Sound waves(The behavior of soundboards)

[Robin Hufford]

Phil,
     The bogging down of this thead in semantic distinctions would be substantially,
lessened, in my opinion, should a proper approach to the nature of motion, and,  in
particular, wave propagation, be had.  It is possible that a body may experience a
change in the velocity of its particles, atoms, molcules, whatever, and yet not move
bodily.  Increase or decrease of heat is, as I continue to point out,  an example of
this; another is the induction of stress waves, sound or what have you, in the body
through resonance or through the "mutually induced strain" I referred to last
month.  Once again we have come to the nature of loading and, once again, I am sure,
the implications of loading will be disregarded.
      Infofar as the quote offered last week,  now submitted again and properly
attested,  it is from Den Hartog p. 128-129;
"If an alternating force acts on a mass of an n-degree-of freedom system, there will
be n-1 frequencies at which the mass will stand still while the rest of the system
vibrates."
      This is taken  from the section on forced vibrations and is a generalization
of a simpler analysis performed in the case of a string with three masses.  Those
making the efforts necessary to understand the analyses offered in this book, or
likewise, in other, similar  approaches, and upon which a real understanding of
these matters can be predicated,  and who are not simply, rambling on based upon the
limited approaches  of "high school physics",   will understand the concept of
"distributed mass" on a string,  as an extension of simpler case used in analysis;
that is,  of a string with one, two, three or n masses, and hence, the utility, of
this generalization.
      Although the statement itself is unambiguous, Den Hartog immediately qualifies
it by urging care be taken in its application which has to do with a system in which
a resonant frequency  approaches the "vibration absorber frequency".  Nevertheless,
as the string and soundboard are, essentially, systems of infinite degrees of
freedom, then obviously, vibrations of an infinite number of frequencies may occur
while some part of the system may stand still.  Of course, no such thing, in
particular as regards an infinite number of frequencies, can exist, I merely urge
the point, as I have done,  that motion of the string does not, necessarily imply
motion of the bridge as the string termination at the bridge is a node.
Regards, Robin Hufford


Phillip L Ford wrote:

> >
> >Apparently so. I certainly don't see that it is logically reconcilable that
> >a body can experience a change in velocity at a measurable rate without
> >having moved.
> >
> >Ron N
> >
> Right.  This is like saying we were talking about force and everyone knows
> you can't measure force with a strain gauge.  OK, strictly speaking, that's
> right.  A strain gauge measures strain.  But the presence of strain indicates
> the presence of force.  I hope this bogged down thread isn't going to bog
> down further in discussions of semantics.
>
> Phil F