Sound waves(The behavior of soundboards)

[John Delacour]

At 10:35 PM -0600 12/19/01, Ron Nossaman wrote:
>Sorry John, but I have one more question I forgot to add to that last post.
>
>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.

Since Del and you have picked up the irrelevant part of my answer to 
this question -- I was not talking of resonance and should have used 
another example -- here is the answer to your question, which really 
contains its own answer.

The waves indeed _travel_ through the elastic medium of the bridge 
wood.  Travelling takes time, and if you doubt that, we have several 
train companies in England that will prove it to you.  In talking of 
the wine glass I was considering only the time taken for the 
compression wave to travel from the singer's throat to the glass, so 
enough of that.

The wave that is sound is ALWAYS a compression wave

At 1:47 AM -0800 12/19/01, Robin Hufford wrote:
>This then, is the paradigm for the string/bridge/soundboard 
>interaction and that is:  driven flexion, stress trajectory; and 
>induced flexion.  The string is driven into flexion by the hammer; 
>it is held clamped by the bridge, bridge pin and relative massivity 
>of the bridge/soundboard assembly which causes the flexion  to be 
>transduced to stress and stress trajectory; induced flexion then 
>occurs as a result of the superposition and recurrency of effect 
>described earlier.

Robin has expressed in other words precisely the series of phenomena 
that I have been  opposing to your theories of rocking bridges, 
rippling bridges, moving bridges and the like.  The transverse 
excursions of the string are terminated at the two ends of the 
speaking length, that is to say at the stud and at the bridge. 
Neglecting the stud end, a pressure wave is initiated at the point 
where the string meets the bridge.  This wave then travels through 
the beech or maple at the acoustic velocity of the material.  The 
molecules of the beech oscillate in turn as the wave travels. 
*Considering only the vertical plane*, at any moment some molecules 
will be at their position of rest, some will be higher and some will 
be lower.  The disturbance at the top of the bridge thus travels 
(vertically) as a wave of pressure between the end of the string and 
the soundboard where it sets up transverse movement of the board, 
which in turn leads to the compression wave in the air that reaches 
our ears as audible sound. All this takes time.  At the moment the 
wave is initiated by the string at the top of the bridge, there is 
molecular disturbance at this point and not at the point where the 
bridge meets the soundboard, which remains undisturbed until the wave 
has travelled there.  The wave moves continuously in a certain 
direction (and back by reflection) whereas the molecules that 
constitute the vibration, oscillate to and fro within the medium, 
some upwards, some downwards.

The vision you and your co-thinkers have of the bodily movement of 
the bridge, not only up and down but also in a rocking motion, 
strikes me as completely absurd and impossible.  Throughout this 
thread and previous related threads you have persistently poo-pooed 
any question of waves except the sort of wave or ripple that you 
associate with the ripples on the surface of a body.  Similarly the 
only movement you seem to be able to comprehend is movement of a body 
as a whole.

Whereas there are many sources on the Web that will provide serious 
information about vibration, impedance and the like, you will be 
quite unable to give us one serious URL that lends any credence to 
your declarations.  When a little while ago I challenged you to 
substantiate a borrowed equation from Conklin relating to impedance, 
you backed off at the speed of light and made it quite clear you had 
no understanding of the theory of it and refused even to demonstrate 
a practical application of this received wisdom.

JD