What physical effects cause ageing/worn guitar strings to change their tone?

Question

Guitar strings wear out over time and are prone to oxidation due to sweat. As a result, they change their tone usually sounding more "bright" when they are new and "dull" when aged.

There's been a question on this site that goes into what changes happen to the strings i.e. that they oxidate and grease starts to settle in: Why does the quality of the sound of guitar strings decay over time?

However, no one has gone into how grease and oxidation changes the vibration characteristics of a string. In particular, I'd assume that it changes the way that a string is able to produce overtones.

I've heard strings would change their stiffness, but haven't seen a scientific comparison between the stiffness properties of new and worn properties anywhere. Does oxidation change the stiffness of a string too? How does stiffness affect the overtones generated by the string?

Also, it'd be interesting to know why changing strings is so particularly relevant on the guitar but seems to be less relevant on e.g. the Piano (to be honest, I've never heard of a string change on a Piano).

Answer 1

One effect that is relevant for the wear on metal cored strings is work-hardening -- the repeated bending of the string through use causes microscopic changes in the crystalline structure of the strand of steel, affecting its stiffness and ultimately leading to breakage. Similar to bending a paper clip back and forth until it breaks. This affects the tone because string stiffness leads to inharmonicities and wave dispersion in the string motion. There are inharmonicities even in fresh strings, but they change as the stiffness of the string changes.

I'm also not 100% convinced that "oxidation" is a significant effect, but if it were it would go something like this: the patches of oxidized metal on the strings are stiff (inelastic), so they to affect the ability of the string to freely bend back and forth, and thus affect the sustain and overtone characteristics of the string in a manner similar to the work-hardening referred to in the previous paragraph.

A third effect is the build up of fatty/gummy gunk (oils from your skin mixed with dust from the air), either on the surface of the string or in the grooves of the wound strings. This has the effect of adding additional internal friction to the string (sort of like smearing a slinky with Vaseline), resulting in less sustain. This makes the notes seem like a pitched "thud" instead of having ringing sustain.

I'm not sure how (or if) these considerations apply to non-metallic strings.

Answer 2

I believe the effects are:

• The oil & muck from your fingers getting into the string :"A third effect is the build up of fatty/gummy gunk (oils from your skin mixed with dust from the air), either on the surface of the string or in the grooves of the wound strings. This has the effect of adding additional internal friction to the string (sort of like smearing a slinky with Vaseline), " - credit Dave's answer for that. This affects the wound strings (nylon or steel) far more than the unwound ones, although it can collect under the thinner strings too.

• Work-hardeining of the string which will affect the stiffness of the metal. Work-hardening occurs when metal is bent to and fro repeatedly. This happens as the string vibrates, notably at the bridge (especially when you bend a string). The metal becomes brittle and eventually will break. I don't believe this will affect the tension in the string but it may affect the tone. Nylon strings don't seem to suffer from the same effect although nylon itself does harden after a while, but this can take a much longer time.

• Lastly and also significant is oxidation. This affects all the strings but in my experience it affects thinner (unwound) string more. Acid in your sweat sets on the strings and the metal rusts. This is why unwound steel stings go a bit black after a while. Eventually bits of the metal in the string actually rust away, and the string gets thinner - but it'll do this usually unevenly giving rise to problems with intonation (you just can't get the string in tune anymore). Obviously the thinner the string gets, the more chance of it breaking. This doesn't affect nylon strings at all, but the windings of wound nylon strings may suffer a similar effect. The inner part of a nylon string is ... nylon, so it won't end up breaking, just sounding strange.

This last effect will differ depending on who plays the guitar. Some people have VERY acid sweat, others not to. I have experience of this : my fingers don't rust guitar strings too much; they last quite a while but for me the work hardening effect is what makes them break in the end. However I have lent a gutar to a friend for a weekly jam and been amazed how, week by week, the strings literally went ginger with rust. He says all his guitars do that as he seems to have very acid sweat.

You can prevent some of this taking hold, to an extent, by wiping down the strings after playing so that the moisture from your hands doens't sit on the string. It'll also help get rid of the oily muck.