the A floats

[Farrell]

I can provide my observations on how an unribbed panel changes size with
varying relative humidity (RH).

Over time I have used several methods to monitor spruce moisture content
(MC) when preparing a Sitka panel for ribbing. I have settled on the simple
method of monitoring environmental factors in my hot box (temperature and
RH) and the size of the panel. I choose two points on the panel that define
a line roughly perpendicular to the grain direction that are near maximum
distance apart (usually close to 100 mm).

I keep my shop just below 50% RH at all times. My spruce sits in that
environment for a minimum of many months (most of it for at least a year or
two) - so it is be pretty darn stable. This equates to wood having a MC of
approximately 9%. My target MC for ribbing prep is 6%. That requires me to
get the RH in my hot box down to 30% or just a tad below. So my observations
are based on a 20% change in RH. Over this RH/MC change, a panel will shrink
about 3 mm.

I find that it pretty consistently requires 3 to 4 days for this dimension
change to occur. The change in size is clearly an asymptotic function, with
maybe half the change in size occurring in the first day.

And of course, change in panel MC is directly related to change in pitch.

Except for the fact that most in-use soundboards have a finish on them, I
should think my numbers could be directly applied to answer your question. I
don't know how significant an effect a couple coats of lacquer or varnish or
shellac or whatever will have on the rate of MC change in a soundboard.
Certainly a finish will tend to slow the moisture exchange down a bit, but I
don't know how much. My guess is that most finishes will not slow it down a
lot (but I don't know what "a lot" means).

Hope this helps.

Terry Farrell

www.farrellpiano.com

David wrote:

SNIP

>   But no matter, the main question I have is:  how long does it take the
> soundboard to react to that 30% drop (or rise) in humidity?
>
>     People have asked me on rainy days if it's a bad idea to tune the
piano.
> I've always told them that just one rainy day isn't enough to throw the
> piano out of tune.  Unless all their doors and windows are open.  In
> general, I don't experience many customers all calling because their
pianos
> have gone out of tune unless we've gotten 2 or 3 weeks of rain almost
every
> day.  And when people turn on their furnaces in the fall, the pianos don't
> drop in pitch in one day.  It takes a few weeks.
>
>     You'd think there'd be scientific charts available, what with all the
> research and technology going on nowadays.  Heck, even since the beginning
> of the industrial revolution -- not much longer after the piano was
> invented.
>
>     David Nereson, RPT