Hi all, While I haven't been following this post by post. Jim wrote. > > Does the plate "flex" or "compress"...well yes without a doubt it does.... >>to some extent......... the question is 'which way'? >-------------------------------- >> For the sake of question No. 1 let's assume that the bridge/soundboard does >>'not' flex....period. > >I'd say that since the struts are above the string plane, they will bow up >in the middle - but that's not the flex that makes the pitch of tuned >strings drop as you raise pitch on the rest. The plate flexes in the >direction of string tension (in addition to any up, down, or rotational >flex - it gets shorter), and flexes more between the struts than at the >struts since the struts are trying to do their job of not allowing the >string tension to collapse the plate. I think Newton was talking about >doing a normal one pass pitch raise, measuring the resulting pitch of every >string, and plotting the results against what the pitch should be. The >graph will show least pitch drop at the struts, and the most half way >between. I haven't tried this myself, but I have no reason to doubt the >reports of those who have, and my math on soundboard deflection changes >don't account for anywhere near the drop we actually see in practice. Some >of this drop may be strings rendering through the bridge too, but that >should be a fairly uniform progression from one end of the scale to the >other, with no anomalies at the struts. There isn't much left besides plate >compression that will account for it. Sure, and the increased drop mid section tells us that plate rigidity between sections should be a significant design factor. Which also leads me to be wary of using an open plank design in pianos with higher total string tensions and higher numbers of notes per string section. The I beam or Z beam factor which Phil Ford wrote about last week on another topic is a significant factor for plate rigidity across the pin block. > > For question No. 2 we will add the 'flexibility' of the sounding board. >------------------------------------------ >>......For the sake of the question let's suppose a >>static temperature and also suppose that a soundboard rises just enough to >>change speaking length tension but not enough to overcome "friction gain" at >>the front bridge pin. In this situation the pitch of the speaking length will >>rise, as will the pitch of the standing end of the string...except that > >because of equal tension and differing speaking lengths the pitch of the > >standing end will be proportionately higher than the speaking >length...right? > >Right. > > > > In this situation does the flexing/compressing of the plate follow the rise >>of the soundboard or does the plate remain static and realtively unaffected >>by this change?............... >>Ponder points only. :-) >>Jim Bryant (FL) > >I'd say it doesn't follow the bridge up and down enough to be a factor. The >downbearing angles aren't extreme enough for that. However, since we're >talking about possibly 20 cent+ pitch changes from season to season, The >plate would almost certainly have to compress like it would with a pitch >raise with the high humidity cycle, and relax/rebound like it would with a >pitch lowering in the dry cycle. This is an interesting point I hadn't >considered. The plate is automatically partially compensating for humidity >induced pitch changes by being flexible! If it didn't flex under string >tension, the pitch swings would be dramatically worse within any given >humidity limits than they now are! Now this is cool. Good show. > >Ok, here's one for you. I think I've demonstrated reasonably well that >humidity swings push strings up and down bridge pins. With a 20° pin angle >and 10° stagger, how much of the pitch increase with humidity increase is >from soundboard rise, and how much is from the fact that the string is >higher up the bridge pins and taking a longer path across the bridge >(stretching as necessary to do so)? > >I'm beginning to suspect that we went through a lot of years not knowing >squat about why and how pianos go out of tune. > >Ron N I have always considered sound board rise (or fall) to be the biggest factor in seasonal pitch changes, and bridge rise relative to the sound board must surely compound the effect. Furthermore, this is also why the tuning is more unstable in those pianos with the greatest deviation in the percentage of breaking strain. If the breaking strain is lower as it often is in the lower tenor, or lower and higher adjacent to the plate struts, the swings due to 'board rise and fall will be greater - as they are in practice. Poor scale design = Poor tuning stability. Ron O -- ______________________________ Website: http://www.overspianos.com.au Email: mailto:ron@overspianos.com.au ______________________________
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