longitudinal mode vibrations

[Ron Nossaman]

>Ron,
>
>Let me amplify my question a bit.  Let's forget wound strings for a
>moment.  I have always heard, and it agrees with my experience, that
>torque can be a cause of false beats, especially in the upper treble. 
>For this reason, we try to avoid putting torque in the plain strings,
>and some manufacturers such as Yamaha go to great lengths in this
>regard.
>
>Why?  My guess - and it's only a guess - is that when there is torque in
>the string and the string is in motion, the torque will not remain
>evenly distributed along the length of the string.  When energy is is
>added to the bearing point, where the torque stops, it will momentarily
>increase the torque, causing the string to roll slightly on the bearing,
>and then roll back as tension is equalized again.  This will cause a
>torque wave to travel from the bearing point down the string to the
>opposite bearing point - i.e. a longitudinal torque wave.  My question
>is, How does this affect other longitudinal mode vibrations, or other
>string dynamics, for that matter?
>
>In the case of wound strings, the torque vibrations would be partly
>(hopefully almost entirely) dissipated by the mass of the windings, but
>I am not a fan of putting torque in them anyway.
>
>Paul S. Larudee, RPT
>Richmond, CA
>


Hi Paul,
Well, string torque probably is somewhat different in different sections of
the same string no matter what lengths the installer goes to prevent it. I
don't believe that a string will roll on the bearing point simply because
strings are bent around bearing points and the least likely place for
torsional movement in the whole string should be at the bends. If this did
happen, it seems reasonable to assume that it would affect the longitudinal
mode as you've described. It also seems reasonable that the wrap of the bass
strings would absorb the phenomenon through friction in the wrap, but I
don't see how this torquing under play could happen unless the string was
out of round and the torquing originated at the strike point. 

We discussed the importance of not twisting strings on installation a while
back on the List. No one had any direct evidence that it made any
difference, though everyone had heard that it would, and Del reported that
he had done a series of experiments with straight, coiled, twisted, and not,
plain and wound strings and concluded that there was no discernable or
measurable difference. That's the only instance I know of where any sort of
scientific method has been applied to the assumption. If there is any more
real evidence out there, I would sure like to hear it. 

Out of curiosity, how did you arrive at the conclusion that false beats in
the treble were caused by wire twist?

 Ron