Re: Woodwind Shrinkage

[Dana S Emery <emery@onyx.si.edu>]

>From: CLARK FOBES <reedman@ix.netcom.com>
>
>>From: Scott Hirsch <WQEditor@aol.com>

>>The author claims that when an instrument looses moisture, it contracts
>>into itself, which would increase the instruments bore diameter.

>I believe that the condition that Mr Naylor describes is true. However,
>forget about the tube for a moment and imagine a flat board on edge. If
>this board loses moisture equally it will shrink into the center losing
>dimension on both sides.

ok so far, the board is shrinking into its physical center.

Note that the rate of shrinkage is not uniform, which is consistant with
the fact that wood is not homogenous.  The cellular structure of the wood
causes tangential, radial, and longitudinal shrinkage rates to differ.
Longitudinal shrinkage is generally unmeasurable.  For many hardwoods the
difference between radial and tangential shrinkage is significant and
causes the cupping one sees in flatsawn boards.  Generally the tangential
fibers shrink more than the radial ones, commonly 2:1.  In the case of our
board on edge this differential movement will not only affect the cros
sectional area of our hole, it will also cause it to go oval.  It will only
be round when it is at the exact same relative humidity as when it was
bored.

>Now 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.

That is _not_ the way I have seen it described.  According to Dr. Bruce
Hoadley (Understanding Wood, Taunton press) that wood movement (ie,
shrinkage) is the same with or without the hole, ie, the id and the od both
shrink to the center of the _instrument_ (not its walls).

>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.

Interesting, have you actually measured that increase, or are you surmising
it based on the observed pitch lowering?

The analysis I have seen of old recorder and flute measurements has always
spoken of adjustments which compensate for bore _shrinkage_, not expansion.
(cf. Bob Marvin, Galpin Society Journal, title escapes my memory but
mentions english flutes and recorders).

--
Dana S. Emery
Smithsonian Institution
Laboratory of Molecular Systematics
MRC-534, MSC A2000
Washington DC 20560

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