Klarinet Archive - Posting 000773.txt from 1998/09

From: "Cox, Graeme" <Graeme.Cox@-----.nz>
Subj: RE: [kl] Thermal effects
Date: Tue, 22 Sep 1998 00:44:14 -0400

-----Original Message-----
From: Dee Hays [SMTP:deerich@-----.net]
Sent: Tuesday, September 22, 1998 12:02 PM
To: klarinet
Subject: [kl] Thermal effects

Dee Hays wrote...

Let me preface my comments by saying that somewhere there very well =
could be a detailed study of the thermal mechanisms affecting instrument =
durability and sound characteristics. However here is some further food =
for thought.

While I was unable to find the material data for grenadilla, I did find =
the information for red oak today. Red oak has a coeffienct of thermal =
expansion of 0.0000019 inches/inch/F (which is substantially less than =
any of the metals). If it is built to be in tune at 72 degrees and you =
cool it by 40 degrees down to 32 degrees, the change in dimensions will =
be only 0.0076% shrinkage. If you heat it from 72 degrees to 112 =
degrees, the change in dimensions will be 0.0076% expansion. The =
manufacturers can't build them this accurately (on a 0.575 inch bore =
diameter, that is only 0.0000437 inches). There will be more variation =
horn to horn than there will be due to temperature. Dimensional changes =
in the instrument are too small to affect the pitch and they are in the =
wrong direction. Cold means shrinkage (which would drive the pitch up) =
and hot means expansion (which would drive the pitch down). So we are =
lucky that the dimensions just don't change that much.

Now the temperature of the instrument can affect the air stream and as =
the instrument warms during playing, you will have constantly changing =
transient effects. Engineers have nightmares trying to pin down the =
effects of transients, by the way, in any field. They aren't as easy to =
calculate as steady state conditions.

Another interesting calculation is thermally induced stress. The stress =
induced in the wood by this expansion or contraction is only about 100 =
psi. Even in its weakest direction, wood has a strength of over 1000 =
psi. So it is not the thermal expansion and contraction alone that =
leads to cracking. However if there are pre-existing stresses or weak =
spots or flaws in the wood, this change could be the straw that breaks =
the camel's back. Or it could be that temperature has nothing to do =
with cracking and that an entirely different root cause exists for this =
problem. Personally I plan to keep my horn at reasonable temperatures, =
swab it out well and since it is old I will oil it whenever it appears =
to be developing dryness at the surface.
=============================================

Nice analysis, Dee !!!!

Now, what scientific data is there about expansion versus moisture content
in hard woods? Especially wood that was soaked in oil some years ago? What
stresses might be setup in a tube consisting of a layer of damp wood
surrounded by a layer that is relatively dry?

Graeme Cox
Graeme.Cox@-----.nz

-------------------------------------------------------------------------