Posted by Rick Denney on August 25, 2003 at 13:13:20:
In Reply to: Tuba metallurgics, carbon sousas posted by SHS Tubamaster on August 24, 2003 at 20:36:16:
I've wondered about the metallurgy and its effects on the sound for a long time, and have never really narrowed it down to anything really clear.
Most of the advantages of brass have to do with how easy it is to machine and form. It is a good combination of strong, ductile, malleable, and pretty. It welds easily and solder sticks to it well. It's easy to polish and to keep polished, and it's pretty resistant to corrosion, especially in some of its alloys. I think this workability more than anything makes it popular for brass instruments. And there is a historical context, too. Most of the alternative materials one might consider have been developed in the last 75 years or so, and most of them are still far more expensive than brass. Refining aluminum requires lots of electricity, and we didn't really know how to alloy titanium to make it strong until the last 30 years or so. Bronze is almost identical to brass, except that it is harder to work. Chuck has already told you about copper, and as he mentions, there is no such thing as pure brass--all brass (and bronze, too) is an alloy of copper and something else. Other alternatives might be pure silver, stainless steel, aluminum, and magnesium. Magnesium and aluminum are pretty similar in their mechanical properties. Aluminum can be spun, and it can be really strong. You'd probably want to attach it with epoxy, because welding makes repairs too difficult and it isn't easy to solder. I could see it used on bells, where it would be stronger than brass. You can also polish it. To add to what Chuck said, titanium and stainless don't polish easily, and highly polished titanium is even more extremely expensive than brushed titanium. Solid silver might seem prohibitively expensive, and it would be harder to work than brass in any case. But it might seem cheap compared to titanium, heh, heh.
Metals have different strength and stiffness, and both of those parameters affect the acoustical result. Metal that is stronger than brass can be made thinner with the same durability, but that thinness may lead to more acoustic vibration, depending on the stiffness. Stiffer metal at the same thickness as brass would resonate at higher frequencies--the ring would be more tinny--unless it was also a lot heavier. When you tapped an aluminum tuba, the ring would be quite different than when you tapped a brass tuba, I expect.
Just how much this affects the sound is another matter entirely. The sound coming from a tuba is a mix of a fundamental and a series of overtones. The highest overtones (in the range of the ringing of the brass) do influence the sound to some extent in theory, but I don't know of anybody who has determined by how much, especially with tubas that have little sound content in those high frequencies. But it seems clear from what we do know that the effect is subtle at best, and probably inaudible to most listeners.
The situation is a little more complicated with plastic composites. We have had plastic composite sousaphones for a long time, with fiberglass. Fiberglass is actually quite similar to carbon fiber composites. It uses spun glass fibers imbedded in a matrix of resin plastic, while carbon composites use carbon fibers imbedded in a matric of epoxy plastic. The plastic is pretty similar (and may be the same) in both cases--I'm no expert on plastics. Carbon fibers are much stronger than spun glass fibers. It's the fibers that carry the tensile load placed on the material, while the plastic carries the compressive loads.
Plastic damps high frequencies pretty effectively. I have seen the carbon composite bell and bottom bow mentioned in another post, and when I tapped it all I got was a dull thud instead of the usual clang. The instrument was not yet playable when I saw it, so I can't report on its sound. The player, who is a top performer, praised its playing, so I would say that the damping of the high-frequency sibilance is probably not causing much of a problem, if any. That leads me to believe that such high-frequency resonance between different metals would be no problem at all; those differences must be dwarfed by the difference between metals and plastic composites.
As with all composites, how you orient the fibers has a strong effect on its strength. There may be a way to preload the fibers to provide more resonance early on, but I would be worried that the plastic would creep in the resulting compression over time.
Would a carbon composite sousaphone be lighter than a fiberglass sousaphone? Yes, if you use high-strength epoxy. I have a carbon-fiber bicycle frame that weighs 2.5 pounds, which is half the weight of a similarly stiff steel bike frame. But the carbon bike damps high-frequency vibrations, and it therefore does have a different feel than metal bikes (I prefer the feel of metal bikes). Thus, I'd bet that a carbon-composite tuba would have a different feel than a metal tuba. But that would not necessarily be detrimental.
Rick "just musing on the possibilities" Denney