<HTML><HEAD> <META charset=UTF-8 http-equiv=Content-Type content="text/html; charse= t=utf-8"> <META content="MSHTML 6.00.2730.1700" name=GENERATOR></HEAD> <BODY style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; BACKGROUND-COLOR: #fffff= f"> <DIV>        Hi John</DIV> <DIV>        Great post. I= didn't get a chance to comment last friday when it came in as I was on my w= ay out of town. I've interspersed some comments below.</DIV> <DIV>John Hartman wrote</DIV> <BLOCKQUOTE style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue = 2px solid"> My point is that once the= soundboard is assembled the ribs attached to a Panel Crowned (PC) board = contribute to the stiffness of the structure. If this were not true then = a PC board would not have any more resistance to deflection that a panel = without ribs. I have handled many soundboards some of which were built wi= th the PC method. When the ribs are attached to the whole structure notic= eably stiffer. So were does this stiffness come from if not from the ribs= ? >>>> I see this as one of several stiffness fac= tors, whether PC or Rc , that contributes to an overall symbiotic effect tot= aling a soundboard structures overall stiffness quotient  &= nbsp;    >> Furthermore, the idea that the small additi= onal compression of the panel       >> due to bea= ring will stress the panel of a PC soundboard more than a RC   &n= bsp;   >> board is just not right.       ><B= R>      >     &= nbsp; >Ron wrote -compression? You mean in addition to the = compression       > necessary to force crown in the rib= s before the string bearing is even       > applied.= How then, are the ribs supporting most of the bearing load in a &nbsp= ;     > CC board when the panel is already supporting the ribs?= That's the       ></STRO= NG> question I'm addressing. John I am just going on = the evidence I see. When you apply down bearing the board moves down abou= t 3mm. You can compute how much more (in addition to compression of crown= ing) compression the panel is holding by figuring how much this straighte= ning of the crown compresses the panel. If you do this you will find that= bearing does not compress the panel to any significant degree. This is t= rue for PC and RC boards alike. At any time we can find the compression l= evels of a soundboard panel by looking at how much change in dimension (c= ross grain) there is when we apply bearing. >>>>>I= agree. Depending on how stiff the soundboard structure is I find that typic= ally the board flattens appox 2.5 to 3.5 mm measured at the central str= ut approx., 2ish mm at the first capo strut, bout 1mm or less a th= e top & about 1.5 At the bottom tenor. During the pre-stressing for bear= ing(wedging the board down at the struts) setup I find the board gets v= ery stiff & resistent to further deflectionat this point. This is a= nother indicator of overall soundboard stiffness due to each = facet of the component stiffening effect meaning  rib to board int= erface, bridge crown, panel stiffness, rim effect etc.</FONT= ></BLOCKQUOTE> <DIV>    =  As you say It doesn'= t seem the panel is going to compress significantly with these minor in= creases from downbearing. I typically dry Sitka  panels befor= e ribbing between 5.5% to 6% EMC out here in Calif. Many go to the bay = area with higher average moisture levels than the central valley where I am.=  I have followed many of these pianos over timed see no sign of compres= sion ridges, cracks bulging etc. With other kinds of spruce with less streng= th across grain , or containing too much soft spring/early wood, it could be= a problem. </DIV> <DIV>   <B= R>        On that 48" rib you mentioned, = the cross grain       > dimension of a panel that is 48= " at 4.5%MC, will be approximately 48.5"       > at = 12%MC. It takes half an inch of compression in a 48" panel just to &n= bsp;     > form the crown in a flat rib. That's a bit over 1% o= f the total width.       > Immediate and permanent c= ompression set occurs at anything over 1%,       > acco= rding to the literature.</DIV> <DIV>>&gt= ;> So then it looks like a safe level of E.M.C for panel crowning could r= ange from 5.5 % to 6%emc</DIV> <BLOCKQUOTE style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue = 2px solid">While I believe that any board mad= e this way is at risk to long term deterioration the PC method as used at= the current Steinway factory does not compress the panel to the point of= damage (above the elastic limit). From my own experience with examin= ing boards I have pulled from relatively new (under 20 years) Steinways y= ou do see most of the crown intact (Steinway does not use a lot of cro= wn). </BLOCKQUOTE> <DIV>  = John I'm Curious . Why were these boards taken out?</DIV> <BLOCKQUOTE style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue = 2px solid">My crude attempts to find how much= compression is involved indicates that about 1/3 the elastic limit is re= ached when a PC board is crowned and reaches 6.5% moisture content. This = is about 200 psi to create the crown. Yes this is a concern and I would n= ot build a board this way but the panel is not at the point were were = it can't hold the crown with some leeway for increased moisture (it doesn= 't begin to get damaged until its above 11%EMC and even then in can st= ill support the load). As you know cross grain spruce can withstand about= 580 psi before it starts to fail. The 1/3 figure I found correspond well= to what we know about the moisture levels used to rib a board with the P= C method in comparison to the moisture level used with the RC method. It = is a difference of 2 percent which if viewed as the stress in a constrain= ed soundboard panel comes out to be a little less than 200psi. <ST= RONG>>>.. This is interesting and good to know If you have= experiments or data to contrast with this I would like to see them. <= BR> What does bearing add? Typically more than     &nbsp= ; > what it takes for ribs of that dimension (no panel) to be deflected<B= R>flat       > if they had been machine crowned. So the= bearing adds at least as much       > load as it to= ok to bend the ribs. No I don't think bearing ads mush compressio= n to the panel for the reasons I went into above. Ron Little st= ress on the panel, and the       > ribs carrying the ma= jority of the bearing load? I think not. And when     &nbsp= ; > you force that panel flat, so the rib is again straight, the rib is<B= R>then       > under no stress at all except at the top= edge. What is it that is       > pushing back on the f= orce it takes to push it flat? It's the panel       > c= ompression.   So please explain how the ribs are supporting most<B= R>of the       > bearing, with the panel supporting ver= y little. John The best way I can find to look at this is to exami= ne the evidence. We have a PC board with ribs that are shallower than wha= t is necessary to hold the bearing. We put the strings on and they push t= he board down. We find that the soundboard has not collapsed and holds th= e bearing load more than we predicted looking at the ribs. We look at the= panel and find that it has not been significantly compressed from the be= aring. We recheck the deflection of the ribs and find that they are not h= olding all of the load ether. Jumping to conclusions we think the panel m= ust be holding it - this is sloppy thinking and flies in the face of the = evidence. I think that the model that a few have been using to explai= n how the soundboard works is, well wrong. We have been fed this idea tha= t the ribs are like floor joists and the panel is like the floor boards. = The ribs hold all the weight and the panel does none of the work. May I<B= R>propose an alternate model? The ribs and the panel and the bridge form a<B= R>single crowned unit that has pretty much the same stiffness along the r= ibs as it does  along the grain of the panel. It is like a crowned<B= R>diaphragm. When the soundboard is attached to the rim all of the compon= ents form one structure well suited to support bearing. If you want to ha= ve a complete picture of how bearing is supported you must consider the c= ontribution of the rim. As the diaphragm is pushed down the rim holds= it in and provides the added stiffness to resist bearing. This theory at= least makes some attempt to show how crown can improve soundboard perfor= mance.</BLOCKQUOTE> <DIV>>&gt= ;>>Not only that but it does help to understand where the overall&nbsp= ;stiffness in the soundboard models we've inherited comes from. The soundboa= rd is somewhat of a laminated structure afterall. I posted on the increased = stiffness that the board takes on after it's glued to the rim last year but = the idea was minimized, However any one who goes to the trouble of building = more than a few boards quickly realizes the dynamic Increase in&nb= sp;a soundboards stiffness once glued to the rim compared to what it wa= s out of the piano. It obviously becomes a homogenous unit at this point.&nb= sp;What I'm not saying is the success of the whole system is depen= dent on this one thing. It's part of a whole</DIV> <DIV> </= DIV> <DIV>  &nb= sp;  John</DIV> <DIV> With the floor joist= model all that can be said for crown is that it compresses the panel. I = say So What! compressing the panel can do nothing to improve tone. <ST= RONG>>>>>Hmmm. Maybe not the panel alone but the whole assembly.= I always thought of this as something like tightening a drum head & tha= t by doing so a light weight object becomes more efficient at producing more= sound thru increased air movement. Or basically what we all refer to a= s an impedance increase. The soundboard we inherited has evolved= to have equal stiffness both along the ribs and along the grain of the p= anel. This makes sense because it assures that the vibration modes of the= soundboard will be fairly circular in shape to best utilize the soundboa= rd area. If you start to design the ribs to be more stiff than the panel = the vibrating areas of the soundboard will begin to elongate in the direc= tion of the ribs. At some point if you continue to design the ribs to sup= port the bearing load, all on there own, you will make a soundboard less = capable of tone projection with small area vibrating out of phase. This w= ould defeat the whole purpose of supporting bearing in the first place.</= FONT></DIV> <DIV>>>>>> </FON= T></DIV> <DIV>One things for sure.  I have e= njoyed the discussion of older & redesigned sound boards, wood stre= ngths and various aspects of the craft on the list.  One thing I want t= o say is I know  rebuilders around the country that are using crowned r= ibs, with or without significant  increases in rib height dimensio= ns and are  somewhat modified copies of original of soundboards th= at sound very,very good. </DIV> <DIV>   If anyone has had the = idea that only a highly modified board is an exceptable and legitimate attem= pt at quality piano rebuilding just isn't so. The proof is in the sound. Bui= ld a few boards and draw your own conclusions.</DIV> <DIV>   DAle Erwin<BR= > John Hartman RPT John Hartman Pianos www.pianos.hartm= anstudios.net Rebuilding Steinway and Mason & Hamlin Grand Pianos = Since 1979 Piano Technicians Journal Journal Illustrator/Contribut= [link redacted at request of site owner - Jul 25, 2015] <DIV></DIV></BODY></HT= ML>