Sound waves

[John Delacour]

At 12:28 PM -0600 12/6/01, Ron Nossaman wrote:

>I see we're right back where we started when I tried to separate internal
>compression waves from ripples in a plane. These are different things.
>Let's try another analogy. If you hang a blanket on a clothes line, pinned
>along one edge, and smack the center of it sharply with something
>relatively light, you will see a ring of deformation, a wave, propagating
>from the point of excitation to the edges. This is not an internal
>compression wave, nor is it a surface wave that doesn't disturb anything
>below the surface. The plane is deformed by the initial excitation
>displacement, and the resulting deformation propagates across the plane
>from the point of origin. After the initial wave reaches the edges, the
>overall organization of the plane tends to fall into larger more general
>waves of a similar type, though less specific. This is caused by the
>boundary conditions, mostly, but is affected by the stiffness and internal
>friction of the medium too, as well as the size and shape of the plane
>relative to the propagation speed of the wave(s). A soundboard acts in a
>very similar manner, except that the plane is stiffer, making the waves
>shallower, and the propagation faster. The reflections from the rim of the
>piano will affect the resonance patterns as the "echoes" interact with the
>initial waves in a sort of vibrational Moire pattern where some waves will
>be reinforced, and some canceled by these reflections. It may be sound if
>that's the consensus, but it isn't an internal compression wave. There will
>be internal compression waves present, but they aren't the primary driver
>of the system.
>
>Is that any better a description?

Well I can certainly visualize a blanket behaving like that from your 
description and I have observed waves at sea and ripples in pools and 
there seems to be a similarity between the two -- that is to say that 
the movement of the blanket is analogous to the movement of the top 
few millimetres of our sea-wave.  The "wave" moves forward while the 
"particles that successively compose it" oscillate in a direction 
perpendicular to the perceived movement.

My first problem is that the blanket is neither an elastic medium nor 
a conductor of sound and its "particles" as it were, are relatively 
free, but then you might argue that a thick sheet of rubber would 
have less freedom and yet move similarly and that if we now hang a 
large sheet of plywood from the line or a theatre thunder, then the 
nature of the wave will be the same but the frequency higher, so that 
the thunder's oscillations will actually become audible.  If that's 
right so far, then I see that a ripple of a certain frequency will 
cause an audible disturbance in the air.  I'm putting reasoning in 
your mouth here, so I may well be going in the wrong direction.

Now if we go even further and glue down the perimeter of an even 
stiffer stuff, say a soundboard, and thump its middle, sure enough we 
get the phenomenon they're talking about over the other side of the 
room -- namely the drum-like behavior of the free and the restrained 
board.  If I thump my Steinway O by the tenor, I get one fundamental 
note and if I thump it by the mid treble, I get the same note but 
louder.  I can still at this advanced stage connect with the blanket 
and the ripple.

But there I stop -- or do I?  No, John, you don't, because you've 
seen stretched skins used with strings and bridges on banjos and 
primitive things.....

Am I moving in the right direction?

JD