Ryan and list, >Is it the actual height that is responsible for the increased >sustain or is it, perhaps, that the increased mass of a taller >bridge raises impedence and increases the sustain? I've seen sustain >increase significantly in pianos where weight has been added to the >bridge. > >Ryan Sowers, RPT >Puget Sound Chapter, #985 >Olympia, WA Well its probably a combination of the two in practice, when considering a fixed bridge width. While the increased mass will certainly raise the impedance and hence the sustain also, raising the bridge height will add to the stiffness of the board along the direction of the long bridge. This also will contribute to longer sustain. Therefore, if you were two test two separate bridges with the same sectional area on the same 'board, say a first bridge 35 mm wide by 30 mm high, and a second 35 mm high by 30 mm wide, they both would have an identical sectional areas and therefore mass, yet the taller bridge would exhibit longer sustain on account of the increased stiffness (it would generally also tend to have a 'cleaner' more pure tone quality). This I believe, is because the taller stiffer bridge will tend to force the panel to move more as a homogeneous diaphragm, rather than flexing all over the place as a whole bunch of little individual sub-sound-boards - causing distortion. In reality of course, if you position a microphone over a long bridge at a certain note, even with a tall bridge board, you will find that the sound pressure level will remain higher at the note directly under the microphone. Whereas, if the increased bridge stiffness of the taller bridge was capable of really forcing the board to move as a unit, the sound pressure level immediately under the microphone should not be so pronounced, as it will still tend to be. Nonetheless, I still have no doubt that the above mentioned tendency of taller bridges to encourage the body of the board to move more homogeneously has some truth to it. I therefore have developed a dislike of shorter shocky toned bridges which encourage such afflicted pianos to present with 'ducks disease', ie. they quack like a duck, instead of singing like a bird. This I believe, is one reason why Bösendorfer pianos tend to have rather satisfactory sustain regardless of the fact that their rims are very weak (the rather more generous plate sectional area of the Böse will tend to offset the negative effect of a weak rim also). They tend to have very tall not so wide bridges, which enhance sustain via their stiffness. Certainly the 9'6" Imperial shouldn't have the sustain it has on account of the sectional size of the soundboard ribs. Sure they're heavy enough since their nominal size is about 30 mm wide by 23 mm high, but with those dimensions there is not much stiffness in those low 23 mm high ribs. But the long bridge height is very often around 40 mm by about 32 mm wide, which makes up for a lot of rib weakness. Of course as Ron N and Del have pointed out on previous occasions, it is not possible to have tall sound board ribs with a CC board (CC boards require that the ribs must be weak to allow for crown formation as the panel tries to return to its normal dimension). It's only when using RC board construction that taller ribs become a possible design option (and a better one of course). I suspect that many CC boards are already loaded past their recommend maximum stress in extreme fibre, even before the first string is given tension. Now hows that for a digression? Regards, Ron O -- ______________________________ Website: http://www.overspianos.com.au Email: mailto:ron@overspianos.com.au ______________________________
This PTG archive page provided courtesy of Moy Piano Service, LLC