Posted by Aaron Eichelberger on December 04, 1998 at 19:28:58:
In Reply to: Re: Re: Re: false tones posted by ken on December 04, 1998 at 11:23:33:
Be careful when you say that just because everything high can be played open that everything low should be playable open too.
The reason that all the high notes can be played with virtually any combonation of valves is because that is the natural overtone series of standing waves in an open tube. Any given open ended (or closed for that matter) has a "fundamental" resonance. On the tuba, this should be the pedal Bb 6 leger lines below the staff. The next harmonic which will resonate (for any tube, not just the tuba) is an octave higher. The next is a fifth higher again. And it continues on to smaller and smaller intervals (although not in a nice linear fashion) until we get to a point where the next harmonic of the tube is a whole step above the previous, and then a few higher and we get half steps, and so on.
So in fact, there is no reason at all to believe that just because all the high notes can be played without valves means that the low ones can too. The valves are there to alter the length of tubing so as to create a harmonic overtone series which sits at a different fundamental frequency, thus allowing us to hit all the notes.
Admittedly, there is more going on with brass instruments than just a standing wave in an open tube. That's why we can lip notes as much as a half step or more. That's also why there are some false tones down low.
I don't honestly know what causes the existence of the "false partial" in the low register. If I had to guess, I'd say that perhaps it is coming about because of a change in the open/closed status of the ends of the horn. I think that the normal partials for brass instruments are based on a half-open tube (bell representing open end and mouthpiece representing closed end). If that's the case, then the fake partial in the low range could result from resonating the horn as if it were a double-open tube by changing the standing wave inside the horn to not have a node at the mouthpice. I don't know for sure, but it's my best guess as a grad student in physics.
Then again, maybe I'm just bitter because I can't play those false tones... :)