longitudinal mode vibrations

[Paul S. Larudee]

Ron Nossaman wrote:
> 
> * My thinking was that the type of termination shouldn't make any
> difference. It's the bearing angle across the v-bar, bridge pin, or agraffe
> that I was considering. The two legs of the formed angle act like outriggers
> to prevent torsional movement at the bearing point. I agree that a torsional
> wave, once started, would probably affect the longitudinal mode. I just
> don't see how it's going to start at the bearing points under these
> circumstances.

I'm not saying it starts at the bearing points, but rather that it gets
reflected, just like a transverse wave.  Also, like a transverse wave,
some of the energy travels beyond the bearing, resulting in torsional
movement at and beyond the bearing, despite the stabilizing angle in the
wire ("roll").
> 
> >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.
> 
> * I don't understand why this would impart any torque. Why would a string
> try to "straighten" itself (I assume you mean "untwist" here) where a hammer
> hits it? I suspect that the overall torque would increase with an increase
> in string tension, but I see no reason for a localized change in torque from
> a hammer impact. Please explain.

Yes, untwist.  My thought was that just as the hammer blow induces
changes in string tension which result in transverse waves, the
consequential changes in torque might result in torsional waves.  I'm
starting to rethink it, however.  The changes in overall torque will
occur with each cycle, but without emanating from a particular point. 
If the only reason the transverse waves are induced by the hammer blow
is the tranverse direction of the blow (which seems logical), then
perhaps only torsional force can impart a torsional wave.  In other
words, the hammer would have to rub sideways across the string to induce
a torsional wave.
> 
> >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
> 
> * Aye, there's the rub! There are too many unknown variables in a field
> situation to blame the string twist for false beats. No one that I know of
> seems to have done any controlled experimentation that indicates that
> twisted strings (within reason) sound any different than straight. In fact,
> the only experimentation I'm aware of indicated that there was no
> discernable difference. We've all heard it said by nearly everyone, lo these
> many years, but if it's that critical, it ought to be pretty easily
> demonstrated, shouldn't it? If it was, I bet we'd have seen an Institute or
> Seminar class (or six) proving it by now. If it's common knowledge, why
> haven't I seen any indication that it's true?
> 
> I wonder how one would go about measuring longitudinal partials anyway. Can
> this be done with an ETD? If so, someone could set it up fairly easily and
> whack the bejeebers out of a string to get a first hand look at what happens
> to the pitch of the longitudinal. Where are our instrumented experimenters
> out there?
> 
>  Ron

I agree that experimental studies are the only way to go any farther,
but I have a feeling there aren't too many grants to be had for this
sort of research - at least not with pianos as the main application. 
Hope I'm wrong.  Nice discussion, though.  Anyone else have any ideas?

Paul