Effects Of Horizontal Distance Between Air Hole And Tongue Tip

[RAc]

Hi there,

It appears to be a rule with both radial chambered and parallel chambered reed pans that the air hole is positioned on top not of the reed's tongue tip but quite contrary above the fixed end of the reed (ie the end where the reed is clamped into the reed shoe).

My theory is that this is so that the sound first needs to resonate through the chamber in order to "build up sound" before it leave the chamber. This interpretation is sort of backed by a very interesting discussion found here :

"Unlike other reed instruments (e.g. clarinet, sax, etc.) the vibration of a concertina and accordion reed itself hardly produces any sound. In fact, laboratory tests have shown that the sound produced by the reed itself is neglectable."

The discussion then goes on:

"The first obstacle [for the sound wave] is the reed chamber. The sound waves produced by the reed will literally bounce of the walls of the chamber. The walls of the chamber can act as a filter or amplifier of particular sound waves. A course surface will absorb more of the higher frequencies that a smooth and hard surface. [...] The size of the chamber also has an effect on the reed performance. In a large and deep reed chamber the sound waves will reflect more than a shallow one and therefore will absorb more of the higher frequencies."


To come back to my original question then, would an air hole placed right above the tongue tip allow the reed's vibration to "escape through the hole before it had a chance to travel through the reed chamber and therebye generate resonance?"

I also noted that reed chambers appear to be designed such that the chamber is as "snug" around the reed as possible, meaning that the length of the chamber is as close to the length of the longer of the two chamber's reeds - probably in order to reduce "wave loss?" Would a larger chamber produce a weaker sound? If so, wouldn't it be logical then to put additional separating walls between the two reeds in the same chamber so that effictively two chambers exist which merely share an "outlet" (the air hole)?

Thanks in advance for all answers and apologies if this should be a trivial question.

Edited March 15, 2015 by Ruediger R. Asche

[Jake Middleton-Metcalfe]

I noticed this as well, all instruments, hybrid or traditional always have the hole above the base of the reed. I have heard it said that an accordion reeded concertina behaves very strangely when the hole is above the tip of the reed. It apparently makes a very very horrible sound. As to why - no idea. Henrik told me about this happening to him when he put a reed back in the wrong way.

[Jake Middleton-Metcalfe]

 

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Edited March 15, 2015 by Jake of Hertford

[wayman]

I can testify to the horribleness of the sound if a reed is on the wrong-way-round in a hybrid. I'm not sure how to characterize it in words. It's just ... really bad.

 

For extremely low notes (beyond, say, an octave-and-a-half below middle C), a chamber longer than the reed (with the reed tip near the end of the chamber) is beneficial. That is, you're further increasing the distance between air pad and reed tip. But don't ask me about the physics (fluid mechanics, etc); what I've learnt has been through lots of geometry, statistics, and experimentation.

[JimLucas]

...don't ask me about the physics (fluid mechanics, etc); what I've learnt has been through lots of geometry, statistics, and experimentation.

 

You mention "fluid mechanics", and certainly one aspect of that is air flow. I have no idea why/how it might be the cause of the observed effects, but:

• If the opening is over the moving tip of the reed, the (bulk of the?) air flow would be directly from the opening down through the tip end of the reed frame, with the bulk of the chamber largely stagnant.
• If the opening is at or behind the base of the reed, a great deal (most?) of the air will flow along the length of the reed before flowing through the frame, and some -- I suspect rather more than if the opening is over the tip -- will flow around the sides of the reed and through the frame well back of the tip, even if the bulk still flows through the frame near the swinging tip.

I believe it's also been said -- somewhere, sometime in the past -- that the reed will not speak quickly if the hole is over the reed's tip. If this is true, I have no basis for speculating whether or not that would be caused by what I've described above, but it might be relevant to the discussion elsewhere of how a reed begins to vibrate.

 

Edited to add: I don't have the tools to do it, but maybe somebody here could build a chamber long enough that the opening could be placed as far forward of the reed tip as it is currently placed back of the reed tip? That would, I think, to some extent separate these factors:

1. Distance between the opening and the reed tip
2. Whether or not the air is flowing along the reed and some of it down past the sides of the reed before it reaches the vibrating reed tip

Edited March 15, 2015 by JimLucas

[JimLucas]

"Unlike other reed instruments (e.g. clarinet, sax, etc.) the vibration of a concertina and accordion reed itself hardly produces any sound. In fact, laboratory tests have shown that the sound produced by the reed itself is neglectable."

 

I suspect a misunderstanding here. The vibration of the reed itself produces very little sound, but it modulates the airflow passing through the aperture in which it vibrates (the reed frame), and this produces significant amplification in a manner similar to that in a vacuum tube or transistor.

 

The reed chambers in a concertina are far too small to have any resonant amplification effect in the frequency range of the perceived notes produced by the concertina. E.g., the wavelengths of the notes in the octave from middle C upward are (very roughly) 100 times as great as their associated chamber dimensions. So while the chamber dimensions might have a "resonant" filtering effect on very high harmonics, and thus on the tone quality, they should have no discernable effect on the volume of the fundamental frequency or lower harmonics.

[JimLucas]

Another thought just occurred to me: If the opening is directly above the tip of the reed, the reed itself will be swinging directly against and with (alternately) the air flow itself, and at high volume (of sound) the tip might even closely approach the opening itself. Could this be the cause of the reported "horrible" sound? With the opening distant from the tip, the swing of the reed would be approximately at right angles to the air flow and it wouldn't be sharing geometry with the air hole itself..

 

As I've suggested in (an addition to) an above post, having a chamber which is extended forward of the reed tip and where the pad opening is distant from but forward of the reed tip should make it possible to separate some theoretical factors and see if they have independent effects. Another possible variation might be to place a pad opening above the center (base to tip) of the reed and what effects that has.

 

 

[RAc]

Thanks for the input so far!

 

When reading through the answers, there is one thing that crossed my mind which provides even more food for thought:

 

All the deliberations more or less concern only "inside reeds," which is reeds that respond to a push of the bellows. In that direction, the air goes from the bellows through the slot in the reed chamber, passing the reed and causing it to do whatever it does, then travelling through the reed chamber and leaving the system through the air hole.

 

So far, so good (though yet unclear).

 

Now what about the outside reeds? When PULLING the bellows, the air flow direction is reversed - the air comes in through the air hole, somehow traverses the reed chamber with the reed being the only possible way to pass thorugh - yet the rules are completly different here because the chamber is long gone when the vibration starts, and all of a sudden, instead of of a closed chamber that determines everything, we are now talking about the inner reed pan which is shared by ALL inner reeds? Is this a condrundrum or what? Why should the same physics apply - or do they?

[JimLucas]

All the deliberations more or less concern only "inside reeds," which is reeds that respond to a push of the bellows. In that direction, the air goes from the bellows through the slot in the reed chamber, passing the reed and causing it to do whatever it does, then travelling through the reed chamber and leaving the system through the air hole.

 

Huh? In my posts, what I was picturing and what I thought I described was in fact your "outside" reeds, the ones with their "tops" exposed inside the chambers and the air flowing from the pad hole to the reed and thence into the bellows. I was picturing the flow constrained within the chamber walls.

 

Aside from that, you do have a good point in questioning why the reeds on the two sides of the reed pan -- one with a surrounding chamber and the other without -- should seem to have essentially identical acoustic behavior. One early maker (blanking on the name at the moment) experimented with building reed pans with chambers on both sides, but that seems not to have made a significant difference.

 

The only thing I can speculate -- and I'm not confident about this "explanation" -- is that while the direction of airflow is opposite for the two reeds, maybe the path of the flow is not significantly different.

[RAc]

 

 

All the deliberations more or less concern only "inside reeds," which is reeds that respond to a push of the bellows. In that direction, the air goes from the bellows through the slot in the reed chamber, passing the reed and causing it to do whatever it does, then travelling through the reed chamber and leaving the system through the air hole.

Huh? In my posts, what I was picturing and what I thought I described was in fact your "outside" reeds, the ones with their "tops" exposed inside the chambers and the air flowing from the pad hole to the reed and thence into the bellows. I was picturing the flow constrained within the chamber walls.

 

Don't worry, we're perfectly in sync, only we happen look at the terms "inside" and "outside" from a different perspective. To me "inside" in this context meant "inside the reed chamber," not "on the inside of the reed pan with respect to the bellows." Sorry for creating confusion!

 

One corollary of a design w/ separate reed chambers to me would be the elimination of the need for a valve (which would be truly revolutinoary), right? The big technical difficulty then would be the challenge to build a button that opens both an "inside" and "outside" air hole with the same movement. It feels rather tricky to come up with a solution to THAT problem...

[JimLucas]

Don't worry, we're perfectly in sync, only we happen look at the terms "inside" and "outside" from a different perspective. To me "inside" in this context meant "inside the reed chamber," not "on the inside of the reed pan with respect to the bellows." Sorry for creating confusion!

Except that you said:

All the deliberations more or less concern only "inside reeds," which is reeds that respond to a push of the bellows. In that direction, the air goes from the bellows through the slot in the reed chamber, passing the reed and causing it to do whatever it does, then travelling through the reed chamber and leaving the system through the air hole.

And the reeds that sound on the push are the ones on the bellows side of the reed pan, not the ones "inside" the chambers. Whence my confusion.

 

One corollary of a design w/ separate reed chambers to me would be the elimination of the need for a valve (which would be truly revolutinoary), right?

 

An extra wall separating the two chambers might prevent sympathetic vibration of the "inactive" reed, but without a valve there would still be air leakage through that reed assembly as long as the pad leaves the chamber open.

 

The big technical difficulty then would be the challenge to build a button that opens both an "inside" and "outside" air hole with the same movement. It feels rather tricky to come up with a solution to THAT problem...

But if you're proposing a mechanism where the action of one button would open only one or the other of two chambers depending on the direction of pressure difference/air flow, I can't imagine anything simpler than the passive valves in the current design.

[Paul_Hardy]

... One early maker (blanking on the name at the moment) experimented with building reed pans with chambers on both sides, but that seems not to have made a significant difference.

George Case certainly built some with internal chambers - see my page on Case 3087, and this image .

[JimLucas]

... One early maker (blanking on the name at the moment) experimented with building reed pans with chambers on both sides, but that seems not to have made a significant difference.

George Case certainly built some with internal chambers - see my page on Case 3087, and this image....

 

Thanks, that's the name I was trying to remember.

 

I've seen a couple of double-chambered Case concertinas, but on those the board covering the chambers extended to the walls of the bellows end. Instead of exposed ends to the chambers, there were teardrop-shaped holes cut in the covering board. I presume the one design preceded the other. I wonder which came first.

[wayman]

Jim Lucas wrote "I believe it's also been said -- somewhere, sometime in the past -- that the reed will not speak quickly if the hole is over the reed's tip. If this is true, I have no basis for speculating whether or not that would be caused by what I've described above, but it might be relevant to the discussion elsewhere of how a reed begins to vibrate."

 

Yes, that's certainly true for the extremely low reeds, in my experience.

 

I *think* the "air pad directly over reed tip" badness is true for both inside and outside reeds. At least, that's my recollection, but I can't swear to it, as it's been some while since I've messed around with this; and I can't do a test on this for some weeks yet as I'm several thousand miles from the workshop at the moment (In England; attending the upcoming Kilve weekend.)

[Geoffrey Crabb]

I re-submit the attached (first posted 1 year ago but received no response). Although primarily written to address the subject of double chambered reed pans (Case) some content may be relevant.

 

 

 

Geoffrey

 

 

 

 

 

 

 

 

 

 

[JimLucas]

I re-submit the attached (first posted 1 year ago but received no response). Although primarily written to address the subject of double chambered reed pans (Case) some content may be relevant.

 

Double chambered reed pans.doc

 

Geoffrey

 

Very interesting. Thanks for that, Geoffrey.

[RAc]

I agree. Thanks for that! I just got a little confused as the point a) in the introduction does not appear to state a requirement. Could you clarify?

[RAc]

 

Don't worry, we're perfectly in sync, only we happen look at the terms "inside" and "outside" from a different perspective. To me "inside" in this context meant "inside the reed chamber," not "on the inside of the reed pan with respect to the bellows." Sorry for creating confusion!

Except that you said:

All the deliberations more or less concern only "inside reeds," which is reeds that respond to a push of the bellows. In that direction, the air goes from the bellows through the slot in the reed chamber, passing the reed and causing it to do whatever it does, then travelling through the reed chamber and leaving the system through the air hole.

And the reeds that sound on the push are the ones on the bellows side of the reed pan, not the ones "inside" the chambers. Whence my confusion.

 

Actually, you are right (no surprise). I somehow confused the air flow directions. Reflecting on this error sort of sheds some light on the issue; in the push direction, the air must pass the reed first and then travel through the reed chamber before being allowed to escape. The pull direction forces the air through the pad hole and then on the way out via the reed that sits INSIDE the chamber, thus possibly forcing the chamber to resonate via a "back draft" (in lack of a better term). It's still asymmetrical though.