String motions

[Robert Scott]

Jerry Anderson wrote:
>
>A number of years ago I witnessed Rick Baldassin, in a convention
>class, demonstrate that a plucked string emits a different set of
>frequencies according to whether it is plucked in a horizontal
>(in the plane of the strings) or vertical (toward the soundboard)
>direction.
>
>It  would seem that the vertical plucking would most approximate
>the principal mode of a string struck by a hammer.


  Again I must refer to that most excellent article in January 1979
Scientific American, pg 118, by Gabriel Weinreich.  In that article
he details his studies of string motions and their coupling to the
soundboard and to each other.  He found by direct measurement
that the initial vertical motion of the string precesses into a
round-and-round motion with both vertical and horizontal components.
This precession affects the decay too.  When plotting the decaying
amplitude as a function of time, Weinreich observed that the inital
rate of decay transformed into a somewhat slower rate of decay.  He
found that this correlated with the vertical vs. round-and-round motion
of the string.  When the string is initially struck, the motion is vertical.
Vertical motion has maximum coupling of energy to the soundboard,
so the amplitude decay rate is set by how fast energy is leaving the
string and going into the soundboard.  But when the motion
precesses to round-and-round, half of the energy is in vertical motion
and half is in horizontal motion.  The horizontal motion does not
couple to the soundboard nearly as well, so the energy stays in the
string and the decay rate is not as great.

This is only one of the interesting conclusions of Weinreich's article.
If your library has back issues on microfilm or something, I strongly
recommend taking a look at it.

Bob Scott
Ann Arbor, Michigan