Soundboard drydown for installation

[John Delacour]

At 00:18 +0100 21/1/08, Richard Brekne wrote:

>...Then there was JD's post which I am still in my mind trying to 
>fit into this picture.  I'm not sure what he was saying there... but 
>it seemed like he was stating that a rib crowned panel glued into 
>place would if anything move in the direction of reverse crown upon 
>taking on humidity. Much of the middle part of the rib was above his 
>two <<force lines>>

I certainly did not intend to suggest that in any circumstances the 
crown would diminish with increase in moisture content, though, 
re-reading my post I must admit it's not unambiguous -- put it down 
to the late hour and my concentration on the SVG drawing.

If two pieces of wood are glued together and one side of the sandwich 
is allowed to take on moisture, the sandwich will become convex on 
the damp side, no matter what the nature and grain direction of the 
woods are.  In the case of a soundboard, even if both the rib and the 
board take on equal degrees of moisture, convexity will develop on 
the soundboard side owing to the fact that this expands across the 
grain and the rib expands only a tiny amount in its length.  A visual 
representation of this would be to press the spine of a comb against 
the side of a saucepan, imagining that the bending force is created 
at the end of the teeth.

When the complete assembly is glued to the inner rim and clamped down 
by the metal frame, even if the rim is angled to follow the ultimate 
curve of the board, as is usual, the resistive force at the edges of 
the board that develops as the board takes on moisture will to some 
extent resist the development of the crown and lead to some degree of 
compression of the summer growth of the spruce.  When the pressure of 
the strings is applied to the crowned board the compression of the 
summer growth is considerably increased as the crown is forced down...

...but all this is pretty obvious and not terribly inforrmative and I 
must get out to the farm and make some strings.

JD