Orchestral Equipment Survey with a Poll

[Rick Denney]

...that 18 feet or so of tubing can't form a stable C.

Don't underestimate the pitch variability allowed by the widely tapered bore of the tuba. It is not an organ pipe by any means.

Also, the player's lips and the mouthpiece each have an impedance curve that describes the resonance space. Those combine with the impedance curve of the tuba to produce sound.

And don't assume that all overtones must be harmonic. We frequently get a noise buzz in our sound when things aren't working well. If it was harmonic, it wouldn't be noise. We also get spurious high frequences from clipping when the vibration of the lips runs into physical constraints, either from lack of air flow or from lack of embouchure quality.

So, with some players and tubas it is indeed possible to play a C using 18 feet of tubing. Sound quality is another matter.

And, of course, middle C resonates in 18 feet of tubing as the ninth partial.

Rick "who thinks all generalizations are false but some are useful" Denney

[windshieldbug]

But when one plays a fundimental D, the instrument is the same length through valve tubing, whether it's an F or an Eb, CC or BBb...

[Z-Tuba Dude]

But when one plays a fundimental D, the instrument is the same length through valve tubing, whether it's an F or an Eb, CC or BBb...

I hope I am reading your comment correctly, but if you are talking about the D just below the staff, the tube length will be the same for the BBb, and C tubas. That same note will involve less tubing for the Eb, and F tubas (which is the same length of tubing, for each).

That is true for the D, an octave lower, as well.

[windshieldbug]

I may not have stated it well, but that is exactly what I meant; (F and Eb), and likewise (CC and BBb)

[Donn]

...that 18 feet or so of tubing can't form a stable C.

...
So, with some players and tubas it is indeed possible to play a C using 18 feet of tubing. Sound quality is another matter.
...
Rick "who thinks all generalizations are false but some are useful" Denney

Wasn't intended as a generalization. Context seems to make it pretty clearly the 4 foot C in the bass staff. As an overtone. (Not) in an 18 foot tuba.