Rocking bridges

[John Delacour]

At 11:27 PM +0100 12/26/01, Richard Brekne wrote:

>Do the same experiment except actually excite the strings by playing a key. In
>fact. JD and Robin do the same thing. Then if the results are at all 
>the same for
>similiar pressure....

Richard, I think if this discussion is going to be at all useful to 
anyone, we must simplify the issue as much as possible. To be frank, 
"playing a key" could open a whole new can of worms because it would 
entail so many imponderables, such as the impact of the hammer, the 
stiffness of the string etc. and things would get even more muddied. 
Besides, I can't think of a reliable test along those lines and I'm 
not sure anything could be shown for wither party if I could.

RN has explicitly confirmed that my summing up of his view was a fair 
one, and RO by what he has written recently and repeated at greater 
length today seems to have implicitly acquiesced in that view.

What is at issue is how the sound excited in the string comes to be 
radiated from the soundboard which is separated from the termination 
of the string by a bridge.  Robin and I are independently of a 
similar mind on how this happens, though each of us is probably 
clearer about certain details than the other as we work towards a 
proper understanding of this interesting question.

Let's get the problem of the mobility of the bridge clear once and 
for all.  I have already, for the time being, conceded the 
unprovability of my hypothesis that the soundboard moves the bridge 
and not vice versa, though RN seems not to have read that and is 
still baying for blood, so that gets one red herring off the slab.  I 
have also, of course never doubted the mobility of the 
bridge/soundboard (how could one?!).  Robin hinted at the 
desirability of minimizing or even eliminating movement and in reply 
I gave reasons and examples why this was actually not desirable, but 
that the _proper_ mobility should be the aim.  I'm pretty sure Robin 
will see that this is so.

We all know subjectively from practical experience roughly what range 
that 'proper mobility' lies in and we know that it would be different 
for the belly of a cello, for example.  The fact that the bridge 
sinks as we put on the strings is sufficient proof of the mobility of 
the system.  However, I have deliberately avoided throughout these 
threads any discussion of the 'movement' or of the vibration of the 
soundboard, because I need to understand a lot more about that.  You 
will have noticed that I took no part in the discussion of modal 
vibration -- neither, I'm afraid, did I learn much from it.  All I'm 
sure of is that the sound excited in the string is radiated from the 
soundboard -- and also from the rim and other things to a lesser 
degree.  My understanding of the soundboard is mainly empirical and 
not as good as I hope it shall be.

Ron claims that "The bodily movements of the bridge cause the 
soundboard to move and to produce the sound of the string into the 
air" and Robin and I say that this is impossible.  We have adduced 
various examples such as

1) The radiation from the soundboard of the sound of a tuning fork 
attached to the bridge by a loose copper wire.

2) Ditto of a tuning fork pressed to the end of a six foot aluminium 
rod pressing against an upright bridge

3) The frequently occurring case of equal and opposite forces acting 
upon the bridge from two sources and preventing any added movement 
and yet not preventing the sound from the sources from being radiated 
from the board.

3)  My illustrated model that demonstrates that the frequency of any 
movement of the bridge is independent of the frequency of the sound 
and that it is impossible for the bodily movement of the bridge to 
excite that sound in the soundboard.

etc.

To put it at its simplest, Robin and I insist that sound is 
propagated in the piano in precisely the way that sound is propagated 
everywhere else in this universe and Ron says that there is a special 
law applying to pianos, which involves "sort of ripples" and things 
like that.  So far as I can tell, both Ron O and Del think similarly, 
so they may be onto something.

Happy holidays to everyone!

JD