Soundboard recrowning and rib re-arching.

[David Ilvedson]

Wouldn't the top cells be stretching outwards and the bottom cells would be compressing.

David Ilvedson, RPT
Pacifica, CA  94044


----- Original message ----------------------------------------
From: "Ric Brekne" <ricbrek at broadpark.no>
To: pianotech at ptg.org
Received: 8/18/2006 3:11:06 PM
Subject: Soundboard recrowning and rib re-arching.



>Thumpy writes:

>    Hey, Terry, yore a  real smart guy, so I know, that you know that
>    when a  board is bent, 1 side goes into compression, the other into
>    tension.

>Everything is relative there Thumpy. You have to remember that we are 
>not talking about a plain ol bent board.  Its a board which is far more 
>compressed then it would be if it was just plain ol bent.  In a bent 
>board the part of the board that is neither compressed or tension is 
>along the exact center line thickness wise.  But if the board is 
>subjected to some degree of compression independent of plain ol bending 
>compression... then this has to be added into the situation as it were.  
>If you add enough of this independently caused compression then the 
>whole panel will be under compression no matter how much you bend it.  
>Now the top half of such a compressed panel would be less compressed as 
>it were then the bottom half to be sure.  Thats where the relativity bit 
>comes it... you could look at things like a number line where you move 0 
>around to fit the middle point.  In that case you could say the top half 
>is in tension relative to the bottom half... but you cant forget to take 
>into consideration the absolute degree of compression there is (or isnt) 
>if you are going to speak in absolute terms at all. 

>Just how much compression is created in a compression soundboard and 
>whether that is enough to put the entire panel in a state of compression 
>despite the relative state of affairs when the bend is taken into 
>account I dont know.  But no doubt the center line is no longer halfway 
>between the bottom and top sides... its been moved up towards the top 
>and most certainly more then half of the panel is under compression.


>    If it is then glued to a panel, and said panel is smashed flatter
>    over the years, this rib must also go intocompression, somewhat, or
>    no crown would be lost.

>I'm not at all sure I buy this.  I'll be the first to argue that a rib 
>is tensioned somewhat as part of the process of the panel taking on 
>humidity and compression... I'll even go so far as to suggest that 
>perhaps the rib tensions a bit more then it would if it was just plain 
>bent... i.e. its center line drops just a hair leaving more then half 
>its mass under tension and less then half under compression....  but 
>when a compression panel flattens out to the degree that it would stay 
>flat in the abscence of any bearing... well in that case the ribs are 
>simply going to relax me thinks.

>Cheers
>RicB