At 7:11 PM +0000 13/2/02, Phillip L Ford wrote: >On Tue, 12 Feb 2002 22:49:25 > John Delacour wrote: > >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. >John, >I've seen this expressed before in other places; since the board is >orthotropic and the speed of sound is less across the grain than >along and waves don't propogate in the same way across the grain >than along the grain, etc., etc. the ribs need to provide a cross >grain path for the sound or waves. I wonder how important this is >to soundboard behavior. I'm not convinced that if you turned the >grain of the soundboard 90 degrees to the current norm and left the >ribs as they normally are (in other words the grain of the board >parallel to the line of the ribs) that the board wouldn't function >just as well, even though the ribs aren't providing a cross grain >path (although admittedly the bridge would be). I think it would be >an interesting experiment. Well, I've seen perfectly well-sounding pianos (eg. Collard and Collard) with the soundboard grain running orthogonally to the bridge and the ribs roughly parallel with the bridge, but I don't see the point of putting girders where they are not needed, that is to say parallel with the stiff plane of the board. You know what a piano sounds like when the ribs come loose -- apart from the buzzing, the whole tone of the thing collapses and I guess you'd get a similar or even worse effect if you barred the board with the grain. The way I see it, the barring of the traditional soundboard is designed to get some approach to isotropy with a material that has excellent wave-bearing properties in one direction but rather poor performance in the other direction. Apart from that, the soundboard needs to have the best range possible of natural frequencies. One of the reasons a small piano performs so badly is that the gravest modes are too high and there is not enough overlap of modal frequencies. Conditions can be optimized in several ways, but it's never good enough. To get a desirable sound, the soundboard must be large enough. >I'm wondering if the main purpose of the ribs isn't structural (just >acting as a girder as you put it). Structurally it seems they serve >two purposes: > >1. To reinforce the board so that it can resist the downward force >of the strings >and do so without adding the weight that would be necessary if the board was >thickened enough to sustain the load without ribs. > >2. To change the vibrational modes of the board. The ribs 'break >up' the board, so to speak, and change its natural modes, which is >probably essential in an instrument with a large frequency range >over which it has to work. > >Perhaps these functions are the primary ones and carrying waves, >damping, etc. are secondary or insignificant to the soundboard's >function. No, I don't think the importance of waves can be underestimated -- in fact the sole purpose of the soundboard is to convert mechanical energy into acoustic radiation and the efficiency with which it is able to do that is crucial. The design of the strings and their coupling to the board can be changed to your heart's content, but if the soundboard does not radiate the results of your endeavours, they are wasted. >Another aspect of this is that some people seem to feel that there >is something magical about the board having orthotropic properties >and that wood is the choice for musical instrument boards partly >because it is orthotropic. I'm unconvinced about this. So you should be! What's magical about good soundboard wood is the way it behaves in _one_ direction. Choice of ring density, quartering and proper ribbing are the maker's best attempts to to make good its severe deficiency in the other direction. >I think a soundboard needs to have high stiffness to density and >some level of internal damping. When pianos were developed the only >material available with these properties was wood (softwood). Now >it just so happens that this material is orthotropic. Some people >have concluded that this means that the soundboard needs to be >orthotropic. Perhaps they're right but I'm unconvinced. I've seen >pianos with laminated boards (which I've taken to be essentially >isotropic) that seem to serve the same function (whether they do it >as well as an orthotropic board is still an open question). Ron >Overs's piano which he displayed in Reno is an example. I don't >know enough about the way the board is constructed to know if it is >truly isotropic. Perhaps Ron can comment on that. I'm sure he will. All configurations have been tried 100 years and more ago and satisfactory results have been achieved with the most unlikely designs -- I don't mean to reflect on Ron's designs in saying this. If you haven't got Dolge's book, I'll type out some of his findings in the good old days of Mathusek and company; it's really quite interesting. I would still regard a laminated spruce soundboard as anisotropic or at best orthotropic, even if the same bending stiffness is achieved in both planes, and unless such a board is unusually thick, it's still going to need ribs, so I don't see the rationale of it. I am just building the rim for my latest soundboard and should have the prototype in testing by the summer. At the moment the board is free-standing and has one covered string just a metre long sounding bottom C better than it would sound in any piano I've come across with strings that short. Not only do I hope to get far better fundamental in the bass but also to bring down the critical frequency perhaps as much as an octave, and that means thinking waves, which you will have gathered I have been! One board will go into a 6' Ibach and the other into the old 1870's straight strung Brinsmead that I've just acquired -- that's just for testing while the "real thing" takes shape. Here's part of an interesting reply I got from a professor of acoustics at one of your universities, who nevertheless _is_ an expert in structural acoustics: >- I am *not* an expert in musical acoustics, but believe piano sound >is very likely dominated by flexural (bending) waves in the sounding >board, amplified at frequencies of resonance of interior cavity >modes that happen to couple well with the board. Discontinuities, >such as stiffening members or the terminations at the bridge, will >couple longitudinal waves (which move fast and radiate very well) to >the bending waves (which usually move slowly and radiate poorly), >increasing radiation efficiency and therefore sound. Of course, >sometimes discontinuities break up the shapes of strongly radiating >modes and reduce sound! I've found that it is in the field of structural acoustics that the most information is to be had, since it's such an important field as regards architects' profits and all sorts of people's pockets. Some quite significant work has been done in the last fifteen years. JD
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