Soundboard question

[Will Truitt]

Ric,

I'm going to second Terry on this one, although my evidence is anecdotal -
it comes from working in the Lakes Region of New Hampshire, where the
conditions that piano soundboards live in throughout the year can vary from
a bathtub to the Sahara Desert.  I have customers where the lake is
literally 15 feet out the back door, all windows and doors are open during
the summer.  RH can get into the 90's at that time, particularly this year
which has been a very rainy one.  Some of these same customers heat with a
wood stove in the winter, which can be long, cold, and very dry (the lake
freezes for about 4 months of the year). RH at this end can be 20% or lower.
It is my regular experience in these conditions to see compression ridges
develop in the panels in the summer time, and the flip side is that these
compression ridges will develop cracks in the winter after a few seasons.
This experience is one that I see all the time after over 30 years of living
here, and no doubt would be seconded by my fellow compatriots who see pianos
under similar conditions.

What I regularly see is exactly what Terry says occurs, "compression set
damage would occur locally - most likely in linear bands of lower wood
strength (resistance to crushing). So you'd end up with 90% of the panel
more-or-less undamaged and 10% of it severely damaged and prone to cracking
very easy, etc."  

Not claiming to be an expert, I will hazard a guess that once this localized
crushing occurs, those wood cells have lost their elasticity and are LESS
able to accommodate the changes in EMC.  Hence, the cracks develop in that
area once things dry out. So, strictly speaking you are correct. " The areas
that were weak (with regards to compression strengths) would be quite a bit
less reactive to dimension change...  The wood that was stronger from the
get go would be pretty much as reactive before the process and would account
for most of the change in dimension from cross grain expansion / contraction
with climate changes." But the net effect is the opposite of your claim - it
will be MORE prone to cracking.  The crushed wood expands and contracts
relatively little, the healthy wood moves a great deal more, and the
differences between the two are sufficiently great at certain times of the
year that the tension at the interface of the two becomes so great that a
failure occurs - it cracks.  Thus it would seem that your process of
deliberately introducing compression set would make it more likely that the
panel will crack rather than less, after it goes out into the imperfect real
world.  

And I think your claim that the panel overall would be less reactive to
humidity changes is at best trivially true. The piano is still going to go
out of tune for all the reasons that they do go out of tune. If your panel
is 1% less reactive, or 10%, or even 25%; so what?  What practical
difference would that make to most of us, or our customers?  Those
Thermomezzonucleursealed soundboards still go out of tune. All the other
elements that contribute to tuning instability are still there.

Respectfully,

Will Truitt



 



-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: Monday, August 18, 2008 5:47 PM
To: pianotech at ptg.org
Subject: Soundboard question

I'm not sure I see why it would be prone to cracking...  The areas that 
were weak (with regards to compression strengths) would be quite a bit 
less reactive to dimension change for climate change. Old panels that 
are de-ribbed and brought back together and rib crowned rarely ever 
crack again and are quite stable in the face of climate change compared 
to fresh wood.  The wood that was stronger from the get go would be 
pretty much as reactive before the process and would account for most of 
the change in dimension from cross grain expansion / contraction with 
climate changes.  You say yourself the panel would be less reactive to 
fluctuating RH.  I would think that the  areas that have experience the 
most compression set would account for this for the most.  What am I 
missing here ?

Cheers
RicB



    I really don't know for sure, but my guess is that the compression
    set damage would occur locally - most likely in linear bands of
    lower wood strength (resistance to crushing). So you'd end up with
    90% of the panel more-or-less undamaged and 10% of it severely
    damaged and prone to cracking very easy, etc. It might be a bit more
    stable with fluctuating RH I suppose, but I suspect you'd be a lot
    better off by simply starting with a laminated panel to begin with.

    Terry Farrell