At 10:49 PM 9/6/97 -0700, you wrote: >Michael J. Wathen wrote: >> >> So, as I understand your reasoning (ddf's) the angle that the strings pass >> through after leaving the V bar or agraffee determines how much of the >> string energy is reflected back in to the standing wave on the speaking >> length. > >Yes, that's basically correct. It's a matter of the length of string in contact with the agraffe or V bar that kills the bleed through into the front duplex, correct? When the radius of the contour of the termination is smaller than the string would naturally assume at a given entry/exit angle, you increase the angle, diminish the se-saw effect at the bearing point and minimize the bleed through. Ok, wouldn't the same result come from increasing the radius of the bearing surface to increase contact area with the string? We're pretty well locked in to (sort of) standard agraffe configurations, but most of these problems occur in the V bar'd sections anyway. I wouldn't see this as a viable rebuild option, where increasing the angle *is*, but designers and manufacturers have had a lot of years and a lot of chances to play with it. Why are we still braiding front duplexes in new pianos to squelch that front duplex Banshee? > > >> Would you use the same argument for greater side bearing at the >> terminating bridge pin? Why not? > >Yes. There are several other factors involved here, though. The bridge >pin is typically angled with reference to the bridge surface. This helps >to clamp the string against the top surface of the bridge. There is a >second bridge pin located 15 to 25 mm behind the first that also helps. >In the case of the V-bar its best to stop all of the wave energy and >keep it in the string. At the bridge, the idea is to actually get some >amount of energy to couple through the bridge and move the soundboard >assembly. >> Gotta comment here. Greater stagger angle across the bridge means the string is more reluctant to render through and makes a stable tuning that much less likely. It also increases the likelihood that the bridge will crack from excessive side bearing pressures. There is plenty of contact surface across the top of the bridge to absorb anything that bleeds past the front bridge pin. >> Yes, your interpretation of the picture http://ucccm56.ccm.uc.edu/duplex.jpg >> is indeed correct. Since the board was over a hundred I felt that I need to >> control the amount of bearing across the back side of the bridge and reduce >> it to a bare minimum. The ability to select the correct diameter and the >> lubraciousness of the copper wrapping led me in this direction. >> >> Originally the Chickering had felt glued to the plate just in front of the >> hitch pins. Would you have left the felt there? If so why? > >Actually, I'd have replaced the soundboard and capped the bridges. There >is no way that a soundboard that old can have the necessary amount of >stiffness that it needs to provide the proper impedance load to the >strings. Both sustain and tone quality will have suffered. Assuming that >wasn't an option for you, I'd suggest that you still need a certain >minimum amount of string load on the soundboard. Forget about crown, >there isn't going to be much left after a hundred years, if any. But >without some sting load against the bridge (i.e., downbearing) you're >going to have a tough time realizing even what little tone performance >the old board does have left to give. > > >--ddf > I agree with Del that the board should have been replaced. I disagree that you need more bearing. If you don't have crown, adding bearing isn't going to make it well. Ron Nossaman
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