Rocking bridges

[John Delacour]

At 1:48 PM -0600 12/22/01, Ron Nossaman wrote:

>
>John, Robin, try this with a strung piano.
>
>...strike the key as if you were demonstrating
>the sustain characteristics of one of your pianos to a skeptic... 
>Now press down on the strings of the unison adjacent to the gage one 
>about the same amount and observe the indicator dial. It will 
>move....Oh, but that's not a fair test because you have to push too 
>hard to get that 0.0005" deflection. Fine. Push less, get less 
>deflection, and produce less air displacement, which would result in 
>less volume from the instrument if the string were then allowed to 
>vibrate from that displacement amplitude. No mystery there I 
>trust....


>All sorts of compression waves and molecular level stress disturbances can
>be immediately moving through every part of the bridge from the first
>movement of the string, but they're clearly not what moves the bridge. The
>strings move the bridge, the bridge moves the soundboard.

All you seem to be saying is that a downward pressure on the strings 
(or presumably the bridge) will cause the bridge and consequently the 
soundboard to sink.  I have absolutely no doubt of that and don't 
need to carry out any experiment.  I also accept that an 
infinitessimal weight placed on a unison or on the bridge will cause 
an infinitessimal downward movement of the bridge and of the 
soundboard.  If I load a feather into a Cadillac, the same phenomenon 
will be observable.  There is no question of this because it's so 
obvious.

>Do that 880 times per second at 440 cycles (displaces up, and down 
>you know), you will move 6.336 cubic inches of air per second just 
>from that average square foot of moving soundboard.

Ah! Now you think that's a logical leap.  I know you do from all 
that's gone before.  Now let's take this a step at a time and keep it 
as brief as possible as we go.

First, may we get it clear that you and others have mentioned three 
types of movement in relation to the bridge, namely a) rocking 
movement (of the top towards and away from the stud or hitchpin)  b) 
up and down movement and c) rippling movement.

Secondly, that it is the string that is the _direct_ cause or driver 
of these bodily movements and

Thirdly, that these bodily movements of the bridge cause the 
soundboard to move (as it must, since the two are glued together) and 
to produce the sound of the string into the air.

I use the word "bodily" to distinguish these movements from molecular 
disturbances within or at the surface of the objects in question 
which result in no displacement of the object as a whole.  I would 
allow that a movement of a part of the object would also constitute 
bodily movement, so it is not necessary for every inch if the bridge 
to be displaced.

So long as I am not twisting your words or misinterpreting your 
stand, then we can continue; so please confirm that the above is your 
view or modify it as necessary.

If you agree, then I would like first to consider the "rocking 
motion" of the bridge that you and Ron Overs (and probably Del) have 
referred to.  We have to begin somewhere and that's as good a place 
as any.  It would be best if we get our positions clear at every 
point as we progress.  How about it?

JD