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Dirty-South Blues Harp forum: wail on! >
Calling Perfessor Yerxa, and other geniuses
Calling Perfessor Yerxa, and other geniuses
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mlefree
416 posts
Sep 25, 2015
12:12 PM
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My friend Jp Pagan has posed a harmonica paradox for which I have no intelligent response. I'll call it the Jp Bending Paradox. It's simply stated but I can't find an answer even in Harmonica for Dummies, and I'm too much of a dummy to answer it myself to answer it. Every reference I can find cites the need to "pinch" your tongue to get a tiny resonant chamber for blow bends but none goes into the physics required to address the JP Bending Paradox.
Jp asks: "Both low draw bends and high blow bends lower the pitch of the unbent tone of a given hole. If draw bends increase the volume of the resonant cavity to lower their pitches, why do blow bends decrease the resonant volume to lower theirs?"
When I learned to blow bend it never occurred to me to even wonder about this.
I'm stumped. Are you? 8^)
Thanks,
Michelle
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WinslowYerxa
963 posts
Sep 25, 2015
6:15 PM
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JP asked the same question on Slidemeister but expressed it differently:
"why, when bending a draw note down, do we usually increase the space in our resonance chamber, but to bend a blow note down, we make the chamber smaller?"
Then he further clarified: "In both cases, the pitch goes down, but on blow notes you make your air chamber smaller, and on draw notes you make the chamber bigger. Opposite changes in chamber size, same downward movement in pitch. That's how it seems to me, anyway."
When I tried it, I notice that I was engaging in the same habit: moving the tongue in the direction of the airflow, back when I was pulling air in, forward when I was pushing air out.
So I tried two experiments. In both, I used the note F in Hole 2 of a 12-hole chromatic, which can be played as blow or draw (with the slide in). Both notes bend down so should require the same tongue placement to create the sweet spot that effects the bend. Then I located and memorized the sweet spot for that bend.
In the first experiment, I didn't move the tongue forward or back. I simply raised it into position. Blow bend and draw bend played the same, with no change in tongue placement.
In the second experiment, knowing where the sweet spot was, I played blow abend while sliding the tongue both forward and backward into the sweet spot. Then I played the draw bend, again approaching the sweet spot by both sliding backward and forward to approach it. In both cases, it made no difference whether I was blowing, drawing, moving the tongue forward or back. As long as I landed on the sweet spot, then note bent.
Conclusion: moving the tongue forward when blow bending and back when drawing are habits that have no effect on bending effectiveness.
Why do we habitually do these things?
Two possible reasons come to mind:
1) We're moving the tongue sympathetically in the same direction as the airflow.
2) We're used to bending high blow notes, which require a small resonant chamber and a forward tongue placement, and so we've formed the habit of moving forward into blow bends even when they're not high pitched. We're also used to bending low draw notes which require a larger chamber and a backward tongue placement, and we've formed the habit of sliding the tongue back when draw bending.
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Winslow
Check out my blog and other goodies at winslowyerxa.com
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Harp Study
140 posts
Sep 25, 2015
7:32 PM
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I don't have an answer, but I have often wondered the same thing. It always seemed strange to me; since, as JP pointed out, you're lowering the note in both cases.
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mlefree
418 posts
Sep 25, 2015
7:45 PM
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I knew you would sink your teeth into this substantive question, and I thank you for it, Winslow. You da man!
But your experiment was for single reed bends. Since I'm almost exclusively a double reed player, that is the domain in which I posed the question.
In another thread you cited the scholarly (and intimidating) "Bending Physics" paper by Robert B. Johnston. I'd seen that before but it had been so long since I studied physical acoustics and such related high-falutin' math as Fourier transforms that I admit that in the past my eyes have glazed over when I read it. But I tonight I was sufficiently intrigued by this specific question that I ~think~ I've ferreted out the answer, at least in the case of double reed harmonicas. As you know, Johnston also delves into single reed bending but I'll let you concentrate on that.
Bending Physics
Here's what I've gleaned. The answer is rooted in the concepts of opening and closing reeds.
From the article:
"It is helpful to distinguish, following Helmholtz, two ways in which a reed that is coupled to a distributed linear acoustic system [I.e., the player or the variable-length tube model of the experiment.] can act as a sound generator. In one mode the reed gap is reduced when the reed moves in the direction of the air flow and in the other it is increased when it moves in the direction of the air flow. We shall call the first a closing reed and the second an opening reed..."
Now I'll distill what I read from the rest of the article:
When playing a blow note the top reed is a closing reed and the bottom reed is an opening reed. The roles are reversed for a draw note.
The acoustic impedance ("resistance" if you will, but that is an oversimplification) of the closing reed is far lower than that of the opening reed. This is why the harmonica plays at all. The closing reed sounds at low air pressure, the opening reed doesn't.
One can only can bend when the higher pitch is the closing reed and the lower pitch is the opening reed. These conditions are satisfied only on the low draw and high blow notes. In these conditions, acoustic admittance (inverse of impedance) is negative only between the two reed resonant frequencies. That negative admittance is what allows the bends to occur and defines the limit of the flattening. But it turns out that the phase function of the acoustic admittance is ~discontinuous~ (like the tangent function compared to the sine function). I'll refer any interested parties to the graphs in the article.
When operating in the region of continuous variation in pitch with length (of tube in the experimental model or in the human analog), increasing the length of the tube lowers pitch. However, the pitch can also be lowered by crossing the that discontinuity in acoustic phase with a ~decrease~ in tube length. This accounts for the apparent contradictory technique on high blow notes, and their discontinuous change in pitch.
We've all experienced that discontinuity in those pesky high blow bends, for me especially on hole 10 where I've had to do a lot of practice to sustain the bent pitches. If I'm not really careful the bend will suddenly "pop" into the wrong pitch. Very small changes in embouchure can result in big swings in pitches and that is a manifestation of those discontinuities.
So there we go. As Paul Harvey used to say, "The Rest Of The Story."
Thanks again, Winslow!
Michelle
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Last Edited by mlefree on Sep 26, 2015 8:51 AM
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WinslowYerxa
964 posts
Sep 27, 2015
9:40 AM
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I wrote of identical experiences bending F as both a blow note and a draw note in Hole 2 of a 12-hole chromatic in C. I could raise my tongue to the same sweet spot (same location of tongue in the mouth) for both bends. And I could approach each bend by moving my tongue both forward and backward in my mouth to reach the sweet spot.
Michelle replied: “But your experiment was for single reed bends. Since I'm almost exclusively a double reed player, that is the domain in which I posed the question.”
OK, I chose that chromatic simply because it was close at hand and a convenient way of accessing the same bendable note as both blow and draw.
So I repeated the experiment with dual-reed bends using an XB-40 in A, bending Draw 2 (E) and Blow 3 (also E). The XB-40 is designed so that all notes are bendable as dual-reed bends.
My experience with these dual-reed bends was identical to my experience with single-reed bends on the chromatic. I could raise my tongue to the same sweet spot for both bends and I could approach the sweet spot for both bends by sliding my tongue to it from in front or from behind.
Again, breath direction had no effect on either chamber size or on the direction from which one could approach the sweet spot by moving the tongue forward or back.
Conclusion: sliding the tongue backwards for draw bends and forward for blow bends is an artifact. Once pitch is eliminated as a factor, breath direction neither indicates a difference in chamber size for the bend nor any requirement to approach it from a particular direction. This is true for both single-reed and dual-reed bends.
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Winslow
Check out my blog and other goodies at winslowyerxa.com
Harmonica For Dummies, Second Edition with tons of new stuff
Join us in 2016 for SPAH on the San Antonio River Walk!
Last Edited by WinslowYerxa on Sep 27, 2015 9:41 AM
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mlefree
433 posts
Sep 27, 2015
10:40 AM
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Very interesting, Winslow. I hadn't thought of that. I don't think Johnston did either. He only approached the "sweet spot" from the lower end by decreasing the volume of his tube model. It may have been impractical for him to approach it from the low-volume side. I know it is for me and I applaud your super-human bending skills for even thinking of it, let alone executing it!
And thanks for trying it on the XB-40. I know little about that harp but I know it has a 3rd "responder" reed. In my ignorance I assume that an XB-40 draw bend only uses 2 reeds. I guess I'm curious why you didn't just use a regular diatonic.
What interested me most about Johnston's article was that it addressed the "Jp Paradox" directly and yielded another unexpected discovery as a dividend.
The passage, "When operating in the region of continuous variation in pitch with length (of tube in the experimental model or in the human analog), increasing the length of the tube lowers pitch. However, the pitch can also be lowered by crossing the that discontinuity in acoustic phase with a ~decrease~ in tube length. This accounts for the apparent contradictory technique on high blow notes, and their discontinuous change in pitch." explains a lot.
For me the kernel of my discovery in Johnston's article was the of the role that the "phase of acoustic admittance," that complicated and esoteric mathematical function that cropped up in Johnston's analysis (his Figure 6). It was key to my understanding about why high blow bends behave so differently from draw bends. It explained both the apparent paradoxical need to make the resonant chamber smaller on those high bends as well as why the "frets" of control up there, as you describe them, are so narrow. This is why if you aren't very careful your target bent tone can suddenly "pop" to a different, undesired one.
I'm once again indebted for your contributions here, Sensei. Muchas gracias!
Michelle
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Last Edited by mlefree on Sep 27, 2015 10:41 AM
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WinslowYerxa
965 posts
Sep 27, 2015
12:18 PM
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Actually, the XB-40 has third AND fourth responder reeds. Each blow note and each draw note has its own dedicated responder reed. That's the "40" in XB-40: 20 regular reeds, 20 responder reeds.
Why didn't I use a regular diatonic?
Because you can't bend Blow 3 down on a regular diatonic. Therefore you can't test the exact same pitch as both a blow bend and as a draw bend. I wanted to eliminate differences in pitch as a skewing factor in the test, so I chose harps that would let me bend the same note as both a blow note and a draw note.
The chromatic lets you test F and C as single-reed bends on the same pitch as both blow and draw, while the XB-40 lets you do the same with dual-reed bends on Draw 2 and Blow 3. The SUB30 is the only other harmonica that allows for this, and my XB-40s were closer to hand.
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Winslow
Check out my blog and other goodies at winslowyerxa.com
Harmonica For Dummies, Second Edition with tons of new stuff
Join us in 2016 for SPAH on the San Antonio River Walk!
Last Edited by WinslowYerxa on Sep 27, 2015 12:20 PM
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mlefree
436 posts
Sep 28, 2015
11:12 AM
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I see.
I understand the idea of keeping the pitch constant in your tests. But since the Johnston article dealt only with single and double reed chambers, I wonder if the findings of his article can be applied to your XB-40 results. That's why I asked about using a regular diatonic. Are the 3rd and 4th responder reeds in an XB-40 inactive in high blow bends?
I don't know if it would be even possible -- definitely not with ~my~ bending skills -- to try to hit the sweet spot from the high side on a regular diatonic harmonica. I know that if Roberts could have done it with his tubular experimental apparatus he didn't mention it in his article.
Do you think it would be even remotely humanly possible to take, say, a really low keyed diatonic and try to approach a high blow bend sweet spot pitch from above? Or is that effectively what your XB-40 test showed?
Again Many Thanks for your input here!
Michelle
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SilverWing Leather - Custom leather creations for musicians and other eccentrics.
Last Edited by mlefree on Sep 28, 2015 11:16 AM
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arzajac
1685 posts
Sep 28, 2015
2:44 PM
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"Conclusion: sliding the tongue backwards for draw bends and forward for blow bends is an artifact. Once pitch is eliminated as a factor, breath direction neither indicates a difference in chamber size for the bend nor any requirement to approach it from a particular direction. This is true for both single-reed and dual-reed bends."
I think I have something to add. To summarize Winslow's conclusion: it doesn't matter if you start off with a slightly bigger air pocket and shrink it down to the size required for resonance or start with a slightly smaller one and grow it to size - once you hit the correct size, resonance happens (and reeds follow by adjusting their pitch).
The diatonic sounds weak unless we develop the proper embouchure. That means we adjust our mouths to resonate at all the pitches we play - bent and unbent. That's one big difference between a player with good acoustic tone and one who doesn't (maybe a beginner...)
Part of developing a proper embouchure is to keep that size constant (resonant) through a wide range of flow and pressure changes. We are contracting muscles to keep the air chamber open all the time. It so happens we need to pull the chamber open when playing draw notes and we need to squeeze it closed on blow notes.
Johnson's tube was of variable volume, but it was rigid. Our vocal tract is made of soft tissue.
When we play draw notes, negative pressure in our vocal tract drawns in air.
When we play blow notes, positive pressure causes air to flow out.
The negative pressure during draw notes also pulls in the soft tissues (cheeks, for example). The resonant chamber in our mouths wants to get smaller. The easiest way to counteract that effect is to exert slight muscular contraction to maintain tone and provide a solid foundation for the air pocket. We pull the air pocket (more) open. (We don't think about this - like a reflex.)
So it's only natural that the easiest way to create a different size pocket is to contract a little more while playing draw notes. We zero-in on the correct size from smaller-to-bigger.
When we play blow notes, the air pocket wants to get bigger. We contract different muscles to keep the air pocket size down. Again, if we want to make a different sized pocket, we find it easier to do it using the muscles we are familiar with while playing blow notes and create the pocket from bigger-to-smaller.
"In both cases, the pitch goes down, but on blow notes you make your air chamber smaller, and on draw notes you make the chamber bigger. "
Correct. That's what comes natural. That's your weapon of choice.
"Opposite changes in chamber size, same downward movement in pitch"
Actually, it's not the case that the chamber needs to be bigger to draw bend or that the air chamber needs to be smaller for blow bends but it's that we prefer to create the air pocket that way because our muscles are already familiar with the activity.
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Last Edited by arzajac on Sep 28, 2015 2:47 PM
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