Sound Propagation

[John Delacour]

[ Too many subject lines!  I've changed this one (was: Re: Violin 
bridges (was The Soundboard according to McFerrin) ]

At 3:04 PM +0100 1/5/02, Richard Brekne wrote:

>I dunno Phil... The more I read on all this the more clear the message comes
>through. No matter what else is happening, compression waves are 
>very very much
>in the picture when ever vibration energy is exchanged from one medium to
>another. I don't see how one can avoid that. That it is difficult to
>conceptualize I will grant you but that just means we have another 
>one of those
>interesting challenges in front of us.

>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, I 
create a disturbance at the point of impact and displace the 
particles.  If you now imagine that rod to be made up of a row of 
little cylindrical magnets in a perspex (plexiglas) tube disposed 
N-S-S-N-N-S-S-N etc. you will have a picture of a "rod" in 
equilibium, the individual cylinders kept apart a certain distance by 
the repulsive force of the similar poles facing each other

X=<>=<>=<>=<>=<>=<>=<>=<>=<>=<>=Y

= is a cylindrical magnet; <> are repulsive forces

If I plug the tube at Y and press X, the magnets will come closer 
together and the "rod" be compressed, shortened.

When I release the pressure at X, the magnetic force will restore the 
"rod" to its original length and the "rod will again be in 
equilibrium.

You can imagine that if I then remove the plug and tap the end of the 
"rod" at X, the magnet closes to Y will fly out, but there will be a 
delay while the pressure at X moves as a "wave" through the length of 
the rod.  You might then consider what might happen if I plug the 
tube at Y with a coil spring, or various other possibilities.

If you now substitute particles for magnets and inter-molecular 
forces for magnetic forces, you have a _one-dimensional_ picture of 
an elastic medium.  The individual magnets in our rod are able to 
move back and forth within a restricted range.  If I disturb the 
equilibrium of the particle at X, it will try to restore its 
eqilibrium by passing on the disturbance to its neighbour, and so on 
down the tube.

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!

>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