Sound Propagation

[Richard Brekne]

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