Posted by Rick Denney on January 31, 2001 at 17:49:59:
In Reply to: Re: Re: Metallurgy posted by Erik on January 30, 2001 at 20:18:28:
You must have me confused for someone who knows what they are talking about.
When you form metal, either by bending it or hammering it, the yield strength of the metal increases, and also its brittleness. This is the result of the molecules being aligned by the work on the material. As a result of working, some material yields fully and some material does not, and these two portions of the material are left in a state of tension against each other. This tension is maintained by the surrounding mass of material that keeps the structure in equilibrium.
Higher residual stress does not affect the stiffness of the resulting structure when the material is elastic. If you hang a half-pound weight on a spring, it may deflect half an inch. If you place a one-pound weight on the spring, it will deflect one inch. If you place an additional half-pound, it will deflect an additional half-inch. This relationship is linear for elastic materials before they yield (read: before they deform permanently). So, if the stress of sound vibration causes a bell to vibrate one thousandth of an inch (just to pull numbers out of the air), then it will move that same amount no matter how much residual stress is in the bell, as long as it is in equilibrium. A horn not in equilibrium is falling apart, so a crack in a tube or a faulty solder joint will destroy this relationship.
Annealing metal causes the molecules to run together, which has the effect of releasing this residual stress. This also eliminates the extra strength gained from cold working, which, in turn eliminates the brittleness associated with that extra strength. This allows you to work the material some more without risk of cracking it due to brittleness. Many people believe that annealing a horn changes its acoustic properties considerably, but I confess I have no idea why that would be the case.
With a French horn, the damping effect of the hand is an external upsetter of equilibrium, and the residual stress in the bell might raise or lower the damping effect of the hand. That is not the case with tubas, unless you add devices to damp vibrations, such as a plastic rim or a belt.
Adding mass also affects the dynamic response of the structure by increasing the energy required to initiate vibration, and by decreasing the resonant frequency of that vibration. So, I have no trouble believing that extreme weight additions such as heavy mouthpieces and valve caps can affect the sibilance of the structure. This is not a damping effect, because elasticity is not attenuated.
Rick "not forgetting the placebo effect" Denney