On Tue, 12 Feb 2002 22:49:25 John Delacour wrote: >At 9:53 PM +0000 12/2/02, Phillip L Ford wrote: > >Spruce, or any wood, particularly softwoods will have very different >values of Young's Modulus according to the direction of the grain. >That's why ribs are needed in the first place! > True. But if we assume we are planning to use fairly tight vertical grain then there should be some basis for comparison between species so that some attempt at preliminary selection for further evaluation could be done. >>Here's a reference for some wood moduli: >> >>http://www.woodbin.com/ref/wood/strength_table.htm > >You will find different figures at different sites. Old soundboard >wood and new that I have tested (and in rather humid conditions at >that) give specific gravities of around 0.35 and nowhere near the >0.40 given here for Sitka spruce. I have no Sitka spruce to test but >I've seen figures as low as 0.33 given in other places. > Granted. This was just a convenient chart to make comparisons of stiffness and density between different types of wood for talking purposes. Different sources will have different numbers. Also individual boards will vary widely from any given published number that is based on an average of several samples. So selection of individual pieces by the maker will always be essential. > >As to balancing stiffness and lightness, it's not just a question of >dividing the elasticity modulus by the density; the desired ratio can >be achieved by making the rib deeper, since the stiffness is >proportional to the width but proportional to the _square_ of the >depth. Actually, for a rectangular section it's proportional to the cube of the depth. I = bh^3/12, but your point is taken. Certainly you can make the rib stiffer and lighter by making it deeper and less wide. But at the same time if you use a material with greater stiffness to weight ratio and are free to vary all dimensional parameters you will still end up with a lighter rib for your desired stiffness than if you used a material with a lower stiffness/density ratio. >And all this presumes that the rib's sole purpose is to act >as a girder, which is not so. The stiffness of any part of the >system will also affect the wave speed of the forced vibrations, not >to speak of the resonant (modal) frequencies of the board as a whole. > >JD OK. The chosen stiffness and mass (and perhaps other parameters that I'm not aware of - like speed of sound through the material, damping, etc.) will affect the way the board vibrates. So, the same board built up in two different ways with ribs of two different materials, those different ribs having the same bending stiffness but other parameters being different would not vibrate in the same way and so would sound different. This gives the designer some choices, although it does make things more complicated. > >PS. One of Wolfenden's pianos that I restored about 18 years ago has >just reappeared in a rather dismal-looking state but behaving as well >as I remember it. It's the only Wolfenden piano (a Weber) I've ever >seen and one of the best English uprights I've ever come across. >I'll have a look and see if he ribbed it with pine before the prices >went up! > Is pine available to you now at reasonable prices? I always hear about Scotch pine? Is that really a pine and would it serve for this purpose? Phil F Go Get It! Send FREE Valentine eCards with Lycos Greetings http://greetings.lycos.com
This PTG archive page provided courtesy of Moy Piano Service, LLC