Why Do Reeds Need To Occur In Pairs?

[rlgph]

Lukasz,

 

I think we are in agreement and have been in agreement about what causes the reed to move ever since you told me the null result of your experiment. But our descriptions have been obscurring this. I, as a physicist, talk in terms of forces, and forces are caused by material objects or fields (such as gravity or EM fields). Suction, e.g., is not a force.

 

The only force that can start the reed moving from its equilibrium position is that due to air molecules hitting the reed. This force is described in an average sense by the difference in pressure between the top of the reed and the bottom times the area of the reed. However, this pressure difference is not simply the difference between the pressure in the bellows and the ambient air pressure. Because of the motion of the air around the end of the reed, a partial vacuum is formed below the reed so that the air pressure difference between top and bottom is much larger than it would be if the air were not moving. My only question at this point is whether turbulence is necessary fo produce this large pressure difference.

Edited February 3, 2015 by rlgph

[Łukasz Martynowicz]

Suction effect in fluid mechanics, as well as lift on the wing of the plane, are two emanations of the same effect, generating force caused by speed difference of particle movement, as you have described above. So I really don't understand what you mean by stating that suction effect is not a force (obciously it is not a force field) and then describing this effect on a microscopic level...

 

And because you can hardly expect an airflow around moving tongue to be laminar, then probably turbulence occuring with sudden cut-off when tongue sinks in the shoe is as significant here as in bumblebee flight, increasing the effect of drawing a tongue into the shoe by airflow.

[rlgph]

Turbulence will undoubtedly exist; my question is whether it's neceesary for the operation. A highly theoretical question, probably of no import, but physicists often worry about such things.

 

As to why i say suction is not a force as physicists use the term force, i refer you to my previous post.

 

I trust that this discussion is now at an end. However, i will be starting a new thread in a day or so with a new proposal fo a bidirectional concertina/accordion reed. I look forward to hearing your comments.

[Łukasz Martynowicz]

I'm very curious about your new bidirectional reed design and looking forward to your thread

[JimLucas]

Suction, e.g., is not a force.

A matter of terminology. "Suction" is a differential pressure (force x area) or resulting flow (I'm used to both usages) in the direction of a measuring device (hypothetical or real), also referred to as "negative pressure". The same force measured from the opposite direction along the force/pressure vector is described as "positive". E.g., the "lift" force from below an airplane wing -- resulting from differential airflow speeds under and over the wing -- could just as well be described as "suction" from above, though it usually isn't. On the other hand, if you "suck" on a hose or straw, it's not normal -- though it is accurate -- to say that the resulting fluid flow is being "pushed" or "pressed" inward by the same differential between the external pressure and that in your mouth/lungs. On a concertina, a pressure differential can be created by either pressing or drawing the bellows (with at least one pad open). On a (mouth) harmonica, the actions with equivalent effects are known as "blow" and "suck".

 

The only force that can start the reed moving from its equilibrium position is that due to air molecules hitting the reed.

True. But they're hitting from both sides. It's when there's a significant differential between the two sides that things start happening.

[JimLucas]

I'm very curious about your new bidirectional reed design and looking forward to your thread

 

I am, too.

 

FWIW, and I know I'm late in bringing this up, my admittedly limited (though non-zero) experience with such reeds in the sheng type of instruments is that they

• have a tone quality quite different from concertinas or accordions,
• are slower to respond than the unidirectional free reeds I'm familiar with, and
• have a much more limited dynamic range.

[Wolf Molkentin]

 

I'm very curious about your new bidirectional reed design and looking forward to your thread

I am, too.

 

FWIW, and I know I'm late in bringing this up, my admittedly limited (though non-zero) experience with such reeds in the sheng type of instruments is that they

• have a tone quality quite different from concertinas or accordions,
• are slower to respond than the unidirectional free reeds I'm familiar with, and
• have a much more limited dynamic range.

Hi Jim,

 

that's interesting news, because what you're reporting is just what we would expect such reeds as suggested here to "behave" (thus not being totally silent), whereas the familiar uni-directional setting provides this great powerful tone we all love - IMO concertina might be the species least capable of such simplification in the flock of free-reed instruments...

 

Best wishes - Wolf

[Łukasz Martynowicz]

From what I have found about asian type reeds on the last couple days, all eastern instruments are mouth blown and require a pipe resonator for reed to work. From my personal experience with a clarinet (which has a beating reed, so physics is a bit different) and from musicians of various wind instruments, I know that you cannot simply attach a bellows to such instrument - to be able to play on them you must first learn how to sound them, how to "manually (mouthually?) jump start" the reed by combination of blow strenght, slight temporal variance and precise direction (probably causing a proper turbulence at the reed point). I don't know for sure that this is a solid fact, but it sounds to me as a very solid reason why bellows driven reeds have to have a different construction and principle (are blown-closing in oposition to blow-opening asian reeds) and are dependant on gap suction mechanism.

[wayman]

I'm a concertina maker; my housemate is a violin maker. When I think about the differences in how his and my instruments produce sound, the violin seems so simple to me, and at times I wish I were building an instrument whose physics I actually have (... or think I have) a decent grasp of!

 

And, as I read this fascinating c-net thread and reflect on concertinas and violins, it calls to mind a passage in The Joy of Cooking (1953 edition) on baking powder:

 

When confronted with the questions growing out of the use of the various forms of baking powder now on the market the puzzled layman is apt to sigh for the good old days when this article was made at home, rather haphazardly, according to a formula handed down from one generation to the other.

 

Had I died in 1933 baking powder [substitute here "free reed physics"!] would have been found written on my heart. Due to the complexity of the problem it became one of life's major issues.

 

When William Beebe's nerves became overwrought due to the study of the personal relationships of birds [substitute here "concertinas"!], he turned from them to occupy himself with the habits of the less emotional fishes [substitute here "violins"!]. I had no such outlet [alas, the plight of the concertina maker!]. I had to battle with many new things, to discover that calcium phosphate baking powder while not necessarily so labeled has a double action, that both calcium and tartrate baking powders may be used in smaller amounts than usually designated in recipes without other harm to the baked product than a slightly smaller volume, that when eggs are added to a batter the usual measurement of 1 teaspoonful baking powder to a cupful of flour may be reduced, etc, etc.

 

I do not pretend to have solved the baking-powder problem scientifically [... I confess, even if this thread yields an absolute consensus on the theoretical physics of how free reeds work, I may not truly, entirely, absolutely understand it...], but endless experiments have enabled me to solve it to my own present satisfaction [... I may not truly get the physics, but at least I know enough to be relatively content building concertinas and not go entirely insane ).

 

I find this passage -- and others from this oft-times surprisingly rambly and philosophical cookbook -- particularly delightful and surprisingly applicable to other realms of my life outside the kitchen.

[rlgph]

Nice quote. Thanks.