longitudinal mode vibrations

[Paul S. Larudee]

Ron,

If there's so little to go on, I guess we're just going to have to
speculate, but let me respond by picking a couple of points in our
correspondence:

> >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.

Perhaps it would be better to say that when energy is imparted to the
string, the distribution of torque will not remain static (vs. "evenly
distributed").

> >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.  

I was thinking more of the V bar with its single horizontal contact
point than of the bridge with two (pin and cap) or the agraffe with a
partial wraparound contact area.  (Of course the string creates a small
groove in the bar, but I think it still allows for more torsional
movement than at the other two types of bearings.  Once started, any
torsional wave travels longitudinally because it releases torque at one
end of the string as it builds at the other, which then needs to be
released in the opposite direction.

> >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.

Actually, I was thinking just the opposite - that the energy imparted by
the hammer will cause the string to try to straighten itself at the
strike point, sending a change in torque towards both ends of the
string.  Just like transverse waves, but in a rotational direction.
> 
> 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

Not much to go on, really, just instances of replacing strings that I
know had some torque because I installed them that way, or showed
torsional movement when broken or cut.  May have been some other factor
that caused the beats to go away.

Paul