I've heard luthiers refer to the weird sounds from a badly adjusted pickup as 'stratitis' (or 'wolf sounds'?).
What are the mechanics of stratitis? What physical phenomenon causes it to occur?
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3... and is "Strat-itis" communicable to other species such as "Bassus Rickenbackus" ?– Carl WitthoftCommented Jul 9, 2020 at 13:10
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1 Answer
Strat-itis is caused when a string is producing overtones which are not in the harmonic series of the fundamental (unlike this). This can be caused by having parts of the strings which are vibrating differently, which obviously sounds dissonant.
This seems to originate with the interaction of the strings with the magnetic field produced by the pickup: if the string can magnetized (paramagnetism maybe) and interacts with the magnetic field produced by the other pickups, this will lead to a force constantly applying to a part of the string. If this force is damping the vibration (which I would say, is likely) this will cause this portion of the string to tend to resonate at a lower frequency.
It would be the same kind of effect as mounting a string which, in some portions, has a different gauge: different parts of the string will have kind of a different fundamental frequency… It would be possible to tune the fundamental, but the harmonics will be mixed up.
According to Seymour Duncan:
The overtones are due to each pickup having the same polarity (3 South or 3 North Polarity). The string acts like a keeper but is magnetized in three locations. As the magnetic field from the center pickup travels down the string, the magnetic field is repelled by the two outside pickups. As the like poles repel each other down the string, the overtones become very distorted sound wise.
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Different frequencies at different sections of a string is difficult, because all the energy is in travelling waves . That part of your answer is partially inaccurate - if the magnetic transition is strong enough, one could view it as a VSWR at a material interface- but Im a bit skeptical that this is a real effect on real guitars. I think the undesired tonality is due to the described effects causing the "wrong" harmonic frequencies to be amplified more than we would like relative to the fundamental. Commented Jul 9, 2020 at 13:15
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@CarlWitthoft "Different frequencies at different sections of a string is difficult, because all the energy is in travelling waves" I do not get why. If the speed of these waves is not constant over the string I would have said that it would be possible… Taking for instance the extreme case of a string with half of it at a very diff gauge, this would behave as two different but coupled strings no? Part of the energy bouncing on the separation…– TomCommented Jul 9, 2020 at 13:18
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That's what I was trying to say, clumsily, in my comment. VSWR "Voltage Standing Wave Ratio" is a term for reflection at an impedance-changing interface. I just think it's unlikely that you can magnetize part of a string enough to make this a measureable effect. Commented Jul 9, 2020 at 14:57
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@CarlWitthoft Frankly I am curious also, I have never experienced this kind of effects and the old type pickups I have on my guitar now are not adjustable in height… So impossible to test to see what is going on… The string still have to generate "non-harmonics" tones, but it is true that even the non-linearity of big strings could be enough for that…– TomCommented Jul 9, 2020 at 15:05