----- Original Message ----- From: "Phillip L Ford" <fordpiano@lycos.com> To: <pianotech@ptg.org> Sent: March 26, 2002 8:01 PM Subject: Re: Varying bridge height (was Re: Floating soundboard) > > > >And why would the 'optimum' bridge height vary from one end to the other? > >Indeed, what is the 'optimum' bridge height? The added mass of the taller > >bridge isn't going to make much, if any, difference. And, beyond a certain > >point, neither will any added stiffness. > > > >Del > > > > > > Optimum is, as ever I suppose, in the eye or ear (or mind) of the beholder. It > would depend on what you believe the purpose of the bridge is and how the > string, bridge, soundboard systems operates. Some of the possibilities that > I have heard, or some that I just made up myself: > > 1. To provide a 'path' for the vibrations imparted at any one spot to 'travel' > to all parts of the board. If this is the function then what are the optimum > dimensions? Is it dependent on frequency? I don't know. Energy--'vibrations'--imparted at a specific spot on the bass bridge aren't expected to travel to 'all parts' of the board. They are expected to cause displacements of the soundboard panel in the bass region. A good argument can be made for extensions off the ends of the bass bridge--basically, impedance controlling devices--and we use these on nearly all of the pianos we remanufacture. But, beyond some minimum height, I don't see any further benefit being realized. And, before you ask, no I don't know what that minimum height might be, but surely 30 or 40 mm is sufficient; assuming reasonable rib spacing, of course. > > 2. To provide something solid to drive the bridge pins into. If this is the main > function then the bridge really only needs to be big enough to hold the pins. Any > extra material would be superfluous. How long do the pins need to be anyway? > Probably not as long as they commonly are. Anything much below the surface > of the bridge is probably not doing much. So practically speaking, the bridge > could be very short if this were its only function. I agree that bridge pins are often longer than required for either their acoustical or mechanical requirements. I've not observed that the currently much-loved 25 mm pins perform any better than the shorter pins they often replace. > > 3. To provide some stiffness smoothing (impedance smoothing?) so that > adjacent notes don't 'see' markedly different local stiffness in the board. If > there were no bridge or a very short bridge, the notes that attach to the > soundboard directly over a rib would see more local stiffness than a note that > attaches to the soundboard between ribs. This seems to me to assume that the > forces applied to the board by the vibrating string (and the subsequent board > movement) are important to the production of sound. As discussions here have > shown, this is a topic of debate. But assuming that this is the function of the > bridge what is the optimum height? Since the strings at the low end of the scale > have more excursion and usually higher tension then the forces they are applying > to the bridge as they vibrate are greater. So it would seem that you would need > more bridge stiffness to provide this 'smoothing'. This could mean a taller bridge > at the bass end. The point of localized stiffness over a rib crossing is a good one, I think. I've known several people over the years who could pick out the rib crossing points in some pianos by listening carefully to a note-by-note comparison of power and sustain. But they could do this only through the mid-tenor and lower treble regions, not the bass. And they could only do it when the ribs were fairly well separated. So, we're back to the basic question: How much stiffness is enough stiffness? If an extremely tall bridge is required to bridge the space between ribs, perhaps the ribs are spaced too far apart. I also find myself thinking about the structure of the bass bridge--typically made out of maple, or some similar hardwood, with a width upwards of 32+ mm and generally well over 50 mm tall--and comparing this with the structure of the ribs in question which are generally spruce or pine and something less than 25 mm in cross-section. > > 4. To provide stiffness to the soundboard, rib, bridge system in addition to that > provided by the board itself and by the addition of ribs to the board. If this is > the primary function then it seems you would want more stiffness at the top end > and less stiffness at the low end. So the bridge might be taller at the top end. > Perhaps items 3 and 4 balance each other out so that the bridge should be the > same height all along. Could you explain this point a bit more? I don't see why you would want more stiffness at the top end and less at the low end. Certainly reduced stiffness is appropriate between the bridge and the inner rim, but I don't see why the bridge itself would need to be more or less stiff at any given point along its length. At least I don't see a practical reason for the notion. > > 5. To change the vibrational response (mode shapes) of the board. I don't know > what the optimum dimensions would be for this but it seems likely that to get > whatever response the designer decided he wanted he might have to use a bridge > that had different stiffness and mass along its length. I don't know of any optimum dimensions either. Some years back I was told there was some optimal proportion between bridge height and width. This proportion just happened to correspond with a particular manufacturers practice. At least it corresponded with what the manufacturer's representatives thought the factory was doing and it may well have been in the design at one point. That this manufacturer's bridges varied considerably in height along their length and were severely undercut along their upper tenor and treble length was apparently unknown to the speaker. As may be, I should think, at the frequencies involved, the bass bridge would have to get pretty thin for this to be a factor. > > 6. To give an observed result. Perhaps if you built several pianos of similar design > with different bridge heights you might decide you liked the top end of the piano > with certain bridge dimensions and the bottom end of the piano with other bridge > dimensions. This might lead you to think that your optimum bridge was one with > varying dimensions from top end to bottom end. Well, this brings me to what I think establishes the 'proper' height of the bass bridge: the need to provide adequate spacing between the bass strings and all the stuff below them. In an overstrung piano the bass strings must be spaced above the tenor strings, bridge and bridge pins far enough to make sure there is no contact between the two during any kind of normal (or abnormal, for that) play. This will vary a bit depending on the particular string configuration, the length of the strings, their tension, various manufacturing tolerances, etc. Typically, this makes the bass bridge somewhere in the vicinity of 15 to 20 mm taller than the tenor bridge. This seems to be adequate. At least I'm not aware of any work done to date that shows either a structural or an acoustical need for any more stiffness along the bass bridge. > > 7. To serve as a marketing tool. As with soundboards and other things I won't > mention bigger must be better, right? The sales people can say that your piano > is better than the competition because it has a bigger bridge. Or they can say > it has a more high tech design than the competition because the bridge changes > dimension and is 'optimized' throughout the scale. It has Patented Acoustic > Construction as one manufacturer says (or used to say). Not my piano. I'll make the bass bridge only as tall as necessary to clear the tenor. With luck and careful control this should be about 12 mm taller than the tenor bridge. And, in response to that marketing department, I would say they are also placing greater stress on the plate; contributing to the piano's instability; making routine tuning--specifically, pitch raises--more difficult; decreasing the efficiency of energy coupling between the strings and the soundboard; contributing to the voice differential across the bass/tenor break (unless, of course, the tenor bridge is also quite tall); etc. Obviously, the marketing department will have the more compelling argument. Del
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