Re: Woodwind Shrinkage

[Cary Karp <ck@nrm.se>]

Scott Hirsch wondered:

> A recent article, Grenadilla Wood and the Environment by Larry Naylor, was
> published in our August issue. The author claims that when an 
> instrument looses moisture, it contracts into itself, which would 
> increase the instruments bore diameter. Is this possible?  I can't 
> quite see how a wooden tube can loose water and increase in diameter.  

Clark Fobes replied:

> I believe that the condition that Mr Naylor describes is true.
> . . . imagine a cross section of a tube with thick side walls (like the 
> clarinet). When this cross section loses moisture it will also shrink 
> to the CENTER OF THE WALL. If it loses thickness dimension on both 
> sides of the wall the O.D. will become smaller and the I.D. will become 
> larger. I have found that if clarinets sit foe several months or years 
> without being played that they drop in overall pitch due to the larger 
> bore diameter. 

Let's find a nice straight tree with a nice round trunk and saw through it
as nearly perpendicularly to its axis as we can. The growth rings seen on
the end grain will appear as concentric circles of different diameters
around the axis of the trunk. As the wood dries it will shrink and each
of these circles will grow smaller. The circles closest to the center of
the trunk will decrease least in diameter and the circles furthest from
the center will decrease most in diameter, but any point on any circle
will move towards the center of the trunk as it dries. This behavior won't
change if we drill a hole through the center. 

The cylindrical tube of a woodwind instrument behaves according to this
model (albeit complicated by the fact that the tube will rarely be
concentric to the axis of the trunk, thus causing the shrinking circles to
become oval). The inner diameter decreases as moisture is lost, and the
outer diameter decreases more, thus resulting in loss of wall thickness.
One example of the consequences of this is seen in wooden flutes with
metal lined headjoints where the wood commonly cracks around the liner as
it shrinks. If the inner diameter increased, we would expect to see the
metal liners loosen. I can provide other examples if anyone would like me
to. 

The lowering of the pitches of clarinets that Clark reports suggests that
their bore diameters have decreased, not increased. Assuming that he is
testing them with ebonite mouthpieces, the diameters of which will not
decrease with moisture loss, the diameter of the clarinet is changing
relative to the fixed diameter of the mouthpiece. In this case, decreasing
the diameter of the wooden portion of the instrument will lower its pitch. 

Earlier in this century, clarinetists oftened flattened high pitch
clarinets by hanging a cord inside the instrument from the barrel
downwards. Anyone interested in testing this may do so by placing a small
diameter dowel against the inner wall of a clarinet from the bell through
the barrel. This will lower the pitch of the instrument. If the dowel is
then pushed into the mouthpiece the pitch will rise.