FREE REED PHYSICS - 1

[Johann]

We never cover all aspects, main difference of DIX is the fact that the reed gup or precision  is less narrow. A tighter fit of tongues to frame results always in more higher overtones.

I only can speak off acordeon DIX reeds the have in average shorter tongues on the comparable tone of modern reeds. Less stiff, thinner frames and geometrie is also different. All of this different dimensions have effects. Some or all have effects on overtone including modes, but the main difference is the reduced precision.  Still if we copy old DIX as good as possible the never sound exactly the same. It is really difficult to copy  old reedsets. If we only would need to know the type of steel and the dimensions then it should be possible but it isn't possible to copy this way. Even if we also copy the shaping the sound not quite equal. Most modern copies are to tight in fitting, so the sound brighter. But there are other factors what makes it difficult to copy DIX or other reedsets. Tom is right that modes do contribute to timbre but we are far away of calculating reed sound by setting up reed material and dimensions and stiffness. I agree that reeds made of different material do sound different because we have to use different length for the same pitch what results in a different mix of modes and overtones.   I also disagree that carbon would be a good material for reed tongues. Carbon conbund is difficult to machine not easy. We also don't know how carbon compound behaves over longtime. 

[ttonon]

18 hours ago, Łukasz Martynowicz said:

but nowhere you adress how the vibration itself starts, which is what David asks for and what is the crux of the the question "how free reed works" and a subject of endless debates on this forume since forever. I don't know if you are aware, but couple of first oscilations of the tongue happen above the upper plane of the frame, with movements yet too small to close the gap entirely (I've been away from this forum for nearly 5 years and may have missed some important arguments; I saw there was at least one topic about turbulent flow being the source of initial vibration while I was absent).

 

Hi Lukasz, please read the following post so that you can get up to date on these details:

 

18 hours ago, Łukasz Martynowicz said:

No model of the free reed will be complete without explaining first weak cycles before the tongue enters the frame.

Not quite.  The analysis behind these posts contains a physical model for the forces acting on the tongue, a mathematical representation of that model, and a solution that is periodic.  The method of solution admits only periodic solutions.  A solution that starts from rest is not periodic.  This is a common practice in the theory of vibrations and in applied mathematics.  Right now, I'm not interested in a transient solution starting from zero initial conditions, which I believe would be more complex than the one I've done, and may not even be amenable to analytic techniques, as opposed to the numerical techniques now in vogue, of course because of necessity and the availability of fast computers.  

18 hours ago, Łukasz Martynowicz said:

Your model should also be able to explain why "tongue tip over hole end of the chamber" produces significantly weaker speaking reed than "rivet over hole end of the chamber" configuration with every other parameter remaining the same.

No it shouldn't, because this is an analysis of solely the tongue vibration.  Hopefully I can use this model and apply it to a coupled chamber, but that's something for the future.  

18 hours ago, Łukasz Martynowicz said:

You might also want to test your model trying to explain why DIX reeds, regardless of materials used, produce less higher harmonics than non-DIX equivalents by the means of only two tiny holes at the tip corners of the shoe.

Can you please explain what DIX reeds are, and maybe give us a picture?  Is it similar to the HOPV (High Output Piccolo Voice) technology?

 

Best regards,

Tom

[Łukasz Martynowicz]

1 hour ago, ttonon said:

 

Hi Lukasz, please read the following post so that you can get up to date on these details:

 

Not quite.  The analysis behind these posts contains a physical model for the forces acting on the tongue, a mathematical representation of that model, and a solution that is periodic.  The method of solution admits only periodic solutions.  A solution that starts from rest is not periodic.  This is a common practice in the theory of vibrations and in applied mathematics.  Right now, I'm not interested in a transient solution starting from zero initial conditions, which I believe would be more complex than the one I've done, and may not even be amenable to analytic techniques, as opposed to the numerical techniques now in vogue, of course because of necessity and the availability of fast computers.  

No it shouldn't, because this is an analysis of solely the tongue vibration.  Hopefully I can use this model and apply it to a coupled chamber, but that's something for the future.  

Can you please explain what DIX reeds are, and maybe give us a picture?  Is it similar to the HOPV (High Output Piccolo Voice) technology?

 

Best regards,

Tom

 

Thank you for this link, I saw this discussion a while earlier than this current thread, before I got steadily back on track of who posts what here nowadays, so I missed that it was your post. It is indeed exactly about what I was asking about and is perfectly in line with my position on the topic from years back. Glad to see that someone worked on theory behind it. The reason why I asked about this is simple - this transient period is far more important part of "why and how free reeds generate sound" question than the focus of this thread and there was a long history of discussions about how free reeds work here on this forum completely ignorant about this transient, usually leading to completely false conclusions.

Now I'm perfectly satisfied with the scope of your answers and understand, that you have divided your works into separate chunks and hopefully at the end of your journey you will indeed come up with what I would call a complete free reed theory.

 

As to DIX reeds, I must apoogize, after 5 years I misremembered my experiments from back then - I have mixed up effects of various shoe materials vs geometry and scaling. I dug up those a moment ago and Johann is right - they have overall different geometry and scaling than their typical accordion counterparts. If you still want to know more about them here's a link to harmonikas.cz with description and schematics https://www.harmonikas.cz/en/dix-1#obsah 

[ttonon]

On 6/15/2019 at 4:08 AM, Johann said:

It all comes down to resonance and coupling, as sayed before the system is very complex we hear the air column vibrating not the reed. Other parts beside camber, boards and casing all participate more or less. The way the reed is fixed to camber or board all - or how the layout of the reeds are set up on the board effect modes and coupling on board. But this is all not really well researched. 

Johann, you are spreading erroneous ideas.  I believe most of us are aware that the sound produced by the Western free reed is not because the reed is coupled to an air column.  This is true for the Asian free reed, an entirely different phenomenon.  The Western free reed used in concertinas, accordions, harmonicas, bandoneons, etc, are only weakly coupled to the cavity.  There are exceptional instances for the highest pitched reeds, and when that happens, the reed has trouble speaking.  In other words, coupling to the cavity is detrimental to the tongue vibration.  However, the cavity does affect the timbre of the musical tone.  I believe that influence can be categorized as reverberation.  Another example of reverberation is to put your ear next to a wall in a noisy room, or to play your accordion very near to the corner of a room.  Near a wall, the acoustic pressure amplitude is twice that of the wave traveling toward the wall.  This must be the case because the reflecting wave must interact with the oncoming wave so that the particle velocity at the wall becomes zero.  The wall is a boundary condition.  It stops the vibratory air motion that's part of the sound wave, because the air can't go through the wall.  Similarly with the cavity.  Sound from the reed strikes the walls of the cavity and rebounds, producing a doubled acoustic pressure (oscillatory) amplitude.  The dimensions of the cavity are small compared to the wavelengths of the vast majority of the musical frequencies - all except for the very highest notes.  The region of this increased "rebounding" pressure extends throughout the cavity and a little beyond.  

 

I know you read my paper describing all this, though others may not be aware of it at:

http://www.concertina.org/archive/pica/pica_2005_2/pdf/reed_cavity_design_resonance.pdf

 

Best regards,

Tom

 

 

Edited June 16, 2019 by ttonon
Correct "lowest" to "highest."

[Wolf Molkentin]

Hi Tom,

 

35 minutes ago, ttonon said:

the sound produced by the Western free reed is not because the reed is coupled to an air column.

 

thank you for straightening that out once again.

 

41 minutes ago, ttonon said:

The dimensions of the cavity are small compared to the wavelengths of the vast majority of the musical frequencies - all except for the very lowest notes.

 

I would have expected the very highest notes being referred to here, possibly an error while typing?

 

Best wishes - ?

[Johann]

Sorry Tom i cant agree, the same ist true for the western free reed as it is to a bigger extent on the eastern type with tuned resonators.

Think about it again. The only fact that it is possible to sound a western reed with a resonator that is very mistuned  still does not change the fact that the reed on its own cant produce a sound. It always needs the air. Also keep in mind that the pitch can be varied by changing chamber parameters as well.  In the eastern type the main sound generator is the air column and the reed is the rather passiv coupled part. With the western type it is the other way round, the reed acts as the main impulse generator and the misstuned resonator as coupled resonator. Johann

 

 

The dimensions of the cavity are small compared to the wavelengths of the vast majority of the musical frequencies - all except for the very lowest notes.

Ok agree but i think you mean the higher notes. Still you agree that we do have a miss tuned cavity. I do a lot of practical tests up to now. Especially on very low notes. Helicon Bassreeds with nots as low as 31 Hz. And changing cavity geometry is one of the major way to change the timbre. And iven on this very misstund situation pitch can be change up to semitone by changing geometry of the cavity.  Johann   

 

 

Edited June 16, 2019 by Johann

[Johann]

Tom i still don't know what your theoretical model includes, only the the reed as vibrating single side mounted beam? Or also the cutting of the air beam through the slot? The western reed is an impulse generator more like sirens not so much like a tuning falk. Johann

[ttonon]

4 hours ago, Wolf Molkentin said:

I would have expected the very highest notes being referred to here, possibly an error while typing?

Yes, thanks Wolf.  It was a long day.  I'll make the correction on the post.

Best regards,

Tom

[ttonon]

3 hours ago, Johann said:

And iven on this very misstund situation pitch can be change up to semitone by changing geometry of the cavity.  Johann   

That's a bit surprising, though keep in mind, with the lowest frequencies, even a minor interactive effect with anything can cause a pitch change.  For instance, around 30 Hz, a change of only one Hz corresponds to a semi-tone. 

 

There are other things that change pitch besides acoustic coupling with a resonator.  Dissipation in itself always produces a lower resonant frequency than the undamped natural frequency.  In the "child on a swing" analogy, with wind resistance, the speed of the swing is reduced, and this lowers the vibration frequency: the period of oscillation is delayed.  The calculations I'm doing confirm that dissipation increases with amplitude, due to their nonlinear nature, and show that frequency drops can be as high as 50%!  This was a surprise to me.  We don't notice such things because we normally cannot excite the reed by means other than an air flow.  An experiment that would give us the undamped natural frequency would be to vibrate a tongue in an evacuated bell jar and excite it with a variable frequency magnetic field.  Of course, we can also calculate the natural frequency for simple shapes using the Euler-Bernoulli results for a vibrating cantilever, which is very accurate.  

 

I'm not saying that you are necessarily observing damping effects, but you may be.  When you tell us your anecdotes without providing documents, you can say pretty much anything you want, so from my perspective, I cannot take much of it too seriously, though I'm interested in what your experience tells you.  I say that because my interest is more of a scientific nature, and speaking undocumented anecdotes is not science, and I'm sure you understand that.    

 

Best regards,

Tom

[Johann]

4 hours ago, ttonon said:

 The calculations I'm doing confirm that dissipation increases with amplitude, due to their nonlinear nature, and show that frequency drops can be as high as 50%!  

Hi Tom, i am well aware of this as well,  also keep in mind that not only with increase of volume the amount the pitch trops with increase of volume depends strongly on the spring stiffness off the reed. Also think in witch way dissipation comes noticeable what is causing the damping?   Inertia material dissipation of the reed?  No it is the air surrounding of the reed within a cavity that is more or less resistant compatible with bass sound boxes. But this is also in relation with resonance and coppeling. Don't see this as a completely different subject. I am electronic  telecommunication engineer as well and i know how things are working and i know very well how LC or RC circuits or series of circuits behave and can be calculated and acoustic circuits don't behave completely different. For me it is not a matter that i have to prove it. It is or more less well known design knowledge.  My be i can't explain it quit well enough because my english is not as good as it should be. Don't put it away think about it.  If you have questions i will tray to explain.  But i am not prepared to present more explanation without detailed further questions.  Also is not for me to get a more intense understanding of this circumstances. At this time in my live (i am 68 Years old now) i am more in building of accordions especially the last teen years. I am retired from my professional work as telecommunication engineer and academy teacher in this field. Johann

Edited June 16, 2019 by Johann

[ttonon]

Hi Johann, I was maybe not clear enough.  I respect your conclusions on observation and I'm eager to hear about them.  It's when you try to explain the physics behind them that I sometimes cannot follow, and it's in the latter area where documentation would help.

Best regards,

Tom

[Johann]

1 hour ago, ttonon said:

Hi Johann, I was maybe not clear enough.  I respect your conclusions on observation and I'm eager to hear about them.  It's when you try to explain the physics behind them that I sometimes cannot follow, and it's in the latter area where documentation would help.

Best regards,

Tom

Ok Tom i think it is mostly because it is very difficult for me to explain it not in my native language. This is why i would prefer that you ask question. You are on a very good way but things are often simplified to much. I know we have to do so to get on. But to understand all a bit better we have to proceed and think about it in a more complex scenario. For the simplified approach you are right, but once you have the simplified modul right you have to take more in account.    Best regards, Johann