Posted by Rick Denney on August 17, 2001 at 11:26:02:
I think we throw around terms like "damping" without really understanding them (and usually without knowing the proper terms, but that is just a pet peeve of mine), and thus often miss the point of what the heavy-weight guys are trying to do.
If the objective is to minimize vibration in the tuba (as opposed to the air within it), then we have two choices. One is to damp them with plastic materials that absorb vibrational energy and turn it into heat (granted, in this case not very much heat). The other is to add mass, which increases the energy required to produce vibration.
But I wonder if the objective is to minimize vibration, or is the objective to direct vibrational energy into the air column. These are two different objectives.
Some random truths:
1. An infinitely stiff material would reflect every bit of energy bounced off it back into the air around it. If we had a tuba interior shape molded into, say, solid rock, then the vibration of that rock resulting from resonance of the air within it would be minimal.
2. A plastic material reduces vibration by absorbing it, not by reflecting it. That energy is lost to heat, which means it is truly lost.
3. An elastic material vibrates sympathetically with the vibration of the air mass inside it. This sympathetic vibration has its own resonance characteristics.
4. Additional elastic material increase stiffness, but it does not decrease elasticity. Therefore, it does not absorb vibrational energy, but it does reflect it to a greater extent.
So, what are we trying to achieve? I think it is different in different cases. In some cases, we may believe that a horn's projection (which, let's say, is related to the efficiency with which vibrational energy is transmitted to the open air) can be enhanced by reducing unwanted resonance in the brass. In other cases, we may believe that a horn has an improved sound if we absorb certain frequencies of vibrations.
Nobody plays a tuba made from massive solid rock, so no tuba approaches effectively infinite stiffness. And the resonance of vibration can take a tiny input force and expand that force hugely, so even the heaviest tuba will vibrate with the sound produced within it.
Here are some of the popular remedies for adjusting the resonance of the brass, plus what I think they are likely to accomplish.
1. The Leather Belt. Many experiment with a leather belt around the bell stack. This is a damping device--it adds a plastic component (leather) to an elastic structure (the bell). If the bell's vibration is robbing the vibrational energy inside the bell, this will not solve the problem. But it will sound different to the player, because much of what the player hears is the ringing of the bell in his left ear. If it affects the sound "out front", it is likely because some high-frequency resonance in the brass has colored the sound, and by damping that frequency, the effect is removed.
2. The Plastic Bell Ring. Others use a vinyl molding around the rim of the bell, often to protect it, and most with sensitive ears report a change in sound. It is a similar effect to the leather belt, though that effect might be different in practice. The description above fits it perfectly.
3. The Bell Attachment Ring. This is NOT a damping device, but a weight device. By adding mass to the bell stack, it changes the energy required to set it vibrating, and also the resonant frequency of that vibration. By increasing the energy require to set it vibrating, it reflects more energy back into the vibration of the air column. So, while the leather belt will not preserve that energy (it is damped--lost for good), the bell ring will preserve that energy in the air column. This may well improve projection. By changing the resonant frequency of the bell stack, it also will affect the color of the sound. It should be noted that for any given tuba, this may or may not be a good thing.
4. Wire, guards, and other stiffeners. These add a little mass, but really work by adding stiffness to the brass. And increased stiffness has much the same effect as adding weight, though the proportions are different. Extra weight primarily increases the energy required to set the brass vibrating, and secondarily changes the frequency of that vibration. Extra stiffness primarly increases the frequency of vibration, and secondarily increase the energy required to drive the vibration. The modificitions Jay Bertolet made to his Cerveny 601 are a good example of this, and I can well believe that they increased the projection of the sound.
5. Monsterweights and heavy valve caps. These are a weight modification, and they work similarly to the bell ring, but at a different part of the horn. The objective is not to absorb, or damp, vibration, but to reflect it back into the air column. I have no trouble believing that in therefore increases projection, and probably response, too, because the embouchure only has to impose changes in the air mass, not in the air mass and the brass.
6. Heavy mouthpieces and mouthpiece weights. These are obviously a weight modification, with no plastic damping of vibration. They reflect energy back into the system. I have no trouble believing that they affect both the projection and response. But I suspect that the closer the weight is to the lips, the more it affects response and the less it affects projection.
7. The host of little stiffness things that we do, from raising the leadpipe to moving braces. These can have varying effects, depending on the vibrational modes of the brass, but there effect is to fine-tune the stiffness of the struction to achieve a better overall balance.
Without doing the math, my gut reaction is that heavy weights at the mouthpiece have the greater effect on response, and heavy weights at the bell have the greater effect on the projection. It may be that the valve caps are effective because they provide a good balance of both.
None of this says anything at all about the value of doing these things for any given horn or player. Some horns may be too responsive (slotty), and other horns not responsive enough. Some horns may ring like a bell, but still sound better doing so out front. Each horn is a balance of these things. The infinitely stiff tuba, even if it were possible, would probably not be very satisfying to play, because the response and projection might come at the expense of other valuable things, like tone and radiation patterns. The infinitely heavy tuba would approach the infinitely stiff tuba, just because adding that much elastic weight would necessarily add considerable stiffness. And the plastic tuba (let say a tuba made from cheese--which has been done--or from butyl rubber) might make sounds, but without any projection or response because it absorbs too much of the sound energy it is there to amplify.
In conclusion, when we talk about valve caps, bell rings, mouthpiece weights, mouthpiece dampers (as in rubber washers--likely to have the opposite effect of metal weights), and leather belts, we need to talk about what we are trying to achieve with a particular horn and whether that is accomplished.
This is an engineer's take on these issues, and I'm curious as to how well they correlate with the experiences people have had. Comments?
Rick "in search of better understanding" Denney