[CAUT] Soundboard Heat Treatment? (LONG)

[Scott E. Thile]

Hello all,

A friend is interested in some research he uncovered indicating the benefits
of treating softwoods with heat or chemical means to increase sound
transmission characteristics and stability. I think the thrust of the
research focuses on using currently "inferior" woods in less expensive
instruments, especially in light of shrinking supplies of top quality
materials and an expanding demand to supply the Asian instrument makers. 

Some of this research seems to indicate treating already excellent materials
could make them better still.

Any of you designer/engineer/scientist types had any experience with this as
it relates to pianos or done any research with it? 

My friend, John, relates the following: 
--------------
"There is a bunch of literature on this, and the indications are that the
treatment would improve several characteristics of the wood.  The best
option seems to be heating the wood in the 200 deg C range for a period of
1/2 to 4 hours using a method that prevents oxygen from getting to the wood.
(in the presence of oxygen the wood degrades pretty quickly when heated).
one method is to heat the wood with steam, this removes the air and protects
the wood from oxygen.  another is to do it in a pure nitrogen gas atmosphere
or in a vacuum.  and there are other methods involving heating in an oil
bath, etc.  the result is that it seems to do what aging naturally does over
a period of decades, although the heat treatment only takes a few hours.
among other things, the wood is 1) darker, 2) stiffer and more brittle, 3)
has a higher modulus of elasticity, i.e. higher sound velocity and therefore
better acoustic qualities, 4) better stability in response to changing
humidity and less tendency to crack with changing humidity, 5) and best of
all the moisture content is about 1/2 of that of untreated wood at any
particular humidity level.  the one downside is that the wood is not as
strong, i.e. it is stiffer and more brittle and will fracture at a lower
bending force.  but this weakening of the wood is tolerable and is a
function of the amount of heat treatment applied. seems like this might be
great option for a piano soundboard and would lead to greater tuning
stability, as long as the board was strong enuf to handle the downbearing
force."
---------------

Here's a link to a presentation from some research at the University of
Dresden: 

http://campus.murraystate.edu/staff/scott.thile/07_59.pdf

My initial reaction is that the best pianos I've heard are relatively new,
or have relatively new soundboards, but older string instruments often have
advantages over new ones. The difference no doubt has to do with the amount
of tension, downbearing, and other forces at play. I think new, properly
seasoned strong wood (weight to strength ratio) has a big advantage in
pianos versus lower tension instruments. And conversely, lighter, older and
more brittle wood has an advantage in lower tension instruments. So I would
think the benefits of this extreme heat treatment would be more noticeable
in lower tension instruments than it is in pianos.

I just removed a perfectly good soundboard out of the Steinway 'O' I'm
rebuilding this summer. I could not detect anything wrong with it at all. I
just wanted to replace it with a new board because it has reacted to 40
years of extreme humidity swings (it's not the original board--the piano is
65 years old). No cracks, good crown and me beating the thing out with a
sledge hammer ;)). Even I had to cringe, but I'm convinced this is the best
strategy here at MSU where we see 60% swings in humidity. Every rebuild gets
a new board now!

I LOVE the sound of the D I just finished rebuilding this January with a new
Bolduc board--it's simply a fantastic board! No extreme heat treatment--just
good wood that has been well constructed into a wonderful soundboard with
more or less traditional techniques. Would it sound better if it had been
extreme heat treated? Perhaps, but how would it age? Is the life span
reduced as the tonality is increased with extreme heat treatment? My guess
is it is. Perhaps like putting the thing in a time machine. Yes, it may
sound better, but for how long? If the board needed to be "beefed up" in
order to compensate for the reduced strength, does that negate the tonal
effect? I bet it does....

The most compelling thing mentioned in the research is that the treated wood
takes on less moisture. Perhaps reducing the expansion and contraction due
to humidity swings, which would help with the situation we have here at MSU
(and many other locations), and that would add as much life as the loss of
strength would reduce, and if it also contributed to better tonality and
more stability, well that really would be interesting.

Sorry this is so long, but I thought it was interesting.

Any of you bright boys in back rooms have a take on this?

Scott

Scott E. Thile, RPT
Piano/Instrument Technician
---------------------------
Dept. of Music, Murray State University
504 Fine Arts Building, Murray, KY 42071
Office Phone: 270-809-4396 
http://campus.murraystate.edu/staff/scott.thile/index.html