AW: Sound Propagation

[Tim]

-----Ursprungliche Nachricht-----
Von: owner-pianotech@ptg.org [mailto:owner-pianotech@ptg.org]Im Auftrag
von Richard Brekne
Gesendet: Montag, 7. Januar 2002 22:14
An: pianotech@ptg.org
Betreff: Re: Sound Propagation


John Delacour wrote:

> RB:
> >The more I get into this "vibration and sound" thing the more evident it
seems
> >to me that we all need to take a closer look at just what is happening
when
> >something is vibrating. It seems like we are too easily caught up in
vibration
> >as something you can touch, see, or feel... the transverse component.
>
> Richard, in all this we are considering the displacement of particles
> (atoms or molecules) at the interior of the elastic media that
> compose the system.  If I lightly tap the end of a long steel
rod,.....snip snip

That, and the rest of what you wrote, seems to be very much in line with all
that I
have run into so far looking through my books and what I can find on the
net.  When
I started in on this discussion about a month ago now I was convinced the
sound
board was more or less a analogous to the diaphragm of a loudspeaker, and I
have to
admit that there is much about that analogy that seems to fit real well. Be
that as
it may, the description of compression wave propagation and about vibration
in
general has left me with no doubt that these simply must play a major role
in the
way the sound board is excited and develops the movement necessary to result
in
sound through the air. Course that may change too if further reading leads
me
hence, but I don't see anything yet that seems to point in that direction
yet.

Two statements in this discussion seem to have caused a lot of trouble for
us,
raising passions and polarizing the discussion. The first was something to
the
effect that "Sound does not travel through the sound board", other was that
"the
bridge does not move".  My natural tendencies are to try and find out on
what plane
both these statements have some validity and then which ways they do not..
Its easy
to find the ways in which they seem absurd. That's the easiest of things to
do,
And its easy to get all hung up about these and end up nowhere in a
discussion. But
both statements were made by individuals I know better then to take lightly.

Let me put it this way. When Del says "Sound does not travel through the
sound
board"  And John Delacour says emphatically the opposite, I start
immediately with
the thought that there is a high probability of these two very informed
individuals
are really speaking somewhat different languages, accenting different
aspects of
this problem that they have come through there experience to feel is very
important
to underline. Chances are that somewhere there is an interpretation of the
problem
that lies somewhere in between that really doesn't conflict with either.

On the other hand it may be that there is a very clear understanding of each
others
perspectives and modes of description and there indeed does exist real
disagreement.  I haven't come to the point in my own reading where I feel
the
compression induced motion model is at odds with the diaphragm like
transverse
motion model.  The "which comes first the impact or the compression" nut
seems like
a sidetrack to me and I don't even see how that could bear on the discussion
of
whether it is compression wave energy exerted at the bridge, or transverse,
any
ways.

I'm left thinking that unless the panel is for some reason too thin to
function as
a 3 dimensional medium, then compression wave propagation has to be a big
part of
the picture. But like I said, I am still just beginning to grasp the basics
here,
and obviously will stumble more then once on that road.

One other thing,,,, what difference does this make ?  Say we are actually
looking
at two very much opposing "theories" for how the panel works... what then
are going
to be some of the consequences of this.  I think Jim touched on something
about
barriers analogous to the rim... though I haven't read close enough to know
where
he is going with that. (grin... aside to see he blames me for the pond
analogy..grin.. what a bud ! :-) ) But it would seem on the top of it from
my
present viewpoint that there should be some implications for the idea about
sound
reflecting off the rim.

Actually, as it seems we may have come far enough for everyone to be
satisfied with
their own positions any ways, that maybe a more hypothetical discussion
based on
what those kind of differences are would be most interesting to pursue.

>
>
> I'll leave it there for the moment, as I have 103 years' worth of
> accumulated coal smoke to remove from a very nice soundboard!

Sounds like fun

> >Conjecture is a good thing... as far as it goes. Makes you think out
loud,
> >bounce ideas, and hopefully stimulate one to looking deeper and deeper to
find
> >better answers to questions we think we know something about.  Think ON
!! :)
>
> Right!
>
> JD




--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no