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Both are obviously in play - when playing brass instruments (which don't feature any holes other than at either end), and I don't know much of anything about air flow dynamics and NEITHER do I know much (ok...a little bit, but not much) about the physics of the vibration of a column of air...BUT...

...I've put way more "weight" (importance) on the vibration of the column of air that I have on the dynamics of air flow...and I still do:

- When blowing into a brass instrument - particularly a huge tuba - we are nearly completely closing our mouths down in order to cause our lips to vibrate at particularly frequencies - which set off enharmonic frequencies within the adjacent air column of the brass instrument (and you needn't pick at in imprecision in this statement, unless you absolutely feel the need).
- Particularly with a tuba, the small amount of air (which we're allowing to past between our lips) just isn't very much air - once that air reaches the bell (due to the huge bore, and the CFM's being so diffused (and go ahead and pick at this if you care to, but it's not my main point).


This (below) is what got me MORE thinking about "air flow":

Last night, I sat in on a band rehearsal (a band which is attracting more-and-more good players, which makes it more-and-more enjoyable for me to sit in) which I often do on Mondays, because those rehearsals are the same time as a Monday meeting that Mrs. bloke attends monthly. As has happened before (even though there are competent/faithful regular tuba players enrolled) I was the ONLY tuba, and - due to that - was actually applauded when I walked in about a half hour after their rehearsal started.
They ran through a full (not watered-down) transcription of the (first) Superman movie "Main Title".
(Interestingly - OR NOT, I was probably the first-ever person to play/record that band transcription, and actually recorded it for one of those cassettes or old "sound sheets" mailed out to band directors by Warner Bros. ...I'm thinking that I actually recorded that - possibly even before the movie had quite been released...?? I recorded it - with some other faculty and students - at the University of Kansas in the fall of 1978, I'm thinking. We also recorded grades 2/3/4/5 versions of that and other pieces from the movie. )

ANYWAY...

I've played that "Main Title" thingie (busy tuba part) MANY times with a C tuba...but (as I've discussed relentlessly) I'm playing B-flat tubas, now...

...I typically play C below the bass clef staff with 1-3 (with the #1 slide pulled just about out to the slide stop, but not quite).

Playing those C's over-and-over forcefully, and with many re-articulations, I felt that slide "jump" under my fingers (of course, it's extraordinarily well-aligned, yet not loose...After all: It's my tuba...) every time I played one of those C's. The slide was trying to be BLOWN OUT farther, each time...

...so there certain IS something to the "air flow" thing, obviously - at least towards the small end of a tuba.

I guess I NOTICED the phenomenon, because...
- The #1 slide floats so nicely on this tuba.
- I've probably never played THAT MANY continuous and LOUD C's below the staff ever before on a B-flat tuba (whereby the pitch, "C", involves using the 1st valve (rather than "no valves down", as with a C tuba).

note:
I'm NOT attempting to set up an argumentative/debate type of thread (as I don't know very much about these sorts of things), but I am posting to ADMIT that there's more (and more to) "compressed" and "strong" (to use very non-scientific terms) air (again - at least, in the small end of even a VERY LARGE tuba) than that of which I was previously aware...

...but had my #1 slide alignment and fit been mediocre (or free-moving based on a loose fit, rather than being precisely fit) I still may have never noticed this phenomenon.

Just as a parenthetical post script...
The tuba (mentioned repeatedly above) does NOT belong in a museum/collection.
Further..
If (??) I'm hoarding B-flat tubas - so far all I have are this one, a stubby one that I built out of surplus junk, a half-completed/"just"playable Besson compensating/recording bell, and an extremely distressed old 14K sousaphone body waiting to be straightened out enough to receive a nicely-rebuilt Conn 36K valveset...(even though I would PREFER a King sousaphone)...and NONE of these (for the record) belong in a collection/museum, either.

Getting the embouchure to work (lips to buzz as desired) is an application of Bernoulli's principle to achieve what is called in physics simple harmonic motion.

As the airflow transitions the throat and backbore of the mouthpiece, the velocity decreases, and the resultant pulses in the air column transition into static wave theory.

The successful application of these concepts by the player are what result in what we perceive as pitch and timbre.

I dig...and (obvious enough to me, now) there's some friggin' air PRESSURE in the valveset of an (at least: at-the-moment - aggressively-played) tuba...even a very large and very large bore tuba...

...at least, when someone's got the gas pedal mashed down pretty far, and (' dunno if this has much to do with it...?) a bunch of very abrupt starts-and-stops of the air.

Also, I see a version for sale which has been transposed down to B-flat...Last night, I was definitely belting out a bunch of C's (though there were some intermediate key changes).

bloke "not arguing nor disagreeing...just reporting experiences"

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Interesting test

As a technician (or, for that matter, as someone who MAY have dipped their finger in a glass of wine and produced nice sounds rubbing over the glass´s edge, occasionally) one is familiar with stick-slip effect as well as static vs. sliding friction.

I´m not saying the following is THE explanation to what you observed, but it MAY contribute.
At least it looks plausible to me.

I assume those slide tubing surfaces are well honed/lapped for easy slide pulling.
Also, they will be well lubricated to the same end.
They will have ALMOST perfect cylindrical shape, close enough to do what they´re made for, but still...

At rest, SOME parts of the neighboring surfaces of inner and outer slide tubes will touch (static friction), other regions will form ever so tiny cavities to accept the lubricating fluid. Only fluid friction in those cavities.
Under the influence of the vibrating air column, there will be SOME resonant vibration induced to both inner and outer tube.
This MAY lessen the static friction of those touching surface regions.
Lubricant MAY even be redistributed, leading to the collapse of static friction altogether. (Big disclaimer here).

Add some raised inner pressure from You rigorously playing your heart out, and the slide may very well set sail and move as described.

If the slide's so free as to move on its own, at what point does that start to resemble a leak? In terms of failure to maintain pressure.

I´d argue that a leak occurs when air passes through the gap between inner and outer slide tube.
As long as a lubricating film is intact and no bubbles show up, no leak is occuring.

On a side note:
I can easily produce an air pressure exceeding 5000 N/m². (just tested blowing through a check valve against a 50mbar spring: Yep, that works.)
Half of that may be happening at loudest of my playing, so let´s assume 2500 N/m² of air pressure at the 1st slide.
That pressure, resting on the two circular cross sections of a tuning slide´s tubes of, say, 20mm diameter, will result in impressive 1.57 N of force.
That´s equivalent to a load of 157g or 0.35 lbs pulling on that slide.

Bloke being the good craftsman he is, I´d assume his 1st slide can move at forces below that.

donn wrote: ↑Wed Aug 23, 2023 1:13 am If the slide's so free as to move on its own, at what point does that start to resemble a leak? In terms of failure to maintain pressure.

They fit like a pair of aceptably-to-very-well-fitting pistons that are perfectly parallel. The valve is vented, so I had to put a stop rod on it as one of the things I did to this instrument, because it would tend to slide out - after I vented the valve - when rested on the bell, simply from gravity. I'm wondering if you're thinking backwards. If the pairs of tubes leaked more than a pair of really well-fit pistons (pistons being little more than slide tubes with stuff on them) why would air pressure push them out, and why wouldn't air just slip around them - due to their leaky fit?
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I'm getting some good information and ideas - here - about actually how much air pressure someone else can produce, and I've also received a good idea for an experiment in the very first reply.

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I'm reading/considering what you're telling me and I'm not questioning it, but I also know that that first valve slide is the first place on the instrument where are the tubing in the instrument moves really freely (again, as well as a pair of really nice feeling valves). I do have a nice-sliding fifth valve slide, but it's quite heavy, and I don't have it fitting surfaces to surfaces quite as exquisitely the first valve slide tubes (since the only time I might ever consider pulling it out is to play a genuine double low C (5234), rather than as a false tone).

The third slide on this instrument is actually on the back of the instrument and is fitted just as well as the #1, but has a spring auto-return (which mucks up any chance for free floating) and I actually push it down with my left wrist/forearm - not my fingertips, so I'm not feeling what might possibly happen sometimes with that slide if it ever would or does compared to what I'm feeling with the first valve slide which is between my thumb and middle finger - where I have lots of nerve endings.

Something else that's helping this #1 slide to jump - when I'm really thrusting air through it suddenly, is that (again) for that 1-3 C below the staff valve combination, the #1 slide is already nearly fully extended, so that is reducing its slide-tube-to-slide-tube friction/resistance even more than normal - making it even easier for that #1 slide to jump when I'm pumping out those Cs below the staff on that Superman theme.

I didn't think I was playing those fast triplets and "Valkyrie rhythm" Cs all that loud, but something tells me that - if the sixty-piece band were taken away, I was likely playing them louder than I suspect, because I naturally play as loud as I need to to make something sound balanced to my ears, and (again) I was the only tuba (as all four of their regulars had to miss that night).

Peterbas wrote

The pressure isn't resting on the cross-section of the slide,

I believe one can trust me on the phenomenon I described, but nobody needs to take my word for it.

As was correctly observed, there will be a pressure distribution along the tubing of a tuba as air flows through.
At the MP this pressure will be highest, gradually decreasing as it approaches the bell, where obviously will reach its lowest value being atmospheric pressure. (Or, in other terms, "Zero")

I also agree that it is perfectly fair to say that the pressure difference between entry and exit of 1st valve slide is tiny enough to be neglected. In fact, in my abbreviated calculation, I did the same.

However, engineering has means to calculate the mechanical stresses induced in the walls of a pressurised vessel (i.e. a tube)
Tangential stress wants to increase the diameter of the tube.
Axial stress wants to increase the tube´s length.

Whether or not the fluid exerting that pressure is moving or at rest, doesn´t matter: The slide can´t tell the difference.
Whether or not the ends of that tube are open won´t matter, either: the slide can´t tell if there´s a sealing lid attached to the bell.
All that counts is that at a certain spot along that tubing, there´s a pressure acting on its surface.

The following is the German Wikipeda entry dealing with that subject.
https://de.wikipedia.org/wiki/Kesselformel
(Sorry, the english version of that page only says "Barlow´s equation" and won´t explain this in detail, so bear with me)...
Scroll down to where it says "Mindestwanddicke".
Right before that you´ll find the following equation:

Sigma(a) x A(Q) = p x A(G,proj)

"Sigma" is a stress, meaning a force "F" applied to a material surface area "A(Q)" (in our case: a ring surface of a brass tube, roughly 20mm in diameter, with a wall thickness of, say, 0.5mm).
The general formula to calculate ANY stress reads: Sigma = F/A.
So whenever You see a term saying "Sigma x A" (as in that formula taken from Wikipedia) that´s just that basic formula resolved for its force F.

Something extremely similar applies to the other side of that formula:
Pressure "p" equals force "F" applied to a surface "A(G,proj)".
Imagine the slide tubing being a hydraulic cylinder with a piston rod of surface area A(G,proj) stuck into it.
Pushing on that rod with force F will create a pressure inside that cylinder: p = F/A(G,proj).
So whenever you see a term "p x A", that´s just the basic pressure formula resolved to calculate the force needed to produce that pressure.

The formula "Sigma(a) x A(Q) = p x A(G,proj)" can therefore be translated to speech as follows:

The axial force pulling the slide tubes apart equals the pressure inside the slide, multiplied by the cross section created by the tube´s inner diameter.

well...

I understand a small bit of that.

I wish I had been an engineering/tech major and a music minor...

...I've never found that I needed to spew biographical info re: Johannes Cesaris, but have had a good bit of need for informal technology/engineering...
Thankfully, most of the things that I've built or "figgured up" (from structures to instrument slide triggers) have been darn good instinctive guesses at either what's "plenty" or what's "enough".

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I have the impression that there may be some misunderstanding, and everyone agrees that pressure difference between inside and outside the slide will be negligible, as a consequence of blowing through the tuba.

What I don't really understand, though, is the sound energy produced within the tuba, and how that might or might not exert force inside. Though not much air comes out, pressure waves do come out with enough physical energy to rattle the china cabinet. In my simple minded idea of how that works, if there's any force there that would push the slide out, in the other half of the cycle it would draw the slide back. But the slide might not be equally free to move in both directions.

You don't feel the slide jump when you just blow in the tuba, right? Especially if you blow at about the same rate that you do if you're playing loud low Cs.

This seems like an AC vs. DC thing. As somebody else pointed out, it's a pretty big tube that gets bigger at the far end, so when you blow steadily into it (DC) there's an imperceptible pressure across any part of the tubing. The tuba presents little resistance to direct air flow, and most of that is at the mouthpiece.

When you play a note, the plain expanding tube can definitely have resistance at resonant frequencies (AC), so you do see (feel) pressure differences, even though there's still very little resistance to the direct air movement.

I haven't done any of the experiments yet. I can post when I'm dead tired - to try to calm my brain, but when but when my arms and legs and back have enough energy, I need to get back out there in the shop and swing hammers at horns. I'll get around to those experiments that were suggested above in this thread.

All I know so far is that undeniably the first slide was jumping every time I started another one of those low C's- none of which lasted but an instant - due to the tempo and velocity of the events.

Considering that the tuba is going from no signal to lots of signal to no signal again, akin to a binary…

https://en.m.wikipedia.org/wiki/Gibbs_phenomenon

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peterbas wrote: ↑Fri Aug 25, 2023 6:38 am And this is relevant to this topic how?

I believe he's referring to how those pitches are being pulsed on and off so quickly (playing that particular piece) and how it might possibly be a factor.