Klarinet Archive - Posting 000696.txt from 1998/09

From: "BRENT A ERESMAN" <BERESMAN@-----.net>
Subj: [kl] bore size changes and pitch
Date: Sat, 19 Sep 1998 22:14:52 -0400

Bill Hausmann wrote:

>Right. The change thermal expansion causes is NOT so much in the length
of
>the instrument, but in the diameter of the bore. When the wood expands,
>the diameter SHRINKS. Only a very small change is required to be
>significant, relative to overall length.

This is not likely to be true. If the instrument expands due to a
temperature change, then the every dimension gets larger, including the
inside diameter (bore) of the tube. I know that this doesn't seem to make
sense at first; i thought the diameter should get smaller too, until i saw
the reasoning in, i think, finite elements class in engineering school. I
don't remember for sure; it's been a few years.

Anyway, consider a fiber of the wood somewhere in the wall of the
instrument. it expands toward the outer part, making the outside diameter
larger, and toward the inside, making the inside diameter smaller. But, it
also expands towards its neighbors, pushing them away. All of the fibers
in that ring around the wall of the instrument do the same, making that
whole ring larger, and as a result the inside diameter expands. This
effect is greater than the other, so the bore actually increases in size.

A picture might be easier, but i can't figure out how to do the neat ones
that others have done here in the past.

Brent Eresman

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