Sound waves(The behavior of soundboards)

[Ron Nossaman]

>I now string one note only of my piano with two strings, tune them to 
>the same pitch and strike them with a hammer in such a way that while 
>one of the strings is tending to move the bridge upwards the other is 
>tending to move the bridge downwards.  You say that this will be 
>impossible and unprovable, and I answer that this state of affairs is 
>likely to exist in any case for a proportion of the time the note is 
>sustained, so in fact I do not have to strike the strings in any 
>special way.  The more often I hit the two strings, the more often I 
>will be able to observe a situation where the upward force of the 
>vibration in one string is exactly counterbalanced by the downward 
>force of that in the other.  Under these conditions the bridge cannot 
>be moved by the strings, since a body only moves in reaction to a 
>force and in these circumstances there is a zero force acting on the 
>bridge.

It doesn't matter if it's possible or not. Regardless of whether the
strings are moving in the same or opposite directions, there will be cyclic
changes in string tension as the vibrating strings pass from their far
limit of excursion, through a relatively straight line neutral, to the
other far limit and back. The tension swings will necessarily pull the
bridge forward and back, and the bearing load changes will push it up and
down. The bridge will move, and the soundboard will follow.


>I say that the sound emitted from the soundboard will be practically 
>identical whether the two strings (according to your theories) are 
>acting in concert to move the soundboard in one direction or whether 
>they are counteracting each other and therefore quite unable to 
>produce any movement.
>
>Answer me that.
>
>JD

If the strings moving in opposition could be kept exactly out of phase,
that could very well be correct for the reasons I gave above.

Ron N