I said just a while ago that I didn't know why there was a difference between fork activated bridge and the tack hammer experiment - but I have some ideas. It's the "Fiddle-de-dee" that John mentioned. It's the difference between a plucked violin string and a bowed one, and that is continuous excitation. End of fiddle analogy. In both the fork and the hammer test, the results are judged by the sound produced by the movement of the soundboard some time after the impetus was applied to the bridge. We're hearing the result of soundboard movement, not the cause. With the fork, we are hearing the organized result of a regularly spaced series of impulses that have moved bridge and soundboard, and organized into a series of standing waves in the board corresponding to the frequency of the input. It doesn't matter whether the input is on top of the bridge, or on the side, because the frequency will be the same in either case, the bridge will move at that frequency and as a result, the standing wave patterns of the board will produce a sound of that frequency. That's what a soundboard does for a living. The tack hammer produces a single pulse, and the assembly is stiffer horizontally than it is vertically, so what you are hearing is the resonant frequency of the assembly in different directions, not of the input. What sound differences do you get when you tap the end of the handle of a vibrating tuning fork on the bridge side, compared to on top? Guess what, it sounds remarkably like the results from the tack hammer. The experiment is interesting, but meaningless as far as any indication of the soundboard moving before the bridge. I still see no reason to entertain the possibility that this could even happen. Spruce, as with most woods, is not a particularly "resonant" material. It has a fairly high internal friction compared to something like steel, so it doesn't ring like the much hammered upon and stroked steel bar that shows up from time to time in this discussion. Neither maple, nor spruce will propagate an internal compression wave nearly as far, nor maintain it nearly as long as steel. This steel bar will, I'm told, carry a compression wave it's full length (whatever that may be) even if it is embedded in concrete. So a spruce soundboard that is presumably initially driven by internal compression waves that turn into transverse waves after reflection from the rim and subsequently move the hitherto unmoved bridge shouldn't be terribly hampered in the transmission of these compression waves by being glued for a short space on one side to a mere wooden rim, should it? Especially along the grain of the panel, which as we are told, is the direction in which sound is best conveyed by the panel. So why doesn't a tuning fork pressed against the end grain of an installed soundboard panel produce the same sound as when it is pressed on the top of the board a few centimeters away? Ok John, I answered your questions again. Now how about you answering mine? We're still holding at two, listed according to age. 1. How minute does a movement have to be to be nonexistent? 2. Why doesn't touching the fork to the edge of the soundboard not produce the same tone as touching it to the top if it's compression wave driven? Ron N
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