Klarinet Archive - Posting 000761.txt from 1998/09

From: "Dee Hays" <deerich@-----.net>
Subj: [kl] Thermal effects
Date: Mon, 21 Sep 1998 20:01:51 -0400

------@-----.4A77E800
charset="iso-8859-1"

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.

Dee Hays
Canton, SD

------@-----.4A77E800

-------------------------------------------------------------------------
------@-----.4A77E800--