Sound waves(The behavior of soundboards)

[Robin Hufford]

David,
     No argument here.

"David M. Porritt" wrote:

> Robin:
>
> I've seen the "breaking glass" experiment done in a classroom as a
> demonstration.  The teacher had a sweep frequency oscillator,
> amplifier and speaker.  To find the resonant frequency of the glass
> he put a coin in the glass and altered the frequency until the coin
> rattled.  That was the correct resonant frequency.  He then took out
> the coin and increased the volume until the glass broke.  Clearly,
> the glass was vibrating in sympathy with the speaker.
>
> dave
>
> *********** REPLY SEPARATOR  ***********
>
> On 12/23/01 at 11:30 AM Robin Hufford wrote:
>
> >Ron N, Del,  and others,
> >
> >     Isn't it easy to see in the events described below that the
> process
> >whereby
> >the glass, although acquiring sufficient energy to subsequently
> shatter, is
> >acquiring energy from the sound pressure incident upon it and that
> this  is
> >molecular and does not require "physical, substantial, motion" to
> occur?
> >How
> >difficult is this to see?   Were one holding the glass at the stem
> one
> >would
> >feel nothing, no sense of motion as the glass, through resonance,
> acquired
> >this
> >energy - it would simply burst in one's hand.  It is obvious that a
> >soundboard/bridge is markedly more flexible than a glass: but the
> >comparison
> >still holds utility for analysis of the mechanism of energy
> transfer.
> >Sure,
> >there is some kind of motion but it is essentially molecular.
> >     Should you impose a coordinate frame to get a grasp on the
> glass's
> >position
> >in space its coordinates would be unchanged until fracture, although
> I do
> >believe one is likely, at some point, to sense vibration in the
> glass.  The
> >point is that the accumulating energy in the glass is obviously the
> result
> >of
> >energy transfer on a molecular level, or now are you going to argue
> that
> >the
> >glass itself  moves in a manner similar to that  you assert occur at
> the
> >soundboard/string interface which, by the way, appears to
> progressively
> >decreasing in these discussions?
> > If the answer is yes, that what would anyone you suppose to be the
> >magnitude of
> >such motion?  Please don't offer observations  that there is mass
> hence
> >acceleration, or that the air is not a string, or that the glass is
> not a
> >bridge, or that a tuning fork is not a string.  It is an analysis of
> the
> >process
> >of energy transfer itself that is the question at hand
> >Ron Nossaman wrote:
> >
> >> >>When these compression waves supposedly travel down through the
> bridge
> >to
> >> >>the soundboard, moving the board before the bridge moves, how
> does this
> >> >>manage to happen with the board attached to the bridge at
> exactly the
> >spot
> >> >>that these waves are supposed to move the board? I'd love to
> know the
> >> >>mechanics behind this.
> >> >
> >> >For the same reason that the wine glass does not shatter the
> moment
> >> >the lady sings the top C but a moment afterwards.  The glass is
> >> >unaffected while the wave is travelling towards it.  Surely this
> is
> >> >obvious.
> >> >
> >> >JD
> >>
> >> Hardly the same thing, as is surely at least as obvious. This
> still
> >leaves
> >> the perpetually unanswered question of how minute a bridge
> movement is no
> >> movement at all. I had presumed that your post immediately
> following this
> >> one would clear that up for me and put this increasingly tedious
> exercise
> >> to rest, but it hasn't happened. I would like an answer to this
> please,
> >> since it is the basis of your stand in this discussion.
> >>
> >> Ron N
>
> _____________________________
> David M. Porritt
> dporritt@mail.smu.edu
> Meadows School of the Arts
> Southern Methodist University
> Dallas, TX 75275
> _____________________________