Venturi - again.

[2ba4t]

Please could someone help me with the VGR (Venturi Gap Receiver) It completely baffles me.

If you squeeze a hose pipe end with your thumb, the water squirts further. A brilliant physicist called Venturi first realised all the implications of this. Your thumb has created a [Venturi] restriction, the water travels faster through the narrowed gap and therefore can squirt further. Just behind your thumb, nearer to the tap, the water is pushing to get through and the pressure is high. Just past your thumb only a narrow (even though fast) stream of water is travelling through, but the restriction (your thumb’s pressure) is over and the pressure within the water drops, as it sprays out of the hose. See https://www.youtube.com/watch?v=oUd4WxjoHKY" target="_blank" target="_blank. If you have a strong stomach you could watch https://www.youtube.com/watch?v=Qz1g6kqvUG8" target="_blank" target="_blank.

Now how does this apply to us? We make a buzz in our mouthpiece and the narrowing of the mouthpiece creates a Venturi effect. It gradually narrows to that small hole and then gradually opens up along the shank of the mouthpiece. The restriction speeds up the movement of the sound wave – the shank slows it down.
It is vital to know that gradual changes in bore create far less ‘turbulence’ and irregularities of the sound wave than a sudden step up or down.
BUT there is, as I understand it, something also called the ‘Venturi’ in our brass players’ world. This is the internal distance, within the mouthpipe, across it- immediately after the inserted mouthpiece shank is ended, but before the inner lining of the mouthpipe starts. This gap has of course a sharp edge at the end of the mouthpiece and at the beginning of the lining. One would imagine that a perfect fit would be best. That is that the end of the mouthpipe shank slides in exactly enough to meet the internal edge of the mouthpipe lining which is meant to meet it, seamlessly. Then there is no inconsistency in the air column.

However this is rarely the case. But do you notice a difference with different mouthpiece shanks which must go in different distances? What puzzles me is that some have made a virtue of this, stating that this gap is very important to our playing. A longer gap is said to make the notes slot very firmly but restrict slurring up and down. A shorter gap is said to improve tone and slurring etc. See https://www.youtube.com/watch?v=8xqK4LHY6Ao" target="_blank" target="_blank

Now this really puzzles me. First, Venturi emphasises that disruption occurs if the narrowing is a sudden one. An example would be a simple solid cylinder wedged into a pipe. This blocks the entire flow. Then you drill a hole through that cylinder. Upstream there is ‘normal’ pressure and slower speed. Just before the cylinder, there a sudden rise in pressure as the passage is partially blocked. In that tube through the cylinder, the water travels faster, but at a lower pressure. Just past the cylinder there is massive turbulence within the suddenly released air.

Surely we need the end of the mouthpiece shank to butt seamlessly up against the internal mouthpipe lining, to avoid any ‘Venturi’ gap.

Indeed why do our mouthpipes not have absolutely no stepped lining end, but rather a smooth blending to the internal mouthpipe walls just about where our mouthpiece shanks end. Then our mouthpiece shanks could similarly taper to a fairly thin (but tough enough) end which would not create any ‘step’ as they sit in the mouthpipe.

Extremely grateful for all I have learnt from you all so far. I wish I had done engineering and stuff.

[timothy42b]

We make a buzz in our mouthpiece and the narrowing of the mouthpiece creates a Venturi effect. It gradually narrows to that small hole and then gradually opens up along the shank of the mouthpiece. The restriction speeds up the movement of the sound wave – the shank slows it down.

That seems intuitively logical but it isn't really what happens. The movement of the sound wave and the air flow are two different things. The air flow is so slow you may consider it essentially stopped. So I don't think there is any way you can figure this out by physics. I think you need to experiment.

And there is the difficulty. Our playing varies from day to day and probably note to note on a bad day, and our perceptions are fraught with error. Some people "clock" their mouthpieces, marking them and then trying different rotations. Others worry about the gap. Others play a mouthpiece until it wears out, trying to learn how to get the best out of it. That's my strategy. I'm now back on the mouthpiece that came with the horn in 1971.

[Matt Walters]

Please could someone help me with the VGR (Venturi Gap Receiver) It completely baffles me.

This would be a 2 beer long discussion. Highlighted points would be:

1) Air velocity when playing, is below the speed of sound. Your air is just used to create the buzz on the lips that create a sound. The mouthpiece modifies the shape or tonality of the sound and condenses it into a very dense buzz that the rest of the horn amplifies.
2) Subsonic, a constriction in a Venturi raises the air velocity but up to a max of sonic. (The exhaust nozzle of a subsonic airliner gets smaller.)
3) Sonic and faster, an expansion from the Venturi is what it takes to increase the air velocity. (The exhaust nozzle of a rocket flairs out wide.)
4) Since you can't blow your air at the speed of sound, (Though many people play loud and obnoxious like a sonic boom.) the Venturi effect happening in the mouthpiece and mouthpiece receiver via gap control is basically just helping you shape the sound wave via sensory feedback so that the giant megaphone called your tuba or Euphonium can amplify it.
5) So if you have a heavier or lighter than average articulation style, and/or the mouthpiece you choose to play creates problems, you can negate some of that by changing the gap in the receiver area to get a cohesive sound wave entering the leadpipe to be amplified.

Many, many more fine points could be discussed here, but I am aware that individual perception and/or ignorance will always trump logic and science in topics like this posted online. If someone wants to debate my ideas, please contact me to schedule a time when we can hang so that you can pay for dinner and drinks to discuss this further.

[Reptilian]

My head hurts. I'll be in the shed.

[timothy42b]

Please could someone help me with the VGR (Venturi Gap Receiver) It completely baffles me.

4) Since you can't blow your air at the speed of sound, (Though many people play loud and obnoxious like a sonic boom.)

Sound is defined as an infinitesimal pressure increase over atmospheric. But (and I don't understand this part) there are also times when a shock wave can exist in an instrument. Bloke can put out matches with a tuba blatt, so my guess is that this is what he's doing. Surely no one can blow air through a tuba hard enough to do that. Anyway, a shock wave is a temporary exceedance of the speed of sound, isn't it?