when the humidity went down this week and I picked it up I knew it would be flat, and man was it flat. The good thing is as long as the weather/humidity is fairly constant it does stay in tune pretty well.

I would also think (but have no clue for sure) that the finish has something to do with it, how 'sealed' it is from the elements. as I notice my guitars without finished necks, alot of worn out finish, and one tung oiled seem to contract/expand more than the others.

I usually play for a few min. when I pick up a guitar and then tune it anyways, so as long as it pretty much stays in tune after that Im fine with it.

patonbass
September 13th, 2004, 02:44 PM
i live in minnesota so i know a thing or two about humidity. i have the same problems with tuning on my basses. in my experience, when i take my basses over to my friend's un-air conditioned house, i go sharp within a few hours, not noticably sharp, but you can definetly see it on the tuner. my MIM jazz bass has a thinner neck so on that bass you can actually hear the difference.

off the topic of tuning and more on humidity, my un-air conditioned friend has rust starting to show on both of his MIM teles' tuners :cry:

tdowns
October 23rd, 2004, 02:26 AM
Sharpness is due to thermal contraction of the strings because the ambient temperature is less than body temperature. All the factors mentioned above play a role in the tuning, but your body heat warming up the strings is the biggest contributor. Next time you pick up your guitar, rub your hand up and down the strings quickly several times. You will find the sharpness less. Us old steel players use that trick when coming back from a break in an air conditioned room.

0le FUZZY
October 23rd, 2004, 12:54 PM
<li>I ain't a fizz-zekes majer neither but I know zack-tick-lee watt cher typin about.
<li>I haff had up tew 70 TELECASTERs sittin inna room in all kinds of weather.
<li>Without fail after a month or tew all the strings go jes a liddo sharp.
<li>I alwayz figger its because of the truss rod over ridin the string tension jes a liddo. It is knott subject tew ambient conditions as much as the strings and when yew store it the truss rod issa a hoe bunch stronger than the strings.
<li>I reckon Terry iss kerrect Sharpness is due to thermal contraction of the strings because the ambient temperature is less than body temperature. All the factors mentioned above play a role in the tuning, but your body heat warming up the strings is the biggest contributor. Next time you pick up your guitar, rub your hand up and down the strings quickly several times. You will find the sharpness less. Us old steel players use that trick when coming back from a break in an air conditioned room.

<li>I know fur sure he iss kerrect bout rubbin the strings reeeeeel good before yew tune and whilst yer inna room yer gonna pick in. I haff alwayz done that.

http://personalweb.sunset.net/~barron/sig.gif

Kris
October 25th, 2004, 07:47 AM
... but I play one on TV.

I find that on my electrics the tuning goes sharp after sitting there for a while.

On my steel string acoustic, the tuning goes flat.

Why is that?

Kris

0le FUZZY
October 25th, 2004, 11:04 AM
<li>No truss rod inna acoustic wood be my guess.

0.F.333333333333333+

JimmyZ
October 25th, 2004, 01:32 PM
The combination of temperature and humidity is called enthalpy. As the air in your home or building is heated (by the furnace) it's ability to absorb moisture increases (that's called the saturation level). As that happens, everything that holds moisture is subject to giving up some of the moisture. Like all of your mucus membranes (lips, nasal tissues, etc). Wood absorbs a lot of moisture and, consequently, gives off a lot of moisture. As a guitar is giving off moisture the wood actually shrinks which makes it go sharp. It also affects the neck angle somewhat which means that you may have to adjust the truss rod. As a guitar absorbs moisture in the spring (when humidity levels go up and the furnace goes off) you will find that you have to adjust the neck again. The solution is to keep your guitar in the case along with a good case humidifier. Keep a room type humidifier in the place you store guitars. This affects all wood so pianos are very much affected by moisture and humidity. You can buy inexpensive "jar type" case humidifiers for a few bucks and they work well. If you don't have that, cut a sponge up and soak it in water, shake off the excess and put it in the case.OK, I'm an engineer geek.

Bob Rogers
October 27th, 2004, 08:00 PM
I was a Physics major, but they didn't teach us enough to figure this out. But they teach engineers to make wild ass guesses and say them with great certainty, and I've kind of picked up the skill over the years. My guess is that the answers that people have been giving concerning the strings and truss rod expanding and contracting due to temperature are on the right track. Here (http://www.engineeringtoolbox.com/14_95.html) is a table of thermal expansion coeficients and those for metals are about twice as big as those for wood. Expansion due to moisture is much slower and the sharpening effect seems to take place year round.

JimmyZ
October 28th, 2004, 09:08 AM
Thanks for sharing your obviously superior information. Your ideas may indeed be right, however, why bash someone else in the process? Did I offend you? Was there something wrong with my answer that pissed you off? You must have some sort of jerk complex to send somthing like that back out. I go to this site a lot and most people are really trying to discuss things but it seems that every now and then some jerk has to get on and ruin it. This isn't work it's for fun. You need to lighten up and be a little nicer to people. I don't think there was any reason to reply like you did.

el_extranjero
November 28th, 2004, 03:20 PM
I noticed that no one mentioned the elasticity of the strings. I've noticed the 'sharpening' effect also and attribute it to the strings gradual return to a balanced state.

You can check your tuning after a few days and it's sharp. Stretch the strings slightly and they'll come back in tune. Essentially, elastic materials (most all materials) operate on a curve where they have more tendency to return the more they're stretched. So, once they're in a stable state on your guitar and have been 'pre-stretched', they have very little force to contract. When you play them, you stretch them slightly and tune 'up' to counteract. When you leave them the material will slowly contract to the previous position (minus permanent damage) and sharpen the tuning.

Yes, temperature and humidity most definitely have an effect, but in a stable environment I tend to believe that the elasticity of the strings is a major contributor.

I believe this is also the major contributor to strings going 'dead'. Over time, due to work hardening, the strings lose that elasticity. This means that the force due to bending a small amount (when you strike the string) is less. So the string vibrates less aggressively, thus losing the 'snap' and sounding flatter.

stevedenver
December 1st, 2004, 04:35 PM
But they teach engineers to make wild ass guesses and say them with great certainty, and I've kind of picked up the skill over the years.

jeez, im a lawyer and this is what we do too :wink: guess im not alone :wink:

i always thought that the sharpness came from one string (b or g for me) going a bit flat and making the others seem sharp-but those tables proved me wrong :shock:

el_extranjero
December 1st, 2004, 04:39 PM
But they teach engineers to make wild ass guesses and say them with great certainty, and I've kind of picked up the skill over the years...

:) Actually modern scientific method is based on that exactly! Make a wag, then develop tests to prove or disprove it! We engineers just happen to be better at WAGs than many other professions :))

That's the problem with an issue such as this. We can all speculate on the cause, and I can postulate several tests, but who the hell wants to know that bad!!! :>