Hi Gene Well impedance was part of your equation ...the unknown part, the part that's difficult to track & monitor as there is no impedance scale, no computer program such as there is to determine rib strength per species etc. As I was reading the posts from Del I was thinking about how many variables there exist that are only partially quantifiable except from the empirical outcome. Mass and stiffness in assemblies are one thing from a weight point of view, but what weight? how much mass? How is that measured? Factor into that what strength to weight ratio your spruce species is, the tight -ness of the grain or lack of changes both weight and stiffness. What rib materials are you using and how are they designed & how much of the bearing load are they designed to carry. After its all glued together, bridges on & glued to the rim it takes on another stiffness factor. Then add a certain amount of bearing to the structure and the impedance changes again. Also Factor in that probably no two Techs who routinely build soundboard systems or seem to set down bearing in the same way or even agree or think it all that necessary to the final outcome. So to some real extent the empirical tonal outcome is what folks say wow to... or not. Is it a tone that falls in the Goldilocks range of musical tone acceptability for a broad spectrum of people. After all it is the beauty of the music we want to enjoy. For the pianist he desires a piano sound and touch he can fall in love with every time he sits down to the key board. I think our collective concern as rebuilder is for a reliable outcome that enhances both reputation and musical satisfaction. Personally, like so many who post here, we are tonally driven artisans and as the saying goes....if Daddy ain't happy nobody's is happy. We really want to please our own ears. We desire to create a sound our own ears intuitively have been pursuing on the tonal road less traveled by. My tone described I personally like a tone that can be described as power without noise, a tone that floats, a dark but clear singing spectrum with definition and clarity between registers. I also like a robust powerful sound but I want it to be tonal power and not much ancillary string noise. I've wanted to share this, say this for some time because, as much as we know, we don't have to know everything about soundboard making to be successful. But, the more we know the closer we can come to a design we can intelligently choose, and then hopefully anticipate the outcome. This is where I am and its taken me along time to say this out loud. I have designs that are repeatable, dependably musical and that I can build with confidence. I am certain many of the regular contributors to this subject who weigh in here can attest to the same confidence, and I think they would all agree that its come with lots of trial and error on a steep, steep learning curve. My own personal choice is varied between rib crowned and rib crowned and supported designs. I love them, the tone the dependable-ness. IS that a word? We usually install a bass cut-off bar to shorten those monstrously long ribs and prevent energy losses in the bass corner. (I have never made pure compression crowned board. Nobody has asked.) My recipe on many designs incorporates these 2 soundboard models. The soundboard to me, in my own head and way of thinking is predominately that of a springy non linear diaphragm. It has strength when resisted. It doesn't just take on bearing load and lay there, it should push back and be a dynamic force and a huge part of the equation. It has a push back component against the string tension. I think we all get that. As I have listened to countless Compression crowned systems I have realized that so many sound fantastic. Yes, they have design shortcomings I do not wish to repeat, but they can sound awesome for some period of time and under the right conditions, a very long time. I have some in my regular clientele with text book crown and bearing. A 1941 D at the college and a 1924 B on my web site to name two One of my respected colleagues Chris Robinson has made awesome sounding C.C. boards for decades and his recipe is dialed in carefully unlike many factory processes. His shop, his success, is iconic for tonal outcomes. My point is, it his choice of design. What I have taken a way from the C.C. model is that these boards have a non linear compression response when pre-loading them for stringing... when built right. Pre-loaded means what the board does as it is strung providing the bridge height was cut high enough to allow them to be loaded/compressed in an adequately. Meaning that the more they are compressed the stiffer they get till they won't deflect anymore. This is what makes this model work and when its dialed in to closely watched parameters they can sound amazing. So, I still see the board in my own design as the springy diaphragm model based on non linearity. I do make quite a stiff board, stiff set of ribs & I do pre-load it much the way the factories have done for decades. It works well & the sound is predictable. The baord needs more bearing in the top 2 capos than in the lower end. and if this is not the case then all the crown in the world will be of no use to tone production. Ask me H I K. There is an acquired feel for how much strength/push back the board should have when it is being pre-loaded/loaded. In my recipe if the board felt too stiff in the pre-loading process then the board would have to much impedance. If I can pound it down at the central strut and the panel continues compressing without becoming resistant & just keeps going, then the ribs and panel assembly isn't stiff enough to resists as I desire. AS old school as this sounds, it works and it is predictable. I don't want to get lost in that process right now as I'm sure there will be questions (or derision) LOL...and I've explained it before. The other factor is how much crown is enough and what are we going to do with it once we have it.? In my recipe, it came to me over time, that what ever the unloaded upward deflection of the board was after the board was glued to the rim, that I wanted to compress that amount of crown by roughly half with string bearing. ie the board has 10 mm of crown on the longest rib. I want it to deflect 4-6 mm. and I want it to be strongly pushing back. I want it to be reactive. If I'm using tight grain Sitka spruce I will want to thin the entire panel to 8 ish mm and the thin it at the rib scoops/perimeter and further into the panel. Gene, as to panel compression I dry to 5.5 to 6 % depending on where its going. I'm not concerned about a certain amount panel compression and consider it as a piece of the equation but a much smaller piece than a C.C. board. Also remember that I am using Sitka spruce in the 20 to 28 grains an inch range and it can easily handle this amount of compression in many climates but especially in our western region climate and this is where most of our restorations end up. I've yet to see any compression ridges develop in any of my board made as described. So here is a few divergent impedance thoughts for starters. Dale Dale Erwin... RPT Mason & Hamlin/Steinway/U.S pianos www.Erwinspiano.com 209-577-8397 -----Original Message----- From: Gene Nelson <nelsong at intune88.com> To: pianotech <pianotech at ptg.org> Sent: Sat, May 26, 2012 8:13 pm Subject: Re: [pianotech] [pianotech--Centeringthe bridge--was S&S something er other The questionremains, though, for those designing boards without or with less panelcompression, what level of rib support is required to compensate for areduction in compression in order to achieve the desired impedancecharacteristics especially when the range of outcomes can vary as much as thetwo examples cited. *** I know I am missingthe boat on this one but for the two boards that I designed using radius ribs,I may have had a small amount of compression in the board after gluing on ribs –all at around 6%mc here in the California foothills – but for the ribs, Ijust looked at how much string bearing they would see, spaced them accordingly,used some deflection numbers that I got from a local rebuilder and thereit was. Impedance was not part of it. I would really like to understand how toinclude impedance into rib design. Gene -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://www.moypiano.com/ptg/pianotech.php/attachments/20120529/d563b3f8/attachment.htm>
This PTG archive page provided courtesy of Moy Piano Service, LLC