Soundboard construction methods

[Richard Brekne]

Hi JD.

    JD write:
    That's why I said "take off the bars" :-)  My posting was in
    response  to Ric's quotes from researchers into wood, which I was
    saying were really not very significant for the range of values for
    moisture  content in a working piano. 

Actually the change of strength characteristics I listed are fairly 
linear in the ranges we typically see... certainly from an MC range of 
3.5 % up to 18 % covers our discussion... and in that range the percent 
changes in strengths for changes in moisture are pretty fairly 
consistant throughout.

    If anything the phenomena he describes will _reduce_ the inherent
    stiffness of the wood as it takes on  moisture

I think, if you double check my post.. this is exactly what I said.  
Wood in general gets stronger as it gets dryer.

    (a problem, incidentally, that is noticeable with hammers)  and
    _increases_ the compression and hence the potential crown.  My  test
    will be to measure the _inherent_ stiffness of the wood over a
    realistic range of values for moisture content.

I think you will find that stiffness characteristics may vary somewhat 
depending on the grain orientation you are taking. Since E is found by 
taking dividing the amount of stress by the degree of strain, and since 
(as stated elsewhere) the resultant quotient can vary depending on 
whether or not the piece is loaded tangentially or radially... E sub T 
and E sub R can well be different.  Hoadley cites a general figure of 2 
% change in E for 1 % change in MC.  But he aslo states that FSPL for 
compression perpendicular to the grain are based on an avearging of both 
these... so I would suppose (tho do not know at this point) that values 
in Static bending tables for E are taken  rather likewise.

In pianos tho... we often use strictly tangential loading or strictly 
radial loading.... and it would seem to me to be prudent to be sure of 
any significant differences.

     The results will be interesting, but almost certainly
    insignificant, because what interests us is the resilience of the
    soundboard assembly as a whole.

How these variances in strengths for variances in moisture play out in 
any given ribbed assembly is of course another question entirely... and 
on the surface of it at least a certain degree of complexity would seem 
involved.  You are dealing with wood pieces with opposing grain 
directions glued together with not only strength changes going on, but 
dimensional changes as well.  Its beyond me at present to calculate 
this... grin... perhaps I will find time to think it through.

Cheers
RicB