<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: Soundboard drydown for installation</title></head><body> <div>At 10:45 -0500 19/1/08, Farrell wrote:</div> <div><br></div> <blockquote type="cite" cite>Again, I agree with Ron - the darn thing won't crown anyway from rim constraint.</blockquote> <div><br></div> <div>Quite so, but let's leave opinion aside and look at the reality of the matter, which seems to be something like this.</div> <div><br></div> <div>Consider a section of soundboard 6" wide with one rib.  Consider them first separately.  Which will it be easier to bend a certain amount?  No contest : the 6" strip of board will bend with the slightest effort and the rib with great difficulty.</div> <div><br></div> <div>The picture below is a scale drawing, which you can see (using Firefox or Opera or Safari) as an SVG file which you can enlarge if you wish. Click <a href="http://pianomaker.co.uk/technical/temp/crown.svg">here</a> or on the image to view the file.  How to zoom it will depend on your browser.</div> <div><br></div> <div><br></div> <div><a href="http://pianomaker.co.uk/technical/temp/crown.svg"><img src="cid:p06240808c3b8184e3b7b@[10.0.0.1].1.0"></a></div> <div>I have exaggerated the crown to a radius of 15ft.  The length of the section is 500 mm.</div> <div><br></div> <div>Note that lines of force acting between the two red lines run mainly through the rib, preponderating if anything slightly towards the top of the rib.</div> <div><br></div> <div>When wood takes on moisture it expands, as we all know, but it doesn't expand isotropically -- far from it; the expansion along the grain is practically negligeable in comparison to the expansion across the grain.  I'm not looking at any figures at the moment but I imagine the difference for softwoods is even more marked.</div> <div><br></div> <div>So the assembly takes on moisture... not only does the rib expand as a result by but a negligeable amount but whatever expansion there is will tend if anything to make it bend downwards and reduce the crown.</div> <div><br></div> <div>As you have just pointed out, spruce compresses very easily across the grain has  low elasticity across the grain.  Whatever expansion there is of the soundboard across the grain (left to right in the picture), and there will be significant expansion, will result in compression of the wood within or beyond the yield point, but any power it might have to make any difference to the crown will be completely negated by the action of the rib and the forces to which this is subjected.</div> <div><br></div> <div>Incidentally I have drawn the rib as it would be in a soundboard made in the English fashion, ie. planed to radius before assembly and planed straight on the underside after the assembly has settled down to the normal environment.</div> <div><br></div> <div>JD</div> <div><br></div> <div><br></div> </body> </html>