----- Original Message ----- From: "Richard Brekne" <Richard.Brekne@grieg.uib.no> To: "Östen Häggmark" <43-2@chello.se>; "Pianotech" <pianotech@ptg.org> Sent: October 21, 2003 8:23 AM Subject: Re: Soundboards: Thickness and Area > > Well that may be, but virtually everywhere I look, including the 5 > lectures, the stated purpose of banging on the soundboard is to provide > an excitement for all soundboard frequencies at once, which > accelerometers placed about the bridge and board use to identify the > vibrational mode frequencies of the assembly. 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
This PTG archive page provided courtesy of Moy Piano Service, LLC