Sound waves

[Ron Nossaman]

> The "wave" moves forward while the 
>"particles that successively compose it" oscillate in a direction 
>perpendicular to the perceived movement.

Exactly.


>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.

Ah but the blanket is an elastic medium or it wouldn't propagate the wave.
Right so far, near as I can tell. 


>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.

In an unloaded board, you get a higher note in the treble than in the bass.
By the time your ear sorts it all out and hears the thump, the ripples have
mostly blended together with their own reflections into a larger diaphragm
movement of the assembly. Keeping the ripples going to form the sand
patterns of various resonant frequencies requires continual driving of the
board.   


>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

Yes, I think you are, as I understand soundboards. And with the
introduction of the stretched skin soundboard vs the plywood sheet or
spruce soundboard, you're into the wonderful world of mechanical impedance
matching. Yes, I know, it's an electrical term, but it's also used to
describe energy (sound, ju ju, or whatever) transfer rates between the
driver and the driven and back. Or are we ready for that yet?

Ron N