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PostPosted: Mon Nov 25, 2019 1:28 am 
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Loren wrote:
For those interested, perhaps this will help clarify:

http://www.flute-a-bec.com/acoustiquegb.html

Thanks Loren, that is an interesting article - I will need to digest it in full this evening.

Quick question -- is it implying that the returning pressure wave in the tube is (a part of) the cause of the switch of direction of the airstream?

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PostPosted: Mon Nov 25, 2019 7:51 am 
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DrPhill wrote:
Loren wrote:
For those interested, perhaps this will help clarify:

http://www.flute-a-bec.com/acoustiquegb.html

Thanks Loren, that is an interesting article - I will need to digest it in full this evening.

Quick question -- is it implying that the returning pressure wave in the tube is (a part of) the cause of the switch of direction of the airstream?



I have a short answer and a longer answer, but the shorter is all I have time for before work, so here it is:

I think that yes, it would seem what you suggest is being implied, however I’m not entirely certain. If I have the time/energy later today I will try to elaborate. That said, I definitely don’t have all the answers due to my lack of degrees in fluid dynamics, particle physics, and acoustical engineering :sniffle:


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PostPosted: Mon Nov 25, 2019 9:56 am 
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DrPhill wrote:
Loren wrote:
For those interested, perhaps this will help clarify:

http://www.flute-a-bec.com/acoustiquegb.html

Thanks Loren, that is an interesting article - I will need to digest it in full this evening.

Quick question -- is it implying that the returning pressure wave in the tube is (a part of) the cause of the switch of direction of the airstream?
Based on my reading of the research, the answer is an unqualified, "Yes."

A note about the source... It seems to me, Philippe Bolton has contributed enough to PhD theses to earn one for himself. In particular, he sat on the jury for the 2009 thesis of François Blanc.


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PostPosted: Mon Nov 25, 2019 12:58 pm 
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Thanks both forconfirming the impression I had reading this over breakfast. I have been trying to sort this out in my head, and get the physics straight. I am aware that any explanation is an approximation butstill have a few niggles in my head.

First, I am completely comfortable with the idea that the vibrating air column in the recorder can influence the oscillation of the air across the labium in a way that reinforces the air column vibration. A returning pressure wave will could easily push the blown airflow from the inside to the outside of the labium. It could also be argued that it is the change in airflow across the labium that drives the vibrating air column, but that makes it more difficult for me to see how the frequency is controlled by the open holes. My guess is that the two effects are locked into a coherent whole where neither entirely causes the other.

What I am trying to to figure out how this oscilation starts in the first place. I cannot see the air column in the recorder playing much part - so it must be the direction-switching nature of the airflow across the labium. What causes this? It is a bit like the starter motor on an infernal combustion engine - needed to get things going but irrelevant afterward?

Or am I lookig to deeply?

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PostPosted: Mon Nov 25, 2019 1:47 pm 
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DrPhill wrote:
Thanks both for confirming the impression I had reading this over breakfast. I have been trying to sort this out in my head, and get the physics straight. I am aware that any explanation is an approximation but still have a few niggles in my head.

First, I am completely comfortable with the idea that the vibrating air column in the recorder can influence the oscillation of the air across the labium in a way that reinforces the air column vibration. A returning pressure wave will could easily push the blown airflow from the inside to the outside of the labium. It could also be argued that it is the change in airflow across the labium that drives the vibrating air column, but that makes it more difficult for me to see how the frequency is controlled by the open holes. My guess is that the two effects are locked into a coherent whole where neither entirely causes the other.

What I am trying to to figure out how this oscillation starts in the first place. I cannot see the air column in the recorder playing much part - so it must be the direction-switching nature of the airflow across the labium. What causes this? It is a bit like the starter motor on an infernal combustion engine - needed to get things going but irrelevant afterward?

Or am I looking to deeply?
Second question first: the air stream from the windway is inherently unstable. Any little wobble--and there will always be wobbles--will grow as the stream passes across the windway. The dominant wobbles tend to be up and down (in and out of the window). Section 2.2.1 of Patricio de la Cuadra's thesis describes this, and points all the way back to Lord Rayleigh.

The wobble at the window will propagate down the bore. The bore resonances, which the open tone holes control, will reinforce wobbles that have certain frequencies, until the whole system is oscillating at one of those frequencies.


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PostPosted: Tue Nov 26, 2019 1:05 am 
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Thanks again everyone, for your comments and links. I swear I had a Quena-style flute from an importer about 10 years ago, but it was a cheapy ($12?), nobody at the store had any idea how it worked. I worked at it quite some time and realized I had to block the end of the flute with my lower lip/chin to form a partial seal, and also allow a space to remain between the lip and the blade area. Unfortunately the one I had was probably made as a toy, was high pitched, the intonation was off and the blade area didn't look very well made. It was bamboo. But I liked the type of tone I heard.


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PostPosted: Thu Nov 28, 2019 9:42 pm 
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I've seen several labia that are 'flat' in that they slope in the same direction on the top as on the bottom.

Look at how a basic clarke is made-- the metal tube is folded in to create the labium. The inside edge of the labium slopes toward the inside of the tube, and the outside edge of the labium also slopes in toward the center of the tube. I've made a few 3d printed whistle heads with a labium profile so that toward the window, the top edge slopes inward and the inside edge slopes outward. They're breathy and quiet. I would like to try and make some more that are sloped more like a folded tube whistle.

The feadog whistle I have, as well as the clarke sweetones and generations, follow the slope pattern you'd see in a folded metal tube whistle like a classic clarke. Recorders are often built with the same pattern. The stream is split but both directions of air travel tend to push the air toward the outside of the tube. I suspect this is key to generate more outward air pressure and volume. This inward-sloped labium profile is impossible to make on a bamboo endblown flute without generous carving, but a transverse flute has a similar relation to your breath as directed by your lips. A quena is much easier to make undercut, and I remember seeing a plastic recorder with an undercut inside edge once I sawed the beak off.


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