Sound waves(The behavior of soundboards)

[Phillip L Ford]

On Tue, 08 Jan 2002 23:58:28  
 Robin Hufford wrote:
>     Since the passing wave has been ruled out we must consider the
>possibilities of mechanical action to account for the bridge motion point
>of view.   Considering  the string to be operating as a cable under
>tension,which seems to be part of  view  the bridge motion proponents will
>argue, and  which case implies a shortening of the string length,  a
>fluctuating pitch and other great complexities, I find this to be most
>difficult to accept for reasons stated previously.

Should I take this to mean that you are saying that the string does not move
up and down and is not lengthening and returning to original length as it
vibrates?  If you were to drive the string at its fundamental frequency only
the standing wave would be a continuous curve (half a sine wave) that is
moving up and down.  Since the distance between the supports is essentially
unchanging the string must stretch as it moves away from its rest position.
At the point of maximum excursion of the string the ends of the string are at
an angle to the supports.  The load in the string times the sine of that angle
is an upward (or downward) force at the supports.  I don't see why this is
difficult to accept.

>       As to stress and strain, these terms are used, as is that of
>impedance, in a kind of colloquial form here and I merely wished to point
>out that thinking of force and stress as synonymous can lead to
>problems;   from my view, part of those problems are demonstrated when one
>believes that a string which is undergoing cyclic stress or strain as it
>were can, in the case of a piano soundboard, operate as a force and induce
>the proposed  motion which is under analysis here.
>Regards, Robin Hufford

My colloquial definition of stress is force/unit area and
strain per unit length is stress/Young's modulus.  Both stress and strain
result from force.  To say that the string is undergoing cyclic stress and
strain but that no force is involved doesn't make sense.

Phil F