Delwin. Dont know what to say to this, except that its the first time I've heard this explaintion. Everywhere else I look, and the times I've sat in on demonstrations are as I have previously described. For example...taken from URL http://www.phys.unsw.edu.au/~jw/chladni.html "How are Chladni patterns formed? There are at least three different methods. The plate can be made to resonate by a powerful sound wave which is tuned to the frequency of the desired mode. The plate can be bowed with a violin bow. This is easiest if one choses a point that is a node for most of the modes that one doesn't want, but not for the desired node. The plate can be excited mechanically or electromechanically at the frequency of the desired mode." Or from Conklin at http://www.speech.kth.se/music/5_lectures/conklin/howdoes.html (note the pictures of the fundemental mode, and others and the apparent effect of the presence of a cutoff bar) Soundboards vibrate more readily at their modal or resonance frequencies than at other frequencies. The photos in Figs. 17 - 20 show how a piano soundboard vibrates at some of its modes (resonances). The lowest frequency at which a soundboard can vibrate strongly is called the first mode. In Fig. 17 we see an experiment in which a concert grand piano soundboard has been vibrated at its first mode. The vibration generator, the circular object that can be seen to the left in the photos, has been connected mechanically to the soundboard at a point near its edge. For such a test the procedure is the following: Before being vibrated the soundboard is covered uniformly all over its surface with a mixture of fine particles (in this case sand and "glitter"). Then the vibration generator is turned on and tuned slowly until its frequency coincides approximately with that of a soundboard mode, as will be indicated by a noticeable increase in sound level from the soundboard. Then the generator level is increased until the acceleration of the particles exceeds "1 g" (the acceleration of gravity, 9.8 m/s2) and the particles begin to dance on the soundboard. As they dance, the particles gradually collect in those areas that are not moving at all or are moving with minimum velocity. This produces a pattern called a Chladni figure, so named after the famous German physicist. The first mode of this soundboard occurred at 49 Hz. In this mode, it is the center of the soundboard that moves most violently; the edges, where you see most of the particles, nearly stand still. A piano soundboard rapidly loses its effectiveness as a sound radiator at frequencies below that of the first mode, so notes below the first modal frequency usually do not have very much energy in the first partial. In Figs. 18, 19, and 20, you can see how the soundboard moves at some of its other modes. Remember that the particles collect where the soundboard is moving least. This is how Chladni patterns are produced useing sand. Modal analysis as you describe below is covered by Wogram http://www.speech.kth.se/music/5_lectures/wogram/modal.html ... and is a way of modeling the vibrational modes. But this has nothing at all to do with spreading sand around and watching the dance. Delwin D Fandrich wrote: > > > In the most common modal analysis test setup one accelerometer is used and > the soundboard is struck at a series of precisely located points with a > modal hammer. The hammer impulse is broadband in nature. The soundboard > then responds by vibrating (oscillating) in many different ways and at many > different resonant frequencies. How it vibrates in response to a specific > impact is picked up by the accelerometer and recorded by a computer. With > enough impacts over the face of the soundboard, and after suitable > processing, the result will be a picture of how the soundboard vibrates at > its various resonant modes. Striking the soundboard with the hammer simply > generates broadband noise. > > > > > And the stated method for > > finding the patterns is to exite the disired mode of the piano by > > vibrating the panel at the corresponding resonant frequency. > > When conducting a Chladni test the object is not to find or excite some > desired mode, it is to find out what those modes are. Once those modes are > know they can be studied and analyzed and appropriate action can be taken. > For Chladni patterns they are generally found by driving the soundboard > with a frequency generator/shaker combination of some sort. While this has > the ability to focus the energy to one specific frequency for more exacting > testing and measurement, these patterns can also be seen -- though less > distinctly -- by driving the soundboard with a broadband noise generator or > by striking the soundboard with a hammer will inject the same frequencies. > When being excited with a hammer impact the resonant modes won't be > continuing but will die out as the energy is dissipated into the air in the > form of sound and internally in the form of heat. Striking the soundboard > with a larger and softer plastic hammer will limit the energy injected into > the soundboard panel to the lower frequencies (i.e., the contact time will > be relatively longer). If the hammer is big enough (calibrated modal > hammers are generally quite small) the excursion of the soundboard will > easily be great enough to bounce the sand away from the smaller resonating > modes and move it around to the parameter of the fundamental vibrational > mode of the soundboard. And, as I said earlier, the higher frequency > vibrating modes will be damped out very quickly due to the internal > resistance of the wood. This is not going to be a very precise test. > > > > > Yes, most noteably right in front of the bridge. And I wish too he'd had > > time to go more into just what he was accomplishing. But to be sure, the > > sand collects at the places where the soundboard is not vibrating. > > And you have, of course, just described a rather crude Chladni test. > > Del > > _______________________________________________ > pianotech list info: https://www.moypiano.com/resources/#archives -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway mailto:rbrekne@broadpark.no http://home.broadpark.no/~rbrekne/ricmain.html http://www.hf.uib.no/grieg/personer/cv_RB.html
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