On May 9, 2009, at 8:22 PM, Fred Sturm wrote: > the original topic, which had to do with the theory that string > vibrations could be coupled, through accujust hitchpins, to the > plate, and could make the plate vibrate/resonate in some way. If we > want to try to see if the analogy between a stake driven into the > earth, abraded by a hoe, and a string terminated on a vertical pin, > driven into a hole in a mass of cast iron, has any validity Returning to the original topic, and analyzing it by the model of a driver and a membrane/resonator: With a soundboard, the string is the driver, and it is firmly coupled to the SB assembly via bridge pins. The speaking length of a struck string has quite a bit of energy, and it is driving a relatively light weight material, selected for its capacity to vibrate (a lot better than many other woods, for instance). The vibration of the backscale of the strings is very low in energy, relying entirely on what is fed to it in the way of "sympathetic" vibrations (mostly the bridge conveying energy back from the SB to the back length). The accujust pin is not a firm coupler, in that the string loops around it, and pivots at the loop when it vibrates. Very little of the vibrational energy of the string is transferred to the pin. The plate is very massive, and not a good resonator (you can make it ring if you hit it with a hammer, but it takes a lot of energy to get a significant amount of sound). The analogy with the stake driven into the earth, and rubbing the top of the stake with the hoe? Based on what I gather from Jeff's description, a lot of energy is being expended at the top of the stake (compared to very little at an accujust pin). The stake is somewhat flexible itself, and can be "set vibrating" and those vibrations are conveyed into the earth. The earth is pliant, so the stake can "vibrate within it" and create waves which are analogous to soundwaves. (The accujust pin is set into a material that is not at all pliant, and the pin will be hard pressed to vibrate within the plate hole). Based on Jeff's description, there is a sound produced in the air, though whether or not it is related to the vibration in the earth is subject to doubt (at least on my part). I suspect the sound heard by the "grunter" is produced by the hoe head and the stake, moving air, not by the earth moving air. So the sound change from different heights of stake may have more to do with the portion of the stake vibrating above the earth rather than within it, or so I would picture it (though I guess the vibrational modes would be related in some way). All told, I find it difficult to draw a close parallel between this stake and hoe arrangement and a back length of string and accujust pin arrangement. In short, looking at this analogy, it seems a pretty big stretch. It seems very far fetched to believe that a significant amount of energy is conveyed into the plate by the backlength via the accujust pin, certainly not enough to make that massive plate resonate. At least that is my take. A way of investigating further: strike, pluck, bow, or otherwise activate the back length of the strings, and see what the result is. You will be introducing far more energy into that system than is introduced by playing the piano. Can you make the plate resonate? Regards, Fred Sturm University of New Mexico fssturm at unm.edu
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