Heavy bottom valve caps

[Rick Denney]

There are similarities but it’s not the same. The radiating element in a tuba is the column of air, not what contains it, just as the radiating element in RF is electrons, not copper. The copper just provides the container for the electrons.

A tuba is more like a microwave feed horn, or even waveguide, not like the antenna itself. It amplifies and filters.

Mass affects resonance profoundly. If I increase the weight of a balance wheel on a watch movement, it will oscillate more slowly unless I also strengthen the hairspring. And then it will decrease the amplitude for a given impulse power from the mainspring through the train of wheels to the escapement.

And it does not change the Q by itself. A tuning fork has very high Q—it resonates one frequency generously and damps all others. But a tuba doesn’t ring like a tuning fork, and making it heavier doesn’t make it do so.

If a tuba vibrates sympathetically with the air column, it may enhance that frequency and damp others. But whatever ringing the brass does is at one set of frequencies for all notes played. So, it will enhance some notes and damp others—what I meant by “uneven”.

Rick “our gooey bodies touching the brass an multiples places lowers Q, but of the brass more than the air column” Denney

[peterbas]

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[peterbas]

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[donn]

If I put a 20-meter signal (energy) into an antenna designed for 10 meters, I'll have a mismatch and poor performance.

OK, so I guess you'd transmit on a single exact frequency, and make the antenna length match that frequency.

Whatever. As long as we are clear about what's resonating in a tuba, and it isn't valve caps.

[donn]

In defense of audio measurement - while there sure are some intractable problems with human performance bias, it's a whale of a lot better than "next I played it with that gimmick, and what a difference I thought I heard!"

[peterbas]

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[peterbas]

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[donn]

What the measurement shows in this case - if we exclude the performer bias problem - is that valve caps can have a real effect on the output.

Whether the effect is important is a different question, but of course it's a question only if there is an effect. It would be easy for me to believe there would be no measurable effect, so here's a good step towards improving common knowledge in this area. The next step would probably be to figure out blind controls to reduce the performer bias problem that "Rick Denney" mentions.

The valve assembly will resonate more or less, so heavy caps can make a difference.

I'm following Jim William's use of "resonate", with analogy to the transmitting antenna etc. In this picture, the resonating element is the air column. Since that produces sound vibrations in its entire environment, of course all kinds of things will resonate along, but secondarily to the tuned resonance phenomenon inside the tuba.

Whether semi-solid materials in contact with the air column can significantly affect air column resonance, is to me unproven. Some people claim to hear differences with changes to bracing, believe they can tell the difference between bell materials or rings around the bell, ... and then someone makes a tuba out of plastic, which ought to be really noticeably different, and it really sounds the same. Mouthpieces ... Maybe there's really something going on, but anyway, my point is that the bottom level mechanism through which a tuba produces sound, has nothing to do with this kind of resonance, and wind instruments in general are different in this respect from strings. Resonant properties of the material instrument have little if anything to do with the output.

[bloke]

I will admit to (at least up close) sister models of fiberglass/brass sousaphone bells offering an easily-heard type of "background sound".

I can absolutely tell them apart, and I'd be willing to do a double-or-more-blind test.

From a distance...I'm not sure, because that "plastic" sound-within-the-sound does not dominate the sound qualities/bouquet.

[peterbas]

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[bloke]

Typing in plain-spoken vernacular (no scientific terms/jargon here), it does seem as though - if a thin well instrument's body tends to vibrate slightly more than a thick wall instrument - that (actual physical movement) could slightly stimulate/amplify/add to (again: vernacular) the vibrations which already exist in the air column...I couldn't imagine it being much more...

...though it's obviously SOMETHING.

If anyone thinks it's NOTHING, just go ahead an unscrew a valve cap or lyre screw just a little bit...or fail to oil the bearings of worn rotary valves - allowing them to run dry.


...but what about that signature background sound that a fiberglass sousaphone bell emits?
I feel sure that I'm not the only one who is aware of that background resonance.