Back pressure in a flute

[Cork]

...My question is whether there is any disadvantage of having a flute that is this open? Is this flute going to take more air to play than a similar flute that offered more resistance, perhaps with a smaller embouchure hole, smaller bore or longer foot joint?

In general, a "larger" embouchure hole better allows for the possibility that more air COULD go through it, BUT not necessarily MUST go through it. That is, a "larger" embouchure hole generally gives a player more in the way of air stream options.

OTOH, however, the availability of such options, or flexibility, also calls for a greater embouchure ability on the part of the player, to control the air stream.

So, perhaps a "smaller" embouchure hole could provide a beginner with success more readily than a "larger" embouchure hole, but perhaps a more advanced player could be able to take advantage of a "larger" embouchure hole, to greater effect.

After all, look at the size of the embouchure hole on a typical Boehm flute, and yet a Boehm flute can speak in the softest of whispers while using very little air, and all while in fine tune.

[cadancer]

My original query has to do with the design of cylindrical-bore flutes, where I have a personal interest with my pvc flutes. Say, for example, you have a 6-hole cylindrical-bore flute with a bore diameter of 20 mm and an 10.5mm x 12mm oval embouchure hole with a rather aggressive embouchure cut (letting in a lot of air). Now this flute is not going to offer much in terms of resistance. My question is whether there is any disadvantage of having a flute that is this open? Is this flute going to take more air to play than a similar flute that offered more resistance, perhaps with a smaller embouchure hole, smaller bore or longer foot joint?

Kavals are cylindrical-bored flutes. The embouchure "cut" is the whole end of the pipe (my "C" kaval is 18 mm in diameter). That makes a very large embouchure "cut". It does take more air to play than my "D" kaval which is about 17 mm in diameter and shorter from end to end.

However, as I gained in experience the "C" became perfectly manageable to play, and indeed, I basically stopped playing the "D" unless I needed to play with other musicians. At first, I never thought that I would be able to produce enough air to get a decent sound on the larger flute. That turned out to be wrong.

One thing that has not been mentioned is that air velocity can impact the sense of resistance. I find that a smaller mouth embouchure can be used to focus the same volume of air with a higher velocity. I don't know the physics behind what that would do in a flute. But, it feels like there are "sweet spots" to hit on flutes that respond to higher velocity, tighter focused air flow with very nice sounds.

Right now, my gorgeous McGee Pratton feels like it takes more air to play than my kavals. That, I am certain, will go away as I improve my mouth's embouchure. So, Doug, I think the embouchure cut you are suggesting may "feel" like it needs more air at first and thus may be more difficult to learn to play. I would guess that once a player got used to it, that it would not pose the same difficulties.

Beta testing ?

All the best,

John

[Cork]

...My question is whether there is any disadvantage of having a flute that is this open? Is this flute going to take more air to play than a similar flute that offered more resistance, perhaps with a smaller embouchure hole, smaller bore or longer foot joint?

In general, a "larger" embouchure hole better allows for the possibility that more air COULD go through it, BUT not necessarily MUST go through it. That is, a "larger" embouchure hole generally gives a player more in the way of air stream options.

OTOH, however, the availability of such options, or flexibility, also calls for a greater embouchure ability on the part of the player, to control the air stream.

So, perhaps a "smaller" embouchure hole could provide a beginner with success more readily than a "larger" embouchure hole, but perhaps a more advanced player could be able to take advantage of a "larger" embouchure hole, to greater effect.

After all, look at the size of the embouchure hole on a typical Boehm flute, and yet a Boehm flute can speak in the softest of whispers while using very little air, and all while in fine tune.

Therefore, I'd suggest that you make two models, one having a beginner friendly "small" embouchure, and another having a full tilt boogie "large" embouchure.

Beginners should know who they are, and should place an order for a flute more closely matching their abilities, while those others who eat flutes for breakfast, for instance, could have an option.

The best of both worlds, so to speak.

[Jon C.]

My original query has to do with the design of cylindrical-bore flutes, where I have a personal interest with my pvc flutes. Say, for example, you have a 6-hole cylindrical-bore flute with a bore diameter of 20 mm and an 10.5mm x 12mm oval embouchure hole with a rather aggressive embouchure cut (letting in a lot of air). Now this flute is not going to offer much in terms of resistance. My question is whether there is any disadvantage of having a flute that is this open? Is this flute going to take more air to play than a similar flute that offered more resistance, perhaps with a smaller embouchure hole, smaller bore or longer foot joint?

Kavals are cylindrical-bored flutes. The embouchure "cut" is the whole end of the pipe (my "C" kaval is 18 mm in diameter). That makes a very large embouchure "cut". It does take more air to play than my "D" kaval which is about 17 mm in diameter and shorter from end to end.

However, as I gained in experience the "C" became perfectly manageable to play, and indeed, I basically stopped playing the "D" unless I needed to play with other musicians. At first, I never thought that I would be able to produce enough air to get a decent sound on the larger flute. That turned out to be wrong.

One thing that has not been mentioned is that air velocity can impact the sense of resistance. I find that a smaller mouth embouchure can be used to focus the same volume of air with a higher velocity. I don't know the physics behind what that would do in a flute. But, it feels like there are "sweet spots" to hit on flutes that respond to higher velocity, tighter focused air flow with very nice sounds.

Right now, my gorgeous McGee Pratton feels like it takes more air to play than my kavals. That, I am certain, will go away as I improve my mouth's embouchure. So, Doug, I think the embouchure cut you are suggesting may "feel" like it needs more air at first and thus may be more difficult to learn to play. I would guess that once a player got used to it, that it would not pose the same difficulties.

Beta testing ?

All the best,

John

Hi John,
How have ya been? Been 2 years? Annvil finely gave the Mcgee flute back...
I was thinking of making a test head that would have four different embouchures, the outer piece would rotate on the sleave, and click in to the desired emb. hole. The sleave would have one hole, and maybe isolated with O rings? Just a thought. I set up my CNC Mill to cut the embouchure profile, so i can have lots of different shapes to choose from. Click in the Wylde emb, or the Meyers square emb... click, click!
As far as back pressure, I prefer the long foot, that will give a nice back pressure.

[kkrell]

Jon, you just need a snap in piece with the embouchure hole cut into it.

[Jon C.]

Jon, you just need a snap in piece with the embouchure hole cut into it.

Ya, I remember that idea, but you would loose the little bits, leave them at the pub, etc. Well, when I repair my cooked CNC drivers, (overheated cutting all those concertina parts I will cut out some inserts, but how will they stay in the head? Threaded? set screw? bluetac? With the rotisory idea, you just dial in the choosen emb. hole, click, click click.
In the 70's they came up with a surfboard, that the tail had adjustable rails, it was a great idea, but weighed a ton! Kind of like the patent head...

[Maihcol]

Well, when I repair my cooked CNC drivers, (overheated cutting all those concertina parts I will cut out some inserts, but how will they stay in the head? Threaded? set screw? bluetac?

I've tried to simplify making up heads/embouchures to cut back on the work involved and make those embouchure design tests quicker. The problem I find is that even the slightest, vaguest hint of a leak up at that end of the flute knocks back the performance - a leak which would have little or no noticeable effect further down the instrument. I've been thinking of this as due to a pressure differential in the bore of the flute - high at the top end and quickly dropping off lower down...

The more bits involved, the more joints which can leak - even with all that beeswax, plumbers tape...and then you can't be sure where you are - whether that's how that particular embouchure cut really is or not...Another thing that happens is that you end up having to do more suction tests on the embouchure hole and with the lips getting sucked down tight into that little hole, they get all out of shape and that can give a false impression too when blowing the flute after...

I sometimes make up a short head section with the embouchure hole and which has a tenon on it which can plug into a socket in the lower part of the head - it's just a tenon/socket joint below the embouchure and saves having to make up the whole head and slide for each embouchure test. I find delrin works best for this as a simple delrin to delrin joint can make a perfect seal even without an o-ring. I haven't tested it enough but I suppose it could be a way to have a single delrin head and slide with various plug-in embouchures - a bit cheaper than having a full head for each embouchure.

Garry

[Terry McGee]

I've tried to simplify making up heads/embouchures to cut back on the work involved and make those embouchure design tests quicker. The problem I find is that even the slightest, vaguest hint of a leak up at that end of the flute knocks back the performance - a leak which would have little or no noticeable effect further down the instrument. I've been thinking of this as due to a pressure differential in the bore of the flute - high at the top end and quickly dropping off lower down...

Garry

The odd thing here is that the pressure at both ends of the flute should be zero (they are open to the atmosphere) and the highest pressure (not that it ever gets very high) is at the centre (in low octave notes), or distributed along the tube (for the harmonics). So, in theory, some leakage around the embouchure shouldn't do any harm - it's open anyway. But I agree it does. It's possible that the leakage is creating some effect other than acoustic - perhaps screwing up the jet aerodynamics.

I think I showed this image of experimental heads and embouchures made for Neville Fletcher that I have here:



Terry

[Maihcol]

The odd thing here is that the pressure at both ends of the flute should be zero (they are open to the atmosphere) and the highest pressure (not that it ever gets very high) is at the centre (in low octave notes), or distributed along the tube (for the harmonics).

I expect you're right about this Terry...I'd just been thinking of it in terms of the general flow of air down through the instrument and how that flow would need to have some sort of pressure gradient in the bore in order to happen.

Those embouchure tubes look like they have two embouchure holes in them which is handy, just rotate them into position. I'm not sure I'd like the bit where you have to bore the taper on the inside so that the hole in the wee plastic tube sits precisely over the hole in the metal head. If you really got into it you could make up little reamers for these though...I wouldn't be confident that the plastic tube would make a reliable seal with the metal - I'd be tempted to make these longer - but I suppose these must have worked if they were made up in a university lab. The other thing that would bother me would be the metal step in the bore all around the embouchure hole and the possible turbulence.

Garry

[Terry McGee]

I expect you're right about this Terry...I'd just been thinking of it in terms of the general flow of air down through the instrument and how that flow would need to have some sort of pressure gradient in the bore in order to happen.

It's probably fairer to say the flow creates the pressure in this case. (That's equally legitimate - how does the pressure get into the balloon other than by flow.)

Blow air into the flute carefully avoiding the edge and all you get is a slow flow of air from one end to the other. But let the jet engage the edge, and ka-boom, suddenly we have action. Resonance occurs in the tube and interaction occurs with the jet, and a high alternating pressure forms in the middle. A graph of the pressure along the flute for the fundamental note resembles the skipping rope held by two girls in the playground. No significant displacement at the hands, maximum displacement halfway between the girls. And you've no doubt seen what happens when they swing the rope harder - it breaks into harmonics. Ditto the flute. The next harmonic has no displacement at the hands or the middle, and two maxima at the quarters, in opposing phase (one positive while the other is negative).

Those embouchure tubes look like they have two embouchure holes in them which is handy, just rotate them into position. I'm not sure I'd like the bit where you have to bore the taper on the inside so that the hole in the wee plastic tube sits precisely over the hole in the metal head. If you really got into it you could make up little reamers for these though...I wouldn't be confident that the plastic tube would make a reliable seal with the metal - I'd be tempted to make these longer - but I suppose these must have worked if they were made up in a university lab. The other thing that would bother me would be the metal step in the bore all around the embouchure hole and the possible turbulence.

Garry

Yes, keep in mind that these were teaching tools for Neville to use in his acoustics courses, so they only had to illustrate the broad principles, which they do well. He has several holes of differing sizes on each perspex ring, and different rings for different shapes.

Terry

[Doug_Tipple]

I am just thinking out loud here and not trying to pontificate to strenuously on a subject where I am not an expert. Clearly, the unplayed flute being open on both ends has a zero internal pressure differential. The pressure everywhere is atomospheric pressure. However, once a flow of air from the mouth enters the embouchure hole, whether or not it excites a note in the flute, that flow causes a small pressure gradient in the flute, allowing the air to enter and pass through the flute. You can say that the flow of air from the mouth sets up the pressure differential in the bore of the flute and that the pressure differential allows the flow of air to pass through the flute. I think that both statements are equally true.

An analogy of another common open system is the earth's atmosphere. There are areas of high and low pressure, and the winds blow from the high pressure to the low pressure areas. However, as the wind blows into the low pressure areas, the pressure in those areas increases causing a reduction in the velocity of the wind. It is a dynamic system with all forces interrelated. The flute, of course, is not as complicated as the earth's atmosphere, but it does share some of the same characteristics, where moving air is both the cause and effect of pressure.

[Terry McGee]

I am just thinking out loud here and not trying to pontificate to strenuously on a subject where I am not an expert. Clearly, the unplayed flute being open on both ends has a zero internal pressure differential. The pressure everywhere is atomospheric pressure. However, once a flow of air from the mouth enters the embouchure hole, whether or not it excites a note in the flute, that flow causes a small pressure gradient in the flute, allowing the air to enter and pass through the flute. You can say that the flow of air from the mouth sets up the pressure differentialAgreed, but that in the bore of the flute and that the pressure differential allows the flow of air to pass through the flute. I think that both statements are equally true.

Agreed, but when we ignore the excitation we are only looking at the "DC flow". High pressure in mouth, small gap in lips, high speed jet (but not much flow) into air near flute, some dissipated immediately into the surrounding air, small flow of air through flute into air at other end. We're much more interested in the alternating or AC flow that occurs when the jet engages the edge and subsequently excites a resonance in the tube.

The flute, of course, is not as complicated as the earth's atmosphere ....

Hmmm, I'll need to get back to you about that!

Terry (who has been seriously exceeding the World Health Organisation's Maximum Daily Recommended Limits on flute acoustics over the last few months)

[david_h]

We're much more interested in the alternating or AC flow that occurs when the jet engages the edge and subsequently excites a resonance in the tube.

I'm getting confused. Doug's OP was about back pressure. Are you saying that that is not at all related to DC flow ? Possibly getting my sciences mixed up but I think the term 'pressure' has different shades of meaning between fluid dynamics (? DC) and acoustics (? AC ).

So is back pressure the 'blow air through lips' type pressure at the point at which some desired acoustic state is reached ? Such that a change of geometry way down the flute that won't have much affect on the fluid flow but will have an acoustic effect that will change the amount of pressure we need to apply ?

Just been experimenting blowing air through different sized holes punched into a piece of flat cardboard at various distances. The change in sensation even doing something so simple is hard to describe.

[Jack Bradshaw]

I'm getting confused. Doug's OP was about back pressure. Are you saying that that is not at all related to DC flow ? Possibly getting my sciences mixed up but I think the term 'pressure' has different shades of meaning between fluid dynamics (? DC) and acoustics (? AC ).

Yes, you have static pressure differences w/ steady flows (DC), dynamic differences w/ acoustic resonances (AC) and then there is "sound pressure" on an object due to the reflection/absorption of the momentum of phonons (or alternatively the force due to the entropy change due to waves "breaking" at that surface...the old "why is the water higher beyond the breakers physic question".....take your pick) Lips w/ flow are a pneumatic sensor for all three!

[david_h]

Lips w/ flow are a pneumatic sensor for all three!

Not sure I am following the three. There is pressure because of the number of molecules packed into given volume - in this context I think we mean there are more of em than if we weren't blowing. And then there is something because they are dashing back and forwards in a somewhat synchronised way - at low frequencies we feel that as vibration. There is also something that indicates flow (a temperature effect ? turbulence ?). But I don't think that is your third. Am I combining two ?