Question about the physics behind a guitar string

[billiards]

Ha Ha Ha, this thread rocks! Especially if you can't be bothered to take it seriously (jeezus christ, bless our sacred souls). I was reckoning that the microphone is the same as a guitar pickup, but then I was trying to recompensate for the obvious discrepancy that a microphone doesn't require a vibrating string to stimulate a current (whereas the pickup does); in the end I (think I) realized that actually the microphone does(!), but this "string" isn't the obvious string one would associate with a guitar, perhaps it's some kind of coil? Perhaps not, I dunno, all I know is they are pressure sensors, scalar fields are below my understanding :-).

 

[phlegmy]

IDEALLY the tension should NOT CHANGE when you fret the fretboard.

the frets are spaced out very carfully to shorten the string only.
obvioulsy because you you press the string onto the fretboard your stretching it every so every so slightly which will increase the tension.

i notice that when i play guitar with very light gauge strings that it sometimes sounds out of tune when fretted. this is becasue I'm pressing tooo hard on the fretboard, stretching the string behind the fret.

also to increase the pitch you can bend the string upwards (or downwards!) the change in pitch in this circumstance is due to change in tension [yes there is a fractional very very very small change in length but that's not important!]

 

[leftyguitarjo]

I want to know how a natural harmonic on a guitar works.

 

[phlegmy]

when you pluck an open guitar string the two ends remain stationary [this may sound obvious!] while the rest of the string vibrates or oscisllates up and down.
this oscillation is made up of a number of modes of vibration



http://cnx.org/content/m11118/latest/
the above page showns a diagram of the modes of vibration of a string
have a look at the second set of illustrations on this page,
to play a "harmonic" on a guitar you can lightly touch halfway along the string [above the 12th fret].
by holding the string halfway you are only allowing the 2nd 4th 6th 8th 10th etc modes of vibration because in theese modes, the point you are holding does not vibrate, so you're not affecting them [much]
the others don't sound because where you are lightly pressing the string is where the fundamental, 3rd 5th 7th etc modes want to vibrate, so you're effictivly damping them out.

 

[rcgldr]

IDEALLY the tension should NOT CHANGE when you fret the fretboard.

Or simply placing the frets so that the change in tension is accounted for and the desired note is a combination of the (slightly less) shortened distance and increase in tension. Taking the bridge height, fret board, and targeted gauge of strings into account would allow fior more accurate placement.

I want to know how a natural harmonic on a guitar works.

Part of this depends on where you pluck the string. Pluck the strings near the middle and you get mostly the primary (lowest) harmonic. Pluck a string near the bridge and you get a more treble sound due to a higher ratio of higher harmonics. You can also place your fingers lightly on a string at key points to generate near pure harmonics: over the 12th fret will double the frequency, over the 7th fret will triple the frequency and over the 5th fret will quadruple the frequency. A table from Wiki:

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

Note that the strings don't generate sound directly on an acoustic guitar, instead they just vibrate the bridge which in turn vibrates the main body of the guitar which generates the sound. Stringed instruments require a sound board or the equivalent in order to produce significant sound.

 

[rcgldr]

Too late to edit so replying.

A table from Wiki:
http://en.wikipedia.org/wiki/Guitar_harmonics

The wiki article mentions that the 7th fret produces an overtone of octave+fith. This doesn't mean two tones, but instead a note that is 19 half-notes above the primary harmonic, but at perfect pitch, 3.0000 instead of 2^(19/12) = 2.9966.

 

[Alcape]

Hey guys, when you put your finger down on part of the string of a guitar depending on how hard you push down the wavelength will become shorter hence changing the frequency

 

[mugaliens]

Ok, seeing some of the very outstanding responses, here, I'm only posting to clean up the remaining questions:

1. If you haven't read through the thread, please do, as your answer is already there.

2. If you didn't understand the response, here is guitar 101, as simple as I can make it:

a. You have six strings, all of which have strings of the same length, but of different masses and tensions. It's the masses of the strings and their tensions which determine their tune.

b. The more massive the string, the lower its note.

c. The tighter the string, the higher its note.

d. Put all of these together into a six-string ensemble as determined by hundreds of years of trial and error, and you wind up with a wonderful sounding guitar!

 

[Danger]

you wind up with a wonderful sounding guitar!

In the right hands...

I mucked about with one once for a couple of minutes; it sounded like a rooster being raped by a cement mixer.

 

[richardmmeade]

can you guys help me. I am not really a physicist but i want an equation to help work something out. is there an equation relating string thickness to the frequency of the sound that comes out. but making sure that the tension in the string remains constant??