longitudinal mode vibrations

[Jos]

Richard Moody wrote:
> 
> Hi Jim,
>         Since no one asked, what are longitudinal mode vibrations?  How is it
> detected?  Can it be measured?  Is there an audible quality to it other
> than false beats?.  Is this a phenom. of bass strings? Is this caused by
> the vibrations of the sound board, so that it would appear after the first
> second? Maybe I have been glancing too fast over the discussions on this,
> but once the sound board is set into motion doesn't that affect the way
> the string vibrates? (which set the sb in motion to begin with?) Is this
> what LMV is about?
> 
Hi!

In physics (fluid dynamics, elasto-dynamics) longitudinal waves are
waves, 
in which the medium vibrates in the same direction as the direction in 
which the waves travels. Imagine for instance, a soundwave, travelling 
parallel to the x-axis. When the air particles also travel parallel to
the x-
axis, it is called a longitudinal wave, if the air particles vibrate
perpendicularly
to the x-axis, it is a transversal wave. 

In a string, the wave travels along the string, and because it reflects
at 
the string ends, you get a standing wave. I would think that the
vibrations 
in this case, only take place perpendicular to the wave motion, so that
there 
are only transversal waves. 

Maybe what they mean, is motion perpendicular to the direction in which
the
hammer strikes. Ideally, the string would only vibrate perpendicular to
the 
soundboard-plane, only in the direction in which it strikes. But when
the 
hammer is mal-formed due to extensive use, there may also be sideways
motion, 
in parallel to the plane of the soundboard. Maybe that is what they call 
'longitudinal' motion.

Jos