Sound waves(The behavior of soundboards)

[John Delacour]

At 7:54 PM +0100 1/15/02, Richard Brekne wrote:

>Some of the Violin stuff I have read so far seems to point in a direction of
>these modes being some kind of ... what... "basis resonance framework" which
>the string resonances are sort of superimposed on... ?Perhaps that's clumsily
>worded. But like I say I have to read a few times through before I think I am
>sure of what's being said in these kinds of articles.

Yes, thanks Charles!  Stephen Birkett put me onto these lectures some 
months back, since they are mentioned in his modelling proposal as 
the summit of modern piano knowledge (not _too_ much irony, 
Stephen!), and they are certainly interesting.  I think the Conklin 
article on longitudinal vibrations in the string has been talked out 
in another thread and is not really relevant in this discussion.  As 
to the Wogram article, which touches on this discussion, it seems 
that one of the main aims of the Giordano paper was to disprove 
Wograms findings, since he covers so much of the same ground in a not 
very original way, so we are left with Suzuki and Giordano largely in 
agreement about certain aspects of the natural flexural vibrations of 
the soundboard and impedance, and still left in some doubt about ther 
significance of any of them.  This is not to bring a plague on all 
their houses, because there are some very interesting observations 
and findings both here and in Giordano.

Clumsily worded or not, Richard, you seem to have got the drift of 
these experiments and in a way they are peripheral to our discussion 
in the first instance, but will become very relevant when, as I hope 
we shall, we get to consider the role of impedance in the production 
(I use the word loosely) of sound.  As you say, they seem to be 
talking of the enabling environment or structure that allows the 
sound to be radiated in the desired fashion.  And this is where 
flexibility or mobility does enter into things in a very important 
way.

The flow of sound through the system is regulated by this impedance, 
and I have seen both hydraulic and electrical analogies made.  To 
take a car battery as an example, here we have two terminals between 
which there is a potential difference, or voltage.  If we connect the 
two terminals with a conducting medium, electrical energy flows from 
one terminal to the other.  The energy is always the same in nature, 
but how much energy flows and the effect is has will depend on what 
we use to make the connection.   The ratio of pressure to flow gives 
us the 'resistance' of the conductor, and electrical 'impedance' is a 
similar ratio used in different contexts.

Our energy is sound energy, and how much of it flows and how much is 
held back (absorbed or reflected) is determined by the harmonic 
"framework" so to speak.

At the same time there are the natural flexural vibrations of the 
soundboard to be taken into account -- and there are some interesting 
examples in the articles of how this can be a nuisance.  We do not 
want notes to set up resonances at the natural frequencies of the 
structure...that's putting it very roughly and I don't want to go 
into detail about this because I still need to understand a lot about 
these relationships.

JD