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A Proposal For A Bi-Directional Concertina Reed


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#1 rlgph

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Posted 04 February 2015 - 12:49 PM

Now that i think i understand the physics of operation of concertina reeds (thanks to the back-and-forth with Lukasz) i have a proposal for a reed assembly that i believe will sound on either the pull or push cycle.  If i have not overlooked something (which i'm sure you'll let me know), it should sound pretty much like a usual concertina reed in either case, though it probably won't sound quite like a traditional reed.

 

Moreover, as you will see, it would be more difficult to precisely tune with filing/scraping, so may well not be practical.  I'll let someone with reed making/tuning experience address that issue.

 

So on to the description.  Take a usual (though it may need to be thicker or thinner in practice) reed frame and file part of the inside end down at an angle, as shown in the cross section of Fig 1 in the picture below.  Repeat with a second frame in such a way that it is a mirror image of the first.  Now sandwich a flat reed tongue between the two frames as shown in Fig 2.  The left end of the reed is clamped between the two frames, and the other end is free to oscillate.  Note that when the reed is in its equilibrium position (as shown in Fig 2) there is an air passage between the top and bottom at the free end of the reed tongue.  The setting of the free end of the reed is such that it comes very close to the top and bottom frames at the end of the sideways V so that it will effectively cut any air flow occurring from top to bottom or vice versa.  (In actuality, the V might need to be shaped like and arc.)

BiReed.jpg

This reed, when placed in the appropriate position in the reed pan, should operate pretty much the way a standard reed does.  Imagine the bellows is above the reed in the following description.  As it is squeezed with the appropriate button pushed, the pressure inside the bellows is increased above the ambient air pressure outside the concertina.  This pressure difference is not enough to cause the reed to move significantly, but it is enough to cause air to flow from the top to the bottom.  As this flow of air passes along the bottom of the reed it causes a partial vacuum on the lower side of the reed tongue, so that the pressure difference between the top and bottom of the reed is substantially increased, causing the tongue to move downward.  It will continue to move downward until the air flow is effectively cut off as the reed end comes very close to the inside end of the lower frame.  (Assuming that the frames are such that this cutoff occurs before the spring force gets too large.)  When the air stops flowing, the partial vacuum is no longer maintained and the force due to air pressure is greatly reduced.  The internal (spring) force of the reed tongue will eventually bring the motion to a stop, and then push the reed back toward equilibrium.  As it does, the air passage is re-opened and the pressure difference will increase substantially again.  At some point above the equilibrium plane the force due to pressure and the restoring spring force will bring the reed to a stop and cause it to move back toward equilibrium.  The reed will continue to cycle as long as the button is pushed and the bellows is squeezed.

 

Note that there is an asymmetry in the operation.  When the tongue is below its equilibrium position, the spring force is acting upward and that due to air pressure downward (with the latter decreasing as the air flow slows).  That is, they act in opposite directions. When the tongue is above the equilibrium position, the two forces act in the same direction (downward).

 

If the bellows is drawn (pulled), the process is pretty much the same except the greater pressure will be below the reed and the initial air flow will be from bottom to top, causing the partial vacuum on the upper side of the tongue.

 

Note that if you want the air flow to be cut twice per cycle you need to adjust the thickness of the frames and the V-shaped cutout so that the reed end passes very close to the frames at both ends of the oscillation.  In this case, though, the frequency would be twice what it would be if the cut is made only on the low pressure side.


Edited by rlgph, 04 February 2015 - 05:36 PM.


#2 Łukasz Martynowicz

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Posted 04 February 2015 - 02:33 PM

I must say I was expecting something like this exactly, as this design was the first idea that came to my mind when you wrote about bidirectional reed :)

 

I have only one, but probably "end game" note on this design: you have now effectively two flow cutting points - one in each half-cycle of the swing motion. So while this type of reed will probably vibrate, it will probably has a very low amplitude (only around the central V inset), probably in almost inaudible range. The "agaist the airflow" (I'll call it upper) cutting point works effectively as a very efficient damper. In traditional reed, in the stable oscilation phase, the upward movement above the shoe is almost unrestricted (as we have agreed, that pressure force is much smaller than spring force) and an airflow around the tongue is almost free, and downward movement is accelerated by suction caused by airflow. But now you have introduced a part in the cycle that counteracts the resonant effect which drives the traditional reed. After achieving the upper flow cutting point any further increase in amplitude is IMHO impossible with this design.



#3 rlgph

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Posted 04 February 2015 - 05:33 PM

I must say I was expecting something like this exactly, as this design was the first idea that came to my mind when you wrote about bidirectional reed :)

 

I have only one, but probably "end game" note on this design: you have now effectively two flow cutting points - one in each half-cycle of the swing motion. So while this type of reed will probably vibrate, it will probably has a very low amplitude (only around the central V inset), probably in almost inaudible range. The "agaist the airflow" (I'll call it upper) cutting point works effectively as a very efficient damper. In traditional reed, in the stable oscilation phase, the upward movement above the shoe is almost unrestricted (as we have agreed, that pressure force is much smaller than spring force) and an airflow around the tongue is almost free, and downward movement is accelerated by suction caused by airflow. But now you have introduced a part in the cycle that counteracts the resonant effect which drives the traditional reed. After achieving the upper flow cutting point any further increase in amplitude is IMHO impossible with this design.

In a conventional reed, as well as in this design, the maximum distance of the oscillation from the equilibrium position on the high pressure side is smaller than on the low pressure side because on the high pressure side both the pressure force and the restoring spring force act together to bring the motion to a stop, whereas on the low pressure side (after the air flow is cut off), only the spring force brings the motion to a stop.  Thus, if the frame thicknesses and the V are designed carefully, the reed tip won't reach the cutoff on the high pressure side and the behavior should be essentially the same as in a conventional reed, with the same frequency, and so far as i can see, the same amplitude.

 

(I am going to correct my statement about frequencies in the previous post since it was incorrect.)



#4 Bruce Thomson

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Posted 04 February 2015 - 08:02 PM

Great to watch this, folks.

If it worked it could bring a revolution in concertina compactness, lightness, simplicity of construction, and cost if it worked.

Maybe set up a small box rig that its whether a reed would work that way

Especially note what happens when the push changes to pull on the same note

- I suspect the reed will be 'slow', not responding until the air current fully changes.

Bruce Thomson in New Zealand.



#5 rlgph

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Posted 04 February 2015 - 09:43 PM

Great to watch this, folks.
If it worked it could bring a revolution in concertina compactness, lightness, simplicity of construction, and cost if it worked.
Maybe set up a small box rig that its whether a reed would work that way.

If it works, indeed. I think that will only be determined by experiment, and it may take a while, because if Lukasz is right, the reed unit will have to be adjusted so that the air flow doesn't stop on the high pressure side of the oscillation.

In any case, even if it does work, it won't have much effect on the weight, since it requires two frames. Simplicity and compactness perhaps, but tuning it so that the pitch and amplitude is the same on both the push and pull cycles might be problematical. I don't have a good sense of that. I also don't have a feeling for how sensitive the operation may be in terms of the thickness of the frame and the angle of the filed ends. A number of parameters to investigate before one can decide for sure.

Edited by rlgph, 04 February 2015 - 10:25 PM.


#6 David Hornett

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Posted 05 February 2015 - 01:45 AM

Profiling? Maybe you could try the idea with harmonic reeds, they are not profiledand it should be an easy matter to clamp two plates together, rerivet the reeds and file the toe slot in the manner described, but I suspect you will have no joy because the concertina reed tips angle up from the shoe, and it two shoes are clamped together the reed is only angled one way and be profiled on one side.

 

David



#7 Łukasz Martynowicz

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Posted 05 February 2015 - 05:18 AM

No offence intended, but with this post you have again proven, that you still know very little about observable reed behavior… Some facts then, that you can observe even with a naked eye, actually looking at a sounding reed:

 

Low A=220Hz reed on moderate volume (gravitational pull on the bellows only) has an amplitude of tongue movement of 5mm (10mm total travel) and to a naked eye this amplitude is symetrical and I don't think that there is any significant assymetry even when measured with apropriate instruments. Plate thickness is usually around 2mm, so with your symmetrical design there would be at least 1mm of tongue movement outside the shoe at each side (if your reed would work) . And this is still at moderate volume. Largest bass reeds can have an aplitude easily exceeding 10mm.

 

What you have described in your opening post, about the lift force created on a tongue is finally true - that the tongue is more similiar to a plane wing than a pressure valve and static pressure of the higher pressure side has neglectible direct effect on it's movement. But after carefully analysing your design, I must say that you forget about orders of magnitude of involved effects and in your latest post strongly overrate the effect of a static higher pressure on the tongue. 

 

Again, look at the actual reed - even when no leather valves are present to cut unwanted airflow, only the sounding side reed moves, the other just leaks air, it does not bend in any signifficant way. With your model it should bend proportionally to the pressure gradient in the same way as the sounding reed. Please understand, that the whole process of sucking the tongue into the shoe and resonant amplification of oscilation happens at this tiny area near the point on the tip of the reed tongue and the only significant aerodynamic force act only there. Pressure gradient is needed only for creating wind, which moves almost parallel to the shoe and reed tongue "meeting point".

 

What I have overlooked in my first reply to your proposed design is the flow which you think will drive this design in the first place. Now I think that it won't even start or will have a very slight vibration only. Here is a quick picture, that may help you understand the relative values of forces involved, and an actual airflow shape.

 

reed aerodynamics-01.jpg

 

Little grey arrows represent the higher pressure acting on entire tongue. Blue arrows represent the airflow, which is faster in the areas when the lines are closer together. Green arrows represent the lift force, resulting from the airflow.

 

Fig.1 represents the typicall reed in it's starting position. The lift force is orders of magnitude higher than the force from pressure gradient. The same effect would occur in your design on the upper shoe-edge crossing point if somehow your reed could start.

Fig.2 represents your design. You can see how the lift is created on both sides of the tongue, with values much smaller that on the "classic" assymetric reed. The similiar forces (however even smaller) are created in normal reed due to flow through tongue fiting tolerances. See how the upper flow creates a significant dampening force, which I have mentioned earlier. 

Fig.3 represents an additional lift force created when tongue passes through the shoe. It is again working towards the resting point and propels the resonant oscilation slightly. It is weaker than on the upper side of the reed, because airflow is not instantaneous and it works within a shorter time than on the upper side. 

 

 

To illustrate orders of magnitude of involved forces, I have another quick and simple experiment for you. Take a piece of typical printer paper. Place it at some distance from your mouth, with one of the corners pointing upwards at mouth level (with corner point at a level of the top of your upper lip), holding it stretched firmly in two hands about 5-10 cm from the corner. Then start blowing gently, silently and steady through a small hole between your lips (so the pressure gradient between your lungs and ambient pressure is the same throughout the experiment) and at the same time start closing the paper to your mouth. At some point you will start to hear the slight noise of air accelerating due to obstruction of pathway. Keep closing the paper slowly to your mouth. There is a very distinctive moment (at few mm from the mouth), when closing the paper even slightly closer will rapidly change the direction of  the airflow (down along the paper) resulting in a gentle "pat" of the paper corner on your lips. This is the moment when gap suction aerodynamic effect occur, sucking air from above the corner, creating dynamic underpresure on the closer surface of the paper, moving the paper and closing the gap. Note that there will be no audible change in noise volume (but there will be change in tone from a high pitch whistle to a lower pitched noise due to paper dumpening energy). Then fiddle around with blow direction, paper position and blow strenght and you will find a "sweet spot" when a paper stretched between hands will start flapping or even sounding with a reed-like buzzing sound (in addition to a whistling noise of airflow). You can observe that the paper movement at this distinctive point is much stronger that even slightly further away and that your blow force is bending the paper only slightly outwards. This is the physics of a beating reed ilustrated in a simplest experimental form. The physics of the free reed is different in details but general principles and involved forces remain the same. 

 



#8 rlgph

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Posted 05 February 2015 - 07:52 AM

I freely admit that i have done no observation of the motion of a concertina reed in operation, and i'm surprised that you have, since it requires that reed be sealed off from ambient air pressure on the bellows side for it to work.  I accepted the drawings on the CC website which suggest that the tongue does not go below the frame on the low pressure side.  If the amplitude of the oscillation is of the order of millimeters as you say, the design won't work with a single cutoff point on the low pressure side.  Whether it would work with cutoffs on both sides is an empirical question.
 
Now, for the rest of your discussion, what is clear is that you still do not understand my description of the physics.  From a physicist's point of view, there are only two forces involved -- the force due to the pressure gradient and the internal spring force.  The words "suction" and "aerodynamic force" that you use are just other words describing the force due to the pressure gradient.  However, it is the dynamic pressure gradient, not the static one.  You seem to think whenever i say "pressure gradient" i mean the (static) difference in the pressure inside the bellows and the ambient pressure outside the bellows.  Let me re-emphasize:  The pressure difference between the top of the tongue and the bottom of the tongue is dynamic, and apparently gets a lot larger than the static pressure difference between the inside and outside of the bellows.  As you say, the static pressure difference causes the air to start to flow.  Once the air is flowing, however, the pressure underneath the tongue drops substantially because of the partial vacuum that is formed due to the air flow (perhaps with a turbulent contribution).

Edited by rlgph, 05 February 2015 - 08:23 AM.


#9 rlgph

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Posted 05 February 2015 - 08:05 AM

As a simple experiment to partially test the feasibilty of the design, use a reed assembly as shown in my original Figure 2, except without the top frame.  This should act as a conventional unidirectional reed without bending the tongue.  If it does not, my bi-directional proposal will not work.  It may be necessary to try different angles and depth of filing the end before it works satisfactorily.

 

This of course would not prove that the full design does work.

 

I have to go to the dentist now, so i can't think more about it.  But it would be interesting to know whether the reed without one frame also works upside down.



#10 Łukasz Martynowicz

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Posted 05 February 2015 - 08:23 AM

What I understand from "your physics description" is that in your opinion such terms as lift (your "partial vacuum formed due to airflow") which is the common and precise level of description on academic level of fluid mechanics and aerodynamic studies, are a jibberish, non-physics and unnecessary complication in trying to describe phenomenons emerging from dynamic motion of a fluid flowing around solid objects… And I understand, that there is the same principle behind this approach, as in centrifugal (aparent) vs centripetal (real) forces. But your refusal for aknowledging their existence as useful descriptive tools is a mystery to me. Even if they answer your questions precisely, are constructed in a strict, physically proper way with a clear underlying mechanism (in much simplification, the Bernoulli's principle in case of lift), and solve the mysteries of observable reed behaviour.

 

And yes, dynamic pressure (lift force) on the low pressure side of the tongue is what drives the oscilation, which I was telling you from the very start of this whole debate.

 

And obviously, there is no difference between raising the tongue a bit or filling the shoe a bit to create a gap - the relative position is what matters. As long as there is assymetry in relation to the tongue resting position, the reed will speak as the airflow will be as in my fig.1 image. If I can recall correctly I have even saw a concertina or harmonium reed filled this way.

 

"since it requires that reed be sealed off from ambient air pressure for it to work" - you have clearly never seen a reed tuning rig, have you? Or even took a reed, placed it between your lips in front of a mirror and draw breath through it? 

"If the amplitude of the oscillation is of the order of millimeters as you say, the design won't work with a single cutoff point on the low pressure side." It is and it works just fine and you should finally get "your physics" straight with the observable facts...

This is my last post in this lecture, I wish you best of luck with your understanding how the reed works. Feel free to disprove my statements by actually building a bidirectional reed of your design.

 

[edited to add some clarification to the first paragraph]


Edited by Łukasz Martynowicz, 05 February 2015 - 09:16 AM.


#11 blue eyed sailor

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Posted 05 February 2015 - 08:38 AM

Łukasz,

not being able to judge upon who's right and who's wrong here conclusively (albeit having an idea which I however will keep private) I'm just finding it appropriate to point out that I seem to have learned a lot from your thorough explanations, and say "thank you" for that.

Best wishes - Wolf

#12 rlgph

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Posted 05 February 2015 - 10:35 AM

And yes, dynamic pressure (lift force) on the low pressure side of the tongue is what drives the oscilation, which I was telling you from the very start of this whole debate.

 

And i have agreed with you from the point that you said that you had tried a reed with a flat tongue and it didn't work.  Albeit, our descriptions have used different terminology.

 

 

In my reading of your accounts i do not find any significant difference in our understanding of how a concertina reed works.  I have written my description in the language of Newtonian physics.  In that language "suction" is not a force -- liquids aren't pulled up into a drinking straw by suction, they are pushed up by the pressure on the outside of the straw.

 

I don't think other descriptions are "jibberish"; i just choose to describe the motion in terms of the language that i know best, which is classical Newtonian physics.

 

But i agree with you there is no point in our further discussing how a concertina/accordion reed works.  We speak two very different languages.


Edited by rlgph, 05 February 2015 - 10:58 AM.


#13 rlgph

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Posted 05 February 2015 - 11:28 AM

As a simple experiment to partially test the feasibilty of the design, use a reed assembly as shown in my original Figure 2, except without the top frame.  This should act as a conventional unidirectional reed without bending the tongue.  If it does not, my bi-directional proposal will not work.  It may be necessary to try different angles and depth of filing the end before it works satisfactorily.

 

This of course would not prove that the full design does work.

 

I have to go to the dentist now, so i can't think more about it.  But it would be interesting to know whether the reed without one frame also works upside down.

Meanwhile, back to testing the proposal by using a reed assembly without the top frame of Fig 2 -- that is, a reed assembly with a straight tongue, but a notch in the end of the frame to allow air to flow at the beginning.  In the ordinary operation of the reed, squeezing the bellows will cause air to flow from the high pressure region above to the lower pressure below, which will cause the tongue to move downward.  As the end of the tongue approaches the frame below, the air flow is blocked, so that the downward force due to pressure (or whatever terminology you want to use) is greatly reduced.  The spring force will bring the motion to a stop and send the tongue back toward equilibrium.  It will pass through equilibrium and move into the region above.  In that region both the pressure and the spring force will act in the downward direction, bringing the tongue to a stop at some point above the equilibrium plane, and then accelerate the tongue back toward equilibrium, where the cycle will repeat.

 

Now what happens if the bellows is pulled and there is no leather valve to prevent air flow in the opposite direction?  In this case the tongue will initially move upward due to the large upward pressure force.  With no frame above, the air flow will not be cut off, so the pressure force continues to act upward.  The restoring spring force will act downward.  The opposition of these two forces will cause the reed tongue to remain in the region above the equilbrium plane of the tongue (oscillating weakly as first one force and then the other is larger).  The result is leakage and no sound (other than perhaps a very weak one due to the continuous air flow), as Lukasz described happens to a conventional reed with air going the wrong way.

 

Those are my predictions for what will happen in an experiment with a flat tongue, single notched frame reed.  As i said, such an experiment will not prove that the double frame reed assembly will work.

 

BTW, doing the initial tuning of the reed with just the bottom frame will make it much easier to determine the appropriate depth and angle of the notch.  The top frame could then be made to match the bottom.  Remember, however, if the oscillation amplitudes are such that the air flow is cut at both the top and bottom of the swing, the frequency of the reed with two frames will be twice that of the one with a single frame.



#14 JimLucas

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Posted 05 February 2015 - 12:10 PM

Time, I think, for one of my (many) favorite adages:

"In theory, there's no difference between theory and practice,

but not in practice."  ;)

I would apply that to my own theoretical arguments, as well as everyone else's.

 

I look forward to reports of actual results, whatever they happen to be.  :)



#15 Theo

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Posted 05 February 2015 - 01:21 PM

 I accepted the drawings on the CC website which suggest that the tongue does not go below the frame on the low pressure side. 
 

 

This is false, I suspect that the diagram is just that, a diagram, showing an idealised view and not a scale drawing of an actual reed in operation.   Practical experience of repairing concertinas shows that reed tongues do go well below the reed frame when the reed is sounding.   One of the problems that crops up occasionally is a reed not sound properly because the reed tip strikes the side of the vent in the reed pan.  This can happen when reed tongue and vent are not aligned.



#16 Łukasz Martynowicz

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Posted 05 February 2015 - 01:34 PM

Łukasz,

not being able to judge upon who's right and who's wrong here conclusively (albeit having an idea which I however will keep private) I'm just finding it appropriate to point out that I seem to have learned a lot from your thorough explanations, and say "thank you" for that.

Best wishes - Wolf

 

 

You're welcome Wolf, and thank you for expressing your appreciation of the effort taken :)



#17 rlgph

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Posted 05 February 2015 - 04:58 PM

I accepted the drawings on the CC website which suggest that the tongue does not go below the frame on the low pressure side.

 
This is false, I suspect that the diagram is just that, a diagram, showing an idealised view and not a scale drawing of an actual reed in operation.   Practical experience of repairing concertinas shows that reed tongues do go well below the reed frame when the reed is sounding.   One of the problems that crops up occasionally is a reed not sound properly because the reed tip strikes the side of the vent in the reed pan.  This can happen when reed tongue and vent are not aligned.

By "go below the frame" i meant go below the bottom of the frame, not just into the frame slot. Lukasz says that it actually goes below the bottom of the frame, and i have no reason to doubt that.

#18 rlgph

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Posted 05 February 2015 - 05:00 PM

Time, I think, for one of my (many) favorite adages:

"In theory, there's no difference between theory and practice,

but not in practice."  ;)

I would apply that to my own theoretical arguments, as well as everyone else's.
 
I look forward to reports of actual results, whatever they happen to be.  :)

Me too!




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