My first rule is never do yourself what you can get others to do for you ;-). I find that every time I gather some new bit of information about something it often causes me to revisit areas that I thought I knew. I try to be cautious of my own assumptions. I like to see that everything still checks out. Sometimes it doesn't, sometimes it does. I've done my share of researching of formulas and setting up spreadsheets but, you're right, this is probably worth doing at some point, not critical for now. I understand the circle geometry, I'd have to find the expansion rate of this particular species. It is a point of interest in terms of selecting the EMC to know exactly at what EMC the top of the panel comes under compression: If ribbing at 6.0 is it 6.01 or is it 7.0. I mean, 6.0 is about 30% RH, string bearing adding some compression aside, that doesn't leave a large margin for putting the top of the panel under tension. But I can do the research, it's not the main direction of my inquiry. You've answered the basic question. Thank you. Anyway, I am going somewhere with this so please bear with me. Just a dialogue here--something I hear we've been wanting. So the next question is where does the bulk of soundboard compression damage take place, at the top of the panel or at the bottom of the panel. You'd expect the bottom of the panel to be under greater amounts of compression but the attachment of the ribs to the bottom of the panel would seem to take some of the pressure off, so to speak, at least at the surface. How deeply does that impact go if it's there at all. On one hand you might expect the bottom of the panel to experience less damage, on the other hand the bottom of the panel is undergoing some greater amount of compression (than the top) so you could argue that there would be more damage. Which is it? David Love www.davidlovepianos.com -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Ron Nossaman Here we go. I don't know why this is so universal a question (eventually) but it's been a few years, so let's look at it again. I took the time to find the formulas and do the numbers to work this out to my satisfaction years ago, so I'm not interested in spending the time to do it again, but I highly recommend that anyone interested do so. It answers a lot of questions and puts what is otherwise a theoretical concern into much more practical perspective. The radius doesn't become tighter at a rate that offsets the expansion. Expansion overtakes it right away. Generate some numbers and see. It's been my experience that answers earned by chasing down the information and putting in the hours to generate the numbers tend to stick better than answers handed over, and I highly recommend you dig out the spreadsheet and chase down the appropriate formulas to put numbers to it, but the gist is this. Wood changes dimension at x% per MC% change. Bending a, say 1M panel in a radius appropriate to a piano rib will put the concave under very light compression and the convex under make a very light tension. How much? Do the math. Pick a radius, and a circle segment length. Calculate the segment angle. Use, that angle to calculate segment lengths for radii 4mm shorter, and 4mm longer than the center line radius of the panel. Those numbers indicate the amount the top and bottom of the panel was stretched or compressed to make the bend, and you'll find there isn't much difference between the two. Now calculate how much the panel at it's starting width will change dimension with a given MC% change. You'll find the numbers bigger than the difference between the top and bottom lengths of the curved panel. In other words, it takes very little MC% over that at which the panel was ribbed, to put the entire panel under compression. Now calculate the top surface length of the panel with a radius giving a crown height of about half the unloaded original crown height. Compare that with the original panel width, and the potential for tension on top of the panel becomes even less than it was. In the real world, this is a non issue in any practical way. For RC&S boards, we typically dry the panel to at least somewhat lower an MC than we expect it to experience in the climate it lives in, so it (hopefully) won't crack. Within reason, naturally. There are areas of the country and world that don't even give us a fighting chance. With the vast majority of soundboard installations being either pure CC, or largely panel supported RC assemblies, the entire panel is under considerable compression from birth. Ron N
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