TubaTinker wrote:We need Rick Denny to work this topic over, but my feeling is that adding anything to a tuba would detract from the energy output.... doesn't matter if it's heavy lacquer or extra metal.
No, we don't need me expounding on the subject, but since I do it for fun, you'll have to put up with it anyway.
We buzz our lips, creating a wide range of frequencies of sound. The vibration of that buzz resonates in the cavity of the mouthpiece, which filters some frequencies and amplifies others. Those that leak into the horn through the mouthpiece shank resonate in the cavity of the instrument. Some frequences are filtered, some cancel out, and some are amplified. Those that are amplified escape out the bell.
If we were to describe the frequencies damped and amplified by either our lips, the mouthpiece cavity, or the tuba cavity, we would describe the resistance of the isntrument to amplifiying certain frequencies, and the eagerness of the instrument to allow others to resonate sympathetically so that they are amplified. That description is called the impedance (okay, EE nitpickers keep to yourself, heh, heh).
The brass vibrates as a result of the air within it vibrating. The amplitude of that vibration is quite small, otherwise we'd get a noticeable hand massage every time we blew a note. Also, we damped such vibration by putting the instrument in our fleshy laps and by holding it with our fleshy hands.
As with the lips and the cavities in the mouthpiece and tuba, the brass itself has an impedance, which describes the frequences that it weakens versus those that it strengthens. The effect is not strong--tubas tend to vibrate at frequences well above those that are audible in tuba sound. And the vibration is usually spread over such a wide range of frequencies that it can't have a big effect on any one frequency. For example, hold your instrument in its normal position, and tap it with a fingernail at various places to exite vibration in the brass. You'll almost always just hear a thud, or at most a clang. Rarely will you hear a ring, which identifies a resonant frequency and the ability to filter out all the other frequencies.
The impedance of the tuba brass is controlled by the mass and stiffness of the brass, plus any external damping. Additional mass lowers the resonant frequency (if there is any discernable resonance) while increasing the required excitation force. Additional mass often means additional stiffness, if the mass is added to the general structure (as by making the brass thicker). This increases the stiffness. But you can also increase the stiffness without adding much mass, by adding more bracing. Increase stiffness (with mass held the same) increases the frequency of whatever resonance might be there, while also increasing the required excitation force.
You can demonstrate both principles with this example. I own a Subaru and a motorhome. Both rest on springs. I can stand on the bumper of the Subaru and jump up and down. If I do so at the right frequency and if I remove the damping effect of the shocks (and if I can hang on), I can make the car hop off the ground. If I jump on the bumper of my motorhome, I can hardly make it move, because the extra mass and stiffer springs need higher excitation force than the weight of my body can provide. If I hang a small weight on a spring, I can make it bounce up and down with the smallest input of energy, if I do it at the right frequency. If I make the weight much heavier, it will require more energy, and it will hop up and down at a slower frequency. If I stiffen the spring, it will require more energy, and hop up and down at a faster frequency. You get the idea.
We damp the vibration of the tuba by holding it against our soft, plastic bodies as we play. Some people try to damp vibration by, say, putting a belt around the bell throat. This has a small damping effect, but it also preloads the brass, which increases the force needed to excite vibration.
Okay, what does this all mean?
Massively bracing a tuba adds considerable mass, but even more stiffness. Thus, the net effect on resonant frequencies is to go up, further away from the frequencies where tubas produce sound. This lowers the effect of the brass vibration on that sound. But the vast increas in stiffness also dramatically increases the energy required to excite vibration. Thus, the instrument vibrates much less for a given amount of vibration of the air within it.
Does the vibration of the brass of a tuba affect the vibration of the air? Probably a little, but probably not enough in the frequencies of tuba sound to be audible. The impedance is wrong. But it will reduce the felt vibration in your hands, and for me that is undesirable.
The energy required to set brass to vibration (if it is resonant) is so little that it pulls nothing noticeable from the sound. It's a tiny influence on an effect that most would find negligible in the first place.
Trumpets are likely different. Characteristic trumpet sound includes considerable edge, which sounds like edge because of the very high frequencies that are part of the sound. Massively bracing a trumpet minimizes the effect of the brass on those frequencies. That's a little closer to the frequencies at which the brass clangs, so there's a greater chance of having an audible effect. But I still think the effect is pretty small, and not always desireable.
Rick "who likes his Holton in particular
because the brass vibrates a little" Denney
