Sound waves(The behavior of soundboards)

[David M. Porritt]

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
_____________________________