Ron Nossaman wrote: > So how could a compression wave traveling through the bridge possibly move > the soundboard, and what's the physics behind it? Actually, that bothered me a while as well, but it was this rock in pond example that your camp through out that got me on to it. And I think I understand where that reasoning comes from now. Lets take another look at the pond analogy. You drop a rock into a pond and a compression wave is set up throughout the medium. It propagates throughout the medium in a circular, actually I suppose half sphere pattern and continues to until it either dissipates or meets some border condition and is reflected back into the medium or absorbed or dissipated by the nature of that border condition. The surface is one such border condition, the sides and the bottom are also border conditions. But the sides and bottoms will reflect most of the wave energy that encounters them back whereas the surface condition accepts the wave into a new medium... the air. Remember the direction of force these ripples have is transverse to the surface of the water, despite the fact that the compression component of these is maintained totally within the water medium and remains circular (spherical). Ok... then take the analogy a bit further. A bowl fashioned out of a shallow open ended cylinder and a tightly stretched piece of cellophane to seal one end will suffice. Drop your "stone" into the middle of the water and watch what happens. The compression wave created can be sort of watched actually, some lighting can help. You can easily visualize the wave pattern propagation this way in this 3 dimensional aid/experiment. The same ripple pattern (a bit hampered due to the nature of the artificial "bottom" of this medium) forms. Both (top and bottom) get quickly more complex as the wave reflects off the boundary conditions of both the bottom and the sides. And both ripple patterns are the result of the initial compression wave created by the stone hitting the water. In fact it would seem that these ripples are actually part of those compression waves and differ from the rest of the wave only in that they have met a non reflective border condition and must then interact with two different medium. Though their pattern is one of an expanding circle the direction of their force is transverse in relation to the plane of the water medium. Which means they are pushing outwards against the air forcing it into movement. Ok.... the sound board is also such a 3 dimensional elastic medium. It has to respond to the same rules that govern wave propagation and behavior as all such mediums. So after the little pond experiment above its not hard to visualize the basics of the mechanics involved. It would seem to me that any force applied to the surface of such an elastic medium then would necessarily result in compression waves being formed. Transverse surface waves are the result of these interacting with two different, conductive mediums. Dropping a ball onto the board would result in much the same kind of thing as dropping a stone into the water. Then too its been shown beyond any shadow of a doubt that the sound board assembly accepts longitudinal input which results in sound being created from the assembly. McFerrin quotes an experiment, Robin and JD have cited similar things with tuning forks and other examples, and I have this upright standing in my shop that has no strings... I attached my brass rod clamped in the middle experiment to the bridge and induced longitudinal waves in the rod and boy did the panel scream. All this put together tells me that the string vibrating at the bridge termination does nothing more then exert a force on the surface of a 3 dimensional wave conductive medium, which must react as all such medium do. This, I think... is close to the rationale and it certainly makes perfect sense and fits well with the physics that deal with wave motion through elastic medium. It also seems to my mind of thinking easier to deal with when it comes to this business of the strings partials, or segments. I still don't really see how any of this is really so totally incompatible with the diaphragm idea. As I have said all along I would suspect the real truth to all this lies in some combination of these two rationale and probably some other things we lay folk and for that matter real physicists as well haven't grasped or thought of yet. OK ?? > > >I am, perhaps due to my own > >ignorance or perhaps due to some other attribute, a bit more open to pondering > >differing ideas.... Funny how McFerrins statement about transfer of energy from > >strings through the bridge is nearly, if not out and out, identical to Robins > >though... I mean if this theory of his is so stupid and all.. > > I would greatly appreciate it if you wouldn't > put words like "stupid" in my mouth. Sorry... didn't mean to actually put any words into your mouth. I had understood you found their ideas to be rather less then useful but then its easy to misinterpret all that kind of extraneous stuff found in these posts. In any case it was not my intention put words into your mouth... I know I dislike it when that happens to me as well... so I will be more careful in the future. > Ron N Now I gotta read Robins post, and I look forward to his response as I obviously have embraced the concept of "ripples" as it were and he hasn't been any more comfortable with them then you have been with compression waves resulting in transverse motion. Great fun, and for my part very interesting and stimulating. -- Richard Brekne RPT, N.P.T.F. Bergen, Norway mailto:rbrekne@broadpark.no
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