Soundboard crown

[Phillip Ford]

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>----- Original Message -----
>From: <mailto:Erwinspiano@aol.com>Erwinspiano@aol.com
>To: <mailto:pianotech@ptg.org>pianotech@ptg.org
>Sent: August 16, 2003 9:29 AM
>Subject: Re: Soundboard crown
>
>
>>>Phil-- My thought was that any wood will shrink over time 
>>>especially quartered wood because of it's expansion contraction 
>>>properties and actually that could take some time. However We've 
>>>all seen old stwy board with no cracks ,compression ridging, crown 
>>>or bearing for that matter that make my statement  questionable. 
>>>Or perhaps whatever little compression the wood had originally has 
>>>equalized or was never great enough to cause compression set or 
>>>damage.
>    Do you think that would be enough tension to exceed the tension 
>strength of the board?  I wonder.  I also wonder what causes cracks 
>in boards to occur.  Do you think it's from tension exceeding the 
>nominal allowable strength of the wood?  
>
>
>Wood does not shrink on its own--at least not so you would 
>notice--only as a result of changing environmental conditions, i.e., 
>with changes in the MC of the wood. Kept in an temperature/humidity 
>stable environment what would cause it to shrink?

I was thinking that normal seasonal variations in humidity would 
cause it to shrink.

>
>Soundboards crack when the wood is placed in tension and that 
>tension exceeds the tensile strength of the wood and, in a piano 
>soundboard this may not be--probably is not--the same tensile 
>strength the wood started out with....So, if a board of particularly 
>resilient and tough wood is ribbed at a relatively high moisture 
>content and the piano spends much or all of its life in a relatively 
>stable microclimate it will survive nicely for some considerable 
>time.
>
>
>
>I also wonder how a flat board that is pushed into reverse crown by 
>the downbearing will behave with changing humidity.  With decreasing 
>humidity the board wants to shrink.  It seems to me that this would 
>mean that the board would want to move up, back toward the flat 
>position.  But the ribs are resisting this.  So a moment would be 
>set up between the ribs and board that would want to push the board 
>down.
>
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>  >>> If it's reversed the ribs to my mind would now be acting as  a 
>spring in tension and could behave the way you describe. 
>         The reverse would be true with increasing humidity.  The 
>board wants to grow, which seems to me would mean moving downward. 
>But the moments between ribs and board would be tending to push the 
>board up.  So perhaps this configuration is actually more stable 
>than the conventional configuration.
>
>... I dunno. It could be
>
>
>I assume you're referring to the (so far, unsubstantiated) phenomena 
>of "oil-canning."

I don't think I was.  But I'm not really sure what that is.  Can you 
explain what you mean by this?

>I don't see how this is expected to happen. The same stress 
>interface between the ribs and panel that originally created the 
>crown will still try to force the soundboard assembly up into a 
>crowned configuration. It would be more likely to develop increasing 
>reverse crown during the dry times. The tensile strength of wood is 
>not all that great, especially after it has been damaged by 
>compression stress.

I wasn't really referring to a compression crowned board.  I think 
the configuration that I have in mind would be uncrowned or reverse 
rib crowned, if that makes sense.  I wasn't thinking of a CC board 
that had collapsed.  I was thinking more of a board that was made 
deliberately made to have a reverse crown, such as some of the pianos 
that Dale has seen (assuming they were intended to have reverse 
crown).  So, take an undried panel, glue on some ribs, and load it 
up.  It will reverse crown and the board will be in tension I think. 
Or machine crown some ribs to give the board a reverse crown when 
they are glued on.  Load it up and once again I think the board will 
be in tension.  Now if this board is subjected to a drier environment 
that that at which is was glued up what will happen?  I was assuming 
that the board would take on additional tension.  Enough to overload 
it?  I don't know.  But if not, then when this board is subjected to 
more humid environments than that at which it was glued up it would 
probably be less likely to sustain compression damage than a CC 
board, or even a rib crowned board with positive crown.

I was also speculating about what would happen to this board if it 
was subjected to a drier or more humid environment.  My thought was 
that in a drier environment the board would want to move up.  My 
thought was that the board is describing an arc.  As it dried this 
arc would have to get shorter, so the board would want to flatten 
out, which in this case would mean moving up.  You seem to think it 
would move down.  Why do you think that?

>
>
>
>>
>>With a conventional board having crown and with a downbearing load 
>>I think the board is always in compression.  However, with this 
>>reverse crown board I can imagine that there is some humidity level 
>>where the expansion of the board induces a compression load in the 
>>board which just balances the tension load from the downbearing so 
>>the board is in an essentially unloaded state.  As Del was 
>>conjecturing in another post, the acoustical behavior of a board in 
>>this state may be very different from a conventional board.
>>
>  >>>I agree
>
>I am uncertain what is meant by "a conventional board." Assuming 
>this refers to a compression-crowned soundboard system, the key 
>words in the above are then "having crown." However, a strong 
>argument can be made that this is no longer--if it ever was--the 
>conventional soundboard system....

I wasn't intending to comment on CC vs RC boards here.  By 
conventional, I meant a board that was intended to to have a positive 
crown, which includes every piano I've ever seen (but not every piano 
that Dale's ever seen).  I therefore felt safe in calling this 
'conventional'.

I was pondering a what if scenario.  What if you built a board that 
was intended to have a reverse crown.  How would it behave?  Would it 
have any advantages over a 'conventional' board that is intended to 
have positive crown?

>
>As well, it is quite possible to have a string downbearing load 
>without having positive crown.
>
>Del
>

Yes, I know.  But in this case it would be a deliberate design 
feature rather than an unintended consequence.

Phil Ford


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